Secure Shell (SSH)
was originally developed by Tatu Ylцnen, Finland, as a secure replacement for Telnet
and rsh. Essentially he re-implemented rsh over secure channel. Later the author
introduced some restrictions and the previous freely licensed version of the project
was forked by FreeBSD team to what now became OpenSSH. which become now dominant
SSH implementation. See History. rlogin,
rcp, rsh. It is based on so called
Public-Key Cryptography
Optional compression of traffic is provided.
SSH can use many authentication schemes such as SecurID, Kerberos, S/KEY to provide
a highly secure remote access point to UNIX servers. By default, the OpenSSH server
listens for requests on port 22 and port 6010 for X11 forwarding.
SSH1 was the the first version (protocol v1.2 and v1.5) that was free in the
earlier days, but licensing has become more restrictive and
SSH Communications and
DataFellows are
trying to get people to move to the newer SSH2 (which is commercial).
OpenSSH has been produced by the OpenBSD team and community. It was first integrated
in OpenBSD in 1999. Linux got it much later as a present from OpenBSD community.
OpenSSH intended as a drop-in replacement for the Berkeley "r"-tools (rsh, rlogin,
rcp). It can also tunnel via encrypted tunnel X windows and other TCP/IP application
level protocols.
tar cvzf - . | rsh xxx.xxx.xxx.xxx "( cd $dir; tar xzf - )"
or
tar cvzf - . | ssh xxx.xxx.xxx.xxx "( cd $dir; tar xzf - )"
This is also possible to do in the reverse direction
ssh xxx.xxx.xxx.xxx "( cd $dir; tar cvzf - )" | tar xzf -
Components
Openssh is a leading SSH
implementation (by world famous OpenBSD team). It was first included into
OpenBSD 2.6. The software
was developed outside the USA, using code from roughly 10 countries, and is freely
useable and re-useable by everyone under a BSD license.
This command can store passphrase for
private RSA keys in memory, to respond to challenges (challenge response)
from the server. This simplifies repeated authentication (imitation
of no password authentication)
ssh-add
Tool which adds keys to ssh-agent
sftp
FTP-like program that works over SSH1
and SSH2 protocol
scp
File copy program that acts like rcp
ssh-keygen
This command generates RSA public and
private keys pair
ssh-keyscan
A utility for gathering the public ssh
host keys from a
number of SSH servers. The keys gathered are
displayed on the standard output. This output can then be compared with
the key in the file /etc/ssh/ssh_known_hosts and be included in the
file.
ssh-keysign
Utility for hostbased authentication
sshd
The daemon that permits you to login
sftp-server
SFTP server subsystem (started
automatically by sshd)
OpenSSH supports SSH protocol versions 1.3, 1.5, and 2.0. Only the latest versions
of v. 2.0 have required level of security. Earlier versions have serious security
vulnerabilities.
It's not without drawbacks: since SSH is encrypted, troubleshooting is quite
challenging. There are also multiple hazards that are inherent in the protocol of
that level complexity. Vulnerabilities with SSH, especially were one of the major
exploits that were used against ISPs. Beyond protocol problems, there are architecture
issues by allowing encrypted pipes. It defeats IDS mechanisms.
Secure Shell (SSH) is a rich subsystem used to log in to remote systems, copy
files, and tunnel through firewalls—securely. Since SSH is a subsystem, it offers
plenty of options to customize and streamline its operation. In fact, SSH provides
an entire "dot directory", named $HOME/.ssh, to contain all its data. (Your .ssh
directory must be mode 600 to preclude access by others. A mode other than 600 interferes
with proper operation.) Specifically, the file $HOME/.ssh/config can define lots
of shortcuts, including aliases for machine names, per-host access controls, and
more.
Here is a typical block found in $HOME/.ssh/config to customize SSH for a specific
host:
Host worker
HostName worker.example.com
IdentityFile ~/.ssh/id_rsa_worker
User joeuser
Each block in ~/.ssh/config configures one or more hosts. Separate individual
blocks with a blank line. This block uses four options: Host, HostName,
IdentityFile, and User. Host establishes a nickname for
the machine specified by HostName. A nickname allows you to type ssh
worker instead of ssh worker.example.com. Moreover, the IdentityFile
and User options dictate how to log in to worker. The former option
points to a private key to use with the host; the latter option provides the login
ID. Thus, this block is the equivalent of the command:
A powerful but little-known option is ControlMaster.
If set, multiple SSH sessions to the same host share a single connection. Once the first connection is established, credentials are not required
for subsequent connections, eliminating the drudgery of typing a password each and
every time you connect to the same machine. ControlMaster is so handy,
you'll likely want to enable it for every machine. That's accomplished easily enough
with the host wildcard, *:
Host *
ControlMaster auto
ControlPath ~/.ssh/master-%r@%h:%p
>
As you might guess, a block tagged Host * applies to every host, even those
not explicitly named in the config file. ControlMaster auto tries to reuse
an existing connection but will create a new connection if a shared connection cannot
be found. ControlPath points to a file to persist a control socket for
sharing. %r is replaced by the remote login user name, %h is replaced
by the target host name, and %p stands in for the port used for the connection.
(You can also use %l; it is replaced with the local host name.) The specification
above creates control sockets with file names akin to:
Each control socket is removed when all connections to the remote host are severed.
If you want to know which machines you are connected to at any time, simply type
ls ~/.ssh and look at the host name portion of the control socket (%h).
The SSH configuration file is so expansive, it too has its own man page. Type
man ssh_config to see all possible options. And here's a clever SSH trick:
You can tunnel from a local system to a remote one via SSH. The command line to
use looks something like this:
$ ssh example.com -L 5000:localhost:3306
>
This command says, "Connect via example.com and establish a tunnel between port
5000 on the local machine and port 3306 [the MySQL server port] on the machine named
'localhost.'" Because localhost is interpreted on example.com as the tunnel
is established, localhost is example.com. With the outbound tunnel—formally
called a local forward—established, local clients can connect to port 5000
and talk to the MySQL server running on example.com.
This is the general form of tunneling:
$ ssh proxyhostlocalport:targethost:targetport
Here, proxyhost is a machine you can access via SSH and one that
has a network connection (not via SSH) to targethost. localport
is a non-privileged port (any unused port above 1024) on your local system, and
targetport is the port of the service you want to connect to.
The previous command tunnels out from your machine to the outside world.
You can also use SSH to tunnel in, or connect to your local system from the
outside world. This is the general form of an inbound tunnel:
When establishing an inbound tunnel—formally called a remote forward—the
roles of proxyhost and targethost are reversed:
The target is your local machine, and the proxy is the remote machine. user
is your login on the proxy. This command provides a concrete example:
The command reads, "Connect to example.com as joe, and connect the remote port
8080 to local port 80." This command gives users on example.com a tunnel to Joe's
machine. A remote user can connect to 8080 to hit the Web server on Joe's machine.
In addition to -L and -R for local and remote forwards, respectively,
SSH offers -D to create an HTTP proxy on a remote machine. See the SSH
man page for the proper syntax.
Connecting and transferring files to remote systems is something system administrators do
all the time. One essential tool used by many system administrators on Linux platforms is SSH.
SSH supports two forms of authentication:
Password authentication
Public-key Authentication
Public-key authentication is considered the most secure form of these two methods, though
password authentication is the most popular and easiest. However, with password authentication,
the user is always asked to enter the password. This repetition is tedious. Furthermore, SSH
also requires manual intervention when used in a shell script. If automation is needed when
using SSH password authentication, then a simple tool called sshpass is
indispensable.
What is sshpass?
The sshpass utility is designed to run SSH using the
keyboard-interactive password authentication mode, but in a non-interactive way.
SSH uses direct TTY access to ensure that the password is indeed issued by an interactive
keyboard user. sshpass runs SSH in a dedicated TTY, fooling SSH into thinking it
is getting the password from an interactive user.
Install sshpass
You can install sshpass with this simple command:
# yum install sshpass
Use sshpass
Specify the command you want to run after the sshpass options. Typically, the
command is ssh with arguments, but it can also be any other command. The SSH
password prompt is, however, currently hardcoded into sshpass .
The synopsis for the sshpass command is described below:
-ppassword
The password is given on the command line.
-ffilename
The password is the first line of the file filename.
-dnumber
number is a file descriptor inherited by sshpass from the runner. The password is read from the open file descriptor.
-e
The password is taken from the environment variable "SSHPASS".
Examples
To better understand the value and use of sshpass , let's look at some examples
with several different utilities, including SSH, Rsync, Scp, and GPG.
Example 1: SSH
Use sshpass to log into a remote server by using SSH. Let's assume the password
is !4u2tryhack . Below are several ways to use the sshpass options.
A. Use the -p (this is considered the least secure choice and shouldn't be
used):
You can also use sshpass with a GPG-encrypted file. When the -f
switch is used, the reference file is in plaintext. Let's see how we can encrypt a file with
GPG and use it.
sshpass is a simple tool that can be of great help to sysadmins. This doesn't,
by any means, override the most secure form of SSH authentication, which is public-key
authentication. However, sshpass can also be added to the sysadmin toolbox.
"... For us, fail2ban uses iptables to ban the IP address of the offending system for a "bantime" of 600 seconds (10 minutes). ..."
"... You can, of course, change any of these settings to meet your needs. Ten minutes seems to be long enough to cause a bot or script to "move on" to less secure hosts. However, ten minutes isn't so long as to alienate users who mistype their passwords more than three times. ..."
Security, for system administrators, is an ongoing struggle because you must secure your systems enough to
protect them from unwanted attacks but not so much that user productivity is hindered. It's a difficult balance to
maintain. There are always complaints of "too much" security, but when a system is compromised, the complaints
range from, "There wasn't enough security" to "Why didn't you use better security controls?" The struggle is real.
There are controls you can put into place that are both effective against intruder attack and yet stealthy enough
to allow users to operate in a generally unfettered manner.
Fail2ban
is the
answer to protect services from brute force and other automated attacks.
Note:
Fail2ban can only be used to protect services that require username/password authentication.
For example, you can't protect ping with fail2ban.
In this article, I demonstrate how to protect the SSH daemon (SSHD) from a brute force attack. You can set up
filters, as
fail2ban
calls them, to protect almost every listening service on your system.
Installation and initial setup
Fortunately, there is a ready-to-install package for
fail2ban
that includes all dependencies, if
any, for your system.
Unless you have some sort of syntax problem in your
fail2ban
configuration, you won't see any
standard output messages.
Now to configure a few basic things in
fail2ban
to protect the system without it interfering with
itself. Copy the
/etc/fail2ban/jail.conf
file to
/etc/fail2ban/jail.local
.
The
jail.local
file is the configuration file of interest for us.
Open
/etc/fail2van/jail.local
in your favorite editor and make the following changes or check to
be sure these few parameters are set. Look for the setting
ignoreip
and add all IP addresses to this
line that must have access without the possibility of a lockout. By default, you should add the loopback address,
and all IP addresses local to the protected system.
ignoreip = 127.0.0.1/8 192.168.1.10 192.168.1.20
You can also add entire networks of IP addresses, but this takes away much of the protection that you wish to
engage
fail2ban
for. Keep it simple and local for now. Save the
jail.local
file and
restart the
fail2ban
service.
$ sudo systemctl restart fail2ban
You must restart
fail2ban
every time you make a configuration change.
Setting up a filtered service
A fresh install of
fail2ban
doesn't really do much for you. You have to set up so-called filters
for any service that you want to protect. Almost every Linux system must be accessible by SSH. There are some
circumstances where you would most certainly stop and disable SSHD to better secure your system, but I assume that
every Linux system allows SSH connections.
Passwords, as everyone knows, are not a good security solution. However, it is often the standard by which we
live. So, if user or administrative access is limited to SSH, then you should take steps to protect it. Using
fail2ban
to "watch" SSHD for failed access attempts with subsequent banning is a good start.
Note:
Before implementing any security control that might hinder a user's access to a system, inform
the users that this new control might lock them out of a system for ten minutes (or however long you decide) if
their failed login attempts exceed your threshold setting.
To set up filtered services, you must create a corresponding "jail" file under the
/etc/fail2ban/jail.d
directory. For SSHD, create a new file named
sshd.local
and enter service filtering instructions into
it.
Create the
[sshd]
heading and enter the setting you see above as a starting place. Most of the
settings are self-explanatory. For the two that might not be intuitively obvious, the "action" setting describes
the action you want
fail2ban
to take in the case of a violation. For us,
fail2ban
uses
iptables
to ban the IP address of the offending system for a "bantime" of 600 seconds (10 minutes).
You can, of course, change any of these settings to meet your needs. Ten minutes seems to be long enough to
cause a bot or script to "move on" to less secure hosts. However, ten minutes isn't so long as to alienate users
who mistype their passwords more than three times.
Once you're satisfied with the settings, restart the
fail2ban
service.
What banning looks like
On the protected system (192.168.1.83),
tail
the
/var/log/fail2ban.log
to see any
current ban actions.
2020-05-15 09:12:06,722 fail2ban.filter [25417]: INFO [sshd] Found 192.168.1.69 - 2020-05-15 09:12:06
2020-05-15 09:12:07,018 fail2ban.filter [25417]: INFO [sshd] Found 192.168.1.69 - 2020-05-15 09:12:07
2020-05-15 09:12:07,286 fail2ban.actions [25417]: NOTICE [sshd] Ban 192.168.1.69
2020-05-15 09:22:08,931 fail2ban.actions [25417]: NOTICE [sshd] Unban 192.168.1.69
You can see that the IP address 192.168.1.69 was banned at 09:12 and unbanned ten minutes later at 09:22.
On the remote system, 192.168.1.69, a ban action looks like the following:
You can see that I entered my password incorrectly three times before being banned. The banned user, unless
explicitly informed, won't know why they can no longer reach the target system. The
fail2ban
filter
performs a silent ban action. It gives no explanation to the remote user, nor is the user notified when the ban is
lifted.
Unbanning a system
It will inevitably happen that a system gets banned that needs to be quickly unbanned. In other words, you
can't or don't want to wait for the ban period to expire. The following command will immediately unban a system.
$ sudo fail2ban-client set sshd unbanip 192.168.1.69
You don't need to restart the fail2ban daemon after issuing this command.
Wrap up
That's basically how
fail2ban
works. You set up a filter, and when conditions are met, then the
remote system is banned. You can ban for longer periods of time, and you can set up multiple filters to protect
your system. Remember that
fail2ban
is a single solution and does not secure your system from other
vulnerabilities. A layered, multi-faceted approach to security is the strategy you want to pursue. No single
solution provides enough security.
You can find examples of other filters and some advanced
fail2ban
implementations described at
fail2ban.org
.
By default, the SSH client verifies the identity of the host to which it connects.
If the remote host key is unknown to your SSH client, you would be asked to accept it by
typing "yes" or "no".
This could cause a trouble when running from script that automatically connects to a remote
host over SSH protocol.
Cool Tip: Slow SSH login? Password prompt takes too long? You can easily remove the delay!
Read more
→
This article explains how to bypass this verification step by disabling host key checking
.
The Authenticity Of Host Can't Be Established
When you log into a remote host that you have never connected before, the remote host key is
most likely unknown to your SSH client, and you would be asked to confirm its fingerprint :
The authenticity of host ***** can't be established.
RSA key fingerprint is *****.
Are you sure you want to continue connecting (yes/no)?
If your answer
is 'yes', the SSH client continues login, and stores the host key locally in the file
~/.ssh/known_hosts .
If your answer is 'no', the connection will be terminated.
If you would like to bypass this verification step , you can set the "
StrictHostKeyChecking " option to " no " on the command line:
$ ssh -o "StrictHostKeyChecking=no" user@host
This option disables the prompt and automatically adds the host key to the
~/.ssh/known_hosts file.
Remote Host Identification Has Changed
However, even with " StrictHostKeyChecking=no ", you may be refused to connect with
the following warning message:
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
*****
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending key in /home/user/.ssh/known_hosts:1
RSA host key for ***** has changed and you have requested strict checking.
Host key verification failed.
If you are sure that it is harmless and the remote host key has been changed in a
legitimate way, you can skip the host key checking by sending the key to a null
known_hosts file:
Note: It is one thing to do this to allow a local IP address such as above 192.168.x.x
but it risky to do with a remote host etc.. I would probably just edit ~/.ssh/known_hosts
or wipe the file and start over if I am seeing the messages above.
I need to copy all the *.c files from local laptop named hostA to hostB including all directories. I am using the following scp
command but do not know how to exclude specific files (such as *.out): $ scp -r ~/projects/ user@hostB:/home/delta/projects/
How do I tell scp command to exclude particular file or directory at the Linux/Unix command line? One can use scp command to securely
copy files between hosts on a network. It uses ssh for data transfer and authentication purpose. Typical scp command syntax is as
follows: scp file1 user@host:/path/to/dest/ scp -r /path/to/source/ user@host:/path/to/dest/ scp [options] /dir/to/source/
user@host:/dir/to/dest/
Scp exclude files
I don't think so you can filter or exclude files when using scp command. However, there is a great workaround to exclude files
and copy it securely using ssh. This page explains how to filter or excludes files when using scp to copy a directory recursively.
-a : Recurse into directories i.e. copy all files and subdirectories. Also, turn on archive mode and all other
options (-rlptgoD)
-v : Verbose output
-e ssh : Use ssh for remote shell so everything gets encrypted
--exclude='*.out' : exclude files matching PATTERN e.g. *.out or *.c and so on.
Example of rsync command
In this example copy all file recursively from ~/virt/ directory but exclude all *.new files: $ rsync -av -e ssh --exclude='*.new' ~/virt/ root@centos7:/tmp
SSH tunneling or SSH port forwarding is a method of creating an encrypted SSH connection between a client
and a server machine through which services ports can be relayed.
SSH forwarding is useful for
transporting network data of services that uses an unencrypted protocol, such as VNC or
FTP
, accessing
geo-restricted content or bypassing intermediate firewalls. Basically, you can forward any TCP port and
tunnel the traffic over a secure SSH connection.
There are three types of SSH port forwarding:
Local Port Forwarding. - Forwards a connection from the client host to the SSH server host and
then to the destination host port.
Remote Port Forwarding. - Forwards a port from the server host to the client host and then to the
destination host port.
Dynamic Port Forwarding. - Creates SOCKS proxy server which allows communication across a range of
ports.
In this article, we will talk about how to set up local, remote, and dynamic encrypted SSH tunnels.
Local Port Forwarding
Local port forwarding allows you to forward a port on the local (ssh client) machine to a port on the
remote (ssh server) machine, which is then forwarded to a port on the destination machine.
In this type of forwarding the SSH client listens on a given port and tunnels any connection to that
port to the specified port on the remote SSH server, which then connects to a port on the destination
machine. The destination machine can be the remote SSH server or any other machine.
Local port forwarding is mostly used to connect to a remote service on an internal network such as a
database or VNC server.
In Linux, macOS and other Unix systems to create a local port forwarding pass the
-L
option to the
ssh
client:
[LOCAL_IP:]LOCAL_PORT
- The local machine ip and port number. When
LOCAL_IP
is omitted the ssh client binds on localhost.
DESTINATION:DESTINATION_PORT
- The IP or hostname and the port of the destination
machine.
[USER@]SERVER_IP
- The remote SSH user and server IP address.
You can use any port number greater than
1024
as a
LOCAL_PORT
. Ports numbers
less than
1024
are privileged ports and can be used only by root. If your SSH server is
listening on a
port other than 22
(the default) use the
-p [PORT_NUMBER]
option.
The destination hostname must be resolvable from the SSH server.
Let's say you have a MySQL database server running on machine
db001.host
on an internal
(private) network, on port 3306 which is accessible from the machine
pub001.host
and you
want to connect using your local machine
mysql
client to the database server. To do so you
can forward the connection like so:
Once you run the command, you'll be prompted to enter the remote SSH user password. After entering it,
you will be logged in to the remote server and the SSH tunnel will be established. It is a good idea to
set up an SSH key-based
authentication
and connect to the server without entering a password.
Now if you point your local machine database client to
127.0.0.1:3336
, the connection
will be forwarded to the
db001.host:3306
MySQL server through the
pub001.host
machine which will act as an intermediate server.
You can forward multiple ports to multiple destinations in a single ssh command. For example, you have
another MySQL database server running on machine
db002.host
and you want to connect to both
servers from your local client you would run:
To connect to the second server you would use
127.0.0.1:3337
.
When the destination host is the same as the SSH server instead of specifying the destination host IP
or hostname you can use
localhost
.
Say you need to connect to a remote machine through VNC which runs on the same server and it is not
accessible from the outside. The command you would use is:
The
-f
option tells the
ssh
command to run in the background and
-N
not to execute a remote command. We are using
localhost
because the VNC and the SSH server
are running on the same host.
If you are having trouble setting up tunneling check your remote SSH server configuration and make
sure
AllowTcpForwarding
is not set to
no
. By default, forwarding is allowed.
Remote Port Forwarding
Remote port forwarding is the opposite of local port forwarding. It allows you to forward a port on
the remote (ssh server) machine to a port on the local (ssh client) machine, which is then forwarded to a
port on the destination machine.
In this type of forwarding the SSH server listens on a given port and tunnels any connection to that
port to the specified port on the local SSH client, which then connects to a port on the destination
machine. The destination machine can be the local or any other machine.
In Linux, macOS and other Unix systems to create a remote port forwarding pass the
-R
option to the
ssh
client:
[REMOTE:]REMOTE_PORT
- The IP and the port number on the remote SSH server. An empty
REMOTE
means that the remote SSH server will bind on all interfaces.
DESTINATION:DESTINATION_PORT
- The IP or hostname and the port of the destination
machine.
[USER@]SERVER_IP
- The remote SSH user and server IP address.
Local port forwarding is mostly used to give access to an internal service to someone from the
outside.
Let's say you are developing a web application on your local machine and you want to show a preview to
your fellow developer. You do not have a public IP so the other developer can't access the application
via the Internet.
If you have access to a remote SSH server you can set up a remote port forwarding as follows:
The command above will make ssh server to listen on port
8080
and tunnel all traffic from
this port to your local machine on port
3000
.
Now your fellow developer can type
the_ssh_server_ip:8080
in his/her browser and preview
your awesome application.
If you are having trouble setting up remote port forwarding make sure
GatewayPorts
is set
to
yes
in the remote SSH server configuration.
Dynamic Port Forwarding
Dynamic port forwarding allows you to create a socket on the local (ssh client) machine which acts as
a SOCKS proxy server. When a client connects to this port the connection is forwarded to the remote (ssh
server) machine, which is then forwarded to a dynamic port on the destination machine.
This way, all the applications using the SOCKS proxy will connect to the SSH server and the server
will forward all the traffic to its actual destination.
In Linux, macOS and other Unix systems to create a dynamic port forwarding (SOCKS) pass the
-D
option to the
ssh
client:
ssh -R [LOCAL_IP:]LOCAL_PORT [USER@]SSH_SERVER
The options used are as follows:
[LOCAL_IP:]LOCAL_PORT
- The local machine ip and port number. When
LOCAL_IP
is omitted the ssh client binds on localhost.
[USER@]SERVER_IP
- The remote SSH user and server IP address.
A typical example of a dynamic port forwarding is to tunnel the web browser traffic through an SSH
server.
The following command will create a SOCKS tunnel on port
9090
:
Once the tunneling is established you can configure your application to use it.
This article
explains how to configure Firefox and Google Chrome browser to use the SOCKS proxy.
The port forwarding has to be separately configured for each application that you want to tunnel the
traffic thought it.
Set up SSH Tunneling in Windows
Windows users can create SSH tunnels using the PuTTY SSH client. You can download PuTTY
here
.
Launch Putty and enter the SSH server IP Address in the
Host name (or IP address)
field.
<img alt="" src=/post/how-to-setup-ssh-tunneling/launch-putty.jpg>
Under the
Connection
menu, expand
SSH
and select
Tunnels
.
Check the
Local
radio button to setup local,
Remote
for remote, and
Dynamic
for dynamic port forwarding.
If setting up local forwarding enter the local forwarding port in the
Source Port
field and in
Destination
enter the destination host and IP, for example,
localhost:5901
.
For remote port forwarding enter the remote SSH server forwarding port in the
Source Port
field and in
Destination
enter the destination host and IP, for example,
localhost:3000
.
If setting up dynamic forwarding enter only the local SOCKS port in the
Source Port
field.
Click on the
Add
button as shown in the image below.
<img alt="" src=/post/how-to-setup-ssh-tunneling/add-tunnel-putty.jpg>
Go back to the
Session
page to save the settings so that you do not need to enter
them each time. Enter the session name in the
Saved Session
field and click on the
Save
button.
<img alt="" src=/post/how-to-setup-ssh-tunneling/save-session-putty.jpg>
Select the saved session and log in to the remote server by clicking on the
Open
button.
<img alt="" src=/post/how-to-setup-ssh-tunneling/open-session-putty.jpg>
A new window asking for your username and password will show up. Once you enter your username and
password you will be logged in to your server and the SSH tunnel will be started.
Setting up
public
key authentication
will allow you to connect to your server without entering a password.
Conclusion
We have shown you how to set up SSH tunnels and forward the traffic through a secure SSH connection.
For ease of use, you can define the SSH tunnel in your
SSH config file
or create a
Bash alias
that will set up the SSH
tunnel.
If you hit a problem or have feedback, leave a comment below.
"... A classic scenario is connecting from your desktop or laptop from inside your company's internal network, which is highly secured with firewalls to a DMZ. In order to easily manage a server in a DMZ, you may access it via a jump host . ..."
A jump host (also known as a jump server ) is an intermediary host or an SSH gateway
to a remote network, through which a connection can be made to another host in a dissimilar
security zone, for example a demilitarized zone ( DMZ ). It bridges two dissimilar security
zones and offers controlled access between them.
A jump host should be highly secured and monitored especially when it spans a private
network and a DMZ with servers providing services to users on the internet.
A classic scenario is connecting from your desktop or laptop from inside your company's
internal network, which is highly secured with firewalls to a DMZ. In order to easily manage a
server in a DMZ, you may access it via a jump host .
In this article, we will demonstrate how to access a remote Linux server via a jump host and
also we will configure necessary settings in your per-user SSH client configurations.
Consider the following scenario.
SSH Jump Host
In above scenario, you want to connect to HOST 2 , but you have to go through HOST 1 ,
because of firewalling, routing and access privileges. There is a number of valid reasons why
jumphosts are needed..
Dynamic Jumphost List
The simplest way to connect to a target server via a jump host is using the -J
flag from the command line. This tells ssh to make a connection to the jump host and then
establish a TCP forwarding to the target server, from there (make sure you've Passwordless
SSH Login between machines).
$ ssh -J host1 host2
If usernames or ports on machines differ, specify them on the terminal as shown.
$ ssh -J username@host1:port username@host2:port
Multiple Jumphosts List
The same syntax can be used to make jumps over multiple servers.
Static jumphost list means, that you know the jumphost or jumphosts that you need to connect
a machine. Therefore you need to add the following static jumphost 'routing' in
~/.ssh/config file and specify the host aliases as shown.
### First jumphost. Directly reachable
Host vps1
HostName vps1.example.org
### Host to jump to via jumphost1.example.org
Host contabo
HostName contabo.example.org
ProxyJump contabo
Now try to connect to a target server via a jump host as shown.
$ ssh -J vps1 contabo
Login to Target Host via Jumphost
The second method is to use the ProxyCommand option to add the jumphost configuration in
your ~.ssh/config or $HOME/.ssh/config file as shown.
In this example, the target host is contabo and the jumphost is vps1 .
Host vps1
HostName vps1.example.org
IdentityFile ~/.ssh/vps1.pem
User ec2-user
Host contabo
HostName contabo.example.org
IdentityFile ~/.ssh/contabovps
Port 22
User admin
Proxy Command ssh -q -W %h:%p vps1
Where the command Proxy Command ssh -q -W %h:%p vps1 , means run ssh in quiet
mode (using -q ) and in stdio forwarding (using -W ) mode, redirect
the connection through an intermediate host ( vps1 ).
Then try to access your target host as shown.
$ ssh contabo
The above command will first open an ssh connection to vps1 in the background effected by
the ProxyCommand , and there after, start the ssh session to the target server contabo .
That's all for now! In this article, we have demonstrated how to access a remote server via
a jump host. Use the feedback form below to ask any questions or share your thoughts with
us.
Normally, you would forward a remote computer's X11 graphical display to your local computer
with the -X option, but the OpenSSH application places additional security limits on such
connections as a precaution. As long as you're starting a shell on a trusted machine, you can
use the -Y option to opt out of the excess security:
$ ssh -Y 93.184.216.34
Now you can launch an instance of any one of the remote computer's applications, but have it
appear on your screen. For instance, try launching the Nautilus file manager:
remote$ nautilus &
The result is a Nautilus file manager window on your screen, displaying files on the remote
computer. Your user can't see the window you're seeing, but at least you have graphical access
to what they are using. Through this, you can debug, modify settings, or perform actions that
are otherwise unavailable through a normal text-based SSH session.
Keep in mind, though, that a forwarded X11 session does not bring the whole remote session
to you. You don't have access to the target computer's audio playback, for example, though you
can make the remote system play audio through its speakers. You also can't access any custom
application themes on the target computer, and so on (at least, not without some skillful
redirection of environment variables).
However, if you only need to view files or use an application that you don't have access to
locally, forwarding X can be invaluable.
Learn to configure SSH port forwarding on your Linux system. Remote forwarding is also explained.
Regular Linux users know about
SSH
, as it is basically what allows them to connect to any server remotely to be able to manage it via command
line. However, this is not the only thing SSH can provide you for, it can also act as a great security tool to
encrypt your connections even when there is no encryption by default.
For example, let's say you have a remote Linux desktop that you wish to connect via
SMTP
or email but the firewall on that network currently blocks the SMTP port (25) which is very common. Through
a SSH tunnel you would simply connect to that particular SMTP service using another port by simply using SSH without
having to reconfigure SMTP configuration to a different port and on top of that, gaining the encryption capabilities
of SSH.
Configure OpenSSH for port forwarding
In order for
OpenSSH
Server to allow forwarding, you have to make sure it is active in the configuration. To do this, you must
edit your
/etc/ssh/ssh_config
file.
For Ubuntu 18.04 this file has changed a little bit so, you must un-comment one line in it:
By
default this line comes commented, you need to un-comment to allow forwarding
Once un-commented, you need to restart the SSH service to apply the changes:
restart
SSH Daemon to apply changes recently done in its configuration
Now that we have our target configured to allow SSH forwarding, we simply need to re-route things through a port
we know is not blocked. Let's use a very uncommonly blocked port like 3300:
So now we have done this, all traffic that comes to port 25 will automatically sent over to port 3300. From
another computer or client we simply will connect to this server to its port 3300 and we will then be able to
interact with it as it was SMTP server without any firewall restrictions to its 25 port, basically we simply
re-routed its port 25 traffic to another (non blocked) one to be able to access it.
We talked about forwarding a local port to another port, but let's say you want to do it exactly opposite: you
want to route a remote port or something you currently can access from the server to a local port.
To explain it easily, let's use an example similar to the previous one: from this server you access a particular
server through port 25 (SMTP) and you want to "share" that through a local port 3302 so anyone else can connect to
your server to the 3302 port and see whatever that server sees on port 25:
Summing up and some tips on SSH port forwarding
As you can see, this SSH forwarding acts like a very small VPN, because it routes things to given ports. Whenever
you execute these commands, they will open SSH shells, as it understands you need to interact to the server via SSH.
If you don't need this, it will be enough to simply add the "-N" option in them, so they will simply not open any
shell.
Liked the article? Please share it and help us grow :)
About
Helder
Systems Engineer, technology evangelist, Ubuntu user, Linux enthusiast, father and husband.
I have a Ubuntu 12.04 server I bought, if I connect with putty using ssh and a sudoer user
putty gets disconnected by the server after some time if I am idle How do I configure Ubuntu to keep this connection alive indefinitely?
No, it's the time between keepalives. If you set it to 0, no keepalives are sent but you want
putty to send keepalives to keep the connection alive. – das Keks
Feb 19 at 11:46
In addition to the answer from "das Keks" there is at least one other aspect that can affect
this behavior. Bash (usually the default shell on Ubuntu) has a value TMOUT
which governs (decimal value in seconds) after which time an idle shell session will time out
and the user will be logged out, leading to a disconnect in an SSH session.
In addition I would strongly recommend that you do something else entirely. Set up
byobu (or even just tmux alone as it's superior to GNU
screen ) and always log in and attach to a preexisting session (that's GNU
screen and tmux terminology). This way even if you get forcibly
disconnected - let's face it, a power outage or network interruption can always happen - you
can always resume your work where you left. And that works across different machines. So you
can connect to the same session from another machine (e.g. from home). The possibilities are
manifold and it's a true productivity booster. And not to forget, terminal multiplexers
overcome one of the big disadvantages of PuTTY: no tabbed interface. Now you get "tabs" in
the form of windows and panes inside GNU screen and tmux .
apt-get install tmux
apt-get install byobu
Byobu is a nice frontend to both terminal multiplexers, but tmux is so
comfortable that in my opinion it obsoletes byobu to a large extent. So my
recommendation would be tmux .
Also search for "dotfiles", in particular tmux.conf and
.tmux.conf on the web for many good customizations to get you started.
Change the default value for "Seconds between keepalives(0 to turn off)" : from 0 to
600 (10 minutes) --This varies...reduce if 10 minutes doesn't help
Check the "Enable TCP_keepalives (SO_KEEPALIVE option)" check box.
Finally save setting for session
,
I keep my PuTTY sessions alive by monitoring the cron logs
tail -f /var/log/cron
I want the PuTTY session alive because I'm proxying through socks.
The simpler alternative is to route your local network traffic with an encrypted SOCKS proxy tunnel. This
way, all your applications using the proxy will connect to the SSH server and the server will forward all
the traffic to its actual destination. Your ISP (internet service provider) and other third parties will not
be able to inspect your traffic and block your access to websites.
This tutorial will walk you through the process of creating an encrypted SSH tunnel and configuring
Firefox and
Google Chrome
web browsers to use SOCKS proxy.
Prerequisites
Server running any flavor of Linux, with SSH access to route your traffic through it.
Web browser.
SSH client.
Set up the SSH tunnel
We'll create an SSH tunnel that will securely forwards traffic from your local machine on port
9090
to the SSH server on port
22
. You can use any port number greater than
1024
.
Linux and macOS
If you run Linux, macOS or any other Unix-based operating system on your local machine, you can easily
start an SSH tunnel with the following command:
ssh -N -D 9090 [USER]@[SERVER_IP]
The options used are as follows:
-N
- Tells SSH not to execute a remote command.
-D 9090
- Opens a SOCKS tunnel on the specified port number.
[USER]@[SERVER_IP]
- Your remote SSH user and server IP address.
To run the command in the background use the
-f
option.
If your SSH server is listening on a port other than port 22 (the default) use the
-p
[PORT_NUMBER]
option.
Once you run the command, you'll be prompted to enter your user password. After entering it, you will be
logged in to your server and the SSH tunnel will be established.
Windows users can create an SSH tunnel using the PuTTY SSH client. You can download PuTTY
here
.
Launch Putty and enter your server IP Address in the
Host name (or IP address)
field.
Under the
Connection
menu, expand
SSH
and select
Tunnels
.
Enter the port
9090
in the
Source Port
field , and check the
Dynamic
radio button.
Click on the
Add
button as shown in the image bellow.
Go back to the
Session
page to save the settings so that you do not need to enter them
each time. Enter the session name in the
Saved Session
field and click on the
Save
button.
Select the saved session and login to the remote server by clicking on the
Open
button.
A new window asking for your username and password will show up. Once you enter your username and
password you will be logged in to your server and the SSH tunnel will be started.
Configuring Your Browser to Use Proxy
Now that you have open the SSH SOCKS tunnel the last step is to configure your preferred browser to use
it.
Firefox
The steps bellow are the same for Windows, macOS, and Linux.
In the upper right hand corner, click on the hamburger icon
☰
to open Firefox's menu:
Click on the
⚙ Preferences
link.
Scroll down to the
Network Settings
section and click on the
Settings...
button.
A new window will open.
Select the
Manual proxy configuration
radio button.
Enter
127.0.0.1
in the
SOCKS Host
field and
9090
in the
Port
field.
Check the
Proxy DNS when using SOCKS v5
checkbox.
Click on the
OK
button to save the settings.
At this point your Firefox is configured and you can browse the Internet thought your SSH tunnel. To
verify it you can open
google.com
, type "what is my ip" and you should see your server IP
address.
To revert back to the default settings go to
Network Settings
, select the
Use system
proxy settings
radio button and save the settings.
There are also several plugins that can help you to configure Firefox's proxy settings such as
FoxyProxy
.
Google Chrome
Google Chrome uses the default system proxy settings. Instead of changing your operating system proxy
settings you can either use an addon such as
SwitchyOmega
or start Chrome web browser from the command line.
To launch Chrome using a new profile and your SSH tunnel use the following command:
The above command kicks off the SSH Key installation process for users. The -o option
instructs ssh-keygen to store the private key in the new OpenSSH format instead of the old (and
more compatible PEM format). It is highly recommended to use the -o option as the new OpenSSH
format has an increased resistance to brute-force password cracking. In case the -o option does
not work on your server (it has been introduced in 2014) or you need a private key in the old
PEM format, then use the command ' ssh-keygen -b 4096 -t rsa '.
The -b option of the ssh-keygen command is used to set the key length to 4096 bit instead of
the default 1024 bit for security reasons.
Upon entering the primary Gen Key command, users need to go through the following drill by
answering the following prompts:
Enter the file where you wish to save the key (/home/demo/.ssh/id_rsa)
Users need to press ENTER in order to save the file to the user home
The next prompt would read as follows:
Enter passphrase
If, as an administrator, you wish to assign the passphrase, you may do so when prompted (as
per the question above), though this is optional, and you may leave the field vacant in case
you do not wish to assign a passphrase.
However, it is pertinent to note there that keying in a unique passphrase does offer a bevy
of benefits listed below:
1. The security of a key, even when highly encrypted, depends largely on its invisibility to
any other party. I 2. In the likely instance of a passphrase-secure private key falling into
the custody of an unauthorized user, they will be rendered unable to log in to its allied
accounts until they can crack the passphrase. This invariably gives the victim (the hacked
user) precious extra time to avert the hacking bid On the downside, assigning a passphrase to
the key requires you to key it in every time you make use of the Key Pair, which makes the
process a tad tedious, nonetheless absolutely failsafe.
Here is a broad outline of the end-to-end key generation process:
root@server1:~# ssh-keygen -b 4096 -o -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Created directory '/root/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:KBZP/guc7lND8I239zKv8PRziF/5jnA6N0nEocCDlLA root@server1
The key's randomart image is:
+---[RSA 2048]----+
| .o.+ |
| ..o + . |
| . Eo o o o . |
| = .+ o . o |
| o +.S. . . |
| . o oo . . . .|
| +.....o+.+.|
| ... . +==Boo|
| .o.. +O==o|
+----[SHA256]-----+
The public key can now be traced to the link ~/.ssh/id_rsa.pub
The private key (identification) can now be traced to the link-/home/demo/.ssh/id_rsa
3
Step Two: Copying the Public Key
Once the distinct key pair has been generated, the next step remains to place the public key
on the virtual server that we intend to use. Users would be able to copy the public key into
the authorized_keys file of the new machine using the ssh-copy-id command. Given below is the
prescribed format (strictly an example) for keying in the username and IP address, and must be
replaced with actual system values:
Either of the above commands, when used, shall toss the following message on your
system:
The authenticity of host '192.168.0.100 ' can't be established. RSA key fingerprint is
b1:2d:32:67:ce:35:4d:5f:13:a8:cd:c0:c4:48:86:12. Are you sure you want to continue connecting
(yes/no)? yes Warning: Permanently added '192.168.0.100' (RSA) to the list of known hosts.
[email protected]'s password: Now try logging into the machine, with "ssh
'[email protected]'", and check in: ~/.ssh/authorized_keys to make sure we haven't added extra
keys that you weren't expecting.
After the above drill, users are ready to go ahead and log into [email protected] without
being prompted for a password. However, if you have earlier assigned a passphrase to the key
(as per Step 2 above), you will be prompted to enter the passphrase at this point (and each
time for subsequent log-ins.).
Step Three (This Step is Optional): Disabling the Password
to Facilitate Root Login
After users have copied their SSH keys unto your server and ensured seamless log-in with the
SSH keys only, they have the option to restrict the root login, and permit the same only
through SSH keys. To accomplish this, users need to access the SSH configuration file using the
following command:
sudo nano /etc/ssh/sshd_config
Once the file is accessed, users need to find the line within the file that includes
PermitRootLogin , and modify the same to ensure a foolproof connection using the SSH key. The
following command shall help you do that:
PermitRootLogin without-password
The last step in the process remains to implement the changes by using the following
command:
reload ssh
The above completes the process of installing SSH keys on the Linux server.
Converting
OpenSSH private key to new format
Most older OpenSSH keys are stored in the PEM format. While this format is compatible with
many older applications, it has the drawback that the password of a password-protected private
key can be attacked with brute-force attacks. This chapter explains how to convert a private
key in PEM format to one in the new OpenSSH format.
ssh-keygen -p -o -f /root/.ssh/id_rsa
The path /root/.ssh/id_rsa is the path of the old private key
file.
Conclusion
The above steps shall help you install SSH keys on any virtual private server in a
completely safe, secure and hassle-free manner.
If you're a Linux system administrator, chances are you've got more than one machine that
you're responsible for on a daily basis. You may even have a bank of machines that you maintain
that are similar -- a farm of Web servers, for example. If you have a need to type the same
command into several machines at once, you can login to each one with SSH and do it serially,
or you can save yourself a lot of time and effort and use a tool like ClusterSSH.
ClusterSSH is a Tk/Perl wrapper around standard Linux tools like XTerm and SSH. As such,
it'll run on just about any POSIX-compliant OS where the libraries exist -- I've run it on
Linux, Solaris, and Mac OS X. It requires the Perl libraries Tk ( perl-tk on
Debian or Ubuntu) and X11::Protocol ( libx11-protocol-perl on Debian or Ubuntu),
in addition to xterm and OpenSSH.
Installation
Installing ClusterSSH on a Debian or Ubuntu system is trivial -- a simple sudo apt-get
install clusterssh will install it and its dependencies. It is also packaged for use
with Fedora, and it is installable via the ports system on FreeBSD. There's also a MacPorts
version for use with Mac OS X, if you use an Apple machine. Of course, it can also be compiled
from source.
Configuration
ClusterSSH can be configured either via its global configuration file --
/etc/clusters , or via a file in the user's home directory called
.csshrc . I tend to favor the user-level configuration as that lets multiple
people on the same system to setup their ClusterSSH client as they choose. Configuration is
straightforward in either case, as the file format is the same. ClusterSSH defines a "cluster"
as a group of machines that you'd like to control via one interface. With that in mind, you
enumerate your clusters at the top of the file in a "clusters" block, and then you describe
each cluster in a separate section below.
For example, let's say I've got two clusters, each consisting of two machines. "Cluster1"
has the machines "Test1" and "Test2" in it, and "Cluster2" has the machines "Test3" and "Test4"
in it. The ~.csshrc (or /etc/clusters ) control file would look like
this:
clusters = cluster1 cluster2
cluster1 = test1 test2
cluster2 = test3 test4
You can also make meta-clusters -- clusters that refer to clusters. If you wanted to make a
cluster called "all" that encompassed all the machines, you could define it two ways. First,
you could simply create a cluster that held all the machines, like the following:
By calling out the "all" cluster as containing cluster1 and cluster2, if either of those
clusters ever change, the change is automatically captured so you don't have to update the
"all" definition. This will save you time and headache if your .csshrc file ever grows in
size.
Using ClusterSSH
Using ClusterSSH is similar to launching SSH by itself. Simply running cssh -l
<username> <clustername> will launch ClusterSSH and log you in as the
desired user on that cluster. In the figure below, you can see I've logged into "cluster1" as
myself. The small window labeled "CSSH [2]" is the Cluster SSH console window. Anything I type
into that small window gets echoed to all the machines in the cluster -- in this case, machines
"test1" and "test2". In a pinch, you can also login to machines that aren't in your .csshrc
file, simply by running cssh -l <username> <machinename1>
<machinename2> <machinename3> .
If I want to send something to one of the terminals, I can simply switch focus by clicking
in the desired XTerm, and just type in that window like I usually would. ClusterSSH has a few
menu items that really help when dealing with a mix of machines. As per the figure below, in
the "Hosts" menu of the ClusterSSH console there's are several options that come in handy.
"Retile Windows" does just that if you've manually resized or moved something. "Add host(s)
or Cluster(s)" is great if you want to add another set of machines or another cluster to the
running ClusterSSH session. Finally, you'll see each host listed at the bottom of the "Hosts"
menu. By checking or unchecking the boxes next to each hostname, you can select which hosts the
ClusterSSH console will echo commands to. This is handy if you want to exclude a host or two
for a one-off or particular reason. The final menu option that's nice to have is under the
"Send" menu, called "Hostname". This simply echoes each machine's hostname to the command line,
which can be handy if you're constructing something host-specific across your cluster.
Caveats with ClusterSSH
Like many UNIX tools, ClusterSSH has the potential to go horribly awry if you aren't
very careful with its use. I've seen ClusterSSH mistakes take out an entire tier of
Web servers simply by propagating a typo in an Apache configuration. Having access to multiple
machines at once, possibly as a privileged user, means mistakes come at a great cost. Take
care, and double-check what you're doing before you punch that Enter key.
Conclusion
ClusterSSH isn't a replacement for having a configuration management system or any of the
other best practices when managing a number of machines. However, if you need to do something
in a pinch outside of your usual toolset or process, or if you're doing prototype work,
ClusterSSH is indispensable. It can save a lot of time when doing tasks that need to be done on
more than one machine, but like any power tool, it can cause a lot of damage if used
haphazardly.
SSH is one of the most widely used protocols for connecting to remote shells. While there are numerous SSH clients the most-used
still remains OpenSSH's ssh . OpenSSH has been the default ssh client for every major Linux operation, and is trusted by
cloud computing providers such as
Amazon's EC2 services and web hosting companies like
MediaTemple . There is a plethora of tips and tricks that can be used to make
your experience even better than it already is. Read on to discover some of the best tweaks to your favorite SSH client.
Adding
A Keep-Alive
A keep-alive is a small piece of data transmitted between a client and a server to ensure that the connection is still open or
to keep the connection open. Many protocols implement this as a way of cleaning up dead connections to the server. If a client does
not respond, the connection is closed.
SSH does not enable this by default. There are pros and cons to this. A major pro is that under a lot of conditions if you disconnect
from the Internet, your connection will be usable when you reconnect. For those who drop out of WiFi a lot, this is a major plus
when you discover you don't need to login again.
For those who get the following message from their SSH client when they stop typing for a few minutes it's not as convenient:
symkat@symkat:~$ Read from remote host symkat.com: Connection reset by peer
Connection to symkat.com closed.
This happens because your router or firewall is trying to clean up dead connections. It's seeing that no data has been transmitted
in N seconds and falsely assumes that the connection is no longer in use.
To rectify this you can add a Keep-Alive. This will ensure that your connection stays open to the server and the firewall doesn't
close it.
To make all connections from your shell send a keepalive add the following to your ~/.ssh/config file:
KeepAlive yes
ServerAliveInterval 60
The con is that if your connection drops and a KeepAlive packet is sent SSH will disconnect you. If that becomes a problem, you
can always actually fix the Internet connection.
Multiplexing Your Connection
Do you make a lot of connections to the same servers? You may not have noticed how slow an initial connection to a shell is. If
you multiplex your connection you will definitely notice it though. Let's test the difference between a multiplexed connection using
SSH keys and a non-multiplexed connection using SSH keys:
# Without multiplexing enabled:
$ time ssh [email protected] uptime
20:47:42 up 16 days, 1:13, 3 users, load average: 0.00, 0.01, 0.00
real 0m1.215s
user 0m0.031s
sys 0m0.008s
# With multiplexing enabled:
$ time ssh [email protected] uptime
20:48:43 up 16 days, 1:14, 4 users, load average: 0.00, 0.00, 0.00
real 0m0.174s
user 0m0.003s
sys 0m0.004s
We can see that multiplexing the connection is much faster, in this instance on an order of 7 times faster than not multiplexing
the connection. Multiplexing allows us to have a â??controlâ? connection, which is your initial connection to a server, this is then
turned into a UNIX socket file on your computer. All subsequent connections will use that socket to connect to the remote host. This
allows us to save time by not requiring all the initial encryption, key exchanges, and negotiations for subsequent connections to
the server.
Host *
ControlMaster auto
ControlPath ~/.ssh/connections/%r_%h_%p
A negative to this is that some uses of ssh may fail to work with your multiplexed connection. Most notably commands which use
tunneling like git, svn or rsync, or forwarding a port. For these you can add the option -oControlMaster=no . To prevent
a specific host from using a multiplexed connection add the following to your ~/.ssh/config file:
Host YOUR_SERVER_OR_IP
MasterControl no
There are security precautions that one should take with this approach. Let's take a look at what actually happens when we connect
a second connection:
$ ssh -v -i /dev/null [email protected]
OpenSSH_4.7p1, OpenSSL 0.9.7l 28 Sep 2006
debug1: Reading configuration data /Users/symkat/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: Applying options for *
debug1: auto-mux: Trying existing master
Last login:
symkat@symkat:~$ exit
As we see no actual authentication took place. This poses a significant security risk if running it from a host that is not trusted,
as a user who can read and write to the socket can easily make the connection without having to supply a password. Take the same
care to secure the sockets as you take in protecting a private key.
Using SSH As A Proxy
Even Starbucks now has free WiFi in its stores. It seems the world has caught on to giving free Internet at most retail locations.
The downside is that more teenagers with "Got Root?" stickers are camping out at these locations running the latest version of wireshark.
SSH's encryption can stand up to most any hostile network, but what about web traffic?
Most web browsers, and certainly all the popular ones, support using a proxy to tunnel your traffic. SSH can provide a SOCKS proxy
on localhost that tunnels to your remote server with the -D option. You get all the encryption of SSH for your web traffic,
and can rest assured no one will be capturing your login credentials to all those non-ssl websites you're using.
Now there is a proxy running on 127.0.0.1:1080 that can be used in a web browser or email client. Any application that supports
SOCKS 4 or 5 proxies can use 127.0.0.1:1080 to tunnel its traffic.
$ nc -vvv 127.0.0.1 1080
Connection to 127.0.0.1 1080 port [tcp/socks] succeeded!
Using One-Off Commands
Often times you may want only a single piece of information from a remote host. "Is the file system full?" "What's the uptime
on the server?" "Who is logged in?"
Normally you would need to login, type the command, see the output and then type exit (or Control-D for those in the know.) There
is a better way: combine the ssh with the command you want to execute and get your result:
This executed the ssh symkat.com, logged in as symkat, and ran the command uptime on symkat. If you're not using
SSH keys then you'll be presented with a password prompt before the command is executed.
$ ssh [email protected] ps aux | echo $HOSTNAME
symkats-macbook-pro.local
This executed the command ps aux on symkat.com, sent the output to STDOUT, a pipe on my local laptop picked it up
to execute echo $HOSTNAME locally. Although in most situations using auxiliary data processing like grep
or awk will work flawlessly, there are many situations where you need your pipes and file IO redirects to work on the
remote system instead of the local system. In that case you would want to wrap the command in single quotes:
As a basic rule if you're using >>><- or | you're going to
want to wrap in single quotes.
It is also worth noting that in using this method of executing a command some programs will not work. Notably anything that requires
a terminal, such as screen, irssi, less, or a plethora of other interactive or curses based applications. To force a terminal to
be allocated you can use the -t option:
Pipes are useful. The concept is simple: take the output from one program's STDOUT and feed it to another program's STDIN. OpenSSH
can be used as a pipe into a remote system. Let's say that we would like to transfer a directory structure from one machine to another.
The directory structure has a lot of files and sub directories.
We could make a tarball of the directory on our own server and scp it over. If the file system this directory is on lacks the
space though we may be better off piping the tarballed content to the remote system.
What we did in this example was to create a new archive ( -c ) and to compress the archive with gzip ( -z
). Because we did not use -f to tell it to output to a file, the compressed archive was send to STDOUT. We then piped
STDOUT with | to ssh . We used a one-off command in ssh to invoke tar with the extract ( -x
) and gzip compressed ( -z ) arguments. This read the compressed archive from the originating server and unpacked it
into our server. We then logged in to see the listing of files.
Additionally, we can pipe in the other direction as well. Take for example a situation where you with to make a copy of a remote
database, into a local database:
symkat@chard:~$ echo "create database backup" | mysql -uroot -ppassword
symkat@chard:~$ ssh [email protected] 'mysqldump -udbuser -ppassword symkat' | \
> mysql -uroot -ppassword backup
symkat@chard:~$ echo "use backup;select count(*) from wp_links;" | mysql -uroot -ppassword
count(*)
12
symkat@chard:~$
What we did here is to create the database backup on our local machine. Once we had the database created we used a one-off
command to get a dump of the database from symkat.com. The SQL Dump came through STDOUT and was piped to another command. We used
mysql to access the database, and read STDIN (which is where the data now is after piping it) to create the database on our local
machine. We then ran a MySQL command to ensure that there is data in the backup table. As we can see, SSH can provide a true pipe
in either direction.
Using a Non Standard Port
Many people run SSH on an alternate port for one reason or another. For instance, if outgoing port 22 is blocked at your college
or place of employment you may have ssh listen on port 443.
Instead of saying ssh -p443 [email protected] you can add a configuration option to your
~/.ssh/config file that is specific to yourserver.com:
Host yourserver.com
Port 443
You can extrapolate from this information further that you can make ssh configurations specific to a host. There is little reason
to use all those -oOptions when you have a well-written ~/.ssh/config file.
Good Article ! I would like to try to implement Two Factor authentication with Google Authenticator , steps can be followed
here
http://www.digitaljournal.sg
Yes, user@hostname is common in SSH lines. Although, you could say, "-l symkat
symkat.com
" (-l is username), [email protected] works just the same. Anything preceding the @ is the username to submit, and anything following
the @ is the hostname or IP address to connect to.
I'm quite surprised you didn't cover key based authentication.
My favorite trick for key-based authentication is having per-host keys, which gives you an extra layer of theoretical security
in the event your key is leaked.
1. If your public key is leaked, nasty people could ( in theory, but its unlikely ) give you permission to log into their machines
with said key, and then log your actions, which, are you not observant, could be an information leak. ( This is insane paranoia
really ).
2. If your *private* key is leaked, every machine you gave a copy of your public key to is now vulnerable. ( This is a much
more valid concern ).
Having per-host keys makes this much weaker in some respects, because if you have a per-host key, then stealing *a* key will
only give them access to *one* machine instead of several. However, in saying that, chances are, if they get in and steal *one*
key, if you have multiple, they can probably steal *every* key, meaning blocking all those accesses via key deletion becomes much
harder. I'm not sure which is the most sane option really, I still just like per-host keys =P.
Doing this is very similar to setting up per-host auto-master connections.
and then send a copy of [email protected] to the admin of
bar.com to
put in the 'foo' users "authorized keys" file.
It will then JustWork(TM).
And if you can't be arsed having to set up a seperate key for a given host, it tries the per-host one before using the general
key, so you can just send them your common .pub file instead =).
Another option that I like when signing on to remote hosts who's ip changes - like AWS - is to prevent ssh from doing strict
host key checking via
-o StrictHostKeyChecking=no
Here's mine: remote to local mysql backup in one line
ssh user@server "/usr/bin/mysqldump -u user -p password database" | dd of=/where/you/want/the/dump.sql
Here's another one I found useful... Redirect local STDOUT to a file on a remote server.
If in the example above I wanted to create a tar.gz file of contents on the remote machine:
tar -cz contents | ssh [email protected] "cat > contents.tar.gz"
Wow. You must have looked in the wrong place all that time, because it is right there in the manpage:
# man ssh_config
Specifies whether the system should send TCP keepalive messages to the other side. If they are sent, death of the connection
or crash of one of the machines will be properly noticed. This option only uses TCP keepalives (as opposed to using ssh level
keepalives), so takes a long time to notice when the connection dies. As such, you probably want the ServerAliveInterval option
as well. However, this means
that connections will die if the route is down temporarily, and some people find it annoying.
The default is "yes" (to send TCP keepalive messages), and the client will notice if the network goes down or the remote host
dies. This is important in scripts, and many users want it too.
To disable TCP keepalive messages, the value should be set to "no".
...Here's a list of 10 things that I think are
particularly awesome and perhaps a bit off the beaten path.
Update: ( 2011-09-19 ) There are some user-submitted ssh-tricks on the wiki now!
Please feel free to add your favorites. Also the hacker news thread might be helpful for
some.
SSH Config
I used SSH regularly for years before I learned about the config file, that you can create
at ~/.ssh/config to tell how you want ssh to behave.
Consider the following configuration example:
Host example.com *.example.net
User root
Host dev.example.net dev.example.net
User shared
Port 220
Host test.example.com
User root
UserKnownHostsFile /dev/null
StrictHostKeyChecking no
Host t
HostName test.example.org
Host *
Compression yes
CompressionLevel 7
Cipher blowfish
ServerAliveInterval 600
ControlMaster auto
ControlPath /tmp/ssh-%r@%h:%p
I'll cover some of the settings in the " Host * " block, which apply to all
outgoing ssh connections, in other items in this post, but basically you can use this to create
shortcuts with the ssh command, to control what username is used to connect to a given host,
what port number, if you need to connect to an ssh daemon running on a non-standard port. See "
man ssh_config " for more information. Control Master/Control Path
This is probably the coolest thing that I know about in SSH. Set the "
ControlMaster " and " ControlPath " as above in the ssh configuration.
Anytime you try to connect to a host that matches that configuration a "master session" is
created. Then, subsequent connections to the same host will reuse the same master connection
rather than attempt to renegotiate and create a separate connection. The result is greater
speed less overhead.
This can cause problems if you' want to do port forwarding, as this must be configured on
the original connection , otherwise it won't work. SSH Keys
While ControlMaster/ControlPath is the coolest thing you can do with SSH, key-based
authentication is probably my favorite. Basically, rather than force users to authenticate with
passwords, you can use a secure cryptographic method to gain (and grant) access to a system.
Deposit a public key on servers far
and wide, while keeping a "private" key secure on your local machine. And it just
works .
You can generate multiple keys, to make it more difficult for an intruder to gain access to
multiple machines by breaching a specific key, or machine. You can specify specific keys and
key files to be used when connected to specific hosts in the ssh config file (see above.) Keys
can also be (optionally) encrypted locally with a pass-code, for additional security. Once I
understood how secure the system is (or can be), I found my self thinking "I wish you could use
this for more than just SSH." SSH Agent
Most people start using SSH keys because they're easier and it means that you don't have to
enter a password every time that you want to connect to a host. But the truth is that in most
cases you want to have unencrypted private keys that have meaningful access to systems because
once someone has access to a copy of the private key the have full access to the system. That's
not good.
But the truth is that typing in passwords is a pain, so there's a solution: the
ssh-agent . Basically one authenticates to the ssh-agent locally, which
decrypts the key and does some magic, so that then whenever the key is needed for the
connecting to a host you don't have to enter your password. ssh-agent manages the
local encryption on your key for the current session.
SSH Reagent
I'm not sure where I found this amazing little function but it's great. Typically,
ssh-agents are attached to the current session, like the window manager, so that when
the window manager dies, the ssh-agent loses the decrypted bits from your ssh key.
That's nice, but it also means that if you have some processes that exist outside of your
window manager's session (e.g. Screen sessions) they loose the ssh-agent and get
trapped without access to an ssh-agent so you end up having to restart
would-be-persistent processes, or you have to run a large number of ssh-agents which
is not ideal.
Enter "ssh-reagent." stick this in your shell configuration (e.g. ~/.bashrc or
~/.zshrc ) and run ssh-reagent whenever you have an agent session running and
a terminal that can't see it.
ssh-reagent () {
for agent in /tmp/ssh-*/agent.*; do
export SSH_AUTH_SOCK=$agent
if ssh-add -l 2>&1 > /dev/null; then
echo Found working SSH Agent:
ssh-add -l
return
fi
done
echo Cannot find ssh agent - maybe you should reconnect and forward it?
}
It's magic.
SSHFS and SFTP
Typically we think of ssh as a way to run a command or get a prompt on a remote machine. But
SSH can do a lot more than that, and the OpenSSH package that probably the most
popular implementation of SSH these days has a lot of features that go beyond just "shell"
access. Here are two cool ones:
SSHFS creates a
mountable file system using FUSE of
the files located on a remote system over SSH. It's not always very fast, but it's
simple and works great for quick operations on local systems, where the speed issue is
much less relevant.
SFTP, replaces FTP (which is plagued by security problems,) with a similar tool for
transferring files between two systems that's secure (because it works over SSH) and is just as
easy to use. In fact most recent OpenSSH daemons provide SFTP access by default.
There's more, like a full VPN solution in recent versions, secure remote file copy, port
forwarding, and the list could go on. SSH Tunnels
SSH includes the ability to connect a port on your local system to a port on a remote
system, so that to applications on your local system the local port looks like a normal local
port, but when accessed the service running on the remote machine responds. All traffic is
really sent over ssh.
I set up an SSH tunnel for my local system to the outgoing mail server on my server. I tell
my mail client to send mail to localhost server (without mail server authentication!), and it
magically goes to my personal mail relay encrypted over ssh. The applications of this
are nearly endless.
Keep Alive Packets
The problem: unless you're doing something with SSH it doesn't send any packets, and as a
result the connections can be pretty resilient to network disturbances. That's not a problem,
but it does mean that unless you're actively using an SSH session, it can go silent causing
your local area network's NAT to eat a connection that it thinks has died, but hasn't. The
solution is to set the " ServerAliveInterval [seconds] " configuration in the SSH
configuration so that your ssh client sends a "dummy packet" on a regular interval so that the
router thinks that the connection is active even if it's particularly quiet. It's good stuff.
/dev/null .known_hosts
A lot of what I do in my day job involves deploying new systems, testing something out and
then destroying that installation and starting over in the same virtual machine. So my "test
rigs" have a few IP addresses, I can't readily deploy keys on these hosts, and every time I
redeploy SSH's host-key checking tells me that a different system is responding for the host,
which in most cases is the symptom of some sort of security error, and in most cases knowing
this is a good thing, but in some cases it can be very annoying.
These configuration values tell your SSH session to save keys to ` /dev/null (i.e.
drop them on the floor) and to not ask you to verify an unknown host:
UserKnownHostsFile /dev/null
StrictHostKeyChecking no
This probably saves me a little annoyance and minute or two every day or more, but it's
totally worth it. Don't set these values for hosts that you actually care about.
I'm sure there are other awesome things you can do with ssh, and I'd live to hear more . Onward and Upward!
Do you make a lot of connections to the same servers? You may not have noticed how slow an initial connection to a shell is. If
you multiplex your connection you will definitely notice it though. Let's test the difference between a multiplexed connection using
SSH keys and a non-multiplexed connection using SSH keys:
# Without multiplexing enabled:
$ time ssh [email protected] uptime
20:47:42 up 16 days, 1:13, 3 users, load average: 0.00, 0.01, 0.00
real 0m1.215s
user 0m0.031s
sys 0m0.008s
# With multiplexing enabled:
$ time ssh [email protected] uptime
20:48:43 up 16 days, 1:14, 4 users, load average: 0.00, 0.00, 0.00
real 0m0.174s
user 0m0.003s
sys 0m0.004s
We can see that multiplexing the connection is much faster, in this instance on an order of 7 times faster than not multiplexing
the connection. Multiplexing allows us to have a "control" connection, which is your initial connection to a server, this is then
turned into a UNIX socket file on your computer. All subsequent connections will use that socket to connect to the remote host. This
allows us to save time by not requiring all the initial encryption, key exchanges, and negotiations for subsequent connections to
the server.
Host *
ControlMaster auto
ControlPath ~/.ssh/connections/%r_%h_%p
A negative to this is that some uses of ssh may fail to work with your multiplexed connection. Most notably commands which use
tunneling like git, svn or rsync, or forwarding a port. For these you can add the option -oControlMaster=no. To prevent a specific
host from using a multiplexed connection add the following to your ~/.ssh/config file:
Host YOUR_SERVER_OR_IP
MasterControl no
There are security precautions that one should take with this approach. Let's take a look at what actually happens when we connect
a second connection:
$ ssh -v -i /dev/null [email protected]
OpenSSH_4.7p1, OpenSSL 0.9.7l 28 Sep 2006
debug1: Reading configuration data /Users/symkat/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: Applying options for *
debug1: auto-mux: Trying existing master
Last login:
symkat@symkat:~$ exit
As we see no actual authentication took place. This poses a significant security risk if running it from a host that is not trusted,
as a user who can read and write to the socket can easily make the connection without having to supply a password. Take the same
care to secure the sockets as you take in protecting a private key.
Using SSH As A Proxy
Even Starbucks now has free WiFi in its stores. It seems the world has caught on to giving free Internet at most retail locations.
The downside is that more teenagers with "Got Root?" stickers are camping out at these locations running the latest version of wireshark.
SSH's encryption can stand up to most any hostile network, but what about web traffic?
Most web browsers, and certainly all the popular ones, support using a proxy to tunnel your traffic. SSH can provide a SOCKS proxy
on localhost that tunnels to your remote server with the -D option. You get all the encryption of SSH for your web traffic, and can
rest assured no one will be capturing your login credentials to all those non-ssl websites you're using.
Now there is a proxy running on 127.0.0.1:1080 that can be used in a web browser or email client. Any application that supports
SOCKS 4 or 5 proxies can use 127.0.0.1:1080 to tunnel its traffic.
$ nc -vvv 127.0.0.1 1080
Connection to 127.0.0.1 1080 port [tcp/socks] succeeded!
Using One-Off Commands
Often times you may want only a single piece of information from a remote host. "Is the file system full?" "What's the uptime
on the server?" "Who is logged in?"
Normally you would need to login, type the command, see the output and then type exit (or Control-D for those in the know.) There
is a better way: combine the ssh with the command you want to execute and get your result:
This executed the ssh symkat.com, logged in as symkat, and ran the command "uptime" on symkat. If you're not using SSH keys then
you'll be presented with a password prompt before the command is executed.
$ ssh [email protected] ps aux | echo $HOSTNAME
symkats-macbook-pro.local
This executed the command ps aux on symkat.com, sent the output to STDOUT, a pipe on my local laptop picked it up to execute "echo
$HOSTNAME" locally. Although in most situations using auxiliary data processing like grep or awk will work flawlessly, there are
many situations where you need your pipes and file IO redirects to work on the remote system instead of the local system. In that
case you would want to wrap the command in single quotes:
As a basic rule if you're using > >> < - or | you're going to want to wrap in single quotes.
It is also worth noting that in using this method of executing a command some programs will not work. Notably anything that requires
a terminal, such as screen, irssi, less, or a plethora of other interactive or curses based applications. To force a terminal to
be allocated you can use the -t option:
Pipes are useful. The concept is simple: take the output from one program's STDOUT and feed it to another program's STDIN. OpenSSH
can be used as a pipe into a remote system. Let's say that we would like to transfer a directory structure from one machine to another.
The directory structure has a lot of files and sub directories.
We could make a tarball of the directory on our own server and scp it over. If the file system this directory is on lacks the
space though we may be better off piping the tarballed content to the remote system.
What we did in this example was to create a new archive (-c) and to compress the archive with gzip (-z). Because we did not use
-f to tell it to output to a file, the compressed archive was send to STDOUT. We then piped STDOUT with | to ssh. We used a one-off
command in ssh to invoke tar with the extract (-x) and gzip compressed (-z) arguments. This read the compressed archive from the
originating server and unpacked it into our server. We then logged in to see the listing of files.
Additionally, we can pipe in the other direction as well. Take for example a situation where you with to make a copy of a remote
database, into a local database:
symkat@chard:~$ echo "create database backup" | mysql -uroot -ppassword
symkat@chard:~$ ssh [email protected] 'mysqldump -udbuser -ppassword symkat' | mysql -uroot -ppassword backup
symkat@chard:~$ echo use backup;select count(*) from wp_links;" | mysql -uroot -ppassword
count(*)
12
symkat@chard:~$
What we did here is to create the database "backup" on our local machine. Once we had the database created we used a one-off command
to get a dump of the database from symkat.com. The SQL Dump came through STDOUT and was piped to another command. We used mysql to
access the database, and read STDIN (which is where the data now is after piping it) to create the database on our local machine.
We then ran a MySQL command to ensure that there is data in the backup table. As we can see, SSH can provide a true pipe in either
direction.
Do you make a lot of connections to the same servers? You may not have noticed how slow an initial connection to a shell is. If
you multiplex your connection you will definitely notice it though. Let's test the difference between a multiplexed connection using
SSH keys and a non-multiplexed connection using SSH keys:
# Without multiplexing enabled:
$ time ssh [email protected] uptime
20:47:42 up 16 days, 1:13, 3 users, load average: 0.00, 0.01, 0.00
real 0m1.215s
user 0m0.031s
sys 0m0.008s
# With multiplexing enabled:
$ time ssh [email protected] uptime
20:48:43 up 16 days, 1:14, 4 users, load average: 0.00, 0.00, 0.00
real 0m0.174s
user 0m0.003s
sys 0m0.004s
We can see that multiplexing the connection is much faster, in this instance on an order of 7 times faster than not multiplexing
the connection. Multiplexing allows us to have a "control" connection, which is your initial connection to a server, this is then
turned into a UNIX socket file on your computer. All subsequent connections will use that socket to connect to the remote host. This
allows us to save time by not requiring all the initial encryption, key exchanges, and negotiations for subsequent connections to
the server.
Host *
ControlMaster auto
ControlPath ~/.ssh/connections/%r_%h_%p
A negative to this is that some uses of ssh may fail to work with your multiplexed connection. Most notably commands which use
tunneling like git, svn or rsync, or forwarding a port. For these you can add the option -oControlMaster=no. To prevent a specific
host from using a multiplexed connection add the following to your ~/.ssh/config file:
Host YOUR_SERVER_OR_IP
MasterControl no
There are security precautions that one should take with this approach. Let's take a look at what actually happens when we connect
a second connection:
$ ssh -v -i /dev/null [email protected]
OpenSSH_4.7p1, OpenSSL 0.9.7l 28 Sep 2006
debug1: Reading configuration data /Users/symkat/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: Applying options for *
debug1: auto-mux: Trying existing master
Last login:
symkat@symkat:~$ exit
As we see no actual authentication took place. This poses a significant security risk if running it from a host that is not trusted,
as a user who can read and write to the socket can easily make the connection without having to supply a password. Take the same
care to secure the sockets as you take in protecting a private key.
Using SSH As A Proxy
Even Starbucks now has free WiFi in its stores. It seems the world has caught on to giving free Internet at most retail locations.
The downside is that more teenagers with "Got Root?" stickers are camping out at these locations running the latest version of wireshark.
SSH's encryption can stand up to most any hostile network, but what about web traffic?
Most web browsers, and certainly all the popular ones, support using a proxy to tunnel your traffic. SSH can provide a SOCKS proxy
on localhost that tunnels to your remote server with the -D option. You get all the encryption of SSH for your web traffic, and can
rest assured no one will be capturing your login credentials to all those non-ssl websites you're using.
Now there is a proxy running on 127.0.0.1:1080 that can be used in a web browser or email client. Any application that supports
SOCKS 4 or 5 proxies can use 127.0.0.1:1080 to tunnel its traffic.
$ nc -vvv 127.0.0.1 1080
Connection to 127.0.0.1 1080 port [tcp/socks] succeeded!
Using One-Off Commands
Often times you may want only a single piece of information from a remote host. "Is the file system full?" "What's the uptime
on the server?" "Who is logged in?"
Normally you would need to login, type the command, see the output and then type exit (or Control-D for those in the know.) There
is a better way: combine the ssh with the command you want to execute and get your result:
This executed the ssh symkat.com, logged in as symkat, and ran the command "uptime" on symkat. If you're not using SSH keys then
you'll be presented with a password prompt before the command is executed.
$ ssh [email protected] ps aux | echo $HOSTNAME
symkats-macbook-pro.local
This executed the command ps aux on symkat.com, sent the output to STDOUT, a pipe on my local laptop picked it up to execute "echo
$HOSTNAME" locally. Although in most situations using auxiliary data processing like grep or awk will work flawlessly, there are
many situations where you need your pipes and file IO redirects to work on the remote system instead of the local system. In that
case you would want to wrap the command in single quotes:
As a basic rule if you're using > >> < - or | you're going to want to wrap in single quotes.
It is also worth noting that in using this method of executing a command some programs will not work. Notably anything that requires
a terminal, such as screen, irssi, less, or a plethora of other interactive or curses based applications. To force a terminal to
be allocated you can use the -t option:
Pipes are useful. The concept is simple: take the output from one program's STDOUT and feed it to another program's STDIN. OpenSSH
can be used as a pipe into a remote system. Let's say that we would like to transfer a directory structure from one machine to another.
The directory structure has a lot of files and sub directories.
We could make a tarball of the directory on our own server and scp it over. If the file system this directory is on lacks the
space though we may be better off piping the tarballed content to the remote system.
What we did in this example was to create a new archive (-c) and to compress the archive with gzip (-z). Because we did not use
-f to tell it to output to a file, the compressed archive was send to STDOUT. We then piped STDOUT with | to ssh. We used a one-off
command in ssh to invoke tar with the extract (-x) and gzip compressed (-z) arguments. This read the compressed archive from the
originating server and unpacked it into our server. We then logged in to see the listing of files.
Additionally, we can pipe in the other direction as well. Take for example a situation where you with to make a copy of a remote
database, into a local database:
symkat@chard:~$ echo "create database backup" | mysql -uroot -ppassword
symkat@chard:~$ ssh [email protected] 'mysqldump -udbuser -ppassword symkat' | mysql -uroot -ppassword backup
symkat@chard:~$ echo use backup;select count(*) from wp_links;" | mysql -uroot -ppassword
count(*)
12
symkat@chard:~$
What we did here is to create the database "backup" on our local machine. Once we had the database created we used a one-off command
to get a dump of the database from symkat.com. The SQL Dump came through STDOUT and was piped to another command. We used mysql to
access the database, and read STDIN (which is where the data now is after piping it) to create the database on our local machine.
We then ran a MySQL command to ensure that there is data in the backup table. As we can see, SSH can provide a true pipe in either
direction.
"... Now, you need to supply this file and path to sshd daemon so that it can fetch this banner for each user login request. For that open /etc/sshd/sshd_config file and search for line #Banner none Here you have to edit file and write your filename and remove hash mark. It should look like : Banner /etc/login.warn ..."
"... Save file and restart sshd daemon. To avoid disconnecting existing connected users, use HUP signal to restart sshd. ..."
How to display message when user connects to system before login
This message will be displayed to user when he connects to server and before he logged in. Means
when he enter the username, this message will be displayed before password prompt.
You can use any filename and enter your message within. Here we used /etc/login.warn
file and put our messages inside.
Shell
# cat /etc/login.warn !!!! Welcome to KernelTalks test server !!!! This server is meant for testing
Linux commands and tools. If you are not associated with kerneltalks.com and not authorized please
dis-connect immediately.
Now, you need to supply this file and path to sshd daemon so that it can fetch this
banner for each user login request. For that open /etc/sshd/sshd_config file and search
for line #Banner none Here you have to edit file and write your filename and remove hash mark. It should look like :
Banner /etc/login.warn
Save file and restart sshd daemon. To avoid disconnecting existing connected users,
use HUP signal to restart sshd.
Port forwarding
using SSH
tunnels is a convenient way to circumvent well-intentioned firewall rules, or to access
resources on otherwise unaddressable networks, particularly those behind NAT (with addresses
such as 192.168.0.1 ).
However, it has a shortcoming in that it only allows us to address a specific host and port
on the remote end of the connection; if we forward a local port to machine A on the remote
subnet, we can't also reach machine B unless we forward another port. Fetching documents from a
single server therefore works just fine, but browsing multiple resources over the endpoint is a
hassle.
The proper way to do this, if possible, is to have a VPN connection into the appropriate
network, whether via a virtual interface or a network route through an IPsec tunnel. In cases
where this isn't possible or practicable, we can use a SOCKS proxy set up via an SSH connection to delegate
all kinds of network connections through a remote machine, using its exact network stack,
provided our client application supports it.
Being command-line junkies, we'll show how to set the tunnel up with ssh and to
retrieve resources on it via curl , but of course graphical browsers are
able to use SOCKS proxies as well.
As an added benefit, using this for browsing implicitly encrypts all of the traffic up to
the remote endpoint of the SSH connection, including the addresses of the machines you're
contacting; it's thus a useful way to protect unencrypted traffic from snoopers on your local
network, or to circumvent firewall policies.
Establishing the tunnel
First of all we'll make an SSH connection to the machine we'd like to act as a SOCKS proxy,
which has access to the network services that we don't. Perhaps it's the only publically
addressable machine in the network.
$ ssh -fN -D localhost:8001 remote.example.com
In this example, we're backgrounding the connection immediately with -f , and
explicitly saying we don't intend to run a command or shell with -N . We're only
interested in establishing the tunnel.
Of course, if you do want a shell as well, you can leave these options out:
$ ssh -D localhost:8001 remote.example.com
If the tunnel setup fails, check that AllowTcpForwarding is set to
yes in /etc/ssh/sshd_config on the remote machine:
AllowTcpForwarding yes
Note that in both cases we use localhost rather than 127.0.0.1 ,
in order to establish both IPv4 and IPv6 sockets if appropriate.
We can then check that the tunnel is established with ss on GNU/Linux:
# ss dst :8001
State Recv-Q Send-Q Local Address:Port Peer Address:Port
ESTAB 0 0 127.0.0.1:45666 127.0.0.1:8001
ESTAB 0 0 127.0.0.1:45656 127.0.0.1:8001
ESTAB 0 0 127.0.0.1:45654 127.0.0.1:8001
Requesting documents
Now that we have a SOCKS proxy running on the far end of the tunnel, we can use it to
retrieve documents from some of the servers that are otherwise inaccessible. For example, when
we were trying to run this from the client side, we found it wouldn't work:
This is because the example subnet is on a remote and unroutable LAN. If its
name comes from a private DNS server, we may not even be able to resolve its address, let alone
retrieve the document.
We can fix both problems with our local SOCKS proxy, by pointing curl to it
with its --proxy option:
$ curl --proxy socks5h://localhost:8001 http://private.example/contacts.html
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
<html>
<head>
<title>Contacts</title>
...
Older versions of curl may need to use the --socks5-hostname
option:
This not only tunnels our HTTP request through to remote.example.com and
returns any response, it does the DNS lookup on the other end too. This means we can not only
retrieve documents from remote servers, we can resolve their hostnames too, even if our client
side can't contact the appropriate DNS server on its own. This is what the h
suffix does in the socks5h:// URI syntax above.
We can configure graphical web browsers to use the SOCKS proxy in the same way, optionally
including DNS resolution:
Browsers are not the only application that can use SOCKS proxies; many IM clients such as
Pidgin and Bitlbee can use them too, for example.
Making things more
permanent
If this all works for you and you'd like to set up the SOCKS proxy on the far end each time
you connect, you can add it to your ssh_config file in
$HOME/.ssh/config :
If you've
dabbled with SSH much, for example by following the excellent suso.org tutorial a few years ago,
you'll know about adding keys to allow passwordless login (or, if you prefer, a passphrase)
using public key authentication. Specifically, you copy the public key
~/.ssh/id_rsa.pub or ~/.ssh/id_dsa.pub off the machine from which you
wish to connect into the /.ssh/authorized_keys file on the target machine. That
will allow you to open an SSH session with the machine from the user account on the local
machine to the one on the remote machine, without having to type in a password.
However, there's a nice shortcut that I didn't know about when I first learned how to do
this, which has since been added to that tutorial too -- specifically, the
ssh-copy-id tool, which is available in most modern OpenSSH distributions and
combines this all into one less error-prone step. If you have it available to you, it's
definitely a much better way to add authorized keys onto a remote machine.
tom@conan:~$ ssh-copy-id crom
Incidentally, this isn't just good for convenience or for automated processes; strong
security policies for publically accessible servers might disallow logging in via passwords
completely, as usernames and passwords can be guessed. It's a lot harder to guess an entire SSH
key, so forcing this login method will reduce your risk of script kiddies or automated attacks
brute-forcing your OpenSSH server to zero. You can arrange this by setting
ChallengeResponseAuthentication to no in your
sshd_config , but if that's a remote server, be careful not to lock yourself
out!
Quite apart from
replacing Telnet and other insecure protocols as the primary means of choice for contacting and
administrating services, the OpenSSH implementation of the SSH protocol has developed into a
general-purpose toolbox for all kinds of well-secured communication, whether using both simple
challenge-response authentication in the form of user and password logins, or for more complex
public key authentication.
SSH is useful in a general sense for tunneling pretty much any kind of TCP traffic, and
doing so securely and with appropriate authentication. This can be used both for ad-hoc
purposes such as talking to a process on a remote host that's only listening locally or within
a secured network, or for bypassing restrictive firewall rules, to more stable implementations
such as setting up a persistent SSH tunnel between two machines to ensure sensitive traffic
that might otherwise be sent in cleartext is not only encrypted but authenticated. I'll discuss
a couple of simple examples here, in addition to talking about the SSH escape sequences, about
which I don't seem to have seen very much information online.
SSH tunnelling for port
forwarding
Suppose you're at work or on a client site and you need some information off a webserver on
your network at home, perhaps a private wiki you run, or a bug tracker or version control
repository. This being private information, and your HTTP daemon perhaps not the most secure in
the world, the server only listens on its local address of 192.168.1.1 , and HTTP
traffic is not allowed through your firewall anyway. However, SSH traffic is, so all you need
to do is set up a tunnel to port forward a local port on your client machine to a local port on
the remote machine. Assuming your SSH-accessible firewall was listening on
firewall.yourdomain.com , one possible syntax would be:
If you then pointed your browser to localhost:5080 , your traffic would be
transparently tunnelled to your webserver by your firewall, and you could act more or less as
if you were actually at home on your office network with the webserver happily trusting all of
your requests. This will work as long as the SSH session is open, and there are means to
background it instead if you prefer -- see man ssh and look for the -f and
-N options. As you can see by the use of the 192.168.1.1 address
here, this also works through NAT.
This can work in reverse, too; if you need to be able to access a service on your local
network that might be behind a restrictive firewall from a remote machine, a perhaps
less typical but still useful case, you could set up a tunnel to listen for SSH connections on
the network you're on from your remote firewall:
As long as this TCP session stays active on the machine, you'll be able to point an SSH
client on your firewall to localhost on port 5022, and it will open an SSH session
as normal:
$ ssh localhost -p 5022
I have used this as an ad-hoc VPN back into a remote site when the established VPN system
was being replaced, and it worked very well. With appropriate settings for sshd ,
you can even allow other machines on that network to use the forward through the firewall, by
allowing GatewayPorts and providing a bind_address to the SSH
invocation. This is also in the manual.
SSH's practicality and transparency in this regard has meant it's quite typical for advanced
or particularly cautious administrators to make the SSH daemon the only process on appropriate
servers that listens on a network interface other than localhost , or as the
only port left open on a private network firewall, since an available SSH service proffers full
connectivity for any legitimate user with a basic knowledge of SSH tunnelling anyway. This has
the added bonus of transparent encryption when working on any sort of insecure network. This
would be a necessity, for example, if you needed to pass sensitive information to another
network while on a public WiFi network at a café or library; it's the same rationale for
using HTTPS rather than HTTP wherever possible on public networks.
Escape sequences
If you use these often, however, you'll probably find it's a bit inconvenient to be working
on a remote machine through an SSH session, and then have to start a new SSH session or restart
your current one just to forward a local port to some resource that you discovered you need on
the remote machine. Fortunately, the OpenSSH client provides a shortcut in the form of its
escape sequence, ~C .
Typed on its own at a fresh Bash prompt in an ssh session, before any other
character has been inserted or deleted, this will drop you to an ssh> prompt.
You can type ? and press Enter here to get a list of the commands available:
The syntax for the -L and -R commands is the same as when used as
a parameter for SSH. So to return to our earlier example, if you had an established SSH session
to the firewall of your local network, to forward a port you could drop to the
ssh> prompt and type -L5080:localhost:80 to get the same port
forward rule working.
Posted on For system and
network administrators or other users who frequently deal with sessions on multiple machines,
SSH ends up being one of the most oft-used Unix tools. SSH usually works so well that until you
use it for something slightly more complex than
starting a terminal session on a remote machine, you tend to use it fairly automatically.
However, the ~/.ssh/config file bears mentioning for a few ways it can make using
the ssh a client a little easier. Abbreviating hostnames
If you often have to SSH into a machine with a long host and/or network name, it can get
irritating to type it every time. For example, consider the following command:
$ ssh web0911.colo.sta.solutionnetworkgroup.com
If you interact with the web0911 machine a lot, you could include a stanza like
this in your ~/.ssh/config :
This would allow you to just type the following for the same result:
$ ssh web0911
Of course, if you have root access on the system, you could also do this by adding the
hostname to your /etc/hosts file, or by adding the domain to your
/etc/resolv.conf to search it, but I prefer the above solution as it's cleaner and
doesn't apply system-wide.
Fixing alternative ports
If any of the hosts with which you interact have SSH processes listening on alternative
ports, it can be a pain to both remember the port number and to type it in every time:
$ ssh webserver.example.com -p 5331
You can affix this port permanently into your .ssh/config file instead:
Host webserver.example.com
Port 5331
This will allow you to leave out the port definition when you call ssh on that
host:
$ ssh webserver.example.com
Custom identity files
If you have a private/public key setup working between your client machine and the server,
but for whatever reason you need to use a different key from your normal one, you'll be using
the -i flag to specify the key pair that should be used for the connection:
I need to do this for Mikrotik's RouterOS connections, as my own private key structure is
2048-bit RSA which RouterOS doesn't support, so I keep a DSA key as well just for that
purpose.
Logging in as a different user
By default, if you omit a username, SSH assumes the username on the remote machine is the
same as the local one, so for servers on which I'm called tom , I can just
type:
tom@conan:$ ssh server.network
However, on some machines I might be known as a different username, and hence need to
remember to connect with one of the following:
If I always connect as the same user, it makes sense to put that into my
.ssh/config instead, so I can leave it out of the command entirely:
Host server.anothernetwork
User tomryder
SSH proxies
If you have an SSH server that's only accessible to you via an SSH session on an
intermediate machine, which is a very common situation when dealing with remote networks using
private RFC1918 addresses through network address translation, you can automate that in
.ssh/config too. Say you can't reach the host nathost directly, but
you can reach some other SSH server on the same private subnet that is publically accessible,
publichost.example.com :
"... The ssh-agent program is designed as a wrapper for a shell. If you have a private and public key setup ready, and you have remote machines for which your key is authorised, you can get an idea of how the agent works by typing: ..."
Public key authentication has a lot of advantages for connecting to servers,
particularly if it's the only allowed means of authentication, reducing the chances of a brute
force password attack to zero. However, it doesn't solve the problem of having to type in a
password or passphrase on each connection, unless you're using a private key with no
passphrase, which is quite risky if the private key is compromised.
Thankfully, there's a nice supplement to a well-secured SSH key setup in the use of
agents on trusted boxes to securely store decrypted keys per-session, per-user.
Judicious use of an SSH agent program on a trusted machine allows you to connect to any server
for which your public key is authorised by typing your passphrase to decrypt your private key
only once.
SSH agent setup
The ssh-agent program is designed as a wrapper for a shell. If you have a
private and public key setup ready, and you have remote machines for which your key is
authorised, you can get an idea of how the agent works by typing:
$ ssh-agent bash
This will prompt you for your passphrase, and once entered, within the context of that
subshell, you will be able to connect to authorised remote servers without typing in the
passphrase again. Once loaded, you can examine the identities you have by using ssh-add
-l to see the fingerprints, and ssh-add -L for the public keys:
You can set up your .bashrc file to automatically search for accessible SSH
agents to use for the credentials for new connections, and to prompt you for a passphrase to
open a new one if it need be. There are very workable instructions on GitHub for
setting this up.
If you want to shut down the agent at any time, you can use ssh-agent -k .
Where the configuration of the remote machine allows it, you can forward
authentication requests made from the remote machine back to the agent on your workstation.
This is handy for working with semi-trusted gateway machines that you trust to forward your
authentication requests correctly, but on which you'd prefer not to put your private key.
This means that if you connect to a remote machine from your workstation running an SSH
agent with the following, using the -A parameter:
user@workstation:~$ ssh -A remote.example.com
You can then connect to another machine from remote.example.com using your
private key on workstation :
user@remote:~$ ssh another.example.com
SSH agent authentication via PAM
It's also possible to use SSH agent authentication as a PAM method for general
authentication, such as for sudo , using pam_ssh_agent_auth .
It may be the case
that while you're happy to allow a user or process to have public key authentication access to
your server via the ~/.ssh/authorized_keys file, you don't necessarily want to
give them a full shell, or you may want to restrict them from doing things like SSH port
forwarding or X11 forwarding.
One method that's supposed to prevent users from accessing a shell is by defining their
shell in /etc/passwd as /bin/false , which does indeed prevent them
from logging in with the usual ssh or ssh command syntax. This isn't
a good approach because it still allows port forwarding and other
SSH-enabled services.
If you want to restrict the use of logins with a public key, you can prepend option pairs to
its line in the authorized_keys file. Some of the most useful options here
include:
from="<hostname/ip>" -- Prepending from="*.example.com"
to the key line would only allow public-key authenticated login if the connection was coming
from some host with a reverse DNS of example.com . You can also put IP addresses
in here. This is particularly useful for setting up automated processes through keys with
null passphrases.
command="<command>" -- Means that once authenticated, the command
specified is run, and the connection is closed. Again, this is useful in automated setups for
running only a certain script on successful authentication, and nothing else.
no-agent-forwarding -- Prevents the key user from forwarding authentication
requests to an SSH agent on their client, using the -A or
ForwardAgent option to ssh .
no-port-forwarding -- Prevents the key user from forwarding ports using
-L and -R .
no-X11-forwarding -- Prevents the key user from forwarding X11
processes.
no-pty -- Prevents the key user from being allocated a tty
device at all.
So, for example, a public key that is only used to run a script called
runscript on the server by the client [email protected] :
A public key for a user whom you were happy to allow to log in from anywhere with a full
shell, but did not want to allow agent, port, or X11 forwarding:
Use of these options goes a long way to making your public key authentication setup harder
to exploit, and is very consistent with the principle of least privilege .
To see a complete list of the options available, check out the man page for sshd .
Occasionally you
may find yourself using a network behind a firewall that doesn't allow outgoing TCP connections
with a destination port of 22, meaning you're unable to connect to your OpenSSH server, perhaps
to take advantage of a SOCKS proxy for encrypted and
unfiltered web browsing.
Since these restricted networks almost always allow port 443 out, since it's the destination
port for outgoing HTTPS requests, an easy workaround is to have your OpenSSH server listen on
port 443 if it isn't already using the port.
This is sometimes given as a rationale for changing the sshd port completely,
but you don't need to do that; you can simply add another Port directive to
sshd_config(5) :
Port 22
Port 443
After restarting the OpenSSH server with this new line in place, you can verify that it's
listening with ss(8)
or netstat(8)
# ss -lnp src :22
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 :::22 :::*
users:(("sshd",3039,6))
LISTEN 0 128 *:22 *:*
users:(("sshd",3039,5))
# ss -lnp src :443
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 :::443 :::*
users:(("sshd",3039,4))
LISTEN 0 128 *:443 *:*
users:(("sshd",3039,3))
You'll then be able to connect to the server on port 443, the same way you would on port 22.
If you intend this setup to be permanent, it would be a good idea to save the configuration in your
ssh_config(5)
file, or whichever SSH client you happen to use. Posted in SSH Tagged additional ports , multiple ports ,
workaround
SSHGuard is an intrusion prevention
utility that parses logs and automatically blocks misbehaving IP addresses (or their subnets)
with the system firewall. SSHGuard version 2.1 was just released with new blocking services,
the ability to block a configurable-sized subnet, and better log reading capabilities.
For system and network administrators or other users who frequently deal with sessions on
multiple machines, SSH ends up being one of the most oft-used Unix tools. SSH usually works so
well that until you use it for something slightly more complex than
starting a terminal session on a remote machine, you tend to use it fairly automatically.
However, the ~/.ssh/config file bears mentioning for a few ways it can make using
the ssh a client a little easier. Abbreviating hostnames
If you often have to SSH into a machine with a long host and/or network name, it can get
irritating to type it every time. For example, consider the following command:
$ ssh web0911.colo.sta.solutionnetworkgroup.com
If you interact with the web0911 machine a lot, you could include a stanza like
this in your ~/.ssh/config :
This would allow you to just type the following for the same result:
$ ssh web0911
Of course, if you have root access on the system, you could also do this by adding the
hostname to your /etc/hosts file, or by adding the domain to your
/etc/resolv.conf to search it, but I prefer the above solution as it's cleaner and
doesn't apply system-wide.
Fixing alternative ports
If any of the hosts with which you interact have SSH processes listening on alternative
ports, it can be a pain to both remember the port number and to type it in every time:
$ ssh webserver.example.com -p 5331
You can affix this port permanently into your .ssh/config file instead:
Host webserver.example.com
Port 5331
This will allow you to leave out the port definition when you call ssh on that
host:
$ ssh webserver.example.com
Custom identity files
If you have a private/public key setup working between your client machine and the server,
but for whatever reason you need to use a different key from your normal one, you'll be using
the -i flag to specify the key pair that should be used for the connection:
I need to do this for Mikrotik's RouterOS connections, as my own private key structure is
2048-bit RSA which RouterOS doesn't support, so I keep a DSA key as well just for that
purpose.
Logging in as a different user
By default, if you omit a username, SSH assumes the username on the remote machine is the
same as the local one, so for servers on which I'm called tom , I can just
type:
tom@conan:$ ssh server.network
However, on some machines I might be known as a different username, and hence need to
remember to connect with one of the following:
If I always connect as the same user, it makes sense to put that into my
.ssh/config instead, so I can leave it out of the command entirely:
Host server.anothernetwork
User tomryder
SSH proxies
If you have an SSH server that's only accessible to you via an SSH session on an
intermediate machine, which is a very common situation when dealing with remote networks using
private RFC1918 addresses through network address translation, you can automate that in
.ssh/config too. Say you can't reach the host nathost directly, but
you can reach some other SSH server on the same private subnet that is publically accessible,
publichost.example.com :
As a Linux user, we use
ssh command
to log in to remote machines. The more you use ssh command, the more time you are wasting in typing
some significant commands. We can use either
alias
defined in your .bashrc file or functions to minimize the time you spend on CLI. But this is
not a better solution. The better solution is to use SSH-alias in ssh config file.
A couple of examples where we can better the ssh commands we use.
Connecting to ssh to AWS instance is a pain. Just to type below command, every time is complete
waste your time as well.
In this post, we will see how to achieve shorting of your ssh commands without using bash alias
or functions. The main advantage of ssh alias is that all your ssh command shortcuts are stored in
a single file and easy to maintain. The other advantage is we can use same alias for both SSH and
SCP commands alike
Before we jump into actual configurations, we should know difference between /etc/ssh/ssh_config,
/etc/ssh/sshd_config, and ~/.ssh/config files. Below is the explanation for these files.
System-level SSH configurations are stored in /etc/ssh/ssh_config. Whereas user-level ssh configurations
are stored in ~/.ssh/config file.
System-level SSH configurations are stored in /etc/ssh/ssh_config. Whereas system level SSH server
configurations are stored in /etc/ssh/sshd_config file.
... ... ...
Example1: Create SSH alias for a host(www.linuxnix.com)
Edit file ~/.ssh/config with following content
Host tlj
User root
HostName 18.197.176.13
port 22
... ... ...
Examaple5: Resolve SSH timeout issues in Linux. By default, your ssh logins are timed out if you
don't activily use the terminial.
SSH timeouts are one
more pain where you have to re-login to a remote machine after a certain time. We can set SSH time
out right in side your ~/.ssh/config file to make your session active for whatever time you want.
To achieve this we will use two SSH options for keeping the session alive. One ServerAliveInterval
keeps your session live for number of seconds and ServerAliveCountMax will initial session after
session for a given number.
ServerAliveInterval A
ServerAliveCountMax B
Example:
Host tlj linuxnix linuxnix.com
User root
HostName 18.197.176.13
port 22
ServerAliveInterval 60
ServerAliveCountMax 30
We will see some other exiting howto in our next post. Keep visiting linuxnix.com.
UPDATE: This was a good exercise but I decided to replace the script with denyhosts:
http://denyhosts.sourceforge.net/
. In CentOS, just
install the
EPEL
repo first,
then you can install it via yum.
This is one of the problems that my team encountered when we opened up a firewall for SSH
connections. Brute force SSH attacks using botnets are just everywhere! And if you're not
careful, it's quite a headache if one of your servers was compromised.
Lot of tips can be found in the Internet and this is the approach that I came up with based
on numerous sites that I've read.
strong passwords
DUH! This is obvious but most people ignore it. Don't be lazy.
disable root access through SSH
Most of the time, direct root access is not needed. Disabling it is highly recommended.
open
/etc/ssh/sshd_config
enable and set this SSH config to no:
PermitRootLogin no
restart SSH:
service sshd restart
limit users who can log-in through SSH
Users who can use the SSH service can be specified. Botnets often use user names that were
added by an application, so listing the users can lessen the vulnerability.
open
/etc/ssh/sshd_config
enable and list the users with this SSH config:
AllowUsers user1 user2
user3
restart SSH:
service sshd restart
use a script to automatically block malicious IPs
Utilizing SSH daemon's log file (in CentOS/RHEL, it's in /var/log/secure ), a simple script
can be written that can automatically block malicious IPs using tcp_wrapper's host.deny
If AllowUsers is enabled, the SSH daemon will log invalid attempts in this format:
sshd[8207]: User apache from 125.5.112.165 not allowed because not listed in AllowUsers
sshd[15398]: User ftp from 222.169.11.13 not allowed because not listed in AllowUsers
SSH also logs invalid attempts in this format: sshd[6419]: Failed password for invalid
user zabbix from 69.10.143.168 port 50962 ssh2 Based on the information above, I came up
with this script:
#!/bin/bash
# always exclude these IPs
exclude_ips='192.168.60.1|192.168.60.10'
file_log='/var/log/secure'
file_host_deny='/etc/hosts.deny'
tmp_list='/tmp/ips.for.restriction'
if [[ -e $tmp_list ]]
then
rm $tmp_list
fi
# set the separator to new lines only
IFS=$'\n'
# REGEX filter
filter="^$(date +%b\\s*%e).+(not listed in AllowUsers|\
Failed password.+invalid user)"
for ip in $( pcregrep $filter $file_log \
| perl -ne 'if (m/from\s+([^\s]+)\s+(not|port)/) { print $1,"\n"; }' )
do
if [[ $ip ]]
then
echo "ALL: $ip" >> $tmp_list
fi
done
# reset
unset IFS
cat $file_host_deny >> $tmp_list
sort -u $tmp_list | pcregrep -v $exclude_ips > $file_host_deny
I deployed the script in root's crontab and set it to run every minute
This page shows common problems experienced with SSH in general, and when establishing an
SSH tunnel
, and
solutions for each problem.
Tip: Most port-forwarding problems are caused by a basic misunderstanding of how an SSH
tunnel actually works, so it is highly recommended that you read the
SSH Tunnel
page before continuing.
Table of Contents
Connection Problems
Unable to open connection: Host does not
exist
Connection fails with the following error:
Unable to open connection:
Host does not exist
This error occurs when:
The server name cannot be resolved to an IP address. If it could be, a different error
would be displayed (such as Connection refused). Check the server exists and is reachable
using PING.
ping servername
Unable to open connection: gethostbyname: unknown error Connection fails with the
following error:
Unable to open connection:
gethostbyname: unknown error
Connection refused
Connection fails with the following error:
Failed to connect to 100.101.102.103: Network error: Connection refused
Network error: Connection refused
FATAL ERROR: Network error: Connection refused
This error occurs when:
The server name is incorrect. Verify the server exists and is running SSHD.
The port specified with the -P (PLINK/PuTTY) or -p (ssh) argument is incorrect. Verify
that the port is correct.
There is a firewall or other connection problem between the two servers. Try using telnet
to telnet to the server/port.
Failed to add the host to the list of known hosts (/home/USERNAME/.ssh/known_hosts)
Connection works, but the following warning is issued
Failed to add the host to the list of known hosts (/home/USERNAME/.ssh/known_hosts)
This error occurs when:
The user's HOME folder has incorrect permissions
The user's HOME/.ssh folder or HOME/.ssh/known_hosts file has incorrect permissions (such
as when the folder has been copied into location by root, or permissions have been manually
set incorrectly)
To fix, execute these commands (as root) to reset the permissions to their correct values
(replace USERNAME with the appropriate username)
Authentication Problems
When using a key, you are prompted for a password (instead of
automatically authenticating)
This can be caused be:
Providing a passphrase on the key. Verify that you have created the SSH key-pair with no
passphrase
Incorrect setup on the SSH server (key file or security not correctly configured). In
some cases, no error will appear in the SSHD logs on the server.
Incorrect key specified on the client. For example, specifying the public key in the
command line arguments instead of the private key.
Incorrect username specified for the key. For example, the key has been installed for
user "neale" but you are connecting as username "cassie".
Unable to use key file "keys\KEYNAME.ppk" (unable to open file)
This is caused by an inability to open the specified SSH key file.
Verify that the key file exists, and is really at the location you have specified with
the -i argument
Verify that the local user executing the PLINK/ssh command has permissions to read the
key file
Tunnel Problems / Port Forwarding Problems
Note that some of these errors will only appear if verbose-output (-v) is switched on for
the PLINK command or SSH commands. PuTTY hides them, but PLINK can be used with exactly the
same command line arguments, so test with PLINK and the -v command line option.
Forwarded
connection refused by server: Administratively prohibited [open failed], or channel N: open
failed: administratively prohibited: open failed
This error appears in the PLINK/PuTTY/ssh window when:
the port forwarding address does not exist (most common reason, normally a typo)
port-forwarding has been disabled server-wide in /etc/ssh/sshd_config using
AllowTcpForwarding no (default setting is yes)
port-forwarding is limited to specific hosts only (and the one you are connecting to is
not in the list), in the server-wide setting file /etc/ssh/sshd_config under the PermitOpen
option. Note that even if the host is allowed in permitopen in authorized_keys2 (see below),
it still needs to be allowed in sshd_config as well.
you are using a certificate-based connection and port-forwarding has been disabled in
/home/username/.ssh/authorized_keys2 with the option no-port-forwarding
you are using a certificate-based connection and port-forwarding is limited to specific
hosts only (and the one you are connecting to is not in the list), in the
/home/username/.ssh/authorized_keys2 file using the permitopen= option
a DNS problem on the server is preventing the host name from being resolved to an IP
address (error in DNS, or manual entry in /etc/hosts)
For example, you have tried to connect to servername.example.com using an SSH command line
argument such as:
-L 127.0.0.1:3500:servername.example.com:3506
However, servername.example.com does not exist, is not permitted, or cannot be resolved
correctly by the remote server. Unfortunately, the error message is quite vague, and always makes
it look like a security issue. Verify the server name is correct and try again, then check with
your administrator.
When this is the problem the following will appear in the SSH server logs (eg:
/var/log/auth.log in Linux):
Nov 28 17:00:57 server sshd[27850]: error: connect_to servername.example.com: unknown host (Name or service not known)
or
Aug 26 17:48:10 server sshd[24180]: Received request to connect to host servername.example.com port NNNN, but the request was denied.
Forwarded connection refused by server: Connect failed [Connection refused]
This error appears in the PLINK/PuTTY/ssh window, when you try to establish a connection to
the tunnel, and the server cannot connect to the remote port specified.
For example, you have specified that the tunnel goes to servername.example.com:3506 using an
SSH command line argument such as:
-L 127.0.0.1:3500:servername.example.com:3506
When you then try to telnet to 127.0.0.1:3500 on the client machine, this is tunnelled
through to the server, which then attempts to connect to servername.example.com:3506. However, that
that connection between the server and servername.example.com:3506 is refused.
Check the tunnel server:port is correct, or ensure that the server is able to connect to the
specified server:port.
Service lookup failed for destination port ""
This error appears in the PLINK/PuTTY/ssh window, if your tunnel definition is incomplete or
incorrect.
For example, the additional space after "3500:" in the following line will cause this
error:
line which causes error:
-L 127.0.0.1:3500: mysql5.metawerx.net:3506
correct line:
-L 127.0.0.1:3500:mysql5.metawerx.net:3506
Local port 127.0.0.1:nnnnn forwarding to nnn.nnn.nnn.nnn:nnnnn failed: Network error:
Permission denied
This error appears in the PLINK/PuTTY/ssh window, if your PuTTY client cannot listen on the
local port you have specified.
This normally occurs because of another service already running on that port.
For example, the tunnel below will fail if you have a local version of SQL/Server already
listening on port 1433:
-L 127.0.0.1:1433:sql2005-1.metawerx.net:1433
To fix, close the program that is listening on that port (ie: SQL/Server in the example
above).
Advanced: You can also adjust to tunnel from another port, such as 127.0.0.2:1433 or
127.0.0.1:1434. However, with SQL/Server, the Management Console application will only allow
connections to 1433. Additionally, it listens on 0.0.0.0:1433, preventing use of port 1433 on
any other IP address. Therefore, unless you first adjust the SQL/Server registry settings to
listen on a specific IP first, it is not possible to have SQL/Server running at the same time
as a local tunnel.
<some program>: not found
If you have connected successfully, but get errors when you try to enter commands at the
tunnel prompt, this is because you have access to the tunnel itself, but not to an SSH prompt
or any tools on the server. You should not be running these commands at the SSH prompt
itself.
Example errors:
createuser: not found
mysql: not found
If you were trying to establish an SSH tunnel, you have already accomplished this part. Your
tunnel should be listening on 127.0.0.1:<some port>. The commands you are trying to
execute should be performed in a new Command Prompt or Shell.
Remember - the tunnel is providing access to a remote service, on your local machine, as if
the server is your own computer.
You can therefore use any command line or GUI tools at your disposal, and connect directly
to 127.0.0.1:<whatever port>.
If you are confused about how this works, see the
SSH Tunnel
page for diagrams and a full
explanation.
Support Topics
page for examples of setting up remote database connections over SSH
Problem not found / not solved? Something to add?
If your problem is not solved by the above guide, please click Add Comment and specify
the error message or problem you are having. This will allow us improve this guide.
If you have helpful information to add, please feel free to add a comment or register so
that you can edit this page yourself.
Contributors:
Christopher Hollowell, John DeStefano There are a number of problems that can cause failures
when connecting to the RACF. Here are some things to look at and try in order to resolve your
problem.
Have you uploaded your public key to the RACF via the key file upload form
(requires your Kerberos user name and password)?
Are you connecting to our SSH gateway from the same system on which you generated
your key pair? If not, you will have to copy the private key to this additional system. If
this system is using a different SSH client, you may need to convert or import the private
key. See: https://www.racf.bnl.gov/docs/authentication/ssh/sshkeys
and click on "Using SSH keys" to help you out. Do not generate and upload another public key
from this additional system; uploading another public key will overwrite your existing public
key and create more problems. Even using different SSH clients on the same system may require
this conversion/import of the private key.
Are you asked for a passphrase when you connect? If not, then your client is not
using the private key for some reason. It could be the private key doesn't exist, is not in
the default filename, access rights of the file are incorrect, the file is not in a directory
the client is searching, or some other reason.
If you have uploaded another public key , then that key pair is the only usable
pair that will work, and all other pairs are now obsolete. Also, the private key from this
pair must be copied and possibly converted or imported to any other SSH clients you are
using.
Username Issues
If your username on your local system is different from your username at the RACF,
then you must specify your RACF username when you connect to the RACF, using the -l option to
the ssh commmand:
ssh -l [username] [RACF-hostname]
or prepending username@ to the SSH gateway system name (no space between the @ and the SSH
gateway system name):
ssh [username]@[RACF-hostname]
In Windows SSH clients, there is typically a text box in which you type in your
username.
Ownership/Access Rights Issues
If you are using a Linux/UNIX based SSH client, please check the ownership and access
rights of your ~/.ssh/ directory and
the private key file in that directory. Both must be owned by your local user account (not
necessarily the same as your
RACF user account). The rights on your ~/.ssh/ directory should be 700 , and the rights
on the private key file (possibly,
but not definitely, named ~/.ssh/id_rsa ), should be 600 . The important thing here is
that "group" and "other" access rights
must be 00 .
PuTTY Issues
If you are using PuTTY in
Windows , then you have to either import your private key , or somehow tell PuTTY
where the key file is.
In the main PuTTY Configuration, click on SSH and then Auth . The window will have a text
box where you can put the path
of the key or browse for it. See Windows SSH Key Generation
for more information on generating SSH keys for use with PuTTY.
You may also need to forward your private key through a remote gateway machine to another
server. See SSH Agent for more information
on storing and forwarding your private key.
Viewing Your Public Key
You can view the contents of the public key you uploaded to the RACF by directing
your Web client to: https://www.racf.bnl.gov/docs/authentication/ssh/sshkeys
and clicking on SSH Public Key File Viewing
Utility . You can check this against the public key that may be on your local
system (the public key is not required to be on your local system; the private key is required
to be there). If they
are not the same, then the private key on your local system may not paired with the public key
you uploaded to the RACF.
If you have both private and public keys on your local system, check the date/time stamps on
them, as they should be the same. If they are not the same, then the private key on your local
system may not be paired with the public key that you uploaded to the RACF. If you are using
the openssh client, then you
can also check to see if your local private key is paired to the public key that you uploaded
to the RACF. Run the command:
ssh-keygen -y
on your local system. It will ask for the filename of your private key and its passphrase
and will display the public
key (without the trailing comment field) that is paired with it. Check this against the results
of viewing the public key
you uploaded to the RACF as described above.
Frozen Sessions and Terminals
If your connection or session intermittently freezes, try adding a server keep-alive option
to your usual SSH command:
ssh ... -o ServerAliveInterval=120
This ensures that a set of request and acknowledgment packets will be sent between the
connection every two minutes, even when no other data has been requested. You can also add this
option to your SSH configuration file ( ~/.ssh/config ) instead of specifying it
with each SSH command:
ServerAliveInterval 120
Host Key Issues
Sometimes host key problems can close the ssh connection before login completes. If you see
an error like this:
ssh_exchange_identification: Connection closed by remote host
Then you might try removing the offending host key from your ~/.ssh/known_hosts
file and try again.
Error: Agent admitted failure to sign using the key
This error might occur if you accidentally load the wrong SSH identity for a specific key,
if you've uploaded a new public key that hasn't yet been synced with your account (or uploaded
multiple or invalid keys), or if you're trying to load too many SSH identities at one time.
Your best recourse is usually to:
I've helped a few people recently who have had trouble getting
OpenSSH
working properly; I've also had my share of issues over the
years. Generally problems with SSH connections fall into two groups - network related and
server related. Most of these problems can be fixed fairly quickly if you know what to look
for.
Network Related Problems
These will typically be caused by improper routing or firewall configurations. Here are some
things to check.
1. If your SSH server sits behind a firewall or router, make sure the default route of your
internal SSH server points back to that firewall or router. Seems obvious, but it's common to
forget about the return trip packets need to make. This will display your default
gateway:
netstat -rn | grep UG
Sometimes the default gateway is just one of your server interfaces, this is OK as long as
that interface is directly connected to something that knows how to get back to your
client.
2. While you're at it, make sure the incoming SSH packets are actually getting to your SSH
server.
Tcpdump
works
very nicely for this, you'll need to be root to run it on the server:
tcpdump -n -i
eth0 tcp port 22 and host [IP address of client]
Just replace eth0 by your client-facing interface name. If you don't see incoming SSH
packets during connection attempts, it's probably due to a firewall or router access
list.
SSH Server Problems
All of these issues revolve around SSH server configuration settings - not misconfigurations
necessarily, just settings you may not be aware of.
1. Permissions can be a problem - in its default configuration, OpenSSH sets StrictModes to
yes and won't allow any connections if the account you're trying to SSH into has group- or
world-writable permissions on its home directory, ~/.ssh directory, or ~/.ssh/authorized_keys
file. I typically just make the two directories mode 700 and the authorized_keys file mode 600.
The sshd man page suggests this one-liner:
chmod go-w ~/ ~/.ssh
~/.ssh/authorized_keys
2. On Debian or Ubuntu systems, it is possible the keys you are using to connect are
blacklisted. This is only an issue on Debian or Debian-based clients, and stems from this
now-famous vulnerability
in May of 2008
. To detect any such blacklisted keys, run ssh-vulnkey on the client, while
logged into the account you are connecting from. Debian and Ubuntu SSH servers will reject any
such keys unless the PermitBlacklistedKeys directive in the /etc/ssh/sshd_config file is set to
no . I don't recommend you actually leave this security check disabled, but it can be useful to
temporarily disable it during testing.
3. Finally, if all else fails, you can see exactly what the SSH server is doing by running
it in debug mode on a non-standard port:
/usr/sbin/sshd -d -p 2222
Then, on the client, connect and watch the server output:
ssh -vv -p 2222 [Server
IP]
Note the -vv option to provide verbose client output. This alone can sometimes help debug
connection issues (and try -vvv for even more output).
I have user
$USER
which is a system user account with an authorized users file.
When I have SELinux enabled I am unable to ssh into the server using the public key. If I
setenabled 0
,
$USER
can now log in.
What SELinux bool/policy should I change to correct this behaviour without disabling
SELinux entirely?
It's worth noting that
$USER
can login with a password under this default
SELinux configuration, I'd appreciate some insight as to what is happening here, and why
SELinux isn't blocking that. (I will be disabling
A:
Assuming the filesystem permissions are correct on ~/.ssh/*, then check the output of
sealert -a /var/log/audit/audit.log
There should be a clue in an AVC entry there. Most likely the solution will boil down to
running:
restorecon -R -v ~/.ssh
could successfully SSH into my machine yesterday with the exact same credentials I am
using today. The machine is running Centos 6.3 . But now for some reason it is giving me
permission denied. Here is my
-v
print out, sshd_config, and ssh_config
files.
$ ssh -vg -L 3333:localhost:6666 misfitred@devilsmilk
OpenSSH_6.1p1, OpenSSL 1.0.1c 10 May 2012
debug1: Reading configuration data /etc/ssh_config
debug1: Connecting to devilsmilk [10.0.10.113] port 22.
debug1: Connection established.
debug1: identity file /home/kgraves/.ssh/id_rsa type -1
debug1: identity file /home/kgraves/.ssh/id_rsa-cert type -1
debug1: identity file /home/kgraves/.ssh/id_dsa type -1
debug1: identity file /home/kgraves/.ssh/id_dsa-cert type -1
debug1: identity file /home/kgraves/.ssh/id_ecdsa type -1
debug1: identity file /home/kgraves/.ssh/id_ecdsa-cert type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.1
debug1: match: OpenSSH_6.1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.1
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr hmac-md5 none
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ECDSA de:1c:37:d7:84:0b:f8:f9:5e:da:11:49:57:4f:b8:f1
debug1: Host 'devilsmilk' is known and matches the ECDSA host key.
debug1: Found key in /home/kgraves/.ssh/known_hosts:1
debug1: ssh_ecdsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password,keyboard-interacti ve
debug1: Next authentication method: publickey
debug1: Trying private key: /home/kgraves/.ssh/id_rsa
debug1: Trying private key: /home/kgraves/.ssh/id_dsa
debug1: Trying private key: /home/kgraves/.ssh/id_ecdsa
debug1: Next authentication method: keyboard-interactive
debug1: Authentications that can continue: publickey,password,keyboard-interacti ve
debug1: Next authentication method: password
misfitred@devilsmilk's password:
debug1: Authentications that can continue: publickey,password,keyboard-interacti ve
Permission denied, please try again.
misfitred@devilsmilk's password:
debug1: Authentications that can continue: publickey,password,keyboard-interacti ve
Permission denied, please try again.
misfitred@devilsmilk's password:
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: No more authentication methods to try.
Permission denied (publickey,password,keyboard-interactive).
Here is my sshd_config file on devilsmilk:
# $OpenBSD: sshd_config,v 1.80 2008/07/02 02:24:18 djm Exp $
# This is the sshd server system-wide configuration file. See
# sshd_config(5) for more information.
# This sshd was compiled with PATH=/usr/local/bin:/bin:/usr/bin
# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented. Uncommented options change a
# default value.
Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::
# Disable legacy (protocol version 1) support in the server for new
# installations. In future the default will change to require explicit
# activation of protocol 1
#Protocol 2
# HostKey for protocol version 1
# HostKey /etc/ssh/ssh_host_key
# HostKeys for protocol version 2
# HostKey /etc/ssh/ssh_host_rsa_key
# HostKey /etc/ssh/ssh_host_dsa_key
# Lifetime and size of ephemeral version 1 server key
#KeyRegenerationInterval 1h
#ServerKeyBits 1024
# Logging
# obsoletes QuietMode and FascistLogging
#SyslogFacility AUTH
#LogLevel INFO
# Authentication:
#LoginGraceTime 2m
#PermitRootLogin yes
StrictModes no
#MaxAuthTries 6
#MaxSessions 10
#RSAAuthentication yes
#PubkeyAuthentication yes
#AuthorizedKeysFile .ssh/authorized_keys
#AuthorizedKeysCommand none
#AuthorizedKeysCommandRunAs nobody
# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#RhostsRSAAuthentication no
# similar for protocol version 2
#HostbasedAuthentication yes
# Change to yes if you don't trust ~/.ssh/known_hosts for
# RhostsRSAAuthentication and HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes
# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes
# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
#KerberosUseKuserok yes
# GSSAPI options
#GSSAPIAuthentication no
#GSSAPIAuthentication yes
#GSSAPICleanupCredentials yes
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication. Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
#UsePAM no
# Accept locale-related environment variables
#AcceptEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
#AcceptEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
#AcceptEnv LC_IDENTIFICATION LC_ALL LANGUAGE
#AcceptEnv XMODIFIERS
#AllowAgentForwarding yes
AllowTcpForwarding yes
GatewayPorts yes
#X11Forwarding no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PrintMotd yes
#PrintLastLog yes
TCPKeepAlive yes
#UseLogin no
#UsePrivilegeSeparation yes
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#ShowPatchLevel no
#UseDNS yes
#PidFile /var/run/sshd.pid
#MaxStartups 10
#PermitTunnel no
#ChrootDirectory none
# no default banner path
#Banner none
# override default of no subsystems
Subsystem sftp /usr/libexec/openssh/sftp-server
# Example of overriding settings on a per-user basis
#Match User anoncvs
# X11Forwarding no
# AllowTcpForwarding no
# ForceCommand cvs server
And here is my ssh_config file:
# $OpenBSD: ssh_config,v 1.25 2009/02/17 01:28:32 djm Exp $
# This is the ssh client system-wide configuration file. See
# ssh_config(5) for more information. This file provides defaults for
# users, and the values can be changed in per-user configuration files
# or on the command line.
# Configuration data is parsed as follows:
# 1. command line options
# 2. user-specific file
# 3. system-wide file
# Any configuration value is only changed the first time it is set.
# Thus, host-specific definitions should be at the beginning of the
# configuration file, and defaults at the end.
# Site-wide defaults for some commonly used options. For a comprehensive
# list of available options, their meanings and defaults, please see the
# ssh_config(5) man page.
# Host *
# ForwardAgent no
# ForwardX11 no
# RhostsRSAAuthentication no
# RSAAuthentication yes
# PasswordAuthentication yes
# HostbasedAuthentication no
# GSSAPIAuthentication no
# GSSAPIDelegateCredentials no
# GSSAPIKeyExchange no
# GSSAPITrustDNS no
# BatchMode no
# CheckHostIP yes
# AddressFamily any
# ConnectTimeout 0
# StrictHostKeyChecking ask
# IdentityFile ~/.ssh/identity
# IdentityFile ~/.ssh/id_rsa
# IdentityFile ~/.ssh/id_dsa
# Port 22
# Protocol 2,1
# Cipher 3des
# Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
# MACs hmac-md5,hmac-sha1,[email protected],hmac-ripemd160
# EscapeChar ~
# Tunnel no
# TunnelDevice any:any
# PermitLocalCommand no
# VisualHostKey no
#Host *
# GSSAPIAuthentication yes
# If this option is set to yes then remote X11 clients will have full access
# to the original X11 display. As virtually no X11 client supports the untrusted
# mode correctly we set this to yes.
ForwardX11Trusted yes
# Send locale-related environment variables
SendEnv LANG LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE LC_MONETARY LC_MESSAGES
SendEnv LC_PAPER LC_NAME LC_ADDRESS LC_TELEPHONE LC_MEASUREMENT
SendEnv LC_IDENTIFICATION LC_ALL LANGUAGE
SendEnv XMODIFIERS
UPDATE REQUEST 1: /var/log/secure
Jan 29 12:26:26 localhost sshd[2317]: Server listening on 0.0.0.0 port 22.
Jan 29 12:26:26 localhost sshd[2317]: Server listening on :: port 22.
Jan 29 12:26:34 localhost polkitd(authority=local): Registered Authentication Agent for session /org/freedesktop/ConsoleKit/Session1 (system bus name :1.29 [/usr/libexec/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Jan 29 12:36:09 localhost pam: gdm-password[3029]: pam_unix(gdm-password:session): session opened for user misfitred by (uid=0)
Jan 29 12:36:09 localhost polkitd(authority=local): Unregistered Authentication Agent for session /org/freedesktop/ConsoleKit/Session1 (system bus name :1.29, object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Jan 29 12:36:11 localhost polkitd(authority=local): Registered Authentication Agent for session /org/freedesktop/ConsoleKit/Session2 (system bus name :1.45 [/usr/libexec/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Jan 29 12:53:39 localhost polkitd(authority=local): Operator of unix-session:/org/freedesktop/ConsoleKit/Session2 successfully authenticated as unix-user:root to gain TEMPORARY authorization for action org.freedesktop.packagekit.system-update for system-bus-name::1.64 [gpk-update-viewer] (owned by unix-user:misfitred)
Jan 29 12:54:02 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 12:54:06 localhost sshd[2317]: Received signal 15; terminating.
Jan 29 12:54:06 localhost sshd[3948]: Server listening on 0.0.0.0 port 22.
Jan 29 12:54:06 localhost sshd[3948]: Server listening on :: port 22.
Jan 29 12:55:46 localhost su: pam_unix(su:session): session closed for user root
Jan 29 12:55:56 localhost pam: gdm-password[3029]: pam_unix(gdm-password:session): session closed for user misfitred
Jan 29 12:55:56 localhost polkitd(authority=local): Unregistered Authentication Agent for session /org/freedesktop/ConsoleKit/Session2 (system bus name :1.45, object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Jan 29 12:55:58 localhost polkitd(authority=local): Registered Authentication Agent for session /org/freedesktop/ConsoleKit/Session3 (system bus name :1.78 [/usr/libexec/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Jan 29 12:56:29 localhost pam: gdm-password[4044]: pam_unix(gdm-password:auth): conversation failed
Jan 29 12:56:29 localhost pam: gdm-password[4044]: pam_unix(gdm-password:auth): auth could not identify password for [misfitred]
Jan 29 12:56:29 localhost pam: gdm-password[4044]: gkr-pam: no password is available for user
Jan 29 12:57:11 localhost pam: gdm-password[4051]: pam_selinux_permit(gdm-password:auth): Cannot determine the user's name
Jan 29 12:57:11 localhost pam: gdm-password[4051]: pam_succeed_if(gdm-password:auth): error retrieving user name: Conversation error
Jan 29 12:57:11 localhost pam: gdm-password[4051]: gkr-pam: couldn't get the user name: Conversation error
Jan 29 12:57:17 localhost pam: gdm-password[4053]: pam_unix(gdm-password:session): session opened for user misfitred by (uid=0)
Jan 29 12:57:17 localhost polkitd(authority=local): Unregistered Authentication Agent for session /org/freedesktop/ConsoleKit/Session3 (system bus name :1.78, object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Jan 29 12:57:17 localhost polkitd(authority=local): Registered Authentication Agent for session /org/freedesktop/ConsoleKit/Session4 (system bus name :1.93 [/usr/libexec/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Jan 29 12:57:49 localhost unix_chkpwd[4495]: password check failed for user (root)
Jan 29 12:57:49 localhost su: pam_unix(su:auth): authentication failure; logname=misfitred uid=501 euid=0 tty=pts/0 ruser=misfitred rhost= user=root
Jan 29 12:58:04 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 13:16:16 localhost su: pam_unix(su:session): session closed for user root
Jan 29 13:18:05 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 13:21:14 localhost su: pam_unix(su:session): session closed for user root
Jan 29 13:21:40 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 13:24:17 localhost su: pam_unix(su:session): session opened for user misfitred by misfitred(uid=0)
Jan 29 13:27:10 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 13:28:55 localhost su: pam_unix(su:session): session closed for user root
Jan 29 13:28:55 localhost su: pam_unix(su:session): session closed for user misfitred
Jan 29 13:28:55 localhost su: pam_unix(su:session): session closed for user root
Jan 29 13:29:00 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 13:31:48 localhost sshd[3948]: Received signal 15; terminating.
Jan 29 13:31:48 localhost sshd[5498]: Server listening on 0.0.0.0 port 22.
Jan 29 13:31:48 localhost sshd[5498]: Server listening on :: port 22.
Jan 29 13:44:58 localhost sshd[5498]: Received signal 15; terminating.
Jan 29 13:44:58 localhost sshd[5711]: Server listening on 0.0.0.0 port 22.
Jan 29 13:44:58 localhost sshd[5711]: Server listening on :: port 22.
Jan 29 14:00:19 localhost sshd[5711]: Received signal 15; terminating.
Jan 29 14:00:19 localhost sshd[5956]: Server listening on 0.0.0.0 port 22.
Jan 29 14:00:19 localhost sshd[5956]: Server listening on :: port 22.
Jan 29 15:03:00 localhost sshd[5956]: Received signal 15; terminating.
Jan 29 15:10:23 localhost su: pam_unix(su:session): session closed for user root
Jan 29 15:10:38 localhost pam: gdm-password[4053]: pam_unix(gdm-password:session): session closed for user misfitred
Jan 29 15:10:38 localhost polkitd(authority=local): Unregistered Authentication Agent for session /org/freedesktop/ConsoleKit/Session4 (system bus name :1.93, object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Jan 29 15:11:21 localhost polkitd(authority=local): Registered Authentication Agent for session /org/freedesktop/ConsoleKit/Session1 (system bus name :1.29 [/usr/libexec/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Jan 29 15:11:32 localhost pam: gdm-password[2919]: pam_unix(gdm-password:session): session opened for user misfitred by (uid=0)
Jan 29 15:11:32 localhost polkitd(authority=local): Unregistered Authentication Agent for session /org/freedesktop/ConsoleKit/Session1 (system bus name :1.29, object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)
Jan 29 15:11:33 localhost polkitd(authority=local): Registered Authentication Agent for session /org/freedesktop/ConsoleKit/Session2 (system bus name :1.45 [/usr/libexec/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8)
Jan 29 15:15:10 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 15:30:24 localhost userhelper[3700]: running '/usr/share/system-config-users/system-config-users ' with root privileges on behalf of 'root'
Jan 29 15:32:00 localhost su: pam_unix(su:session): session opened for user misfitred by misfitred(uid=0)
Jan 29 15:32:23 localhost passwd: gkr-pam: changed password for 'login' keyring
Jan 29 15:32:39 localhost passwd: pam_unix(passwd:chauthtok): password changed for misfitred
Jan 29 15:32:39 localhost passwd: gkr-pam: couldn't change password for 'login' keyring: 1
Jan 29 15:33:06 localhost passwd: pam_unix(passwd:chauthtok): password changed for misfitred
Jan 29 15:33:06 localhost passwd: gkr-pam: changed password for 'login' keyring
Jan 29 15:37:08 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 15:38:16 localhost su: pam_unix(su:session): session closed for user root
Jan 29 15:38:16 localhost su: pam_unix(su:session): session closed for user misfitred
Jan 29 15:38:16 localhost su: pam_unix(su:session): session closed for user root
Jan 29 15:38:25 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 15:42:47 localhost su: pam_unix(su:session): session closed for user root
Jan 29 15:47:13 localhost sshd[4111]: pam_unix(sshd:session): session opened for user misfitred by (uid=0)
Jan 29 16:49:40 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 29 16:55:19 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
Jan 30 08:34:57 localhost sshd[4111]: pam_unix(sshd:session): session closed for user misfitred
Jan 30 08:34:57 localhost su: pam_unix(su:session): session closed for user root
Jan 30 08:35:24 localhost su: pam_unix(su:session): session opened for user root by misfitred(uid=501)
I agree with fboaventura; The configs look fine; try changing the password for
your user to what you think it should be, also check that it isn't expired/account
locked. And try another user just in case. Also, are you able to log in locally as
that user? i.e. is the error specific to SSH or is it having an error via other
auth mechs. –
Justin
Jan 29 '13 at 22:56
@ fboaventura & Justin I did try another user and I also changed the
password and tried it again with no success. I can login locally just fine and I
can also SSH to localhost just fine. –
Kentgrav
Jan 30 '13 at 13:33
@ John Siu I added the /var/log/secure and I attempted the SSH right before I
copied it. And nothing was added to it. Hope it helps. –
Kentgrav
Jan 30 '13 at 13:41
Yeah I did this already I actually figured out what the problem was. And as I
thought...it was the one thing that should have been blatantly obvious. –
Kentgrav
Jan 30 '13 at 16:08
The problem with this answer is that the defaults are commented out by default
as the comments in the file explain. It doesn't matter if (1) is commented or not
because the default is "yes". The correct answer is below. It's probably a DNS
problem and can easily test by using the IP address instead of the domain name.
–
Colin Keenan
Sep 18 '15 at 4:41
Make sure the permissions on the
~/.ssh
directory and its contents are proper.
When I first set up my ssh key auth, I didn't have the
~/.ssh
folder properly
set up, and it yelled at me.
Your home directory
~
, your
~/.ssh
directory and the
~/.ssh/authorized_keys
file on the remote machine must be writable only by
you:
rwx------
and
rwxr-xr-x
are fine, but
rwxrwx---
is no good¹, even if you are the only user in your group (if you prefer numeric modes:
700
or
755
, not
775
).
If
~/.ssh
or
authorized_keys
is a symbolic link,
the canonical path (with symbolic links expanded) is checked
.
Your
~/.ssh/authorized_keys
file (on the remote machine) must be readable
(at least 400), but you'll need it to be also writable (600) if you will add any more keys
to it.
Your private key file (on the local machine) must be readable and writable only by you:
rw-------
, i.e.
600
.
If you have root access to the server, the easy way to solve such problems is to run sshd in
debug mode, e.g.:
service ssh stop # will not kill existing ssh connections
/usr/sbin/sshd -d # full path to sshd executable needed, 'which sshd' can help
...debug output...
service ssh start
(If you can access the server through any port, you can just use
/usr/sbin/sshd -d
-p <port number>
to avoid having to stop the SSH server. You still need to be
root though.)
In the debug output, look for something like
debug1: trying public key file /path/to/home/.ssh/authorized_keys
...
Authentication refused: bad ownership or modes for directory /path/to/home/
Is your home dir encrypted? If so, for your first ssh session you will have to provide a
password. The second ssh session to the same server is working with auth key. If this is the
case, you could move your
authorized_keys
to an unencrypted dir and change the
path in
~/.ssh/config
.
What I ended up doing was create a
/etc/ssh/username
folder, owned by
username, with the correct permissions, and placed the
authorized_keys
file in
there. Then changed the AuthorizedKeysFile directive in
/etc/ssh/config
to :
AuthorizedKeysFile /etc/ssh/%u/authorized_keys
This allows multiple users to have this ssh access without compromising permissions.
I faced challenges when the home directory on the remote does not have correct privileges. In
my case the user changed the home dir to 777 for some local access with in the team. The
machine could not connect with ssh keys any longer. I changed the permission to 744 and it
started to work again.
We ran into the same problem and we followed the steps in the answer. But it still did not
work for us. Our problem was that login worked from one client but not from another (the .ssh
directory was NFS mounted and both clients were using the same keys).
So we had to go one step further. By running the ssh command in verbose mode you get a lot
of information.
ssh -vv user@host
What we discovered was that the default key (id_rsa) was not accepted and instead the ssh
client offered a key matching the client hostname:
debug1: Offering public key: /home/user/.ssh/id_rsa
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Offering public key: /home/user/.ssh/id_dsa
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Offering public key: user@myclient
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: pkalg ssh-rsa blen 277
Obviously this will not work from any other client.
So the solution in our case was to switch the default rsa key to the one that contained
user@myclient. When a key is default, there is no checking for client name.
Then we ran into another problem, after the switch. Apparently the keys are cached in the
local ssh agent and we got the following error on the debug log:
'Agent admitted failure to sign using the key'
This was solved by reloading the keys to the ssh agent:
ssh-add
,
It would be SSH miss configuration at server end. Server side sshd_config file has to be
edited. Located in
/etc/ssh/sshd_config
. In that file, change variables
'yes' to 'no' for ChallengeResponseAuthentication, PasswordAuthentication, UsePAM
I am not sure whether it is possible to scp a folder from remote to local, but
still I am left with no other options. I use ssh to log into my server and from there I would
like to copy the folder foo to home/user/Desktop (my local). Is there
any command so that I can do this?
To use full power of scp you need to go through next steps:
Then, for example if you'll have this ~/.ssh/config :
Host
test
User
testuser
HostName
test
-
site
.
com
Port
22022
Host
prod
User
produser
HostName
production
-
site
.
com
Port
22022
you'll save yourself from password entry and simplify scp syntax like this:
scp
-
r prod
:/
path
/
foo
/
home
/
user
/
Desktop
# copy to local
scp
-
r prod
:/
path
/
foo test
:/
tmp
# copy from remote prod to remote test
More over, you will be able to use remote path-completion:
scp test
:/
var
/
log
/
# press tab twice
Display
all
151
possibilities
?
(
y or n
)
Update:
For enabling remote bash-completion you need to have bash-shell on both <source>
and <target> hosts, and properly working bash-completion. For more information see related
questions:
"... scp will overwrite the files only if you have write permissions to them. In other words: You can make scp effectively skip said files by temporarily removing the write permissions on them (if you are the files' owner, that is). ..."
"... before running scp (it will complain and skip the existing files). And change them back afterward ( chmod +w to get umask based value). If the files do not all have write permission according to your umask, you would somehow have to store the permissions so that you can restore them. (Gilles' answer overwrites existing files if locally they are newer, I lost valuable data that way. Do not understand why that wrong and harmful answer has so many up votes). I don't get it: how did rsync --ignore-existing cause you to lose data? – ..."
"... Unable to create temporary file Clock skew detected ..."
"... In my case - I could not do this and the solution was: lftp . lftp 's usage for syncronization is below: ..."
"... To copy a whole bunch of files, it's faster to tar them. By using -k you also prevent tar from overwriting files when unpacking it on the target system. ..."
How do I copy an entire directory into a directory of the same name without replacing the content
in the destination directory? (instead, I would like to add to the contents of the destination folder)
ssh rsync scp synchronization
Use rsync , and pass -u if you want to only update files that are newer
in the original directory, or --ignore-existing to skip all files that already exist
in the destination.
rsync -au /local/directory/ host:/remote/directory/
rsync -a --ignore-existing /local/directory/ host:/remote/directory/
(Note the / on the source side: without it rsync would create
/remote/directory/directory .)
@Anthon I don't understand your comment and I don't see an answer or comment by chandra.
--ignore-existing does add without replacing, what data loss do you see? –
Gilles
Nov 27 '13 at 9:59
Sorry, I only looked at your first example that is where you can have data loss (and is IMHO not
what the OP asked for), if you include --ignore-existing data-loss should not happen. –
Anthon
Nov 27 '13 at 10:08
@Gilles: True, but all of the options seems to involve Cygwin DLLs... (The current state of the
MS port of OpenSSH is such that enabling compression on scp is enough to break SCP...) (Getting rsync
functional over Win32-OpenSSH also seems non-trivial - hopefully that improves over time) (Solaris
10 is the other example, where a third party package and --rsync-path is needed) –
Gert van den
Berg
Oct 25 '16 at 13:01
scp will overwrite
the files only if you have write permissions to them. In other words: You can make scp
effectively skip said files by temporarily removing the write permissions on them (if you
are the files' owner, that is).
If you can make the destination file contents read-only:
find . -type f -exec chmod a-wbefore running scp (it will complain and skip the existing files). And change
them back afterward ( chmod +w to get umask based value). If the files do not all
have write permission according to your umask, you would somehow have to store the permissions
so that you can restore them.
(Gilles' answer overwrites existing files if locally they are newer, I lost valuable data
that way. Do not understand why that wrong and harmful answer has so many up votes).
I had a similar task, in my case I could not use rsync , csync , or
FUSE because my storage has only SFTP. rsync could not change the date and time for
the file, some other utilities (like csync ) showed me other errors: " Unable to
create temporary file Clock skew detected ".
If you have access to the storage-server - just install openssh-server or launch
rsync as a daemon here.
In my case - I could not do this and the solution was: lftp . lftp 's usage for
syncronization is below:
scp does overwrite files and there's no switch to stop it doing that, but you can
copy things out the way, do the scp and then copy the existing files back. Examples:
Copy all existing files out the way
mkdir original_files ; cp -r * original_files/
Copy everything using scp
scp -r user@server:dir/* ./
Copy the original files over anything scp has written over:
cp -r original_files/* ./
This method doesn't help when you're trying to pull files over from a remote and pick up where
you left off. I.e. if the whole purpose is to save time. –
Oliver Williams
Dec 1 '16 at 17:58
>To copy a whole bunch of files, it's faster to tar them. By using -k you also prevent tar
from overwriting files when unpacking it on the target system.
It does make a remote connection. First it tar's the source, pipes it into the ssh connection
and unpacks it on the remote system. –
huembi
Aug 22 '16 at 21:17
The following steps needs to be performed in your SSH client, not in the remote server.
To configure the current user, edit SSH config file:
sudo nano ~/.ssh/config
Add the following lines:
Host *
ServerAliveInterval 60
Please ensure you indent the second line with a space . Let me explain what these lines do. Once you added these lines in your
SSH client system, it will send a packet called no-op (No Operation) to your Remote system. The no-op packet will inform
the remote system "Nothing to do". Also it tells the SSH client is still connected with the remote system, hence do not close the
TCP connection and log you out.
Here "Host *" indicates this configuration is applicable for all remote hosts. "ServerAliveInterval 60" indicates the number of
seconds to wait to send a no-op packet.
... ... ...
To apply this settings for all users (globally) in your system, add or modify the following line in /etc/ssh/ssh_config file.
Do you have the need to securely browse an internal-only company webpage
remotely? Well, here is a method for tunnelling your web browser through
an encrypted connection. Please note that this will NOT hide the DNS
queries which can reveal the target site.
Many people don't realize that SSH can emulate a SOCKS proxy. You can
use any server you have SSH terminal access to as your own personal
proxy.
Lazy Linux: 10 essential tricks for admins by Vallard Benincosa Certified Technical
Sales Specialist, IBM
Many times I'll be at a site where I need remote support from someone
who is blocked on the outside by a company firewall. Few people realize
that if you can get out to the world through a firewall, then it is relatively
easy to open a hole so that the world can come into you.
In its crudest form, this is called "poking a hole in the firewall."
I'll call it an SSH back door. To use it, you'll need a machine on
the Internet that you can use as an intermediary.
In our example, we'll call our machine blackbox.example.com. The machine
behind the company firewall is called ginger. Finally, the machine that
technical support is on will be called tech. Figure 4 explains how this
is set up.
Check that what you're doing is allowed, but make sure you ask the
right people. Most people will cringe that you're opening the firewall,
but what they don't understand is that it is completely encrypted. Furthermore,
someone would need to hack your outside machine before getting into
your company. Instead, you may belong to the school of "ask-for-forgiveness-instead-of-permission."
Either way, use your judgment and don't blame me if this doesn't go
your way.
SSH from ginger to blackbox.example.com with the -R flag.
I'll assume that you're the root user on ginger and that tech will need
the root user ID to help you with the system. With the -R flag,
you'll forward instructions of port 2222 on blackbox to port 22 on ginger.
This is how you set up an SSH tunnel. Note that only SSH traffic can
come into ginger: You're not putting ginger out on the Internet naked.
VNC or virtual network computing has been around a long time. I typically
find myself needing to use it when the remote server has some type of graphical
program that is only available on that server.
For example, suppose in Trick 5,
ginger is a storage server. Many storage devices come with a GUI program
to manage the storage controllers. Often these GUI management tools need
a direct connection to the storage through a network that is at times kept
in a private subnet. Therefore, the only way to access this GUI is to do
it from ginger.
You can try SSH'ing to ginger with the -X option and launch
it that way, but many times the bandwidth required is too much and you'll
get frustrated waiting. VNC is a much more network-friendly tool and is
readily available for nearly all operating systems.
Let's assume that the setup is the same as in Trick 5, but you want tech
to be able to get VNC access instead of SSH. In this case, you'll do something
similar but forward VNC ports instead. Here's what you do:
Start a VNC server session on ginger. This is done by running something
like:
The options tell the VNC server to start up with a resolution of
1024x768 and a pixel depth of 24 bits per pixel. If you are using a
really slow connection setting, 8 may be a better option. Using
:99 specifies the port the VNC server will be accessible from.
The VNC protocol starts at 5900 so specifying :99 means the
server is accessible from port 5999.
When you start the session, you'll be asked to specify a password.
The user ID will be the same user that you launched the VNC server from.
(In our case, this is root.)
SSH from ginger to blackbox.example.com forwarding the port 5999
on blackbox to ginger. This is done from ginger by running the command:
Once you run this command, you'll need to keep this SSH session open
in order to keep the port forwarded to ginger. At this point if you
were on blackbox, you could now access the VNC session on ginger by
just running:
thedude@blackbox:~$ vncviewer localhost:99
That would forward the port through SSH to ginger. But we're interested
in letting tech get VNC access to ginger. To accomplish this, you'll
need another tunnel.
From tech, you open a tunnel via SSH to forward your port 5999 to
port 5999 on blackbox. This would be done by running:
This time the SSH flag we used was -L, which instead of
pushing 5999 to blackbox, pulled from it. Once you are in on blackbox,
you'll need to leave this session open. Now you're ready to VNC from
tech!
From tech, VNC to ginger by running the command:
root@tech:~# vncviewer localhost:99 .
Tech will now have a VNC session directly to ginger.
While the effort might seem like a bit much to set up, it beats flying
across the country to fix the storage arrays. Also, if you practice this
a few times, it becomes quite easy.
Let me add a trick to this trick: If tech was running the Windows® operating
system and didn't have a command-line SSH client, then tech can run Putty.
Putty can be set to forward SSH ports by looking in the options in the sidebar.
If the port were 5902 instead of our example of 5999, then you would enter
something like in Figure 5.
Secure Shell (SSH) is a rich subsystem used to log in to remote systems,
copy files, and tunnel through firewalls-securely. Since SSH is a subsystem,
it offers plenty of options to customize and streamline its operation. In fact,
SSH provides an entire "dot directory", named $HOME/.ssh, to contain all its
data. (Your .ssh directory must be mode 600 to preclude access by others. A
mode other than 600 interferes with proper operation.) Specifically, the file
$HOME/.ssh/config can define lots of shortcuts, including aliases for machine
names, per-host access controls, and more.
Here is a typical block found in $HOME/.ssh/config to customize SSH for a
specific host:
Host worker
HostName worker.example.com
IdentityFile ~/.ssh/id_rsa_worker
User joeuser
Each block in ~/.ssh/config configures one or more hosts. Separate individual
blocks with a blank line. This block uses four options: Host, HostName,
IdentityFile, and User. Host establishes a nickname
for the machine specified by HostName. A nickname allows you to type
ssh worker instead of ssh worker.example.com. Moreover, the
IdentityFile and User options dictate how to log in to
worker. The former option points to a private key to use with the host;
the latter option provides the login ID. Thus, this block is the equivalent
of the command:
A powerful but little-known option is ControlMaster.
If set, multiple SSH sessions to the same host share a single connection.
Once the first connection is established, credentials are not
required for subsequent connections, eliminating the drudgery of typing a password
each and every time you connect to the same machine. ControlMaster
is so handy, you'll likely want to enable it for every machine. That's accomplished
easily enough with the host wildcard, *:
Host *
ControlMaster auto
ControlPath ~/.ssh/master-%r@%h:%p
As you might guess, a block tagged Host * applies to every host, even
those not explicitly named in the config file. ControlMaster auto tries
to reuse an existing connection but will create a new connection if a shared
connection cannot be found. ControlPath points to a file to persist
a control socket for sharing. %r is replaced by the remote login user
name, %h is replaced by the target host name, and %p stands
in for the port used for the connection. (You can also use %l; it is
replaced with the local host name.) The specification above creates control
sockets with file names akin to:
Each control socket is removed when all connections to the remote host are severed.
If you want to know which machines you are connected to at any time, simply
type ls ~/.ssh and look at the host name portion of the control socket
(%h).
The SSH configuration file is so expansive, it too has its own man page.
Type man ssh_config to see all possible options. And here's a clever
SSH trick: You can tunnel from a local system to a remote one via SSH. The command
line to use looks something like this:
$ ssh example.com -L 5000:localhost:3306
This command says, "Connect via example.com and establish a tunnel between
port 5000 on the local machine and port 3306 [the MySQL server port] on the
machine named 'localhost.'" Because localhost is interpreted on example.com
as the tunnel is established, localhost is example.com. With the outbound
tunnel-formally called a local forward-established, local clients can
connect to port 5000 and talk to the MySQL server running on example.com.
This is the general form of tunneling:
$ ssh proxyhostlocalport:targethost:targetport
Here, proxyhost is a machine you can access via SSH and one
that has a network connection (not via SSH) to targethost.
localport is a non-privileged port (any unused port above 1024)
on your local system, and targetport is the port of the service
you want to connect to.
The previous command tunnels out from your machine to the outside
world. You can also use SSH to tunnel in, or connect to your local system
from the outside world. This is the general form of an inbound tunnel:
When establishing an inbound tunnel-formally called a remote forward-the
roles of proxyhost and targethost are reversed:
The target is your local machine, and the proxy is the remote machine. user is your login on the proxy. This command provides a concrete
example:
The command reads, "Connect to example.com as joe, and connect the remote
port 8080 to local port 80." This command gives users on example.com a tunnel
to Joe's machine. A remote user can connect to 8080 to hit the Web server on
Joe's machine.
In addition to -L and -R for local and remote forwards,
respectively, SSH offers -D to create an HTTP proxy on a remote machine.
See the SSH man page for the proper syntax.
Martin Streicher is a freelance Ruby on Rails developer and
the former Editor-in-Chief of Linux Magazine.
Martin holds a Masters of Science degree in computer science from Purdue University
and has programmed UNIX-like systems since 1986. He collects art and toys. You
can reach Martin at
[email protected].
libssh is a C library to access SSH services from a program. It can remotely
execute programs, transfer files, and serve as a secure and transparent tunnel
for remote programs. Its Secure FTP implementation
If not option is given, sshpass reads the password from the standard input.
The user may give at most one alternative source for the password:
-p password - The password is given on the command line. Please note
the section titled "SECURITY CONSIDERATIONS".
-f filename - The password is the first line of the file filename.
-d number - number is a file descriptor inherited by sshpass from
the runner. The password is read from the open file descriptor.
-e - The password is taken from the environment variable "SSHPASS".
Security Considerations
First and foremost, users of sshpass should realize that ssh's insistance
on only getting the password interactively is not without reason. It is close
to impossible to securely store the password, and users of sshpass should consider
whether ssh's public key authentication provides the same end-user experience,
while involving less hassle and being more secure.
The -p option should be considered the least secure of all of sshpass's options.
All
system users can see the password in the command
line with a simple "ps"command. Sshpass makes no attempt to hide the password,
as such attempts create race conditions without actually solving the problem.
Users of sshpass are encouraged to use one of the other password passing techniques,
which are all more secure.
In particular, people writing programs that are meant to communicate the
password programatically are encouraged to use an anonymous pipe and pass the
pipe's reading end to sshpass using the -d option.
sshpass Examples
1) Run rsync over SSH using password authentication, passing the password
on the command line:
Do you make a lot of connections to the same servers? You may not have noticed
how slow an initial connection to a shell is. If you multiplex your connection
you will definitely notice it though. Let's test the difference between a multiplexed
connection using SSH keys and a non-multiplexed connection using SSH keys:
# Without multiplexing enabled:
$ time ssh [email protected] uptime
20:47:42 up 16 days, 1:13, 3 users, load average: 0.00, 0.01, 0.00
real 0m1.215s
user 0m0.031s
sys 0m0.008s
# With multiplexing enabled:
$ time ssh [email protected] uptime
20:48:43 up 16 days, 1:14, 4 users, load average: 0.00, 0.00, 0.00
real 0m0.174s
user 0m0.003s
sys 0m0.004s
We can see that multiplexing the connection is much faster, in this instance
on an order of 7 times faster than not multiplexing the connection. Multiplexing
allows us to have a "control"connection, which is your initial connection to
a server, this is then turned into a UNIX socket file on your computer. All
subsequent connections will use that socket to connect to the remote host. This
allows us to save time by not requiring all the initial encryption, key exchanges,
and negotiations for subsequent connections to the server.
Host *
ControlMaster auto
ControlPath ~/.ssh/connections/%r_%h_%p
A negative to this is that some uses of ssh may fail to work with your multiplexed
connection. Most notably commands which use tunneling like git, svn or rsync,
or forwarding a port. For these you can add the option -oControlMaster=no. To
prevent a specific host from using a multiplexed connection add the following
to your ~/.ssh/config file:
Host YOUR_SERVER_OR_IP
MasterControl no
There are security precautions that one should take with this approach. Let's
take a look at what actually happens when we connect a second connection:
$ ssh -v -i /dev/null [email protected]
OpenSSH_4.7p1, OpenSSL 0.9.7l 28 Sep 2006
debug1: Reading configuration data /Users/symkat/.ssh/config
debug1: Reading configuration data /etc/ssh_config
debug1: Applying options for *
debug1: auto-mux: Trying existing master
Last login:
symkat@symkat:~$ exit
As we see no actual authentication took place. This poses a significant security
risk if running it from a host that is not trusted, as a user who can read and
write to the socket can easily make the connection without having to supply
a password. Take the same care to secure the sockets as you take in protecting
a private key.
Using SSH As A Proxy
Even Starbucks now has free WiFi in its stores. It seems the world has caught
on to giving free Internet at most retail locations. The downside is that more
teenagers with "Got Root?"stickers are camping out at these locations running
the latest version of wireshark.
SSH's encryption can stand up to most any hostile network, but what about
web traffic?
Most web browsers, and certainly all the popular ones, support using a proxy
to tunnel your traffic. SSH can provide a SOCKS proxy on localhost that tunnels
to your remote server with the -D option. You get all the encryption of SSH
for your web traffic, and can rest assured no one will be capturing your login
credentials to all those non-ssl websites you're using.
Now there is a proxy running on 127.0.0.1:1080 that can be used in a web
browser or email client. Any application that supports SOCKS 4 or 5 proxies
can use 127.0.0.1:1080 to tunnel its traffic.
$ nc -vvv 127.0.0.1 1080
Connection to 127.0.0.1 1080 port [tcp/socks] succeeded!
Using One-Off Commands
Often times you may want only a single piece of information from a remote
host. "Is the file system full?""What's the uptime on the server?""Who is logged
in?"
Normally you would need to login, type the command, see the output and then
type exit (or Control-D for those in the know.) There is a better way: combine
the ssh with the command you want to execute and get your result:
This executed the ssh symkat.com, logged in as symkat, and ran the command
"uptime"on symkat. If you're not using SSH keys then you'll be presented with
a password prompt before the command is executed.
$ ssh [email protected] ps aux | echo $HOSTNAME
symkats-macbook-pro.local
This executed the command ps aux on symkat.com, sent the output to STDOUT,
a pipe on my local laptop picked it up to execute "echo $HOSTNAME" locally.
Although in most situations using auxiliary data processing like grep or awk
will work flawlessly, there are many situations where you need your pipes and
file IO redirects to work on the remote system instead of the local system.
In that case you would want to wrap the command in single quotes:
As a basic rule if you're using > >> < - or | you're going to want to wrap
in single quotes.
It is also worth noting that in using this method of executing a command
some programs will not work. Notably anything that requires a terminal, such
as screen, irssi, less, or a plethora of other interactive or curses based applications.
To force a terminal to be allocated you can use the -t option:
Pipes are useful. The concept is simple: take the output from one program's
STDOUT and feed it to another program's STDIN. OpenSSH can be used as a pipe
into a remote system. Let's say that we would like to transfer a directory structure
from one machine to another. The directory structure has a lot of files and
sub directories.
We could make a tarball of the directory on our own server and scp it over.
If the file system this directory is on lacks the space though we may be better
off piping the tarballed content to the remote system.
What we did in this example was to create a new archive (-c) and to compress
the archive with gzip (-z). Because we did not use -f to tell it to output to
a file, the compressed archive was send to STDOUT. We then piped STDOUT with
| to ssh. We used a one-off command in ssh to invoke tar with the extract (-x)
and gzip compressed (-z) arguments. This read the compressed archive from the
originating server and unpacked it into our server. We then logged in to see
the listing of files.
Additionally, we can pipe in the other direction as well. Take for example
a situation where you with to make a copy of a remote database, into a local
database:
symkat@chard:~$ echo "create database backup"| mysql -uroot -ppassword
symkat@chard:~$ ssh [email protected] 'mysqldump -udbuser -ppassword symkat' | mysql -uroot -ppassword backup
symkat@chard:~$ echo use backup;select count(*) from wp_links;"| mysql -uroot -ppassword
count(*)
12
symkat@chard:~$
What we did here is to create the database "backup"on our local machine.
Once we had the database created we used a one-off command to get a dump of
the database from symkat.com. The SQL Dump came through STDOUT and was piped
to another command. We used mysql to access the database, and read STDIN (which
is where the data now is after piping it) to create the database on our local
machine. We then ran a MySQL command to ensure that there is data in the backup
table. As we can see, SSH can provide a true pipe in either direction.
When using multiple systems the indispensable tool is, as we all know, ssh.
Using ssh you can login to other (remote) systems and work with them as if you
were sitting in front of them. Even if some of your systems exist behind firewalls
you can still get to them with ssh, but getting there can end up requiring a
number of command line options and the more systems you have the more difficult
it gets to remember them. However, you don't have to remember them, at least
not more than once: you can just enter them into ssh's config file and be done
with it.
For example, let's say that you have two "servers" that you connect to regularly,
one at your house that's behind your firewall. Further, let's say that you use
dyndns to make your home IP address known, and that you've got ssh listening
on port 12022 rather than the default port 22 (and you've got your firewall
forwarding that port to the server). So to connect you need to run:
$ ssh -p 12022 example.dyndns.org
The second system, let's say is local and you just connect with:
$ ssh 192.168.1.15
The second one is not too bad to type, but a name would be easier. You could
put the name in your /etc/hosts file, or you could set up a local DNS
server, but you can also solve this problem using ssh's config file.
To create an ssh config file execute the commands:
$ touch ~/.ssh/config
$ chmod 600 ~/.ssh/config
Now use your favorite text editor to edit the file and enter the following
into it:
Host server1
HostName example.dyndns.org
Port 12022
Host server2
HostName 192.168.1.15
The Host option starts a new "section": all the options that follow
apply to that host till a new "Host" option is seen. The "HostName" option specifies
the "real" host name that ssh tries to connect to (otherwise the "Host" value
is used). The "Port" is obviously the port that ssh tries to connect to, if
you don't specify a port, the default port is used.
Now you can connect much more simply:
$ ssh server1
$ ssh server2
These are just a few of the options that you can set in ssh's config file.
You can also, for example, specify that X11 forwarding be enabled. You can set
up local and remote port forwarding (i.e. ssh's -L and -R
command line options, respectively). Take a look at the man page (man ssh_config)
for more information on the available options.
One of the added benefits of using ssh's config file is that programs like
scp, rsync, and rdiff-backup automatically pick up these options also and work
just as you'd expect (hope).
______________________
Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor
for linuxjournal.com.
Even though the Linux version of the sftp client doesn't offer a direct way
to resume an interrupted transfer, doing so is quite simple by using common
shell tools, as long as you are able to login to the remote server through a
console. Assuming that you are transferring data.zip from source_server to target_server
and the transfer was interrupted, you can do the following:
Connect to target_server using ssh, since you will be required to perform
some operations there. Navigate to the directory containing the partially
transferred file (also called data.zip)
Check the sizes of the original and the partially transferred files.
The easiest way to do that is by using the ls -al data.zip command. Let's
assume that data.txt is 8231129 bytes long, and only 2811110 bytes were
transferred before the interruption
Subtract the size of the partially transferred file from the original,
to get the remaining size in bytes. In this case, it is 5420019 bytes. In
case you didn't know, Linux has a practical command-line calculator, bc,
which comes very handy for quick calculations
In source_server, create a new file consisting of the last 5420019 bytes
of the original. You can do this with the tail command: tail -c 5420019
data.zip >data.tail
Transfer the data.tail file to target_server, using sftp as usually.
Once the transfer is complete, delete data.tail from source_server to
avoid any mistake that would corrupt your original file.
In target_server, use the cat command to append data.tail to the partially
transferred file: cat data.tail >>data.zip (Note the double >>)
This works for both text and binary files. Apparently a better way would
be integrating this ability into the sftp client, which is the way some clients
such as putty and winscp work, but until that happy day you can use the tips
above as a workaround.
Comments
mikeX:
lftp is a very nice command line ftp client,
which supports tab completion, directory mirroring and of course resuming
of interrupted downloads. It can also work as an sftp client,
with the right protocol prefix, e.g. lftp sftp://user@host, and can even
be used in batch mode.
OpenSSH for Windows is a free package that installs a minimal OpenSSH server
and client utilities in the Cygwin package without needing the full Cygwin installation.
The OpenSSH for Windows package provides full SSH/SCP/SFTP support. SSH terminal
support provides a familiar Windows Command prompt, while retaining Unix/Cygwin-style
paths for SCP and SFTP.
Note : This set of instructions has worked for me at our institution.
You should read /usr/share/doc/Cygwin/openssh.README after installing cygwin
and check the cygwin mailing list
if you encounter problems.
SSHMenu adds a button to your
GNOME panel that displays a configurable drop-down list of hosts that you have
might like to connect to with SSH.
SSHMenu is packaged and available in repositories for both Ubuntu (as sshmenu-gnome)
and Fedora (gnome-applet-sshmenu). Other SSHMenu packages available for both
distributions do not include GNOME support. In those, the button for the SSH
menu is started in its own window and an xterm is started when you wish to connect
to a host with SSH. If you install the GNOME-aware SSHMenu packages, you can
add SSHMenu to your panel by right-clicking the panel and choosing "Add to Panel..."
and selecting the "SSH Menu Applet."
When using the GNOME-aware SSHMenu, a gnome-terminal is started to handle
your SSH connections, and you can select the profile gnome-terminal should use
on a per-host basis. That lets you specify a font and background color in the
terminal that can act as a reminder of which host that terminal is connected
with.
SSHMenu is a GNOME panel applet* that keeps all your regular SSH connections
within a single mouse click.
Each menu option will open an SSH session in a new terminal window. You can
organise groups of hosts with separator bars or sub-menus. You can even open
all the connections on a submenu (in separate windows or tabs) with one click.
Here's a killer feature: imagine if every time you connected to a
production server the terminal window had a red-tinted background, to remind
you to tread carefully. Using terminal profiles, SSHMenu allows you to specify
colours, fonts, transparency and a variety of other settings on a per-connection
basis. You can even set window size and position.
Cluster SSH opens terminal windows with connections to specified hosts and
an administration console. Any text typed into the administration console is
replicated to all other connected and active windows.
This tool is intended for, but not limited to, cluster administration
where the same configuration or commands must be run on each node within the
cluster. Performing these commands all at once via this tool
ensures all nodes are kept in sync.
This document covers the SSH client on the Linux Operating System and other
OSes that use OpenSSH. If you use Windows, please read the document
SSH Tutorial for Windows
If you use Mac OS X, you should already have OpenSSH installed and can use this
document as a reference.
This is one of the top tutorials covering SSH on the Internet. It was originally
written back in 1999 and was completely revised in 2006 to include new and more
accurate information. It has been read by over 227,000 people and consistently
appears at the top of Google's search results for SSH Tutorial and Linux SSH.
Belier allows opening a shell or executing a command on a remote computer
through an SSH session. The main feature of Belier is its ability to cross several
intermediate computers before realizing the... job. You can execute commands
with any account available on the remote computer. It is possible to switch
an account on intermediate computers before accessing the final computer, and
Belier will generate one script for each final computer to reach
Silk Tree propagate /etc/passwd and
/etc/group files from a master to a list
of hosts via SSH. Neither the sending nor the receiving end connect to each
other as root. Instead there is a read-only sudo sub-component on the receiver's
side that makes the final modifications in /etc. Many checks are made to ensure
reliable authorization updates. ACLs are used to enforce a simple security policy.
Differences between old and new versions are shown. Two small scripts are included
for exporting LDAP users and groups.
About: MindTerm is a complete ssh-client in pure Java. It can be used
either as a standalone Java application or as a Java applet. Three packages
of importance are provided (terminal, ssh, and security). The terminal package
is a rather complete vt102/xterm-terminal, and the ssh-package contains the
ssh- protocol and also "drop-in" socket replacements to use ssh-tunnels transparently
from a Java application/applet. It also contains functionality to realize a
ssh-server. Finally, the security package contains RSA, DES, 3DES, Blowfish,
IDEA, and RC4 ciphers.
[Mar 2, 2008] From John Hinsley
Mar 29, 2000
Q: I use telnet from my Linux box at home to use the HP_UX boxes at university.
No problems with telnet, but is there a way to get it to export the X display
so that I can use tools other than command line ones?
John Hinsley
Short answer: Use ssh instead.
The default for telnet is to preserve a number of environment settings, including
TERM, and DISPLAY. (Any recent telnet daemon should also perform some sanitization
on these variables to prevent some degenerate values from being propagated through
them to a potentially vulnerable program).
So, if you issue a 'set', 'env' or 'printenv'
command and look you might find that your DISPLAY variable IS set. However,
it's probably set to the wrong thing.
When you run 'startx' on the local system, it sets your DISPLAY
variable to something like: DISPLAY=:0.0 X client programs seeing this
value under Linux or UNIX will attempt to connect to the X server via a local
UNIX domain socket (one of those nodes in the a filesystem whose permissions/type
starts with an "s" in a "long" 'ls' output). That works for
the local processes talking to the local X server.
However, to start a remote process that needs to talk to your local X server
you must set the DISPLAY variable to a hostname and display number. What you
need is something like
DISPLAY=123.45.67.85:0.0 or DISPLAY=foo.bar.not:0.0
Programs that are linked against X libraries will automatically search their
environment for a DISPLAY value. If it specifies a hostname or IP address,
they will attempt to open a TCP connection (Internet domain socket) instead
of a local file/node (UNIX domain socket) connection. Specifically they will
try to connect to port 6000 for :0.0, and 6001 for ...:1.0, etc. (Incidentally,
the .0 in :0.0 or localhost:0.0 refers to a possible display number. Some X
servers support multiple displays/monitors, and these address each of the displays
as 0.0, 0.1, 1.0, 1.1 etc).
So, one solution is to use the following sort of command (assuming that you
are using a Bourne compatible shell like 'bash' which is the Linux
default):
... this variation of the familiar syntax sets this value for the DISPLAY
in the environment of the following command (that is on the same line as the
assignment, and NOT separated with one of the normal command delimiters, like
the semicolon).
Naturally you'd probably put this into whatever function, alias, or shell
script you are using to start these telnet sessions. You could use a more portable
syntax like:
DISPLAY=`hostname`:0.0 telnet ...
... where the backtick (command substitution) expression is used to fill
in the blank. This will allow those shell scripts, etc to adapt to whatever
system you copy them to, and will save you from having to fix all of them if
you change hostname (and ISP).
Of course, these days your machine's hostname might not match anything that
your ISP has set for you. So you might want to extract your IP address and use
that instead of your idea of your hostname. I'll leave the extraction of your
IP address from the output of the 'ifconfig' command using sh,
awk, PERL, TCL or whatever, as an exercise to the reader, it's not
difficult (*).
(Here's an example using just shell builtins for the parsing: '/sbin/ifconfig
eth0 | { read x; IFS=": " read x x a x; echo $a; }' )
Another problem with using straight IP addresses is that you might be going
through some sort of IP masquerading (NAT --- network address translation) between
your local system and the remote.
There is a better way!
USE ssh!
ssh will automatically handle your DISPLAY variable for you. When you establish
a remote shell session using ssh it creates it's own version of the DISPLAY
variable, one which points "localhost:10" (or localhost:11, etc).
What? Yep! You read that right. Your ssh client tells the remote sshd (daemon/server)
to pretend to be the "10th (or later) X server" on the remote system. The the
sshd will listen for X protocol activity on TCP port 6010 (or 6011, 6012, etc)
and relay that through to your local X server. This feature of ssh is called
X11 port forwarding. It is completely transparent and automatic.
On top of that all the traffic between your remote X clients and your local
display server is encrypted from the time it gets to the remote sshd (X proxy)
until it gets to your local ssh client process. It can't be sniffed or spoofed
(not without some heretofore unheard of cryptanalysis or the application of
a WHOLE LOT or brute computing force).
Also, when you install and configure ssh you can put one or more public keys
in the ~/.ssh/authorized_keys on each of the remote systems to which
you want access. So long as you keep the corresponding private keys secure on
your system, you can safely access your remote accounts without a password.
It's as convenient as 'rsh' and as safe as Kerberos (possibly more
so).
You can even publish one or more ssh public identities. Then anyone who wants
to let you access an account on any of their systems can just add that to the
authorized_keys file there. Possession of the public key can let them let you
in, while not directly compromising the security of any other sites to which
you have access.
On top of all that you can also use the 'scp' program as a "secure
'rcp'." That's a way to copy files to and from a remote system using basically
the same syntax as a 'cp' command and without having to start up a
copy of ftp or C-Kermit, etc.
It's also possible to set up ssh tunnels and run any number of common protocols
through them.
There's also an ssh-agent program. This is a way of allowing you
to login, start up one shell under the ssh-agent, give it your passphrase (in
effect unlocking your local private key) and having all your other ssh commands
in all of descendent processes, including those on remote systems all automatically
use the "unlocked" key. When you exit that one ssh-agent shell or X session,
you've effectively "locked" the key back up. (It's actually a rather clever
hack).
Oh, yeah! That X11 forwarding trick works right through any IP masquerading,
NAT, or applications proxying. It's just more traffic between your ssh client
and the remote daemon multiplexed in band with the rest of your session.
It makes no sense to use rsh or telnet in the modern world. We should all
switch to more secure protocols like ssh, Kerberos etc. (Ironically, the emergence
of IPSec and the future of ubiquitously secure DNS may eventually make the 'net
safe for plain telnet and rsh protocols. But that's a different story.)
About: Gnome-sshman is an SSH session manager for GNOME. It is easy and
fast to use, and is useful for system administrators that need to connect to
many SSH servers. Gnome-sshman saves ssh sessions and allows you to open a saved
session with a double click in nautilus.
Changes: The "open sessions folder" button was removed, so nautilus
is now an optional dependency. A session information tool was added to view
session data and attach notes to an ssh session. Telnet support was added. A
warning is given if you are closing a session with opened tabs. A preferences
window was added to change colors, fonts, and set other default options. Gconf
support was added. Two bugs were corrected: a cypher module bug with the hwrandom
module and a bug with GNOME 2.20 nautilus in background mode.
SftpDrive lets your applications access your files from anywhere on the Internet
safely and securely, like a VPN, without the VPN.
SSH is the industry standard for remote access to Linux, Mac OS X,
and UNIX computers because it's safe, secure, and just works from anywhere on
the Internet. SSH servers like OpenSSH and VShell have a powerful system called
SFTP built-in. Unrelated to the archaic FTP protocol, SFTP is a modern, secure
system that gives you the power to treat your network files as if they were
right on your desktop. Stream movies and music. Run programs. Load and save
any file from any application. Best of all, your SSH server is ready to go.
Many of us use the excellent OpenSSH
as a secure, encrypted replacement for the venerable telnet and rsh commands.
One of OpenSSH's (and the commercial SSH2's) intriguing features is its ability
to authenticate users using the RSA and DSA authentication protocols, which
are based upon a pair of complementary numerical "keys". And one of the
main appeals of RSA and DSA authentication is the promise of being able to establish
connections to remote systems without supplying a password. The keychain
script makes handling RSA and DSA keys both convenient and secure. It acts as
a front-end to ssh-agent, allowing you to easily
have one long-running ssh-agent process per system, rather than per login session.
This dramatically reduces the number of times you need to enter your passphrase
from once per new login session to once every time your local machine is rebooted.
Keychain was first introduced in a series of
IBM developerWorks articles.
The
first article introduces the concepts behind RSA/DSA key authentication
and shows you how to set up primitive (with passphrase) RSA/DSA authentication.
The
second
article shows you how to use keychain to set up secure, passwordless
ssh access in an extremely convenient way. keychain also provides a clean,
secure way for cron jobs to take advantage of RSA/DSA keys without having
to use insecure unencrypted private keys.
The
third article shows you how to use ssh-agent's authentication forwarding
mechanism.
Current versions of keychain are known to run on Linux, BSD,
Cygwin,
Tru64 UNIX,
HP-UX,
Mac OS X, and
Solaris using
whatever variant of Bourne shell you have available.
TCP wrappers provides limited, connection-oriented host-based firewall functionality
with which connections can be denied or accepted based on the originating host.
Connection attempts are logged using syslog(3C). OpenSSH uses this functionality
by linking in the libwrap library. TCP wrappers is dependent on the name and
IP address information returned by the name services, such as DNS. It cannot
stop low-level network-based attacks, such as port scanning, IP spoofing, or
denial of service. For those, a packet-based firewall solution such as SunScreenTM
software is necessary. The Solaris 9 OE has TCP wrappers integrated into it,
package SFWtcpd, which is located in the /usr/sfw directory. For the Solaris
8 OE, TCP wrappers can be found on the Software Companion CD (starting in the
Solaris 8 10/00 release). For the Solaris 2.6 and 7 OE releases, TCP wrappers
must be downloaded and built from the source. TCP wrappers is not required to
build OpenSSH.
On modern machines this is not a problem and you can run ssh via intetd/xinetd,
which provides you the ability to use TCP wrapper controls unless ssh is complied
with them.
Yes, you can. No, you generally shouldn't. And boy, do I hate this question
:)
When the Secure Shell daemon is started, it processes its configuration file
and generates a cryptographic key. This can take several seconds, especially
on a slow or busy server, and the startup time can be unacceptably long.
However, as Mike Friedman
writes: "What many people (including me) do is run a
'backup' sshd at a non-standard port out of inetd, for use just when the standalone
sshd has failed. This gives you a way to login to restart the regular sshd (or
to investigate why it won't start!), but the latter would still be what most
users normally connect to (at the standard port 22)."
If you decide to run Secure Shell via inetd:
To reduce the startup time for SSH1, you can reduce the size of the key that
is generated with the -b flag (e.g. "-b 512"). The default keysize is 768 bits,
and a keysize of 512 bits should be small enough to reduce the startup time.
This is not recommended, however, as a 512-bit key is significantly easier to
break than a larger key. The key size cannot be altered at runtime with SSH2;
a new server key must be generated with ssh-keygen2.
When starting sshd from inetd, be sure to pass it the -i flag so
it behaves properly.
FUSE is a Linux kernelmodule also available for FreeBSD, OpenSolaris and
Mac OS X that allows non-privileged users to create their own file systems without
the need to write any kernel code. This is achieved by running the file system
code in user space, while the FUSE module only provides a "bridge"to the actual
kernel interfaces. FUSE was officially merged into the mainstream Linux kernel
tree in kernel version 2.6.14.
You need to use SSHFS to access to a remote filesystem through SSH or even
you can use Gmail account to store files.
Following instructions are tested on CentOS, Fedora Core and RHEL 4/5 only.
But instructions should work with any other Linux distro without a problem.
Step # 1: Download and Install FUSE
Visit fuse home page and download
latest source code tar ball. Use
wget command to download fuse package: # wget http://superb-west.dl.sourceforge.net/sourceforge/fuse/fuse-2.6.5.tar.gz
Untar source code: # tar -zxvf fuse-2.6.5.tar.gz
Compile and Install fuse: # cd fuse-2.6.5
# ./configure
# make
# make install
Step # 2: Configure Fuse shared libraries loading
You need to configure dynamic linker run time bindings using ldconfig command
so that sshfs command can load shared libraries such as libfuse.so.2: # vi /etc/ld.so.conf.d/fuse.conf
Append following path: /usr/local/lib
Run ldconfig: # ldconfig
Step # 3: Install sshfs
Now fuse is loaded and ready to use. Now you need sshfs to access and mount
file system using ssh. Visit
sshfs home page and download
latest source code tar ball. Use
wget command to download fuse package: # wget http://easynews.dl.sourceforge.net/sourceforge/fuse/sshfs-fuse-1.7.tar.gz
Untar source code: # tar -zxvf sshfs-fuse-1.7.tar.gz
Compile and Install fuse: # cd sshfs-fuse-1.7
# ./configure
# make
# make install
Mounting your remote filesystem
Now you have working setup, all you need to do is mount a filesystem under
Linux. First create a mount point: # mkdir /mnt/remote
Now mount a remote server filesystem using sshfs command: # sshfs [email protected]: /mnt/remote
Where,
sshfs : SSHFS is a command name
[email protected]: - vivek is ssh username and rock.nixcraft.in
is my remote ssh server.
/mnt/remote : a local mount point
When promoted supply vivek (ssh user) password. Make sure you replace username
and hostname as per your requirements.
Now you can access your filesystem securely using Internet or your LAN/ WAN: # cd /mnt/remote
# ls
# cp -a /ftpdata . &
To unmount file system just type: # fusermount -u /mnt/remote ...
Updated 13 Sep 04. Nevermind.
phil_g's comment says it well. keychain is the way to go. I'll rewrite this
when I have more time.
Some co-workers turned me on to
GNU
screen last year. It's a handy addition to my toolbox. It became most useful
after I learned how to use it with SSH. The original URL that gave me the solution
appears to be gone (a
message in the now-defunct gnu-screen Yahoo group). So I thought I'd write
this up and see how it fares when people
google
gnu screen ssh.
The solution I settled on is a nested invocation I learned from
Jason White. I
recommend you read
my screenrc
and
my slave screenrc in another window and read along here for commentary.
You run an "outer" screen session (the "slave" session) that in turn runs an
"inner" (or "master") session. You use the regular escape sequence (Ctrl-A d)
to detatch from the master, and you map Ctrl-^ to be the control key for the
slave session. If you press Ctrl-^ while using screen this way, you'll see one
process in the slave session. It's running ssh-agent. That's the key to using
ssh with screen. The slave's only purpose is to run ssh-agent. The master runs
as a child of that. Consequently, all shells in the master session are running
under the ssh-agent. Just run ssh-add from any master shell, and then all shells
have your ssh identity.
Nested Screens Not Necessary phil_g
2004-07-06 08:57 am UTC (link)
You don't need to use nested screens to get this effect. I achieve it by
the use of a simple wrapper script for screen. To attach to a screen session,
I have a single script that I run; it loads the agent before starting screen.
(I use
keychain to ensure that only one agent instance is running, regardless of
how many times I attach to screen.) See
my attach-screen script for specific details.
[Dec 8, 2006] The Connection
Manager A Python-based tool to connect to foreign systems using a variety of
methods.
[Dec 8, 2006] pam_eps A
PAM module for ssh authentication against a remote server.
[Dec 8, 2006] syncpasswd
An Expect script that synchronizes passwords via SSH on multiple platforms.
But what about administrators using SSH on other platforms? Will they just plop
in this tool as a simple FTP replacement, get it to work in that limited role,
and then declare success?
The biggest issues with SSH lie at Layer 8 of the
OSI model-politics and personnel:
One vulnerability issue underlies all SSH implementations: Most administrators
know nothing about SSH's port-forwarding abilities (or choose to ignore
them). They may very well regard the security problems as "a UNIX issue."
So the first risk is proliferation of a naïve SSH security design across
multiple platforms, with little ownership of the big issues.
A second risk is the "convenience at all costs" approach to agent forwarding.
Anyone who has read an SSH man page knows that agent forwarding has known
risks when used in untrusted environments. Do the same vulnerabilities exist
with other operating systems? For that matter, do all client and server
SSH implementations carry the same warnings? We can't answer all of these
questions, but we can make a strong recommendation and review a suggested
Slashdot poster's mitigation.
A third major issue is the port-forwarding
risk that allows an innocent outbound connection (to a remote SSH server)
to become a malicious inbound connection into your company's intranet.
This connection is encrypted and will be very difficult to
monitor, thus adding to the danger.
Security mitigations must do more than suggest technical settings for one
SSH version. (And the technical settings vary by version, anyway, so don't expect
this article to be a primer on SSH server and client security. There are too
many features to discuss, and we must address greater issues than just technical
settings.)
So what can your organization do to help secure multiple versions of SSH
running on multiple operating systems?
The MindTerm
product page describes the MindTerm Java-coded ssh client and provides a
quick download of the latest no-charge version.
If you do decide to build a Web page that relies on a signed version of
the MindTerm applet, be certain to read up on
applet
signing.
"One-Time
Password" is, among other things, an
IETF standard. While it can be combined with ssh, Cameron believes in focusing
his security efforts on other technologies that pay off bigger, including intrusion
detection.
socat does
more than previous generations of network proxies or port forwarders, enough
so that author Gerhard Rieger justly terms it "multipurpose relay."
netcat is a port forwarder that's been in use for years.
sockspy is a scriptable
network proxy that's useful for debugging network connections or adding programmability
to them.
SSH is the descendant of rsh and rlogin, which are non-encrypted programs
for remote shell logins. Rsh and rlogin, like telnet, have a long lineage but
now are outdated and insecure. However, these programs evolved a surprising
number of nifty features over two decades of UNIX development, and the best
of them made their way into SSH. Following are the 11 tricks I have found useful
for squeezing the most power out of SSH.
The
July BluePrints
OnLine magazine includes an
article
on building OpenSSH for Solaris. It talks about compiling the Zlib compression
library, OpenSSL, PRNGD, and OpenSSH using either Forte Compilers or GCC and
the appropriate compilation options. There are also some included scripts to
help build a Solaris software package for easier deployment and a quite useful
and powerful init script.
PuTTY is free software that provides an ssh client, telnet, and several other
things. PuTTY also does both ssh1 and ssh2, and saves settings (e.g., hostnames,
IP addresses, and telnet, ssh, orraw selections), providing me a way to record
(instead of having to remember) the systems that I can connect to. PuTTY also
allows me to change window colors.
This site is operated by the authors of the
O'Reilly book on SSH. The
first edition was published
in February of 2001, by Dan Barrett and Richard Silverman. Joined by Robert
Byrnes, we completed the second edition in May of 2005.
Integrating the Secure Shell Software (May 2003) -by Jason Reid
This article discusses integrating Secure Shell software into an environment.
It covers replacing rsh(1) with ssh(1) in scripts, using proxies to bridge disparate
networks, limiting privileges with role-based access control (RBAC), and protecting
legacy TCP-based applications. This article is the entire fifth chapter of the
upcoming Sun BluePrints book "Secure Shell in the Enterprise" by Jason
Reid, which will be available in June 2003.
Role Based Access Control and Secure Shell--A Closer Look At Two Solaris
Operating Environment Security Features (June 2003) -by Thomas M. Chalfant
To aid the customer in adopting better security practices, this article introduces
and explains two security features in the Solaris operating environment. The
first is Role Based Access Control and the second is Secure Shell. The goal
is to provide you with enough information to make an effective decision to use
or not use these features at your site as well as to address configuration and
implementation topics. This article is targeted to the intermediate level of
expertise.
Building OpenSSH--Tools and Tradeoffs (January 2003) -by Jason M. Reid
This article updates much of the information in the July 2001 Sun BluePrints
OnLine article,
"Building and Deploying OpenSSH for the Solaris Operating Environment".
The article contains information about gathering the needed components, making
the compile-time configuration decisions, building the components, and finally
assembling the OpenSSH environment.
Configuring the Secure Shell Software (April 2003) -by Jason M. Reid
This article provides recommendations for configuring two specific Secure Shell
implementations for the Solaris Operating Environment (Solaris OE): OpenSSH
and the Solaris Secure Shell software. The Solaris Secure Shell software is
a component of the Solaris 9 OE release. OpenSSH is also available for previous
Solaris OE releases. For information on building OpenSSH, consult the January
2003 Sun BluePrints OnLine article,
"Building OpenSSH Tools and Tradeoffs." ...
Configuring OpenSSH for the Solaris Operating Environment (January
2002) -by Jason M. Reid
The network environment was never safe. As more users connect to open networks
for remote access, the risks of compromising systems and accounts increase.
Secure network tools such as OpenSSH counter the threats of password theft,
session hijacking, and other network attacks. These tools require planning,
configuration, and integration. This article deals with server and client configurations,
key management, and integration into existing environments for the Solaris Operating
Environment (OE).
(NOTE - See the Sun BluePrints article
"Configuring Secure Shell Software" by Jason M. Reid, April 2003 for additional
and updated information.)
Building and Deploying OpenSSH on the Solaris Operating Environment
(July 2001) -by Jason M. Reid and Keith Watson
This article describes the build and deployment processes for OpenSSH on Solaris
Operating Environment. There are several components that must be built prior
to building OpenSSH itself. Each necessary component is listed and described
along with recommendations on build options. Openssh itself is a flexible tool
with several options that affect integration into a site's security policy.
These options are explored. Issues of packaging and deployment are also addressed.
The Last but not LeastTechnology is dominated by
two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt.
Ph.D
FAIR USE NOTICEThis site contains
copyrighted material the use of which has not always been specifically
authorized by the copyright owner. We are making such material available
to advance understanding of computer science, IT technology, economic, scientific, and social
issues. We believe this constitutes a 'fair use' of any such
copyrighted material as provided by section 107 of the US Copyright Law according to which
such material can be distributed without profit exclusively for research and educational purposes.
This is a Spartan WHYFF (We Help You For Free)
site written by people for whom English is not a native language. Grammar and spelling errors should
be expected. The site contain some broken links as it develops like a living tree...
You can use PayPal to to buy a cup of coffee for authors
of this site
Disclaimer:
The statements, views and opinions presented on this web page are those of the author (or
referenced source) and are
not endorsed by, nor do they necessarily reflect, the opinions of the Softpanorama society.We do not warrant the correctness
of the information provided or its fitness for any purpose. The site uses AdSense so you need to be aware of Google privacy policy. You you do not want to be
tracked by Google please disable Javascript for this site. This site is perfectly usable without
Javascript.
<img alt="" src=/post/how-to-setup-ssh-tunneling/launch-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/configure-tunnel-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/add-tunnel-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/save-session-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/open-session-putty.jpg>
SSH Jump Host
.
The Sexiest Man in Jamaica 113
<img alt="" src=/post/how-to-setup-ssh-tunneling/launch-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/configure-tunnel-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/add-tunnel-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/save-session-putty.jpg>
<img alt="" src=/post/how-to-setup-ssh-tunneling/open-session-putty.jpg>
