Linux last and lastb commands

Show listing of last logged in users and bad logins

• Introduction
• Last vs lastb
• Three forms of the last command
• Options
• Output format
• Examples

Introduction

Utility last searches through the file /var/log/wtmp (or the file designated by the -f flag) and displays a list of all users logged in (and out) since that file was created in reverse chronological order. Names of users and tty’s can be given, in which case last will show only those entries matching the arguments. Linux version of last also displays IP or DNS, but unless option -a is given DNS names of machines from which user connected are abbreviated. 

Lat command also allows to determine  set of active user. Which is important for sending  announcements about shutdown and similar things.  Generally users who did not login for a specific server for two month  or more should be  disabled. This also can be accomplished by  analyzing last command output. you do not need password expiation for that.

Last vs lastb

Utility lastb produiced the  same set of reports as the last commend but for bad logs. As such it has certain security value as it allows to see probed. We can define probes as attempt to login  from IP that  never has  a successful login.  Usually the number of unsuccessful login attempts is set to some small number such as five or sever (do not use three, unless you need a stream of helpdesk tickets ;-), so there is possibility of denial of service attack if login is possible from any IP. Restriction of IP login space to specific host is a viable and necessary method of protection from such  attack in the current environment.

Unfortunately few people know about  lastb  existence and even fewer use it.  Often it is not even enabled by default and you need manually create the file /var/log/btmp for this command to be operational. It is not created automatically. For example:

# lastb -a
lastb: /var/log/btmp: No such file or directory
Perhaps this file was removed by the operator to prevent logging lastb info.
# touch /var/log/btmp
lastb -a

btmp begins Mon Jul  1 18:57:15 2013

Three forms of the last command

So there are three main forms of last and lastb commands:

• Without parameters (but possibly with options): last -a; lastb -a

• With parameter meaning a user: last root; lastb root   In this case information for particular user is printed

• With parameter meaning tty: The last command can accepts the name of the user or name of tty as a parameter. Names of tty can be abbreviated to digits, thus last 0 is the same as last tty0.  Without parameter information about all users is printed.  For example, last 1; lastb 1

The pseudo user reboot logs in each time the system is rebooted. Thus last reboot will show a log of all reboots since the log file was created. 

Lastb is the same as last, except that by default it shows a log of the file /var/log/btmp, which contains all the bad login attempts.

last [-R] [-num] [ -n num ] [-adiox] [ -f file ] [ -t YYYYMMDDHHMMSS ] [name...]  [tty...]

lastb [-R] [-num] [ -n num ] [ -f file ] [-adiox] [name...]  [tty...]

Options

We will discuss linux implementation options

• -f file Tells last to use a specific file instead of /var/log/wtmp.  If wtmp files are archived  you can unpack archive and run last command on it to see the login activity for a specific period
• -num This is a count telling last how many lines to show.
• -n num The same.
• -t YYYYMMDDHHMMSS Display the state of logins as of the specified time. This is useful, e.g., to determine easily who was logged in at a particular time -- specify that time with -t and look for "still logged in".
• -R Suppresses the display of the hostname field.
• -a Display the hostname in the last column. Useful in combination with the next flag.
• -d For non-local logins, Linux stores not only the host name of the remote host but its IP number as well. This option translates the IP number back into a hostname. This option is default in most distributions.
• -F Print full login and logout times and dates.
• -i This option is like -d in that it displays the IP number of the remote host, but it displays the IP number in numbers-and-dots notation.
• -o Read an old-type wtmp file (written by linux-libc5 applications).
• -w Display full user and domain names in the output.
• -x Display the system shutdown entries and run level changes.

• The files wtmp and btmp might not be found. The system only logs information in these files if they are present. This is a local configuration issue. If you want the command last and lastb to be operational both files should be created with a simple touch(1) command (for example, touch /var/log/wtmp). Most distributions create  /var/log/wtmp) during the installation, but not /var/log/btmp.

• When last catches a SIGINT signal (generated by the interrupt key, usually Ctrl-C) or a SIGQUIT signal (generated by the quit key, usually Ctrl-\), last will show how far it has searched through the file; in the case of the SIGINT signal last will then terminate.

Output format

From the point of view of processing output in scripts, we can distinguish  three output formats for last:

• without options and arguments (bad for script processing as DNS of the client which logged to the server is truncated)
• with option -a (the most suitable for script processing format)

• With the user name, including pseudo user reboot: last reboot (show only reboot)

The most important is output format last -a in which you can see full DNS name of client which logged to the server:

# last -a
user1    pts/3        Mon Jul  1 08:35   still logged in    joe.doer.vpn.firma.com
user1    pts/3        Fri Jun 28 17:20 - 18:24  (01:03)     lap245.firma.net
root     pts/2        Fri Jun 28 02:31 - 02:31  (00:00)
user1    pts/2        Tue Jun 25 14:18 - 16:17  (01:58)     lap245.firma.net
root     pts/3        Wed Jun 12 14:08 - 14:21  (00:13)
root     pts/3        Wed Jun 12 14:06 - 14:08  (00:01)
root     pts/3        Wed Jun 12 14:04 - 14:06  (00:02)
user1    pts/2        Wed Jun 12 12:59 - 14:22  (01:22)     joe.doer.vpn.firma.com
user1    pts/2        Wed Jun 12 12:49 - 12:59  (00:10)     joe.doer.vpn.firma.com
oracle   pts/1        Wed Jun 12 09:02 - 17:45  (08:43)     10.20.23.23
root     pts/1        Wed Jun  5 17:54 - 17:56  (00:02)     nti147.usfp2.basf.net
root     pts/2        Thu May 30 14:57 - 15:20  (00:23)     nti106.firma.com
oracle   pts/1        Thu May 30 10:37 - 17:10  (06:33)     lap719.firma.net
user1    pts/2        Wed May 29 16:58 - 19:10  (02:12)     joe.doer.vpn.firma.com

Output format of lastb

user2    pts/1        Fri Oct 14 10:17 - 10:17  (00:00)     lap316.firma.com
user2    pts/1        Fri Oct 14 10:17 - 10:17  (00:00)     lap316.firma.com
root     ssh:notty    Thu Oct 13 12:08 - 12:08  (00:00)     srv016.firma.com
root     ssh:notty    Thu Oct 13 12:08 - 12:08  (00:00)     srv016.firma.com
root     ssh:notty    Tue Sep 20 16:35 - 16:35  (00:00)     lap316.firma.com
root     ssh:notty    Wed Aug 10 17:05 - 17:05  (00:00)     lap921.firma.com
root     ssh:notty    Fri Jul 15 09:51 - 09:51  (00:00)     lap921.firma.com
root     ssh:notty    Fri Jul 15 09:51 - 09:51  (00:00)     lap921.firma.com
root     ssh:notty    Thu Jun 30 17:09 - 17:09  (00:00)     lusytp02.firma.com
roacle** pts/3        Thu Jun 30 16:58 - 16:58  (00:00)     lapl61.firma.com
root     ssh:notty    Thu Jun 30 12:24 - 12:24  (00:00)     srv029.firma.com
root     ssh:notty    Thu Jun 30 12:24 - 12:24  (00:00)     srv029.firma.com
user1    ssh:notty    Tue Jun 28 14:22 - 14:22  (00:00)     lap921.firma.com

Examples

• Without parameters (but possibly with options): last -a; lastb -a

• With parameter meaning a user: last root; lastb root

• With parameter meaning tty: last 1; lastb 1
• With pseudo user reboot: last reboot (show only reboot)

Files

/var/log/wtmp /var/log/btmp

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Last modified: March, 12, 2019