Like almost all open source sniffing applications tcpdump uses the libpcap library.
People with Windows distributions are best to check the
Windows PCAP page
for references to WinDUMP.
Tcpdump prints verbose information about the sniffed traffic with the -v
option. It can print hex dump of the packet with option -x. For printing
full packet you need to use option -s. One important feature is the
ability to write the "raw packets" to a file using -s0 -w name_of_file.
You can later analyze the capture using any program that understand pcap
raw dump format, for example tcpdump, snort or Ethereal. I think almost all useful
network programs now understand this format and can reuse raw packet tcpdump-generated
files. If you forgot the option -s 0 packets will truncated and you
will not be able to analyze them for example with snort. To print them from
raw tcpdump file you can use option -r. For example tcpdump -r file_with_capture. You can use option r
with -v (verbose) and -vv to get more information.
Tcpdump is often used in remote installation of OS, when you need to determine which of devices
is connected to the network. Sniffing the right device shows packets on the network. for example
tcpdump -vv -i eth0
Here -i eth0 means "Listen on interface eth0." Option -i specifies interface to listen. If unspecified,
tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback). Ties are broken by choosing the earliest match.
-nn blocks recognition of protocol and port as well.
Here are several additional useful options
-i eth0 -- Listen on interface eth0. Option
-i specifies interface to listen. If unspecified,
tcpdump searches the system interface list for the
lowest numbered, configured up interface (excluding loop back).
Ties are broken by choosing the earliest match.
-n -- Don't resolve hostnames.
-nn -- Don't resolve hostnames or port names.
-X -- Show the packet's contents in both
hex and
ASCII.
-v, -vv, -vvv -- Increase the amount of packet information
you get back.
-c -- Only get x number of packets and then
stop.
-s -- Define the size of the capture (use -s0
unless you are intentionally capturing less.)
TCPDUMP will only process packets that match the filter expression. Such a filter
expression can be passed on the command line, or read from a file using the -F filename
parameter.
# tcpdump -F path_to_filter
Tcpdump filter language is similar to snort (actually this language originated
in tcpdump and later was adopted and extended by snort). For more detailed info
on TCPDUMP and filter expressions, please consult the TCPDUMP man page, either via
the man command or
online at tcpdump.org.
Classic example is capturing packets coming and going to particular host:
tcpdump dst host 10.10.10.10 or scr host 10.10.10.10
Here we try to get TCP packets with source port 80
tcpdump src port 80 and tcp
Here are several examples of filters borrowed fromPrelab 2.
Write the syntax of a tcpdump command that captures packets containing
IP datagrams with a source or destination IP address equal to 10.0.1.12.
tcpdump host 10.0.1.12
Write the syntax of a tcpdump command that captures
packets containing ICMP messages with a source or destination IP address equal
to 10.0.1.12.
tcpdump icmp and host 10.0.1.12
Write the syntax of a tcpdump command that captures packets containing
IP datagrams between two hosts with IP addresses 10.0.1.11 and 10.0.1.12, both
on interface eth1.
tcpdump -i eth1 host 10.0.1.11 and host 10.0.1.12
Write a tcpdump filter expression that captures packets containing
TCP segments with a source or destination IP address equal to 10.0.1.12.
tcp and host 10.0.1.12
Write a tcpdump filter expression that, in addition to the constraints
in Question 5, only captures packets using port number 23.
tcp port 23 and host 10.0.1.12
Filter can be more complex, for example:
tcpdump '(host 1.2.3.4 and net 192.168.1) and ((tcp
port 80 or port 443))'
Excluding Things
The default is to capture all packets. You can capture all packets except those
for certain ports, like this:
tcpdump not port 110 and not port 25 and not port 53 and not port 22
A useful option is to de-clutter the display with the -t flag, which
suppresses the timestamps:
# tcpdump -t not port 110 and not port 25
You can speed up performance and decrease clutter further by turning off
DNS lookups with the -n flag:
# tcpdump -tn not port 110 and not port 25
Or you can go nuts and increase the amount of data shown with the -v
and -vv flags. -vv is the most verbose:
# tcpdump -vv
Here is a sample of what you'll see with -vv:
192.168.1.5.35401 > 69.56.234.130.995: R [tcp sum ok] 4522529:394522529(0 win 0 (DF) (ttl 64, id 684, len 40)
12.169.174.5.1985 > 224.0.0.2.1985: [udp sum ok] udp 20 [tos 0xc0] (ttl 2, id 0, len 48)
To print all packets arriving at or departing from sundown:
tcpdump host sundown
To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup: (note that the expression
is quoted to prevent the shell from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'
To print traffic neither sourced from nor destined for local hosts (if you gateway
to one other net, this stuff should never make it onto your local net).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN pack ets) of each TCP conversation
that involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were not sent via ethernet
broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
This article is the final part of my
three-part series covering 18 different
tcpdump
tips
and tricks where I continue to demonstrate features that help you filter and organize the information returned by
tcpdump
.
I recommend reading
parts
one
and
two
before
continuing with the content below.
# tcpdump -i any -c4 -A
tcpdump: data link type LINUX_SLL2
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
21:03:21.363917 wlp0s20f3 Out IP6 kkulkarni > ff02::1:ff0e:bfb6: ICMP6, neighbor solicitation, who has kkulkarni, length 32
`.... :.........Q{AZq..w.................................r.pm.....`.b...
21:03:21.363953 lo In IP6 kkulkarni.45656 > kkulkarni.hostmon: Flags [S], seq 3428690149, win 65476, options [mss 65476,sackOK,TS val 1750938785 ecr 0,nop,wscale 7,tfo cookiereq,nop,nop], length 0
`....,...........r.pm............r.pm....X...].....................
h]4........."...
21:03:21.363972 lo In IP6 kkulkarni.hostmon > kkulkarni.45656: Flags [S.], seq 3072789718, ack 3428690150, win 65464, options [mss 65476,sackOK,TS val 1750938785 ecr 1750938785,nop,wscale 7], length 0
`....(...........r.pm............r.pm......X.'...].................
h]4.h]4.....
21:03:21.363988 lo In IP6 kkulkarni.45656 > kkulkarni.hostmon: Flags [.], ack 1, win 512, options [nop,nop,TS val 1750938785 ecr 1750938785], length 0
`.... ...........r.pm............r.pm....X...]...'.......w.....
h]4.h]4.
4 packets captured
173 packets received by filter
0 packets dropped by kernel
15. Options for extra verbosity
With some Linux programs, it's sometimes
useful to have more verbose output.
tcpdump
uses
-v
,
-vv
,
or
-vvv
to
provide different levels of verbosity. See below for examples with no verbosity to three levels of verbosity.
Default verbosity:
# tcpdump -i any -c1
tcpdump: data link type LINUX_SLL2
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
21:06:00.903186 lo In IP kkulkarni.39876 > kkulkarni.hostmon: Flags [S], seq 1718143023, win 65495, options [mss 65495,sackOK,TS val 1879208671 ecr 0,nop,wscale 7,tfo cookiereq,nop,nop], length 0
1 packet captured
100 packets received by filter
0 packets dropped by kernel
Using the
-v
option:
# tcpdump -i any -c1 -v
tcpdump: data link type LINUX_SLL2
dropped privs to tcpdump
tcpdump: listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
21:06:04.209638 lo In IP6 (flowlabel 0xd17f0, hlim 1, next-header TCP (6) payload length: 44) kkulkarni.33022 > kkulkarni.hostmon: Flags [S], cksum 0x0d5b (incorrect -> 0x6c92), seq 2003870985, win 65476, options [mss 65476,sackOK,TS val 3266653263 ecr 0,nop,wscale 7,tfo cookiereq,nop,nop], length 0
1 packet captured
20 packets received by filter
0 packets dropped by kernel
Here is the
-vv
option:
# tcpdump -i any -c1 -vv
tcpdump: data link type LINUX_SLL2
dropped privs to tcpdump
tcpdump: listening on any, link-type LINUX_SLL2 (Linux cooked v2), snapshot length 262144 bytes
21:06:05.916423 tun0 Out IP (tos 0x0, ttl 64, id 22069, offset 0, flags [DF], proto TCP (6), length 1360)
kkulkarni.37152 > 10.0.115.119.https: Flags [.], cksum 0xe218 (correct), seq 168413028:168414336, ack 944490821, win 502, options [nop,nop,TS val 1351042119 ecr 3391883323], length 1308
1 packet captured
235 packets received by filter
0 packets dropped by kernel
Finally, display the highest level of detail
with the
-vvv
option:
Below are a few options (with examples) that will help you greatly when working
with the tool. They're easy to forget and/or confuse with other types of filters,
i.e. ethereal, so hopefully this page can serve as a reference for you, as it
does me.
First off, I like to add a few options to the tcpdump command itself,
depending on what I'm looking at. The first of these is -n, which requests
that names are not resolved, resulting in the IPs themselves always being displayed.
The second is -X, which displays both hex and ascii content within the
packet. The final one is -S, which changes the display of sequence numbers
to absolute rather than relative. The idea there is that you can't see weirdness
in the sequence numbers if they're being hidden from you. Remember, the advantage
of using tcpdump vs. another tool is getting manual interaction with
the packets.
It's also important to note that tcpdump only takes the first
68 96 bytes of data from a packet by default. If you would
like to look at more, add the -s number option to the mix, where
number is the number of bytes you want to capture. I recommend using
0 (zero) for a snaplength, which gets everything. Here's a short list of the
options I use most:
-i any : Listen on all interfaces just to see if you're seeing
any traffic.
-n : Don't resolve hostnames.
-nn : Don't resolve hostnames or port names.
-X : Show the packet's contents in both
hex and
ASCII.
-XX : Same as -X,
but also shows the ethernet header.
-v, -vv, -vvv : Increase the amount of packet information you
get back.
-c : Only get x number of packets and then stop.
-s : Define the size of the capture (use -s0 unless
you are intentionally capturing less.)
-S : Print absolute sequence numbers.
-e : Get the ethernet header as well.
-q : Show less protocol information.
-E : Decrypt IPSEC traffic by providing an encryption key.
-s : Set the snaplength, i.e. the amount of data that
is being captured in bytes
-c : Only capture x number of packets, e.g. 'tcpdump -c
3'
[ The default snaplength as of tcpdump 4.0 has changed from 68 bytes
to 96 bytes. While this will give you more of a packet to see, it still won't
get everything. Use -s 1514 to get full coverage ]
Another way to process the content from tcpdump is to save the raw network packet
data to a file and then process the file to find and decode the information
that you want.
There are a number of modules in different languages that provide functionality
for reading and decoding the data captured by tcpdump and snoop. For example,
within Perl, there are two modules: Net::SnoopLog (for snoop) and Net::TcpDumpLog
(for tcpdump). These will read the raw data content. The basic interfaces for
both of these modules is the same.
To start, first you need to create a binary record of the packets going past
on the network by writing out the data to a file using either snoop or tcpdump.
For this example, we'll use tcpdump and the Net::TcpDumpLog module: $ tcpdump
-w packets.raw.
Once you have amassed the network data, you can start to process the network
data contents to find the information you want. The Net::TcpDumpLog parses the
raw network data saved by tcpdump. Because the data is in it's raw binary format,
parsing the information requires processing this binary data. For convenience,
another suite of modules, NetPacket::*, provides decoding of the raw data.
For example, Listing 8 shows a simple script that prints
out the IP address information for all of the packets.
Listing 8. Simple script that prints out the IP address info
for all packets
use Net::TcpDumpLog;
use NetPacket::Ethernet;
use NetPacket::IP;
my $log =
foreach my $index ($log->indexes) {
my $packet = $log->data($index);
my $ethernet = NetPacket::Ethernet->decode($packet);
if ($ethernet->{type} == 0x0800) {
my $ip = NetPacket::IP->decode($ethernet->{data});
printf(" %s to %s protocol %s \n",
$ip->{src_ip},$ip->{dest_ip},$ip->{proto});
}
}
The first part is to extract each packet. The Net::TcpDumpLog
module serializes each packet, so that we can read each packet by using the
packet ID. The data() method then returns the raw data for the
enti ket.
As with the output from snoop, we have to extract each of the
blocks of data from the raw network packet information. So in this example,
we first need to extract the ethernet packet, including the data payload, from
the raw network packet. The NetPacket::Ethernet module does this
for us.
Since we are looking for IP packets, we can check for IP packets by looking
at the Ethernet packet type. IP packets have an ID of 0x0800.
The NetPacket::IP module can then be used to extract the IP
information from the data payload of the Ethernet packet. The module provides
the source IP, destination IP and protocol information, among others, which
we can then print.
Using this basic framework you can perform more complex lookups and decoding
that do not rely on the automated solutions provided by tcpdump or snoop. For
example, if you suspect that there is HTTP traffic going past on a non-standard
port (i.e., not port 80), you could look for the string HTTP on ports other
than 80 from the suspected host IP using the script in Listing
9.
$ perl http-non80.pl
Found HTTP traffic on non-port 80
192.168.0.2 (port: 39280) to 168.143.162.100 (port: 80)
GET /statuses/user_timeline.json HTTP/1.1
Found HTTP traffic on non-port 80
192.168.0.2 (port: 39282) to 168.143.162.100 (port: 80)
GET /statuses/friends_timeline.json HTTP/1
In this particular case we're seeing traffic from the host to an external
website (Twitter).
Obviously, in this example, we are dumping out the raw data, but you could
use the same basic structure to decode and the data in any format using any
public or proprietary protocol structure. If you are using or developing a protocol
using this method, and know the protocol format, you could extract and monitor
the data being transferred.
Although, as already mentioned, tools like tcpdump, iptrace and snoop provide
basic network analysis and decoding, there are GUI-based tools that make the
process even easier. Wireshark is one such tool that supports a vast array of
network protocol decoding and analysis.
One of the main benefits of Wireshark is that you can captu kets over
a period of time (just as with tcpdump) and then interactively analyze and filter
the content based on the different protocols, ports and other data. Wireshark
also supports a huge array of protocol decoders, enabling you to examine in
minute detail the contents of the packets and conversations.
You can see the basic screenshot of Wireshark showing all of the packets
of all types being listed in Figure 1. The window is divided
into three main sections: the list of filtered packets, the decoded protocol
details, and the raw packet data in hex/ASCII format.
This report details an investigation of the TCPDUMP utility.
Tcpdump is a powerful tool that allows us to sniff network packets and make
some statistical analysis out of those dumps. One major drawback to tcpdump
is the size of the flat file containing the text output. But tcpdump allows
us to precisely see all the traffic and enables us to create statistical monitoring
scripts.
In our case, looking at an ethernet segment, tcpdump operates by putting
the network card into promiscuous mode in order to capture all the packets going
through the wire. Tcpdump runs using BSD Packet Filter (BPF) which is the method
of collecting data from this network interface running into promiscuous mode.
BPF receives copies from the driver of sent packets and received packets. Before
traveling through the kernel all the way up to the user process the user can
set a filter so only interesting packets go through the Kernel. SUN OS uses
Network Interface Tap (NIT) which only allows to captu kets received from
the interface but no packets sent by the host. Still the SUN OS tcpdump does
the trick but it performs its own filtering at the user process level which
means nel.
We use tcpdump to measure the response time and the packet loss percentages.
It can also tell us about lack of reachability for some distant server.
Using tcpdump we have a view on any TCP/UDP connection Establishment and
Termination. TCP uses a special mechanism to set and close connections (we will
discuss this later on); we measure the time lapse between the packets involved
with this mechanism in order to know how fast some connections operate.
Pretty weak paper. Still some example might be useful ...
The syntax I used to copy all of the packets being sent and received by the
exploit is as follows:
tcpdump -nXvSs 0 ip and host 192.168.1.101 -w sploit_log
The above noted tcpdump filter will copy all packets with a valid IP header
in them, and write them to the binary log called "sploit_log". Lastly the IP
address of the lab machine being tested is 192.168.1.101.
Now that I have the binary log file and the exploit has been executed I can
look at the packets themselves, and see if there is something telling in the
packets themselves. I use the following tcpdump syntax to look at the file that
was generated:
Last week we took a deep dive into decoding TCP headers, so that we would
know what the heck we were looking at when firing up tcpdump. Today we'll
dig into specific tcpdump commands for looking at traffic from specific protocols,
hosts, or ports, and how to save tcpdump's output to various file formats for
analysis with other utilities.
Exporting tcpdumps To File
In these here spyware-infested times, one of the most useful
things you can do with a packet sniffer is monitor the traffic coming
off Windows PCs.
tcpdump merely captures packet headers; it does not have reporting or analysis
capabilities. If you are going to do any kind of extended packet capture and
analysis, it is best to capture it to a file, then use something like Ethereal
or Snort to sort it out. As with all things Linux, there are several ways to
do this. Use the -l switch to create re-directable
output, or as man tcpdump says "Make stdout line buffered."
The following command stores output in webdump.txt, and displays it on
the screen at the same time:
This creates a human-readable ASCII text file. Another way to do this is
to capture the raw data in a binary file. This is a good thing to do when you
want to capture a huge glob of unfiltered traffic and analyze it later with
Ethereal or Snort:
# tcpdump -w rawdump
You can also replay it in tcpdump by naming the file to be read with the
-r switch, then re-directing the output to another file:
# tcpdump -r rawdump > rawdump.txt
tcpdump runs until you stop it with Ctrl+c, so you might want to set a limit
on the number of packets to capture, so that it will stop itself:
# tcpdump -c1000 -w rawdump
tcpdump defaults to the lowest-numbered NIC, which is usually eth0. Use the
-i flag to specify a different NIC:
# tcpdump -i eth1 -c1000 -w rawdump
Common Commands
These two commands are the tcpdump workhorses. The first one captures traffic
on a specific port. The second one captures traffic on a specific host on the
LAN. (See
Part 1, "Hubs Are Blabbermouths" and "Foiled By Switches" for more on how
to do this.)
# tcpdump port nn
# tcpdump host 1.2.3.4
You can select several hosts on your LAN, and capture the traffic that passes
between them:
# tcpdump host workstation4 and workstation11 and workstation13
Or capture all the LAN traffic between workstation4 and the LAN, except for
workstation11:
# tcpdump workstation4 except workstation11
It can be useful to show the
MAC
addresses, especially when you're debugging network problems, because the
MAC address is the definitive identification for a host. Use the -e flag:
# tcpdump -e host workstation4 and workstation11 and workstation13
Yes, astute reader, you read it right- you can spy on the traffic exchanged
between certain computers on your LAN.
Excluding Things
The default is to capture all packets. You can capture all packets except those
for certain ports, like this:
tcpdump not port 110 and not port 25 and not port 53 and not port 22
A useful option is to declutter the display with the -t flag, which
suppresses the timestamps:
# tcpdump -t not port 110 and not port 25
You can speed up performance and decrease clutter further by turning off
DNS lookups with the -n flag:
# tcpdump -tn not port 110 and not port 25
Or you can go nuts and increase the amount of data shown with the -v
and -vv flags. -vv is the most verbose:
# tcpdump -vv
Here is a sample of what you'll see with -vv:
192.168.1.5.35401 > 69.56.234.130.995: R [tcp sum ok] 4522529:394522529(0 win 0 (DF) (ttl 64, id 684, len 40)
12.169.174.5.1985 > 224.0.0.2.1985: [udp sum ok] udp 20 [tos 0xc0] (ttl 2, id 0, len 48)
Some of the additional fields displayed are:
checksum. A checksum is computed for each segment; damaged segments
are discarded.
ttl is time to live. The standard ttl is two minutes; if a packet
is still floating around undelivered after two minutes, it is discarded. In
this example, the limit is 64 seconds.
id is a segment identification number, to assist in re-assembling
datagram fragments.
tos, type of service, contains the values for precedence, delay, throughput,
and reliability. Applications can theoretically set these values; for example,
streaming video wants low latency, which is the delay value. In practice, this
field is mostly useless in IPv4, because routers can be configured to ignore
the TOS field, and there is no way to enforce the sensible use of it. IPv6 promises
to make it meaningful, but again the problem lies in the implementation, not
the standard.
len is the length, in bytes, of the segment.
Filtering On Protocols
A very useful tcpdump filter is the ability to filter on different protocols.
Suppose you want to see only udp traffic:
# tcpdump udp
tcpdump recognizes the keywords ip, igmp, tcp, udp, and icmp.
You can also specify any protocol named in /etc/protocols by using the
proto qualifier. This example captures ospf (Open Shortest Path First)
traffic, which is useful for eavesdropping on routers:
# tcpdump ip proto OSPFIGP
To capture traffic on a specific host and restrict by protocol, do this:
# tcpdump host server02 and ip # tcpdump host server03 and not udp # tcpdump host server03 and ip and igmp and not udp
Wrapping Up
When you look at what goes over the wires after the simple act of clicking
on a URL, or checking email, it is amazing that it works at all. In these here
spyware-infested times, one of the most useful things you can do with a packet
sniffer is monitor the traffic coming off Windows PCs. Nothing can be hidden,
it's all there for your keen probing eye to find.
I wrote this filter for a friend that told me that I couldn't do this in AWK.
I also learned AWK doing this. This is NOT the proper way to do this,
Perl or C is definatly better. But, due to how simple it is, it can quickly
be changed to display the data in any way you want and support more prootcols
too. Right now it only supports IP, TCP, UDP, and ICMP. Drop me a line if you
have any comments or questions.
There is also a few things that can be configured, such as the margins, the
byte order of your machine, and also the starting position of the ip header
in the output. The last option is due to a bug konwn in tcpdump when dumping
on FDDI interfaces. See the script for more information.
Script is at end of examples. Beware, my AWK code looks like C without semicolons.
-sSnarf snaplen bytes of data from each packet rather than
the default of 68 (with SunOS's NIT, the minimum is actually 96).
68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate
protocol information from name server
and NFS packets (see below). Packets truncated
because of a limited snapshot are indicated in
the output with ``[|proto]'', where proto
is the name of the protocol level at which the truncation
has occurred. Note that taking larger snapshots
both increases the amount of time it takes to process packets and, effectively,
decreases the amount of packet buffering. This may cause packets
to be lost. You should limit snaplen
to the smallest number that will capture the protocol information you're interested
in.
Setting snaplen to 0 means use the required length
to catch whole packets.
-C Before writing a raw packet to
a savefile, check whether the file is currently larger than file_size
and, if so, close the current savefile and open a new
one. Savefiles after the first savefile will have the name specified
with the -w flag, with a number after
it, starting at 2 and continuing
upward. The units of file_size are
millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
-i Listen on interface. If unspecified,
tcpdump searches the system interface list for the
lowest numbered, configured up interface (excluding loop back).
Ties are broken by choosing the earliest match.
-w Write the raw
packets to file rather than parsing and printing
them out. They can later be printed with the -r option. Standard
output is used if file is ``-''.
-r Used to print
of view previous capture. For example
tcpdump -r file_with_capture.
There are several useful options that should be used with it.
-n option blocks DNS resolution.
-nn blocks recognition of protocol
and port as well.
-v (verbose) -vv (super-verbose) can be uses to get more information.
Tcpdump prints out the headers of packets on a network interface that
match the boolean expression. It can also be run with the -w flag,
which causes it to save the packet data to a file for later analysis, and/or with
the -b flag, which causes it to read from a saved packet file rather than
to read packets from a network interface. In all cases, only packets that match
expression will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue cap turing
packets until it is interrupted by a SIGINT signal (generated, for example, by typing
your interrupt charac ter, typically control-C) or a SIGTERM signal (typically
generated with the kill(1) command); if run with the -c flag, it will
captu kets until it is interrupted by a SIGINT or SIGTERM signal or the specified
number of packets have been processed.
When tcpdump finishes capturing packets, it will report counts of:
packets ``received by filter'' (the meaning of this depends on the OS on which
you're running tcpdump, and possibly on the way the OS was configured - if
a filter was specified on the command line, on some OSes it counts packets regardless
of whether they were matched by the filter expression, and on other OSes it counts
only packets that were matched by the filter expression and were processed by
tcpdump);
packets ``dropped by kernel'' (this is the number of packets that were dropped,
due to a lack of buffer space, by the packet capture mechanism in the OS on which
tcpdump is running, if the OS reports that information to applications; if
not, it will be reported as 0).
On platforms that support the SIGINFO signal, such as most BSDs, it will report
those counts when it receives a SIG INFO signal (generated, for example, by typing
your ``sta tus'' character, typically control-T) and will continue you have special
privileges:
Under SunOS 3.x or 4.x with NIT or BPF: You must have read access to
/dev/nit or /dev/bpf*.
Under Solaris with DLPI: You must have read/write access to the network
pseudo device, e.g. /dev/le. On at least some versions of Solaris, however,
this is not suffi cient to allow tcpdump to capture in promiscuous mode;
on those versions of Solaris, you must be root, or tcpdump must be installed
setuid to root, in order to capture in promiscuous mode.
Under HP-UX with DLPI: You must be root or tcpdump must be installed
setuid to root.
Under IRIX with snoop: You must be root or tcpdump must be installed
setuid to root.
Under Linux: You must be root or tcpdump must be installed setuid
to root.
Under Ultrix and Digital UNIX: Once the super-user has enabled promiscuous-mode
operation using pfconfig(8), any user may capture network traffic with
tcpdump.
Under BSD: You must have read access to /dev/bpf*.
Reading a saved packet file doesn't require special privileges.
-a Attempt to convert network and broadcast addresses to names.
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check whether the file
is currently larger than file_size and, if so, close the
current savefile and open a new one. Savefiles after the first savefile will
have the name specified with the -w flag, with a number after it, starting
at 2 and continuing upward. The units of file_size are
millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (pre ceded with
a count).
-e Print the link-level header on each dump line.
-E Use algo:secret for decrypting IPsec ESP packets. Algorithms
may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc,
cast128-cbc, or none. The default is des- cbc. The ability
to decrypt packets is only pre sent if tcpdump was compiled with cryptography
enabled. secret the ascii text for ESP secret key. We cannot take arbitrary
binary value at this moment. The option assumes RFC2406 ESP, not RFC1827 ESP.
The option is only for debugging pur poses, and the use of this option with
truly `secret' key is discouraged. By presenting IPsec secret key onto command
line you make it visible to others, via ps(1) and other occasions.
-f Print `foreign' internet addresses numerically rather than symbolically
(this option is intended to get around serious brain damage in Sun's yp server
-- usually it hangs forever translating non- local internet numbers).
-F Use file as input for the filter expression. An additional
expression given on the command line is ignored.
-i Listen on interface. If unspecified, tcpdump searches
the system interface list for the lowest numbered, configured up interface (excluding
loop back). Ties are broken by choosing the earliest match.
On Linux systems with 2.2 or later kernels, an interface argument
of ``any'' can be used to cap tu kets from all interfaces. Note that cap
tures on the ``any'' device will not be done in promiscuous mode.
-l Make stdout line buffered. Useful if you want to see the data
while capturing it. E.g., ``tcpdump -l | tee dat'' or ``tcpdump -l > dat & tail
-f dat''.
-m Load SMI MIB module definitions from file module. This
option can be used several times to load sev eral MIB modules into tcpdump.
numbers, etc.) to names.
-N Don't print domain name qualification of host names. E.g., if
you give this flag then tcpdump will print ``nic'' instead of ``nic.ddn.mil''.
-O Do not run the packet-matching code optimizer. This is useful
only if you suspect a bug in the optimizer.
-pDon't put the interface into promiscuous mode. Note that
the interface might be in promiscuous mode for some other reason; hence, `-p'
cannot be used as an abbreviation for `ether host {local-hw- addr} or ether
broadcast'.
-q Quick (quiet?) output. Print less protocol infor mation so output
lines are shorter.
-R Assume ESP/AH packets to be based on old specifica tion (RFC1825
to RFC1829). If specified, tcpdump will not print replay prevention field.
Since there is no protocol version field in ESP/AH speci fication, tcpdump
cannot deduce the version of ESP/AH protocol.
-r Read packets from file (which was created with the -w option).
Standard input is used if file is ``-''.
-S Print absolute, rather than relative, TCP sequence numbers.
-s Snarf snaplen bytes of data from each packet rather than
the default of 68 (with SunOS's NIT, the mini mum is actually 96). 68 bytes
is adequate for IP, ICMP, TCP and UDP but may truncate protocol infor mation
from name server and NFS packets (see below). Packets truncated because of a
limited snapshot are indicated in the output with ``[|proto]'', where
proto is the name of the proto col level at which the truncation has
occurred. Note that taking larger snapshots both increases the amount of time
it takes to process packets and, effectively, decreases the amount of packet
buffer ing. This may cause packets to be lost. You should limit snaplen
to the smallest number that will capture the protocol information you're inter
ested in. Setting snaplen to 0 means use the required length to catch
whole packets.
-T Force packets selected by "expression" to be inter cedure
Call), rtp (Real-Time Applications proto col), rtcp (Real-Time
Applications control proto col), snmp (Simple Network Management Protocol),
vat (Visual Audio Tool), and wb (distributed White Board).
-tDon't print a timestamp on each dump line.
-tt Print an unformatted timestamp on each dump line.
-ttt Print a delta (in micro-seconds) between current and previous
line on each dump line.
-tttt Print a timestamp in default format proceeded by date on each
dump line. -u Print undecoded NFS handles.
-v (Slightly more) verbose output. For example, the time to live,
identification, total length and options in an IP packet are printed. Also enables
additional packet integrity checks such as verify ing the IP and ICMP header
checksum.
-vv Even more verbose output. For example, additional fields are
printed from NFS reply packets, and SMB packets are fully decoded.
-vvv Even more verbose output. For example, telnet SB ...
SE options are printed in full. With -X telnet options are printed
in hex as well.
-w Write the raw packets to file rather than parsing and printing
them out. They can later be printed with the -r option. Standard output is used
if file is ``-''.
-x Print each packet (minus its link level header) in hex. The smaller
of the enti ket or snaplen bytes will be printed.
-X When printing hex, print ascii too. Thus if -x is also
set, the packet is printed in hex/ascii. This is very handy for analysing new
protocols. Even if -x is not also set, some parts of some packets may
be printed in hex/ascii.
Filter expression selects which packets will be dumped. If no expression
is given, all packets on the net will be dumped. Otherwise, only packets for which
expression is `true' will be dumped. ber) preceded by one or more
qualifiers. There are three different kinds of qualifier:
type qualifiers say what kind of thing the id name or number refers
to. Possible types are host, net and port. E.g., `host
foo', `net 128.3', `port 20'. If there is no type qualifier, host is
assumed.
dir qualifiers specify a particular transfer direction to and/or
from id. Possible directions are src, dst, src or dst
and src and dst. E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'.
If there is no dir qualifier, src or dst is assumed. For `null' link
layers (i.e. point to point pro tocols such as slip) the inbound and
out bound qualifiers can be used to specify a desired direction.
proto qualifiers restrict the match to a particu lar protocol. Possible
protos are: ether, fddi, tr, ip, ip6,
arp, rarp, decnet, tcp and udp. E.g., `ether
src foo', `arp net 128.3', `tcp port 21'. If there is no proto qualifier, all
protocols consistent with the type are assumed. E.g., `src foo' means `(ip or
arp or rarp) src foo' (except the latter is not legal syntax), `net bar' means
`(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
[`fddi' is actually an alias for `ether'; the parser treats them identically
as meaning ``the data link level used on the specified network interface.'' FDDI
headers contain Ethernet-like source and destination addresses, and often contain
Ethernet-like packet types, so you can filter on these FDDI fields just as with
the analogous Ethernet fields. FDDI headers also contain other fields, but you cannot
name them explicitly in a filter expression.
Similarly, `tr' is an alias for `ether'; the previous paragraph's statements
about FDDI headers also apply to Token Ring headers.]
In addition to the above, there are some special `primitive' keywords that don't
follow the pattern: gateway, broadcast, less, greater
and arithmetic expressions. All of these are described below.
E.g., `host foo and not port ftp and not port ftp-data'. To save typing,
identical qualifier lists can be omitted. E.g., `tcp dst port ftp or ftp-data or
domain' is exactly the same as `tcp dst port ftp or tcp dst port ftp-data or tcp
dst port domain'.
dst hosthost True if the IPv4/v6 destination field of the
packet is host, which may be either an address or a name.
src hosthost True if the IPv4/v6 source field of the packet
is host.
hosthost True if either the IPv4/v6 source or desti nation
of the packet is host. Any of the above host expressions can be prepended
with the keywords, ip, arp, rarp, or ip6 as in:
ip hosthost which is equivalent to: ether proto\ipand hosthost If host is a name with multiple IP addresses,
each address will be checked for a match.
ether dstehost True if the ethernet destination address is
ehost. Ehost may be either a name from /etc/ethers or a number
(see ethers(3N) for numeric format).
ether srcehost True if the ethernet source address is
ehost.
ether hostehost True if either the ethernet source or desti
nation address is ehost.
gatewayhost True if the packet used host as a gateway.
I.e., the ethernet source or destination address was host but neither
the IP source nor the IP destination was host. Host must be a
name and must be found both by the machine's host-name-to-IP-address resolution
mechanisms (host name file, DNS, NIS, etc.) etc.). (An equivalent expression
is ether hostehostand not hosthost which can
be used with either names or num bers for host/ehost.)
This syntax does not work in IPv6-enabled configuration at this moment.
dst netnet True if the IPv4/v6 destination address of the
packet has a network number of net. Net may be either a name from
/etc/networks or a network number (see networks(4) for details).
src netnet True if the IPv4/v6 source address of the packet
has a network number of net.
netnet True if either the IPv4/v6 source or desti nation
address of the packet has a network number of net.
netnetmasknetmask True if the IP address
matches net with the specific netmask. May be qualified with
src or dst. Note that this syntax is not valid for IPv6 net.
netnet/len True if the IPv4/v6 address matches
net with a netmask len bits wide. May be qualified with src
or dst.
dst portport True if the packet is ip/tcp, ip/udp, ip6/tcp
or ip6/udp and has a destination port value of port. The port
can be a num ber or a name used in /etc/services (see tcp(4P) and
udp(4P)). If a name is used, both the port number and protocol are checked.
If a number or ambiguous name is used, only the port number is checked (e.g.,
dst port 513 will print both tcp/login traf fic and udp/who traffic,
and port domain will print both tcp/domain and udp/domain traffic).
src portport True if the packet has a source port value of
port. True if either the source or destination port of the packet is
port. Any of the above port expressions can be prepended with the keywords,
tcp or udp, as in: tcp src portport which matches
only tcp packets whose source port is port.
lesslength True if the packet has a length less than or equal
to length. This is equivalent to: len <=length.
greaterlength True if the packet has a length greater than
or equal to length. This is equivalent to: len >=length.
ip protoprotocol True if the packet is an IP packet (see
ip(4P)) of protocol type protocol. Protocol can be a number
or one of the names icmp, icmp6, igmp, igrp,
pim, ah, esp, vrrp, udp, or tcp. Note
that the identifiers tcp, udp, and icmp are also keywords
and must be escaped via backslash (\), which is \\ in the C-shell. Note that
this primitive does not chase the protocol header chain.
ip6 protoprotocol True if the packet is an IPv6 packet of
pro tocol type protocol. Note that this primi tive does not chase the
protocol header chain.
ip6 protochainprotocol True if the packet is IPv6 packet,
and con tains protocol header with type protocol in its protocol header
chain. For example, ip6 protochain 6 matches any IPv6 packet with TCP
protocol header in the protocol header chain. The packet may contain, for example,
authentica tion header, routing header, or hop-by-hop option header, between
IPv6 header and TCP header. The BPF code emitted by this primi tive is complex
and cannot be optimized by BPF optimizer code in tcpdump, so this can
be somewhat slow.
ip protochainprotocol Equivalent to ip6 protochainprotocol, but True if the packet is an ethernet broadcast packet. The
ether keyword is optional.
ip broadcast True if the packet is an IP broadcast packet. It checks
for both the all-zeroes and all-ones broadcast conventions, and looks up the
local subnet mask.
ether multicast True if the packet is an ethernet multicast packet.
The ether keyword is optional. This is shorthand for `ether[0] & 1
!= 0'.
ip multicast True if the packet is an IP multicast packet.
ip6 multicast True if the packet is an IPv6 multicast packet.
ether protoprotocol True if the packet is of ether type
protocol. Protocol can be a number or one of the names
ip, ip6, arp, rarp, atalk, aarp,
decnet, sca, lat, mopdl, moprc,
iso, stp, ipx, or netbeui. Note these identifiers
are also keywords and must be escaped via backslash (\).
[In the case of FDDI (e.g., `fddi protocol arp') and Token Ring (e.g.,
`tr protocol arp'), for most of those protocols, the pro tocol identification
comes from the 802.2 Logical Link Control (LLC) header, which is usually layered
on top of the FDDI or Token Ring header.
When filtering for most protocol identifiers on FDDI or Token Ring, tcpdump
checks only the protocol ID field of an LLC header in so-called SNAP format
with an Organizational Unit Identifier (OUI) of 0x000000, for encapsulated Ethernet;
it doesn't check whether the packet is in SNAP format with an OUI of 0x000000.
The exceptions are iso, for which it checks the DSAP (Destination
Service Access Point) and SSAP (Source Service Access Point) fields of the LLC
header, stp and netbeui, packet with an OUI of 0x080007 and the
Appletalk etype.
In the case of Ethernet, tcpdump checks the Ethernet type field for
most of those proto cols; the exceptions are iso, sap, and
netbeui, for which it checks for an 802.3 frame and then checks
the LLC header as it does for FDDI and Token Ring, atalk, where it checks
both for the Appletalk etype in an Ethernet frame and for a SNAP-format packet
as it does for FDDI and Token Ring, aarp, where it checks for the Appletalk
ARP etype in either an Ethernet frame or an 802.2 SNAP frame with an OUI of
0x000000, and ipx, where it checks for the IPX etype in an Eth ernet
frame, the IPX DSAP in the LLC header, the 802.3 with no LLC header encapsulation
of IPX, and the IPX etype in a SNAP frame.]
decnet srchost True if the DECNET source address is host,
which may be an address of the form ``10.123'', or a DECNET host name. [DECNET
host name support is only available on Ultrix systems that are configured to
run DECNET.]
decnet dsthost True if the DECNET destination address is
host.
decnet hosthost True if either the DECNET source or destina
tion address is host.
ip, ip6, arp, rarp, atalk, aarp,
decnet, iso, stp, ipx, netbeui Abbreviations
for: ether protop where p is one of the above protocols.
lat, moprc, mopdl Abbreviations for: ether protop where p is one of the above protocols. Note that tcpdump
does not currently know how to parse these protocols.
vlan[vlan_id] True if the packet is an IEEE
802.1Q VLAN packet. If [vlan_id] is specified, only encountered
in expression changes the decod ing offsets for the remainder of
expression on the assumption that the packet is a VLAN packet.
tcp, udp, icmp Abbreviations for: ip protopor ip6 protop where p is one of the above protocols.
iso protoprotocol True if the packet is an OSI packet of
pro tocol type protocol. Protocol can be a num ber or one of
the names clnp, esis, or isis.
clnp, esis, isis Abbreviations for: iso protop where p is one of the above protocols. Note that tcpdump
does an incomplete job of pars ing these protocols.
exprrelopexpr True if the relation holds, where
relop is one of >, <, >=, <=, =, !=, and expr is an arithmetic
expression composed of integer constants (expressed in standard C syntax), the
normal binary operators [+, -, *, /, &, |], a length operator, and special packet
data accessors. To access data inside the packet, use the following syntax:
proto[expr:size]Proto
is one of ether, fddi, tr, ip, arp, rarp, tcp, udp, icmp or ip6,
and indicates the protocol layer for the index operation. Note that tcp,udp and other upper-layer protocol types only apply to IPv4, not IPv6
(this will be fixed in the future). The byte offset, relative to the indicated
pro tocol layer, is given by expr. Size is optional and indicates
the number of bytes in the field of interest; it can be either one, two, or
four, and defaults to one. The length operator, indicated by the keyword
len, gives the length of the packet.
For example, `ether[0] & 1 != 0' catches all multicast traffic. The
expression `ip[0] & 0xf != 5' catches all IP packets with options. The
expression `ip[6:2] & 0x1fff = 0' catches only unfragmented datagrams
and frag zero of fragmented datagrams. This always means the first byte of the
TCP header, and never means the first byte of an intervening fragment.
Some offsets and field values may be expressed as names rather than as numeric
values. The following protocol header field offsets are available: icmptype
(ICMP type field), icmpcode (ICMP code field), and tcpflags (TCP
flags field).
The following ICMP type field values are available: icmp-echoreply,
icmp-unreach, icmp-sourcequench, icmp-redirect, icmp-echo,
icmp-routeradvert, icmp-routersolicit, icmp- timxceed,
icmp-paramprob, icmp-tstamp, icmp- tstampreply, icmp-ireq,
icmp-ireqreply, icmp-maskreq, icmp-maskreply.
The following TCP flags field values are available: tcp-fin, tcp-syn,
tcp-rst, tcp- push, tcp-push, tcp-ack, tcp-urg.
Primitives may be combined using:
A parenthesized group of primitives and operators (parentheses are special to
the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation have equal precedence
and associate left to right. Note that explicit and tokens, not juxtaposition,
are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword is assumed.
For example,
not host vs and ace
is short for not host vs and host ace
which should not be confused with not ( host vs or ace )
Expression arguments can be passed to tcpdump as either a single argument
or as multiple arguments, whichever is more convenient. Generally, if the expression
contains Shell metacharacters, it is before being parsed.
The output of tcpdump is protocol dependent. The following gives a brief
description and examples of most of the formats.
Link Level Headers
addresses, protocol, and packet length are printed.
On FDDI networks, the '-e' option causes tcpdump to print the `frame control'
field, the source and destination addresses, and the packet length. (The `frame
control' field governs the interpretation of the rest of the packet. Normal packets
(such as those containing IP data grams) are `async' packets, with a priority value
between 0 and 7; for example, `async4'. Such packets are assumed to contain
an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is
not an ISO datagram or a so-called SNAP packet.
ARP/RARP Packets
Arp/rarp output shows the type of request and its argu ments. The format is
intended to be self explanatory. arp who-has csam tell rtsg arp reply csam is-at
CSAM The first line says that rtsg sent an arp packet asking for the ethernet
address of internet host csam. Csam replies with its ethernet address (in this example,
ether net addresses are in caps and internet addresses in lower case).
This would look less redundant if we had done tcpdump-n: arp
who-has 128.3.254.6 tell 128.3.254.68 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump-e, the fact that the first packet is broadcast
and the second is point-to-point would be visible: RTSG Broadcast 0806 64: arp
who-has csam tell rtsg CSAM RTSG 0806 64: arp reply csam is-at CSAM For the
first packet this says the ethernet source address is RTSG, the destination is the
ethernet broadcast address, the type field contained hex 0806 (type ETHER_ARP) and
the total length was 64 bytes.
The general format of a tcp protocol line is: src>dst:flagsdata-seqnoackwindowurgentoptionsSrc and dst are the source and destination IP addresses and ports.
Flags are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a
single `.' (no flags). Data-seqno describes the portion of sequence space
covered by the data in this packet (see example below). Ack is sequence number
of the next data expected the other direc tion on this connection. Window
is the number of bytes of receive buffer space available the other direction on
this connection. Urg indicates there is `urgent' data in the packet. Options
are tcp options enclosed in angle brack ets (e.g., <mss 1024>).
Src,dst and flags are always present. The other fields
depend on the contents of the packet's tcp protocol header and are output only if
appropriate.
Here is the opening portion of an rlogin from host rtsg to host csam.
rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024> csam.login > rtsg.1023:
S 947648:947648(0) ack 768513 win 4096 <mss 1024> rtsg.1023 > csam.login: . ack
1 win 4096 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096 csam.login > rtsg.1023:
P 1:2(1) ack 21 win 4077 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1 The first line says that
tcp port 1023 on rtsg sent a packet to port login on csam. The S indicates
that the SYN flag was set. The packet sequence number was 768512 and it contained
no data. (The notation is `first:last(nbytes)' which means `sequence numbers
first up to but not including last which is nbytes bytes of user
data'.) There was no piggy-backed ack, the available receive window was 4096 bytes
and there was a max-segment- size option requesting an mss of 1024 bytes.
Csam replies with a similar packet except it includes a piggy-backed ack for
rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no flags were set. The packet
con tained no data so there is no data sequence number. Note that the ack sequence
number is a small integer (1). The first time tcpdump sees a tcp `conversation',
it prints the sequence number from the packet. On subsequent pack ets of the conversation,
the difference between the cur rent packet's sequence number and this initial sequence
number is printed. This means that sequence numbers after the first can be interpreted
as relative byte positions in the conversation's data stream (with the first data
byte each direction being `1'). `-S' will override this fea ture, causing the original
sequence numbers to be output.
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the
rtsg -> csam side of the conversation). The PUSH flag is set in the packet. On the
7th line, csam says it's received data sent by rtsg up to but not includ ing byte
21. Most of this data is apparently sitting in the socket buffer since csam's receive
window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in
this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data
to rtsg.
If the snapshot was small enough that tcpdump didn't cap ture the full
TCP header, it interprets as much of the header as it can and then reports ``[|tcp]''
to indicate the remainder could not be interpreted. If the header contains a bogus
option (one with a length that's either too small or beyond the end of the header),
tcpdump reports it as ``[badopt]'' and does not interpret
any further options (since it's impossible to tell where they start). If the header
length indicates options are pre sent but the IP datagram length is not long enough
for the options to actually be there, tcpdump reports it as ``[badhdrlength]''.
There are 8 bits in the control bits section of the TCP header:
CWR|ECE|URG|ACK|PSH|RST|SYN|FIN
Let's assume that we want to watch packets used in estab lishing a TCP connection.
Recall that TCP uses a 3-way handshake protocol when it initializes a new connection;
the connection sequence with regard to the TCP control bits is
1) Caller sends SYN 2) Recipient responds with SYN, ACK 3) Caller sends ACK
Now we're interested in capturing packets that have only the SYN bit set (Step
1). Note that we don't want packets from step 2 (SYN-ACK), just a plain initial
SYN. What we need is a correct filter expression for tcpdump.
Recall the structure of a TCP header without options:
015 31
-----------------------------------------------------------------
| source port | destination port |
-----------------------------------------------------------------
| sequence number |
-----------------------------------------------------------------
| acknowledgment number |
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| TCP checksum | urgent pointer |
-----------------------------------------------------------------
A TCP header usually holds 20 octets of data, unless options are present. The first
line of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bits are contained in octet 13:
These are the TCP control bits we are interested in. We have numbered the bits in
this octet from 0 to 7, right to left, so the PSH bit is bit number 3, while the
URG bit is number 5.
Recall that we want to captu kets with only SYN set. Let's see what happens
to octet 13 if a TCP datagram arrives with the SYN bit set in its header:
Looking at the control bits section we see that only bit number 1 (SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in network byte order,
the binary value of this octet is
We're almost done, because now we know that if only SYN is set, the value of the
13th octet in the TCP header, when interpreted as a 8-bit unsigned integer in network
byte order, must be exactly 2.
This relationship can be expressed as
tcp[13] == 2
We can use this expression as the filter for tcpdump in
order to watch packets which have only SYN set:
tcpdump -i xl0 tcp[13] == 2
The expression says "let the 13th octet of a TCP datagram
have the decimal value 2", which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but
we don't care if ACK or any other TCP control bit is set
at the same time. Let's see what happens to octet 13 when
a TCP datagram with SYN-ACK set arrives:
Now we can't just use 'tcp[13] == 18' in the tcpdump fil ter expression,
because that would select only those pack ets that have SYN-ACK set, but not those
with only SYN set. Remember that we don't care if ACK or any other con trol bit
is set as long as SYN is set. In order to achieve our goal, we need to logically
AND the binary value of octet 13 with some other value to preserve the SYN bit.
We know that we want SYN to be set in any case, so we'll logically AND the value
in the 13th octet with the binary value of a SYN:
00010010 SYN-ACK00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010
We see that this AND operation delivers the same result
regardless whether ACK or another TCP control bit is set.
The decimal representation of the AND value as well as the
result of this operation is 2 (binary 00000010), so we
know that for packets with SYN set the following relation
must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'
Note that you should use single quotes or a backslash in the expression to hide
the AND ('&') special character from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp data
Some UDP services are recognized (from the source or destination port number)
and the higher level protocol infor mation printed. In particular, Domain Name
service requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS.
Name server requests are formatted as src>dst:idop?flagsqtypeqclassname(len)h2opolo.1538
> helios.domain: 3+ A? ucbvax.berkeley.edu. (37) Host h2opolo asked the
domain server on helios for an address record (qtype=A) associated with the
name ucbvax.berkeley.edu. The query id was `3'. The `+' indi cates
the recursiondesired flag was set. The query length was 37 bytes,
not including the UDP and IP protocol headers. The query operation was the normal
one, Query, so the op field was omitted. If the op had been anything else,
it would have been printed between the `3' and the `+'. Similarly, the qclass was
the normal one, C_IN, and omitted. Any other qclass would have
been printed immedi ately after the `A'.
A few anomalies are checked and may result in extra fields enclosed in square
brackets: If a query contains an answer, authority records or additional records
section, ancount, nscount, or arcount are printed as `[na]',
`[nn]' or `[nau]' where n is the appropriate count. If any
of the response bits are set (AA, RA or rcode) or any of the `must be zero' bits
are set in bytes two and three, `[b2&3=x]' is printed, where x is
the hex value of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as src>dst:idoprcodeflagsa/n/autypeclassdata(len)helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273) helios.domain
> h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo
with 3 answer records, 3 name server records and 7 additional records. The first
answer record is type A (address) and its data is internet address 128.32.137.3.
The total size of the response was 273 bytes, excluding UDP and IP headers. The
op (Query) and response code (NoError) were omitted, as was the class (C_IN) of
the A response code of non-existent domain (NXDomain) with no answers, one name
server and no authority records. The `*' indicates that the authoritativeanswer bit was set. Since there were no answers, no type, class or data were
printed.
Other flag characters that might appear are `-' (recursion available, RA,
not set) and `|' (truncated message, TC, set). If the `question' section doesn't
contain exactly one entry, `[nq]' is printed.
Note that name server requests and responses tend to be large and the default
snaplen of 68 bytes may not capture enough of the packet to print. Use the
-s flag to increase the snaplen if you need to seriously investigate name
server traffic. `-s 128' has worked well for me.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decod ing for data
on UDP/137, UDP/138 and TCP/139. Some primi tive decoding of IPX and NetBEUI SMB
data is also done.
By default a fairly minimal decode is done, with a much more detailed decode
done if -v is used. Be warned that with -v a single SMB packet may take up a page
or more, so only use -v if you really want all the gory details.
If you are decoding SMB sessions containing unicode strings then you may wish
to set the environment variable USE_UNICODE to 1. A patch to auto-detect unicode
srings would be welcome.
For information on SMB packet formats and what all te fields mean see www.cifs.org
or the pub/samba/specs/ directory on your favourite samba.org mirror site. The SMB
patches were written by Andrew Tridgell ([email protected]).
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed as: src.xid>dst.nfs:lenopargssrc.nfs>dst.xid:replystatlenopresults
In the first line, host sushi sends a transaction with id 6709
to wrl (note that the number following the src host is a transaction id,
not the source port). The request was 112 bytes, excluding the UDP and IP
headers. The operation was a readlink (read symbolic link) on file han dle
(fh) 21,24/10.731657119. (If one is lucky, as in this case, the file handle
can be interpreted as a major,minor device number pair, followed by the inode number
and gen eration number.) Wrl replies `ok' with the contents of the link.
In the third line, sushi asks wrl to lookup the name `xcolors'
in directory file 9,74/4096.6878. Note that the data printed depends on the operation
type. The format is intended to be self explanatory if read in conjunction with
an NFS protocol spec.
If the -v (verbose) flag is given, additional information is printed. For example:
(-v also prints the IP header TTL, ID, length, and frag mentation fields, which
have been omitted from this exam ple.) In the first line, sushi asks
wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl
replies `ok'; the packet shown on the second line is the first fragment of the reply,
and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments,
but these fragments do not have NFS or even UDP headers and so might not be printed,
depending on the filter expression used). Because the -v flag is given, some of
the file attributes (which are returned in addition to the file data) are printed:
the file type (``REG'', for regu lar file), the file mode (in octal), the uid and
gid, and the file size.
If the -v flag is given more than once, even more details are printed.
Note that NFS requests are very large and much of the detail won't be printed
unless snaplen is increased. Try using `-s 192' to watch NFS traffic.
NFS reply packets do not explicitly identify the RPC oper ation. Instead,
tcpdump keeps track of ``recent'' requests, and matches them to the replies
using the trans action ID. If a reply does not closely follow the corre
Transarc AFS (Andrew File System) requests and replies are printed as:
elvis.7001 > pike.afsfs: rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc" pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike. This was a RX data packet
to the fs (fileserver) service, and is the start of an RPC call. The RPC call was
a rename, with the old directory file id of 536876964/1/1 and an old filename of
`.newsrc.new', and a new directory file id of 536876964/1/1 and a new filename of
`.newsrc'. The host pike responds with a RPC reply to the rename call (which was
successful, because it was a data packet and not an abort packet).
In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs
have at least some of the arguments decoded (generally only the `interesting' arguments,
for some definition of interesting).
The format is intended to be self-describing, but it will probably not be useful
to people who are not familiar with the workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets and additional
header information is printed, such as the the RX call ID, call number, sequence
number, serial number, and the RX packet flags.
If the -v flag is given twice, additional information is printed, such as the
the RX call ID, serial number, and the RX packet flags. The MTU negotiation information
is also printed from RX ack packets.
If the -v flag is given three times, the security index and service id are printed.
Error codes are printed for abort packets, with the excep tion of Ubik beacon
packets (because abort packets are used to signify a yes vote for the Ubik protocol).
Note that AFS requests are very large and many of the arguments won't be printed
unless snaplen is increased. Try using `-s 256' to watch AFS traffic.
ation. Instead, tcpdump keeps track of ``recent'' requests, and matches them
to the replies using the call number and service ID. If a reply does not closely
follow the corresponding request, it might not be parsable.
KIP Appletalk (DDP in UDP)
Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped
as DDP packets (i.e., all the UDP header information is discarded). The file
/etc/atalk.names is used to translate appletalk net and node numbers to names.
Lines in this file have the form numbername
1.254 ether 16.1 icsd-net 1.254.110 ace
The first two lines give the names of appletalk networks. The third line gives
the name of a particular host (a host is distinguished from a net by the 3rd octet
in the number - a net number must have two octets and a host number must
have three octets.) The number and name should be sepa rated by whitespace (blanks
or tabs). The /etc/atalk.names file may contain blank lines or comment lines
(lines starting with a `#').
Appletalk addresses are printed in the form net.host.port144.1.209.2
> icsd-net.112.220 office.2 > icsd-net.112.220 jssmag.149.235 > icsd-net.2 (If
the /etc/atalk.names doesn't exist or doesn't contain an entry for some appletalk
host/net number, addresses are printed in numeric form.) In the first example, NBP
(DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220
of net icsd node 112. The second line is the same except the full name of the source
node is known (`office'). The third line is a send from port 235 on net jssmag node
149 to broadcast on the icsd-net NBP port (note that the broadcast address (255)
is indi cated by a net name with no host number - for this reason it's a good idea
to keep node names and net names distinct in /etc/atalk.names).
NBP (name binding protocol) and ATP (Appletalk transaction protocol) packets
have their contents interpreted. Other protocols just dump the protocol name (or
number if no name is registered for the protocol) and packet size.
NBP packets are formatted like the following examples: techpit.2 >
icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186 The first line
is a name lookup request for laserwriters sent by net icsd host 112 and broadcast
on net jssmag. The nbp id for the lookup is 190. The second line shows a reply for
this request (note that it has the same id) from host jssmag.209 saying that it
has a laserwriter resource named "RM1140" registered on port 250. The third line
is another reply to the same request saying host techpit has laserwriter "techpit"
registered on port 186.
Jssmag.209 initiates transaction id 12266 with host helios by requesting up to
8 packets (the `<0-7>'). The hex num ber at the end of the line is the value of
the `userdata' field in the request.
Helios responds with 8 512-byte packets. The `:digit' following the transaction
id gives the packet sequence number in the transaction and the number in parens
is the amount of data in the packet, excluding the atp header. The `*' on packet
7 indicates that the EOM bit was set. Jssmag.209 then requests that packets 3 &
5 be retransmit ted. Helios resends them then jssmag.209 releases the transaction.
Finally, jssmag.209 initiates the next request. The `*' on the request indicates
that XO (`exactly once') was not set.
IP Fragmentation
Fragmented Internet datagrams are printed as (fragid:size@offset+)
(fragid:size@offset) (The first form
indicates there are more fragments. The second indicates this is the last fragment.)
offset (in bytes) in the original datagram.
The fragment information is output for each fragment. The first fragment contains
the higher level protocol header and the frag info is printed after the protocol
info. Fragments after the first contain no higher level protocol header and the
frag info is printed after the source and destination addresses. For example, here
is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connec tion that
doesn't appear to handle 576 byte datagrams: arizona.ftp-data > rtsg.1170: .
1024:1332(308) ack 1 win 4096 (frag 595a:328@0+) arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses in the 2nd line don't
include port numbers. This is because the TCP protocol information is all in the
first fragment and we have no idea what the port or sequence numbers are when we
print the later fragments. Second, the tcp sequence information in the first line
is printed as if there were 308 bytes of user data when, in fact, there are 512
bytes (308 in the first frag and 204 in the second). If you are looking for holes
in the sequence space or trying to match up acks with packets, this can fool you.
A packet with the IP don'tfragment flag is marked with a trailing
(DF).
Timestamps
By default, all output lines are preceded by a timestamp. The timestamp is the
current clock time in the form hh:mm:ss.frac and is as accurate as the kernel's
clock. The timestamp reflects the time the kernel first saw the packet. No attempt
is made to account for the time lag between when the ethernet interface removed
the packet from the wire and when the kernel serviced the `new packet' interrupt.
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
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