tcpdump

One of the early diagnostic tools for TCP/IP that was written by Van Jacobson, Craig Leres, and Steven McCanne. Tcpdump can be used to capture and decode packets in real time, or to capture packets to files (in "libpcap" format, see below), and analyze (decode) them later.

There are now more elaborate and, in some ways, user-friendly packet capturing programs, such as Wireshark (formerly called Ethereal), but tcpdump is widely available, widely used, so it is very useful to know how to use it.

Tcpdump/libpcap is still actively being maintained, although not by its original authors.

libpcap

Libpcap is a library that is used by tcpdump, and also names a file format for packet traces. This file format - usually used in files with the extension .pcap - is widely supported by packet capture and analysis tools.

Selected options

Some useful options to tcpdump include:

  • -s snaplen capture snaplen bytes of each frame. By default, tcpdump captures only the first 68 bytes, which is sufficient to capture IP/UDP/TCP/ICMP headers, but usually not payload or higher-level protocols. If you are interested in more than just headers, use -s 0 to capture packets without truncation.
  • -r filename read from an previously created capture file (see -w)
  • -w filename dump to a file instead of analyzing on-the-fly
  • -i interface capture on an interface other than the default (first "up" non-loopback interface). Under Linux, -i any can be used to capture on all interfaces, albeit with some restrictions.
  • -c count stop the capture after count packets
  • -n don't resolve addresses, port numbers, etc. to symbolic names - this avoids additional DNS traffic when analyzing a live capture.
  • -v verbose output
  • -vv more verbose output
  • -vvv even more verbose output

Also, a pflang expression can be appended to the command so as to filter the captured packets. An expression is made up of one or more of "type", "direction" and "protocol".

  • type Can be host, net or port. host is presumed unless otherwise speciified
  • dir Can be src, dst, src or dst or src and dst
  • proto (for protocol) Common types are ether, ip, tcp, udp, arp ... If none is specifiied then all protocols for which the value is valid are considered.
Example expressions:
  • dst host <address>
  • src host <address>
  • udp dst port <number>
  • host <host> and not port ftp and not port ftp-data

Usage examples

Capture a single (-c 1) udp packet to file test.pcap:

: root@diotima[tmp]; tcpdump -c 1 -w test.pcap udp
tcpdump: listening on bge0, link-type EN10MB (Ethernet), capture size 96 bytes
1 packets captured
3 packets received by filter
0 packets dropped by kernel

This produces a binary file containing the captured packet as well as a small file header and a timestamp:

: root@diotima[tmp]; ls -l test.pcap
-rw-r--r-- 1 root root 114 2006-04-09 18:57 test.pcap
: root@diotima[tmp]; file test.pcap
test.pcap: tcpdump capture file (big-endian) - version 2.4 (Ethernet, capture length 96)

Analyze the contents of the previously created capture file:

: root@diotima[tmp]; tcpdump -r test.pcap
reading from file test.pcap, link-type EN10MB (Ethernet)
18:57:28.732789 2001:630:241:204:211:43ff:fee1:9fe0.32832 > ff3e::beac.10000: UDP, length: 12

Display the same capture file in verbose mode:

: root@diotima[tmp]; tcpdump -v -r test.pcap
reading from file test.pcap, link-type EN10MB (Ethernet)
18:57:28.732789 2001:630:241:204:211:43ff:fee1:9fe0.32832 > ff3e::beac.10000: [udp sum ok] UDP, length: 12 (len 20, hlim 118)

More examples with some advanced tcpdump use cases.

References

-- SimonLeinen - 2006-03-04 - 2016-03-17

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Topic revision: r11 - 2016-03-17 - SimonLeinen
 
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