Taking a packet trace on Linux using tcpdump

This document (7001152) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Desktop
SUSE Linux Enterprise Server

Situation

How to take a packet trace (LAN trace, packet capture) from Linux using tcpdump.

Resolution

tcpdump -i any -s0 -w /tmp/capture.cap

-i any = Capture on all interfaces

-s0 = Captures maximum size of packets, without this packets will possibly be truncated.
-w /tmp/capture.cap = Output file for capture

[Please note, this is not an exhaustive list of options, but it covers the basics. Check the man pages for a complete list]

After capturing the packets is complete, press <Ctrl><c> to stop capturing.

Additional Information

Advanced:

To create a rolling trace, for situation where you are trying to capture a random issue without filling up the partition space, various command options are available. We recommend the following (these are case-sensitive):

-C file_size (in millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

-W count -- Used in conjunction with -C, this will limit the number of files created to the specified number, overwriting files from the beginning, thus creating a rotating buffer.

tcpdump -i <eth device> -s 0 -C <file_size> -W <number of files> -w </path/to/trace.cap>

For roughly 9.6 MB files, rotating through three files, the following command is used:

tcpdump -i eth0 -s 0 -C 10 -W 3 -w /home/myTrace.cap

You will see the following traces being created:

myTrace.cap0

myTrace.cap1

myTrace.cap2

After myTrace.cap2 is full, the tcpdump command will overwrite myTrace.cap0

For roughly 955 MB files, rotating through 8 files, use the following:

tcpdump -i eth0 -s 0 -C 1000 -W 8 -w /some/other/location/serverTrace.cap

You will see the following traces created:

serverTrace.cap0

serverTrace.cap1

.... and so on

serverTrace.cap8

When dealing with larger traces, and trying to capture a specific issue, it is usually not necessary to send in all of the traces to SUSE Support. Typically the issue will be in the last one or two traces (depending on the settings used and the amount of traffic on the wire at the time).

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Ethereal or Wireshark can also be used on Linux to take a packet trace. See the following TID for more detail:
3892415 - How to use Wireshark to capture a packet trace

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.