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tcpreplay – Resend PCAP Packets for Performance or Functional Testing

The tcpreplay command is a tool that allows you to resend previously captured network traffic in the form of PCAP packets. This can be useful for a variety of purposes, including performance testing, functional testing, and network troubleshooting.

Overview

The basic syntax for using tcpreplay is as follows:

tcpreplay [options] <pcap file(s)>

Here, options refers to any additional command-line options you want to specify, and <pcap file(s)> refers to the name(s) of the PCAP file(s) you want to replay.

For example, to replay a single PCAP file named capture.pcap, you would use the following command:

tcpreplay capture.pcap

If you have multiple PCAP files that you want to replay, you can specify them all on the command line:

tcpreplay capture1.pcap capture2.pcap capture3.pcap

By default, tcpreplay will replay packets at their original speed, which may not be desirable if you are trying to simulate high network traffic loads. To speed up the replay process, you can use the -t option to specify a speedup factor. For example, to replay packets at twice their original speed, you would use the following command:

tcpreplay -t 2 capture.pcap

Another useful option is -l, which loops the replayed packets indefinitely. This can be useful for testing network devices under sustained loads. For example:

tcpreplay -l capture.pcap

Options

The following table lists the available options for the tcpreplay command:

Option Description
-c <count> Only replay the first count packets from the PCAP file(s).
-C Continuously replay packets until interrupted.
-D <device> Send packets out the specified network interface.
-i Show packet information while replaying.
-K Use kernel bypass mode for higher performance.
-l Loop the PCAP file(s) indefinitely.
-q Quiet mode – do not display status messages.
-t <factor> Speed up packet replay by a factor of factor.
-x Enable checksum recalculation.

Troubleshooting Tips

  • If you receive an error message stating that tcpreplay cannot find the specified PCAP file(s), double-check the file path and make sure you have read permissions for the file(s).
  • If you are experiencing slow replay speeds, try using the -K option to enable kernel bypass mode.
  • If you are experiencing packet loss during replay, try using the -x option to enable checksum recalculation.

Notes

  • tcpreplay can be a powerful tool for testing network devices and applications, but it should be used with caution. Make sure you are not replaying sensitive or confidential data over a live network.
  • Always test tcpreplay on a non-production network before using it in a production environment.
  • tcpreplay is available on most Linux distributions and can be installed using the package manager of your choice.

Related commands