Last updated on November 18, 2020 by Dan Nanni
If you are a network admin who is involved in testing an intrusion detection system or network access control policy, you may often rely on offline analysis using collected packet dumps. When it comes to storing packet dumps,
libpcap's packet dump format (
pcap format) is the most widely used by many open-source packet sniffing and capture programs. If
pcap files are used as part of penetration testing or any kind of offline analysis, there's often need for manipulating
pcap files before injecting them into the network.
In this tutorial, I am going to introduce useful
pcap manipulation tools and show their use cases.
Wireshark, the most popular GUI-based packet sniffer, actually comes with a suite of very useful command-line tools. Among them are
mergecap. The former is a versatile
pcap editor which can filter or split a
pcap file in various fashions. The latter allows you to merge multiple
pcap files into one. This tutorial is based on these Wireshark CLI tools.
If you already have Wireshark installed, these tools are already available for you. If not, go ahead and install Wireshark command-line tools on Linux. Note that on Debian-based distributions, you can install Wireshark command-line tools without installing Wireshark GUI, while on Red Hat based distributions, you need to install the whole Wireshark package.
$ sudo apt-get install wireshark-common
$ sudo yum install wireshark
Once you install Wireshark CLI tools, you can start using
editcap allows you to filter an input
pcap file in various fashions, and save the result in a new
First of all, you can filter an input
pcap file based on start time and/or end time.
-A <start-time> and
-B <end-time> options are used to capture only those packets whose arrival time falls within a specific time range (e.g., between 2:30pm and 2:35pm). The time format to use is
$ editcap -A '2014-12-10 10:11:01' -B '2014-12-10 10:21:01' input.pcap output.pcap
If you want to extract specific
N packets from an input
pcap file, you can also do that. The command below extracts
100 packets (from 401 to 500) from
input.pcap and save them as
$ editcap input.pcap output.pcap 401-500
If you want to filter out duplicate packets in a
pcap file, use
-D <dup-window> option. This will compare each packet against the previous (
<dup-window> - 1) packets in terms of packet length and MD5 hash, and discard the packet if any match is found.
$ editcap -D 10 input.pcap output.pcap
37568 packets seen, 1 packets skipped with duplicate window of 10 packets.
Alternatively, you can define
<dup-window> in terms of time interval. If you use
-w <dup-time-window> option, it will compare each packet against all the packets which arrived within
<dup-time-window> seconds to determine its duplicity.
$ editcap -w 0.5 input.pcap output.pcap
50000 packets seen, 0 packets skipped with duplicate time window equal to or less than 0.500000000 seconds.
editcap can be also useful if you want to split a large
pcap file into multiple smaller
To split a
pcap file into multiple
pcap files of the same packet count:
$ editcap -c <packets-per-file> <input-pcap-file> <output-prefix>
pcap file will have the same packet count, and be named as
To split a
pcap file into multiple
pcap files with the same time interval:
$ editcap -i <seconds-per-file> <input-pcap-file> <output-prefix>
If you want to combine multiple
pcap files into one,
mergecap is handy.
mergecap, by default, relies on per-packet timestamp information in
pcap files to sort packets in chronological order.
$ mergecap -w output.pcap input.pcap input2.pcap [input3.pcap . . .]
If you want to ignore timestamp information, and simply merge multiple
pcap files in their order in the command line, use
For example, the following command will write all packets from
output.pcap, followed by all packets in
$ mergecap -a -w output.pcap input.pcap input2.pcap
In this tutorial, I presented several use cases of
pcap file manipulation using
mergecap. Besides these, there are other
pcap related tools out there, for example,
reordercap for reordering packets,
text2pcap for text to
pcap-diff for performing
pcap files, etc. Some of these
pcap tools can be really handy along with packet injection tools for network penetration testing and various network troubleshooting purposes, so better to know they exist!
Do you use any
pcap tool out there? If so, what is your use case?
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