mergecap Man page

MERGECAP(1) The Wireshark Network Analyzer MERGECAP(1)


mergecap – Merges two or more capture files into one


mergecap [ -a ] [ -F ] [ -h ] [ -I ] [ -s ] [ -v ] [ -V ] -w |- [ …]


Mergecap is a program that combines multiple saved capture files into a
single output file specified by the -w argument. Mergecap knows how to
read pcap capture files, including those of tcpdump, Wireshark, and
other tools that write captures in that format.

By default, Mergecap writes the capture file in pcap format, and writes
all of the packets from the input capture files to the output file.

Mergecap is able to detect, read and write the same capture files that
are supported by Wireshark. The input files don’t need a specific
filename extension; the file format and an optional gzip compression
will be automatically detected. Near the beginning of the


section of wireshark or
is a detailed
description of the way Wireshark handles this, which is the same way
Mergecap handles this.

Mergecap can write the file in several output formats. The -F flag can
be used to specify the format in which to write the capture file,
mergecap -F provides a list of the available output formats.

Packets from the input files are merged in chronological order based on
each frame’s timestamp, unless the -a flag is specified. Mergecap
assumes that frames within a single capture file are already stored in
chronological order. When the -a flag is specified, packets are copied
directly from each input file to the output file, independent of each
frame’s timestamp.

The output file frame encapsulation type is set to the type of the
input files if all input files have the same type. If not all of the
input files have the same frame encapsulation type, the output file
type is set to WTAP_ENCAP_PER_PACKET. Note that some capture file
formats, most notably pcap, do not currently support
WTAP_ENCAP_PER_PACKET. This combination will cause the output file
creation to fail.


-a Causes the frame timestamps to be ignored, writing all packets from
the first input file followed by all packets from the second input
file. By default, when -a is not specified, the contents of the
input files are merged in chronological order based on each frame’s

Note: when merging, mergecap assumes that packets within a capture
file are already in chronological order.

Sets the file format of the output capture file. Mergecap can write
the file in several formats; mergecap -F provides a list of the
available output formats. The default is to use the file format of
the first input file.

-h Prints the version and options and exits.

Sets the Interface Description Block (IDB) merge mode to use during
merging. mergecap -I provides a list of the available IDB merge

Every input file has one or more IDBs, which describe the
interface(s) the capture was performed on originally. This includes
encapsulation type, interface name, etc. When mergecap merges
multiple input files, it has to merge these IDBs somehow for the
new merged output file. This flag controls how that is
accomplished. The currently available modes are:

* ‘B‘: no merging of IDBs is performed, and instead all IDBs are
copied to the merged output file.
* ‘B‘: IDBs are merged only if all input files have the same number
of IDBs, and each IDB matches their respective entry in the
other files. This is the default mode.
* ‘B‘: Any and all duplicate IDBs are merged into one IDB, regardless
of what file they are in.

Note that an IDB is only considered a matching duplicate if it has
the same encapsulation type, name, speed, time precision, comments,
description, etc.

Sets the snapshot length to use when writing the data. If the -s
flag is used to specify a snapshot length, frames in the input file
with more captured data than the specified snapshot length will
have only the amount of data specified by the snapshot length
written to the output file. This may be useful if the program that
is to read the output file cannot handle packets larger than a
certain size (for example, the versions of snoop in Solaris 2.5.1
and Solaris 2.6 appear to reject Ethernet frames larger than the
standard Ethernet MTU, making them incapable of handling gigabit
Ethernet captures if jumbo frames were used).

-v Causes mergecap to print a number of messages while it’s working.

-V Print the version and exit.

-w |-
Sets the output filename. If the name is ‘-‘, stdout will be used.
This setting is mandatory.

To merge two capture files together, 100 seconds apart use:

capinfos -aeS a.pcap b.pcap

(Let’s suppose a.pcap starts at 1009932757 and b.pcap ends at
873660281. 1009932757 – 873660281 – 100 = 136272376 seconds.)

editcap -t 136272376 b.pcap b-shifted.pcap
mergecap -w compare.pcap a.pcap b-shifted.pcap


pcap(3), wireshark, tshark(1), dumpcap, editcap, text2pcap,
pcap-filter(7) or tcpdump(8)

Mergecap is based heavily upon editcap by Richard Sharpe
and Guy Harris .

Mergecap is part of the Wireshark distribution. The latest version of
Wireshark can be found at .

HTML versions of the Wireshark project man pages are available at:

Original Author
——– ——
Scott Renfro

Bill Guyton

2.0.2 2016-02-28 MERGECAP(1)