Provided by: wireshark-common_3.6.7-1_amd64 
NAME
mergecap - Merges two or more capture files into one
SYNOPSIS
mergecap [ -a ] [ -F <file format> ] [ -h ] [ -I <IDB merge mode> ] [ -s <snaplen> ]
[ -v ] [ -V ] -w <outfile>|- <infile> [<infile> ...]
DESCRIPTION
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 and pcapng capture files,
including those of tcpdump, Wireshark and other tools that write captures in those
formats.
By default, Mergecap writes the capture file in pcapng 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, zstd or lz4 compression will be automatically detected. Near the
beginning of the DESCRIPTION section of wireshark(1) or
https://www.wireshark.org/docs/man-pages/wireshark.html 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.
OPTIONS
-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 timestamp.
Note: when merging, mergecap assumes that packets within a capture file are already in
chronological order.
-F <file format>
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. By
default this is the pcapng format.
-h
Prints the version and options and exits.
-I <IDB merge mode>
Sets the Interface Description Block (IDB) merge mode to use during merging. mergecap
-I provides a list of the available IDB merge modes.
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:
none: No merging of IDBs is performed, and instead all IDBs are copied to the merged
output file.
all: 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.
any: 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.
-s <snaplen>
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 <outfile>|-
Sets the output filename. If the name is '-', stdout will be used. This setting is
mandatory.
EXAMPLES
To merge two capture files together into a third capture file, in which the last packet of
one file arrives 100 seconds before the first packet of another file, use the following
sequence of commands.
First, use:
capinfos -aeS a.pcap b.pcap
to determine the start and end times of the two capture files, as seconds since January 1,
1970, 00:00:00 UTC.
If a.pcap starts at 1009932757 and b.pcap ends at 873660281, then the time adjustment to
b.pcap that would make it end 100 seconds before a.pcap begins would be 1009932757 -
873660281 - 100 = 136272376 seconds.
Thus, the next step would be to use:
editcap -t 136272376 b.pcap b-shifted.pcap
to generate a version of b.pcap with its time stamps shifted 136272376 ahead.
Then the final step would be to use :
mergecap -w compare.pcap a.pcap b-shifted.pcap
to merge a.pcap and the shifted b.pcap into compare.pcap.
SEE ALSO
pcap(3), wireshark(1), tshark(1), dumpcap(1), editcap(1), text2pcap(1), pcap-filter(7) or
tcpdump(8)
NOTES
Mergecap is based heavily upon editcap by Richard Sharpe <sharpe[AT]ns.aus.com> and Guy
Harris <guy[AT]alum.mit.edu>.
This is the manual page for Mergecap 3.6.7. Mergecap is part of the Wireshark
distribution. The latest version of Wireshark can be found at https://www.wireshark.org.
HTML versions of the Wireshark project man pages are available at
https://www.wireshark.org/docs/man-pages.
AUTHORS
Original Author
Scott Renfro <scott[AT]renfro.org>
Contributors
Bill Guyton <guyton[AT]bguyton.com>
2022-07-28 MERGECAP(1)