mergecap - Man Page

Merges two or more capture files into one

Synopsis

mergecap [ -a ] [ -F <file format> ] [ -I <IDB merge mode> ] [ -s <snaplen> ] [ -V ] -w <outfile>|- <infile> [<infile> ...]

mergecap -h|--help

mergecap -v|--version

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|--help

Print the version number and options and exit.

-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. (Only the IDBs that occur at the beginning of the files, before any packet blocks, are compared. IDBs that occur later in the files are merged with duplicates iff the initial IDBs were merged.) 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|--version

Print the full version information and exit.

-V

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

-w <outfile>|-

Sets the output filename. If the name is '-', stdout will be used. If the --compress option is not given, then the filename extension is used to deduce the desired compression method, if any; e.g., if the name has the extension '.gz', then the output file is compressed to a gzip archive. This setting is mandatory.

--compress <type>

Compress the output file using the type compression format. --compress with no argument provides a list of the compression formats supported for writing. The type given takes precedence over the extension of outfile.

Diagnostic Options

--log-level <level>: Set the active log level. Supported levels in lowest to highest order are "noisy", "debug", "info", "message", "warning", "critical", and "error". Messages at each level and higher will be printed, for example "warning" prints "warning", "critical", and "error" messages and "noisy" prints all messages. Levels are case insensitive.
--log-fatal <level>: Abort the program if any messages are logged at the specified level or higher. For example, "warning" aborts on any "warning", "critical", or "error" messages.
--log-domains <list>: Only print messages for the specified log domains, e.g. "GUI,Epan,sshdump". List of domains must be comma-separated. Can be negated with "!" as the first character (inverts the match).
--log-debug <list>: Force the specified domains to log at the "debug" level. List of domains must be comma-separated. Can be negated with "!" as the first character (inverts the match).
--log-noisy <list>: Force the specified domains to log at the "noisy" level. List of domains must be comma-separated. Can be negated with "!" as the first character (inverts the match).
--log-fatal-domains <list>: Abort the program if any messages are logged for the specified log domains. List of domains must be comma-separated.
--log-file <path>: Write log messages and stderr output to the specified file.

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.

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 4.4.1. 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.