Merging multiple capture files into a single capture file with mergecap

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 libpcap capture files, including those of tcpdump. In addition, Mergecap can read capture files from snoop (including Shomiti) and atmsnoop, LanAlyzer, Sniffer (compressed or uncompressed), Microsoft Network Monitor, AIX's iptrace, NetXray, Sniffer Pro, RADCOM's WAN/LAN analyzer, Lucent/Ascend router debug output, HP-UX's nettl, and the dump output from Toshiba's ISDN routers. There is no need to tell Mergecap what type of file you are reading; it will determine the file type by itself. Mergecap is also capable of reading any of these file formats if they are compressed using gzip. Mergecap recognizes this directly from the file; the '.gz' extension is not required for this purpose.

By default, it writes the capture file in libpcap format, and writes all of the packets in both input capture files to the output file. The -F flag can be used to specify the format in which to write the capture file; it can write the file in libpcap format (standard libpcap format, a modified format used by some patched versions of libpcap, the format used by Red Hat Linux 6.1, or the format used by SuSE Linux 6.3), snoop format, uncompressed Sniffer format, Microsoft Network Monitor 1.x format, and the format used by Windows-based versions of the Sniffer software.

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.

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).

If the -T flag is used to specify an encapsulation type, the encapsulation type of the output capture file will be forced to the specified type, rather than being the type appropriate to the encapsulation type of the input capture file. Note that this merely forces the encapsulation type of the output file to be the specified type; the packet headers of the packets will not be translated from the encapsulation type of the input capture file to the specified encapsulation type (for example, it will not translate an Ethernet capture to an FDDI capture if an Ethernet capture is read and '-T fddi' is specified).

hagbard@hagbard:~/build/src/ethereal/doc$ mergecap -h
mergecap version 0.8.19
Usage: mergecap [-h] [-v] [-a] [-s <snaplen>] [-T <encap type>]
          [-F <capture type>] -w <outfile> <infile> [...]

  where	-h produces this help listing.
       	-v verbose operation, default is silent
       	-a files should be concatenated, not merged
       	     Default merges based on frame timestamps
       	-s <snaplen>: truncate packets to <snaplen> bytes of data
       	-w <outfile>: sets output filename to <outfile>
       	-T <encap type> encapsulation type to use:
       	     ether - Ethernet
       	     tr - Token Ring
       	     slip - SLIP
       	     ppp - PPP
       	     fddi - FDDI
       	     fddi-swapped - FDDI with bit-swapped MAC addresses
       	     rawip - Raw IP
       	     arcnet - ARCNET
       	     atm-rfc1483 - RFC 1483 ATM
       	     linux-atm-clip - Linux ATM CLIP
       	     lapb - LAPB
       	     atm-sniffer - ATM Sniffer
       	     null - NULL
       	     ascend - Lucent/Ascend access equipment
       	     lapd - LAPD
       	     v120 - V.120
       	     ppp-with-direction - PPP with Directional Info
       	     ieee-802-11 - IEEE 802.11 Wireless LAN
       	     linux-sll - Linux cooked-mode capture
       	     frelay - Frame Relay
       	     chdlc - Cisco HDLC
       	     default is the same as the first input file
       	-F <capture type> capture file type to write:
       	     libpcap - libpcap (tcpdump, Ethereal, etc.)
       	     rh6_1libpcap - Red Hat Linux 6.1 libpcap (tcpdump)
       	     suse6_3libpcap - SuSE Linux 6.3 libpcap (tcpdump)
       	     modlibpcap - modified libpcap (tcpdump)
       	     nokialibpcap - Nokia libpcap (tcpdump)
       	     ngsniffer - Network Associates Sniffer (DOS-based)
       	     snoop - Sun snoop
       	     netmon1 - Microsoft Network Monitor 1.x
       	     netmon2 - Microsoft Network Monitor 2.x
       	     ngwsniffer_1_1 - Network Associates Sniffer (Windows-based) 1.1
       	     default is libpcap

A simple example merging dhcp-capture.libpcap and imap-1.libpcap into outfile.libpcap is shown below.

hagbard@hagbard:~/captures$ mergecap -w outfile.libpcap dhcp-capture.libpcap imap-1.libpcap