D.7. mergecap: Merging multiple capture files into one

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).

Example D.4. Help information available from mergecap

$ mergecap -h
Mergecap version 0.99.6
Merge two or more capture files into one.
See http://www.wireshark.org for more information.

Usage: mergecap [-hva] [-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
             arcnet_linux - Linux ARCNET
             atm-rfc1483 - RFC 1483 ATM
             linux-atm-clip - Linux ATM CLIP
             lapb - LAPB
             atm-pdus - ATM PDUs
             atm-pdus-untruncated - ATM PDUs - untruncated
             null - NULL
             ascend - Lucent/Ascend access equipment
             isdn - ISDN
             ip-over-fc - RFC 2625 IP-over-Fibre Channel
             ppp-with-direction - PPP with Directional Info
             ieee-802-11 - IEEE 802.11 Wireless LAN
             prism - IEEE 802.11 plus Prism II monitor mode header
             ieee-802-11-radio - IEEE 802.11 Wireless LAN with radio information
             ieee-802-11-bsd - IEEE 802.11 plus BSD WLAN header
             ieee-802-11-avs - IEEE 802.11 plus AVS WLAN header
             linux-sll - Linux cooked-mode capture
             frelay - Frame Relay
             frelay-with-direction - Frame Relay with Directional Info
             chdlc - Cisco HDLC
             ios - Cisco IOS internal
             ltalk - Localtalk
             pflog-old - OpenBSD PF Firewall logs, pre-3.4
             hhdlc - HiPath HDLC
             docsis - Data Over Cable Service Interface Specification
             cosine - CoSine L2 debug log
             whdlc - Wellfleet HDLC
             sdlc - SDLC
             tzsp - Tazmen sniffer protocol
             enc - OpenBSD enc(4) encapsulating interface
             pflog - OpenBSD PF Firewall logs
             chdlc-with-direction - Cisco HDLC with Directional Info
             bluetooth-h4 - Bluetooth H4
             mtp2 - SS7 MTP2
             mtp3 - SS7 MTP3
             irda - IrDA
             user0 - USER 0
             user1 - USER 1
             user2 - USER 2
             user3 - USER 3
             user4 - USER 4
             user5 - USER 5
             user6 - USER 6
             user7 - USER 7
             user8 - USER 8
             user9 - USER 9
             user10 - USER 10
             user11 - USER 11
             user12 - USER 12
             user13 - USER 13
             user14 - USER 14
             user15 - USER 15
             symantec - Symantec Enterprise Firewall
             ap1394 - Apple IP-over-IEEE 1394
             bacnet-ms-tp - BACnet MS/TP
             default is the same as the first input file
        -F <capture type> capture file type to write:
             libpcap - libpcap (tcpdump, Wireshark, 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)
             lanalyzer - Novell LANalyzer
             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
             ngwsniffer_2_0 - Network Associates Sniffer (Windows-based) 2.00x
             visual - Visual Networks traffic capture
             5views - Accellent 5Views capture
             niobserverv9 - Network Instruments Observer version 9
             default is libpcap
      
-h

Prints the version and options and exits.

-v

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

-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.

-s

Sets the snapshot length to use when writing the data.

-w

Sets the output filename.

-T

Sets the packet encapsulation type of the output capture file.

-F

Sets the file format of the output capture file.

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

Example D.5. Simple example of using mergecap

$ mergecap -w outfile.libpcap dhcp-capture.libpcap imap-1.libpcap