MERGECAP(1) The Ethereal Network Analyzer MERGECAP(1)
mergecap - Merges two capture files into one
mergecap [ -hva ] [ -s snaplen ] [ -F file format ] [ -T encapsulation type ] -w outfile infile ...
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, Ethereal, and other tools that write captures in that format. In addition,
Mergecap can read capture files from snoop and atmsnoop, Shomiti/Finisar Surveyor, Novell LANalyzer, Network General/Network Associates
DOS-based Sniffer (compressed or uncompressed), Microsoft Network Monitor, AIX's iptrace, Cinco Networks NetXRay, Network Associates Win-
dows-based Sniffer, AG Group/WildPackets EtherPeek/TokenPeek/AiroPeek, RADCOM's WAN/LAN analyzer, Lucent/Ascend router debug output,
HP-UX's nettl, the dump output from Toshiba's ISDN routers, the output from i4btrace from the ISDN4BSD project, the output in IPLog format
from the Cisco Secure Intrusion Detection System, pppd logs (pppdump format), the output from VMS's TCPIPtrace utility, the text output
from the DBS Etherwatch VMS utility, traffic capture files from Visual Networks' Visual UpTime, and the output from CoSine L2 debug. 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, the format used by Windows-based versions of the
Sniffer software, and the format used by Visual Networks' 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).
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 for-
mats, most notably libpcap, do not currently support WTAP_ENCAP_PER_PACKET. This combination will cause the output file creation to fail.
If the -T flag is used to specify a frame encapsulation type, the encapsulation type of the output capture file will be forced to the spec-
ified type, rather than being the type appropriate to the encapsulation type of the input capture files. 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 encapsu-
lation 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).
-w Sets the output filename.
-F Sets the file format of the output capture file.
-T Sets the packet encapsulation type of the output capture file.
-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.
-v Causes mergecap to print a number of messages while it's working.
-s Sets the snapshot length to use when writing the data.
-h Prints the version and options and exits.
tcpdump(8), pcap(3), ethereal(1), editcap(1)
Mergecap is based heavily upon editcap by Richard Sharpe <firstname.lastname@example.org> and Guy Harris <email@example.com>.
Mergecap is part of the Ethereal distribution. The latest version of Ethereal can be found at http://www.ethereal.com.
Scott Renfro <firstname.lastname@example.org>
0.9.8 2002-08-08 MERGECAP(1)