1MERGECAP(1) MERGECAP(1)
2
6 mergecap - Merges two or more capture files into one
7
9 mergecap [ -a ] [ -F <file format> ] [ -I <IDB merge mode> ]
10 [ -s <snaplen> ] [ -V ] -w <outfile>|- <infile> [<infile> ...]
11 mergecap -h|--help
13 mergecap -v|--version
15
17 Mergecap is a program that combines multiple saved capture files into a
18 single output file specified by the -w argument. Mergecap knows how to
19 read pcap and pcapng capture files, including those of tcpdump,
20 Wireshark and other tools that write captures in those formats.
21 By default, Mergecap writes the capture file in pcapng format, and
23 writes all of the packets from the input capture files to the output
24 file.
25 Mergecap is able to detect, read and write the same capture files that
27 are supported by Wireshark. The input files don’t need a specific
28 filename extension; the file format and an optional gzip, zstd or lz4
29 compression will be automatically detected. Near the beginning of the
30 DESCRIPTION section of wireshark(1) or
31 https://www.wireshark.org/docs/man-pages/wireshark.html is a detailed
32 description of the way Wireshark handles this, which is the same way
33 Mergecap handles this.
34 Mergecap can write the file in several output formats. The -F flag can
36 be used to specify the format in which to write the capture file,
37 mergecap -F provides a list of the available output formats.
38 Packets from the input files are merged in chronological order based on
40 each frame’s timestamp, unless the -a flag is specified. Mergecap
41 assumes that frames within a single capture file are already stored in
42 chronological order. When the -a flag is specified, packets are copied
43 directly from each input file to the output file, independent of each
44 frame’s timestamp.
45 The output file frame encapsulation type is set to the type of the
47 input files if all input files have the same type. If not all of the
48 input files have the same frame encapsulation type, the output file
49 type is set to WTAP_ENCAP_PER_PACKET. Note that some capture file
50 formats, most notably pcap, do not currently support
51 WTAP_ENCAP_PER_PACKET. This combination will cause the output file
52 creation to fail.
53
55 -a
56 Causes the frame timestamps to be ignored, writing all packets from
58 the first input file followed by all packets from the second input
59 file. By default, when -a is not specified, the contents of the
60 input files are merged in chronological order based on each frame’s
61 timestamp.
62 Note: when merging, mergecap assumes that packets within a capture
64 file are already in chronological order.
65 -F <file format>
67 Sets the file format of the output capture file. Mergecap can write
69 the file in several formats; mergecap -F provides a list of the
70 available output formats. By default this is the pcapng format.
71 -h|--help
73 Prints the version and options and exits.
75 -I <IDB merge mode>
77 Sets the Interface Description Block (IDB) merge mode to use during
79 merging. mergecap -I provides a list of the available IDB merge
80 modes.
81 Every input file has one or more IDBs, which describe the
83 interface(s) the capture was performed on originally. This includes
84 encapsulation type, interface name, etc. When mergecap merges
85 multiple input files, it has to merge these IDBs somehow for the
86 new merged output file. This flag controls how that is
87 accomplished. The currently available modes are:
88 none: No merging of IDBs is performed, and instead all IDBs are
90 copied to the merged output file.
91 all: IDBs are merged only if all input files have the same number
93 of IDBs, and each IDB matches their respective entry in the other
94 files. (Only the IDBs that occur at the beginning of the files,
95 before any packet blocks, are compared. IDBs that occur later in
96 the files are merged with duplicates iff the initial IDBs were
97 merged.) This is the default mode.
98 any: Any and all duplicate IDBs are merged into one IDB, regardless
100 of what file they are in.
101 Note that an IDB is only considered a matching duplicate if it has
103 the same encapsulation type, name, speed, time precision, comments,
104 description, etc.
105 -s <snaplen>
107 Sets the snapshot length to use when writing the data. If the -s
109 flag is used to specify a snapshot length, frames in the input file
110 with more captured data than the specified snapshot length will
111 have only the amount of data specified by the snapshot length
112 written to the output file. This may be useful if the program that
113 is to read the output file cannot handle packets larger than a
114 certain size (for example, the versions of snoop in Solaris 2.5.1
115 and Solaris 2.6 appear to reject Ethernet frames larger than the
116 standard Ethernet MTU, making them incapable of handling gigabit
117 Ethernet captures if jumbo frames were used).
118 -v|--version
120 Print the version and exit.
122 -V
124 Causes mergecap to print a number of messages while it’s working.
126 -w <outfile>|-
128 Sets the output filename. If the name is '-', stdout will be used.
130 This setting is mandatory.
131
133 --log-level <level>
134 Set the active log level. Supported levels in lowest to highest
135 order are "noisy", "debug", "info", "message", "warning",
136 "critical", and "error". Messages at each level and higher will be
137 printed, for example "warning" prints "warning", "critical", and
138 "error" messages and "noisy" prints all messages. Levels are case
139 insensitive.
140 --log-fatal <level>
142 Abort the program if any messages are logged at the specified level
143 or higher. For example, "warning" aborts on any "warning",
144 "critical", or "error" messages.
145 --log-domains <list>
147 Only print messages for the specified log domains, e.g.
148 "GUI,Epan,sshdump". List of domains must be comma-separated.
149 --log-debug <list>
151 Force the specified domains to log at the "debug" level. List of
152 domains must be comma-separated.
153 --log-noisy <list>
155 Force the specified domains to log at the "noisy" level. List of
156 domains must be comma-separated.
157 --log-file <path>
159 Write log messages and stderr output to the specified file.
160
162 To merge two capture files together into a third capture file, in which
163 the last packet of one file arrives 100 seconds before the first packet
164 of another file, use the following sequence of commands.
165 First, use:
167 capinfos -aeS a.pcap b.pcap
169 to determine the start and end times of the two capture files, as
171 seconds since January 1, 1970, 00:00:00 UTC.
172 If a.pcap starts at 1009932757 and b.pcap ends at 873660281, then the
174 time adjustment to b.pcap that would make it end 100 seconds before
175 a.pcap begins would be 1009932757 - 873660281 - 100 = 136272376
176 seconds.
177 Thus, the next step would be to use:
179 editcap -t 136272376 b.pcap b-shifted.pcap
181 to generate a version of b.pcap with its time stamps shifted 136272376
183 ahead.
184 Then the final step would be to use :
186 mergecap -w compare.pcap a.pcap b-shifted.pcap
188 to merge a.pcap and the shifted b.pcap into compare.pcap.
190
192 pcap(3), wireshark(1), tshark(1), dumpcap(1), editcap(1), text2pcap(1),
193 pcap-filter(7) or tcpdump(8)
194
196 Mergecap is based heavily upon editcap by Richard Sharpe
197 <sharpe[AT]ns.aus.com> and Guy Harris <guy[AT]alum.mit.edu>.
198 This is the manual page for Mergecap 4.0.8. Mergecap is part of the
200 Wireshark distribution. The latest version of Wireshark can be found at
201 https://www.wireshark.org.
202 HTML versions of the Wireshark project man pages are available at
204 https://www.wireshark.org/docs/man-pages.
205
207 Original Author
208 Scott Renfro <scott[AT]renfro.org>
209 Contributors
211 Bill Guyton <guyton[AT]bguyton.com>
212 2023-08-31 MERGECAP(1)