tcpslice - extract pieces of and/or glue together tcpdump files
tcpslice [ -dRrt ] [ -w file ]
[ start-time [ end-time ] ] file ...
Tcpslice is a program for extracting portions of packet-trace files generated using tcp-
dump(1)'s -w flag. It can also be used to glue together several such files, as discussed
The basic operation of tcpslice is to copy to stdout all packets from its input file(s)
whose timestamps fall within a given range. The starting and ending times of the range
may be specified on the command line. All ranges are inclusive. The starting time
defaults to the time of the first packet in the first input file; we call this the first
time. The ending time defaults to ten years after the starting time. Thus, the command
tcpslice trace-file simply copies trace-file to stdout (assuming the file does not include
more than ten years' worth of data).
There are a number of ways to specify times. The first is using Unix timestamps of the
form sssssssss.uuuuuu (this is the format specified by tcpdump's -tt flag). For example,
654321098.7654 specifies 38 seconds and 765,400 microseconds after 8:51PM PDT, Sept. 25,
All examples in this manual are given for PDT times, but when displaying times and inter-
preting times symbolically as discussed below, tcpslice uses the local timezone, regard-
less of the timezone in which the tcpdump file was generated. The daylight-savings set-
ting used is that which is appropriate for the local timezone at the date in question.
For example, times associated with summer months will usually include daylight-savings
effects, and those with winter months will not.
Times may also be specified relative to either the first time (when specifying a starting
time) or the starting time (when specifying an ending time) by preceding a numeric value
in seconds with a `+'. For example, a starting time of +200 indicates 200 seconds after
the first time, and the two arguments +200 +300 indicate from 200 seconds after the first
time through 500 seconds after the first time.
Times may also be specified in terms of years (y), months (m), days (d), hours (h), min-
utes (m), seconds (s), and microseconds(u). For example, the Unix timestamp
654321098.7654 discussed above could also be expressed as 90y9m25d20h51m38s765400u.
When specifying times using this style, fields that are omitted default as follows. If
the omitted field is a unit greater than that of the first specified field, then its value
defaults to the corresponding value taken from either first time (if the starting time is
being specified) or the starting time (if the ending time is being specified). If the
omitted field is a unit less than that of the first specified field, then it defaults to
zero. For example, suppose that the input file has a first time of the Unix timestamp
mentioned above, i.e., 38 seconds and 765,400 microseconds after 8:51PM PDT, Sept. 25,
1990. To specify 9:36PM PDT (exactly) on the same date we could use 21h36m. To specify a
range from 9:36PM PDT through 1:54AM PDT the next day we could use 21h36m 26d1h54m.
Relative times can also be specified when using the ymdhmsu format. Omitted fields then
default to 0 if the unit of the field is greater than that of the first specified field,
and to the corresponding value taken from either the first time or the starting time if
the omitted field's unit is less than that of the first specified field. Given a first
time of the Unix timestamp mentioned above, 22h +1h10m specifies a range from 10:00PM PDT
on that date through 11:10PM PDT, and +1h +1h10m specifies a range from 38.7654 seconds
after 9:51PM PDT through 38.7654 seconds after 11:01PM PDT. The first hour of the file
could be extracted using +0 +1h.
Note that with the ymdhmsu format there is an ambiguity between using m for `month' or for
`minute'. The ambiguity is resolved as follows: if an m field is followed by a d field
then it is interpreted as specifying months; otherwise it specifies minutes.
If more than one input file is specified then tcpslice first copies packets lying in the
given range from the first file; it then increases the starting time of the range to lie
just beyond the timestamp of the last packet in the first file, repeats the process with
the second file, and so on. Thus files with interleaved packets are not merged. For a
given file, only packets that are newer than any in the preceding files will be consid-
ered. This mechanism avoids any possibility of a packet occurring more than once in the
If any of -R, -r or -t are specified then tcpslice reports the timestamps of the first and
last packets in each input file and exits. Only one of these three options may be speci-
-d Dump the start and end times specified by the given range and exit. This option is
useful for checking that the given range actually specifies the times you think it
does. If one of -R, -r or -t has been specified then the times are dumped in the
corresponding format; otherwise, raw format ( -R) is used.
-R Dump the timestamps of the first and last packets in each input file as raw time-
stamps (i.e., in the form sssssssss.uuuuuu).
-r Same as -R except the timestamps are dumped in human-readable format, similar to
that used by date(1).
-t Same as -R except the timestamps are dumped in tcpslice format, i.e., in the
ymdhmsu format discussed above.
-w Direct the output to file rather than stdout.
The original author was:
Vern Paxson, of Lawrence Berkeley Laboratory, University of California, Berkeley, CA.
It is currently being maintained by tcpdump.org.
The current version is available in the ``tcpslice'' module of the CVS tree at tcp-
dump.org; see the tcpdump.org home page at
for information on anonymous CVS access.
The original distribution is available via anonymous ftp:
Please send problems, bugs, questions, desirable enhancements, etc. to:
Please send source code contributions, etc. to:
An input filename that beings with a digit or a `+' can be confused with a start/end time.
Such filenames can be specified with a leading `./'; for example, specify the file
`04Jul76.trace' as `./04Jul76.trace'.
tcpslice cannot read its input from stdin, since it uses random-access to rummage through
its input files.
tcpslice refuses to write to its output if it is a terminal (as indicated by isatty(3)).
This is not a bug but a feature, to prevent it from spraying binary data to the user's
terminal. Note that this means you must either redirect stdout or specify an output file
tcpslice will not work properly on tcpdump files spanning more than one year; with files
containing portions of packets whose original length was more than 65,535 bytes; nor with
files containing fewer than three packets. Such files result in the error message:
`couldn't find final packet in file'. These problems are due to the interpolation scheme
used by tcpslice to greatly speed up its processing when dealing with large trace files.
Note that tcpslice can efficiently extract slices from the middle of trace files of any
size, and can also work with truncated trace files (i.e., the final packet in the file is
only partially present, typically due to tcpdump being ungracefully killed).
21 December 1996 TCPSLICE(8)