tcpslice - Online in the Cloud

PROGRAM:

NAME

tcpslice - extract pieces of and/or merge together tcpdump files

SYNOPSIS

tcpslice [ -DdlRrt ] [ -w file ]
[ start-time [ end-time ] ] file ...

DESCRIPTION

Tcpslice is a program for extracting portions of packet-trace files generated using
tcpdump(l)'s -w flag. It can also be used to merge together several such files, as
discussed below.

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 earliest time of the first packet in any of the input files; 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,
1990.

All examples in this manual are given for PDT times, but when displaying times and
interpreting times symbolically as discussed below, tcpslice uses the local timezone,
regardless of the timezone in which the tcpdump file was generated. The daylight-savings
setting 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),
minutes (m), seconds (s), and microseconds(u). For example, the Unix timestamp
654321098.7654 discussed above could also be expressed as 1990y9m25d20h51m38s765400u. 2
or 4 digit years may be used; 2 digits can specify years from 1970 to 2069.

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 merges the packets from the various
input files into the single output file. Normally, this merge is done based on the value
of the timestamps in the packets in the individual files. (Tcpslice assumes that within
each input file, packets are in timestamp order.) If the -l option is used, the value
used for ordering is the timestamp of a given packet minus the timestamp of the first
packet in the input file in which the given packet occurs.

When merging files, by default tcpslice will discard any duplicate packet it finds in more
than one file. A duplicate is a packet that has an identical timestamp (either relative
or absolute) and identical packet contents (for as much as was captured) as another packet
previously seen in a different file. Note that it is possible for the network to generate
true replicates of packets, and for systems that can return the same timestamp for
multiple packets, these can be mistaken for duplicates and discarded. Accordingly,
tcpslice will not discard duplicates in the same trace file. In addition, you can use the
-D option to suppress any discarding of duplicates.

A different issue arises if a file contains timestamps that skip backwards. tcpslice will
include these in the output, even if they precede the minimum time requested. There
should probably be an option to suppress these.

Another problem relating to backwards timestamps is that tcpslice uses random access to
seek through a file looking for packets corresponding to the desired range of time. While
doing so leads to a major performance benefit for very large trace files, it also means
that in the presence of backwards timestamps tcpslice can fail to find the true earliest
occurrence of a packet matching the time interval criteria. There should probably be an
option to specify not to use random access but just read the file linearly.

OPTIONS

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
specified.

-D Do not discard duplicate packets seen when merging multiple trace files.

-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.

-l When merging more than one file, merge on the basis of relative time, rather than
absolute time. Normally, when merging files is done, packets are merged based on
absolute timestamps. With -l packets are merged based on the relative time between
the start of the file in which the packet is found and the timestamp of the packet
itself. The timestamp of packets in the output file is calculated as the relative
time for the packet within its file plus first time.

-R Dump the timestamps of the first and last packets in each input file as raw
timestamps (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.

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