dumpcap - Online in the Cloud

PROGRAM:

NAME

dumpcap - Dump network traffic

SYNOPSIS

dumpcap [ -a <capture autostop condition> ] ... [ -b <capture ring buffer option>] ...
[ -B <capture buffer size> ] [ -c <capture packet count> ] [ -C <byte limit> ] [ -d ]
[ -D ] [ -f <capture filter> ] [ -g ] [ -h ]
[ -i <capture interface>|rpcap://<host>/<capture interface>|TCP@<host>:<port>|- ] [ -I ]
[ -L ] [ -M ] [ -n ] [ -N <packet limit> ] [ -p ] [ -P ] [ -q ] [ -s <capture snaplen> ]
[ -S ] [ -t ] [ -v ] [ -w <outfile> ] [ -y <capture link type> ]
[ --capture-comment <comment> ]

DESCRIPTION

Dumpcap is a network traffic dump tool. It lets you capture packet data from a live
network and write the packets to a file. Dumpcap's default capture file format is pcap-ng
format. When the -P option is specified, the output file is written in the pcap format.

Without any options set it will use the libpcap/WinPcap library to capture traffic from
the first available network interface and writes the received raw packet data, along with
the packets' time stamps into a pcap file.

If the -w option is not specified, Dumpcap writes to a newly created pcap file with a
randomly chosen name. If the -w option is specified, Dumpcap writes to the file specified
by that option.

Packet capturing is performed with the pcap library. The capture filter syntax follows
the rules of the pcap library.

OPTIONS

-a <capture autostop condition>
Specify a criterion that specifies when Dumpcap is to stop writing to a capture file.
The criterion is of the form test:value, where test is one of:

duration:value Stop writing to a capture file after value seconds have elapsed.

filesize:value Stop writing to a capture file after it reaches a size of value kB. If
this option is used together with the -b option, dumpcap will stop writing to the
current capture file and switch to the next one if filesize is reached. Note that the
filesize is limited to a maximum value of 2 GiB.

files:value Stop writing to capture files after value number of files were written.

-b <capture ring buffer option>
Cause Dumpcap to run in "multiple files" mode. In "multiple files" mode, Dumpcap will
write to several capture files. When the first capture file fills up, Dumpcap will
switch writing to the next file and so on.

The created filenames are based on the filename given with the -w option, the number
of the file and on the creation date and time, e.g. outfile_00001_20050604120117.pcap,
outfile_00002_20050604120523.pcap, ...

With the files option it's also possible to form a "ring buffer". This will fill up
new files until the number of files specified, at which point Dumpcap will discard the
data in the first file and start writing to that file and so on. If the files option
is not set, new files filled up until one of the capture stop conditions match (or
until the disk is full).

The criterion is of the form key:value, where key is one of:

duration:value switch to the next file after value seconds have elapsed, even if the
current file is not completely filled up.

filesize:value switch to the next file after it reaches a size of value kB. Note that
the filesize is limited to a maximum value of 2 GiB.

files:value begin again with the first file after value number of files were written
(form a ring buffer). This value must be less than 100000. Caution should be used
when using large numbers of files: some filesystems do not handle many files in a
single directory well. The files criterion requires either duration or filesize to be
specified to control when to go to the next file. It should be noted that each -b
parameter takes exactly one criterion; to specify two criterion, each must be preceded
by the -b option.

Example: -b filesize:1000 -b files:5 results in a ring buffer of five files of size
one megabyte each.

-B <capture buffer size>
Set capture buffer size (in MiB, default is 2 MiB). This is used by the capture
driver to buffer packet data until that data can be written to disk. If you encounter
packet drops while capturing, try to increase this size. Note that, while Dumpcap
attempts to set the buffer size to 2 MiB by default, and can be told to set it to a
larger value, the system or interface on which you're capturing might silently limit
the capture buffer size to a lower value or raise it to a higher value.

This is available on UNIX systems with libpcap 1.0.0 or later and on Windows. It is
not available on UNIX systems with earlier versions of libpcap.

This option can occur multiple times. If used before the first occurrence of the -i
option, it sets the default capture buffer size. If used after an -i option, it sets
the capture buffer size for the interface specified by the last -i option occurring
before this option. If the capture buffer size is not set specifically, the default
capture buffer size is used instead.

-c <capture packet count>
Set the maximum number of packets to read when capturing live data.

-C <byte limit>
Limit the amount of memory in bytes used for storing captured packets in memory while
processing it. If used in combination with the -N option, both limits will apply.
Setting this limit will enable the usage of the separate thread per interface.

-d Dump the code generated for the capture filter in a human-readable form, and exit.

-D Print a list of the interfaces on which Dumpcap can capture, and exit. For each
network interface, a number and an interface name, possibly followed by a text
description of the interface, is printed. The interface name or the number can be
supplied to the -i option to specify an interface on which to capture.

This can be useful on systems that don't have a command to list them (e.g., Windows
systems, or UNIX systems lacking ifconfig -a); the number can be useful on Windows
2000 and later systems, where the interface name is a somewhat complex string.

Note that "can capture" means that Dumpcap was able to open that device to do a live
capture. Depending on your system you may need to run dumpcap from an account with
special privileges (for example, as root) to be able to capture network traffic. If
"dumpcap -D" is not run from such an account, it will not list any interfaces.

-f <capture filter>
Set the capture filter expression.

The entire filter expression must be specified as a single argument (which means that
if it contains spaces, it must be quoted).

This option can occur multiple times. If used before the first occurrence of the -i
option, it sets the default capture filter expression. If used after an -i option, it
sets the capture filter expression for the interface specified by the last -i option
occurring before this option. If the capture filter expression is not set
specifically, the default capture filter expression is used if provided.

-g This option causes the output file(s) to be created with group-read permission
(meaning that the output file(s) can be read by other members of the calling user's
group).

-h Print the version and options and exits.

-i <capture interface>|rpcap://<host>/<capture interface>|TCP@<host>:<port>|-
Set the name of the network interface or pipe to use for live packet capture.

Network interface names should match one of the names listed in "dumpcap -D"
(described above); a number, as reported by "dumpcap -D", can also be used. If you're
using UNIX, "netstat -i" or "ifconfig -a" might also work to list interface names,
although not all versions of UNIX support the -a option to ifconfig.

If no interface is specified, Dumpcap searches the list of interfaces, choosing the
first non-loopback interface if there are any non-loopback interfaces, and choosing
the first loopback interface if there are no non-loopback interfaces. If there are no
interfaces at all, Dumpcap reports an error and doesn't start the capture.

Pipe names should be either the name of a FIFO (named pipe) or ``-'' to read data from
the standard input. Data read from pipes must be in standard pcap format.

This option can occur multiple times. When capturing from multiple interfaces, the
capture file will be saved in pcap-ng format.

Note: the Win32 version of Dumpcap doesn't support capturing from pipes or stdin!

-I Put the interface in "monitor mode"; this is supported only on IEEE 802.11 Wi-Fi
interfaces, and supported only on some operating systems.

Note that in monitor mode the adapter might disassociate from the network with which
it's associated, so that you will not be able to use any wireless networks with that
adapter. This could prevent accessing files on a network server, or resolving host
names or network addresses, if you are capturing in monitor mode and are not connected
to another network with another adapter.

This option can occur multiple times. If used before the first occurrence of the -i
option, it enables the monitor mode for all interfaces. If used after an -i option,
it enables the monitor mode for the interface specified by the last -i option
occurring before this option.

-L List the data link types supported by the interface and exit. The reported link types
can be used for the -y option.

-M When used with -D, -L or -S, print machine-readable output. The machine-readable
output is intended to be read by Wireshark and TShark; its format is subject to change
from release to release.

-n Save files as pcap-ng. This is the default.

-N <packet limit>
Limit the number of packets used for storing captured packets in memory while
processing it. If used in combination with the -C option, both limits will apply.
Setting this limit will enable the usage of the separate thread per interface.

-p Don't put the interface into promiscuous mode. Note that the interface might be in
promiscuous mode for some other reason; hence, -p cannot be used to ensure that the
only traffic that is captured is traffic sent to or from the machine on which Dumpcap
is running, broadcast traffic, and multicast traffic to addresses received by that
machine.

This option can occur multiple times. If used before the first occurrence of the -i
option, no interface will be put into the promiscuous mode. If used after an -i
option, the interface specified by the last -i option occurring before this option
will not be put into the promiscuous mode.

-P Save files as pcap instead of the default pcap-ng. In situations that require pcap-ng,
such as capturing from multiple interfaces, this option will be overridden.

-q When capturing packets, don't display the continuous count of packets captured that is
normally shown when saving a capture to a file; instead, just display, at the end of
the capture, a count of packets captured. On systems that support the SIGINFO signal,
such as various BSDs, you can cause the current count to be displayed by typing your
"status" character (typically control-T, although it might be set to "disabled" by
default on at least some BSDs, so you'd have to explicitly set it to use it).

-s <capture snaplen>
Set the default snapshot length to use when capturing live data. No more than snaplen
bytes of each network packet will be read into memory, or saved to disk. A value of 0
specifies a snapshot length of 65535, so that the full packet is captured; this is the
default.

This option can occur multiple times. If used before the first occurrence of the -i
option, it sets the default snapshot length. If used after an -i option, it sets the
snapshot length for the interface specified by the last -i option occurring before
this option. If the snapshot length is not set specifically, the default snapshot
length is used if provided.

-S Print statistics for each interface once every second.

-t Use a separate thread per interface.

-v Print the version and exit.

-w <outfile>
Write raw packet data to outfile. Use "-" for stdout.

-y <capture link type>
Set the data link type to use while capturing packets. The values reported by -L are
the values that can be used.

This option can occur multiple times. If used before the first occurrence of the -i
option, it sets the default capture link type. If used after an -i option, it sets
the capture link type for the interface specified by the last -i option occurring
before this option. If the capture link type is not set specifically, the default
capture link type is used if provided.

--capture-comment <comment>
Add a capture comment to the output file.

This option is only available if we output the captured packets to a single file in
pcap-ng format. Only one capture comment may be set per output file.

CAPTURE FILTER SYNTAX

See the manual page of pcap-filter(7) or, if that doesn't exist, tcpdump(8), or, if that
doesn't exist, <https://wiki.wireshark.org/CaptureFilters>.

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