DUMPCAP(1) DUMPCAP(1)

NAME

dumpcap - Dump network traffic

SYNOPSIS

dumpcap [ -a|--autostop <capture autostop condition> ] ...
[ -b|--ring-buffer <capture ring buffer option> ] ...
[ -B|--buffer-size <capture buffer size> ]
[ -c <capture packet count> ] [ -C <byte limit> ] [ -d ]
[ -D|--list-interfaces ] [ -f <capture filter> ] [ -g ] [ -h|--help ]
[ -i|--interface <capture interface>|rpcap://<host>:<port>/<capture interface>|TCP@<host>:<port>|- ]
[ -I|--monitor-mode ]
[ -k <freq>,[<type>],[<center_freq1>],[<center_freq2>] ]
[ -L|--list-data-link-types ] [ -M ] [ -n ] [ -N <packet limit> ]
[ -p|--no-promiscuous-mode ] [ --ifdescr <description> ]
[ --ifname <name> ] [ -P ] [ -q ]
[ -s|--snapshot-length <capture snaplen> ] [ -S ] [ -t ]
[ -v|--version ] [ -w <outfile> ]
[ -y|--linktype <capture link type> ] [ --capture-comment <comment> ]
[ --list-time-stamp-types ] [ --time-stamp-type <type> ]

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 pcapng 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, Npcap, or 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 capture file.

If the -w option is not specified, Dumpcap writes to a newly created
capture 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|--autostop <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. Floating point values (e.g. 0.5) are allowed.

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

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.

packets:value Stop writing to a capture file after value packets
have been written. Same as -c <capture packet count>.

-b|--ring-buffer <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_20240714120117.pcapng,
outfile_00002_20240714120523.pcapng, ...

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.
Floating point values (e.g. 0.5) are allowed.

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,
interval 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.

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.

interval:value switch to the next file when the time is an exact
multiple of value seconds. For example, use 3600 to switch to a
new file every hour on the hour.

packets:value switch to the next file after it contains value
packets.

printname:filename print the name of the most recently written
file to filename after the file is closed. filename can be stdout
or - for standard output, or stderr for standard error.

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

-B|--buffer-size <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. Same as -a packets:<capture packet count>.

-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|--list-interfaces

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 (UNIX systems lacking ifconfig -a or Linux systems lacking
ip link show). The number can be useful on Windows systems, where
the interface name might be a long name or a GUID.

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.

Pre-defined capture filter names, as shown in the GUI menu item
Capture->Capture Filters, can be used by prefixing the argument
with "predef:". Example: -f "predef:MyPredefinedHostOnlyFilter"

-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|--help

Print the version and options and exits.

-i|--interface <capture interface>|rpcap://<host>:<port>/<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",
"ifconfig -a" or "ip link" 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. On Windows systems,
pipe names must be of the form "\\pipe\.*pipename*". Data read
from pipes must be in standard pcapng or pcap format. Pcapng data
must have the same endianness as the capturing host.

"TCP@<host>:<port>" causes Dumpcap to attempt to connect to the
specified port on the specified host and read pcapng or pcap
data.

This option can occur multiple times. When capturing from
multiple interfaces, the capture file will be saved in pcapng
format, even if -P is specified.

--ifdescr> <description>

Use description as the description in the capture file for the
interface or pipe specified before it with -i.

--ifname> <name>

Use name as the name in the capture file for the the interface or
pipe specified before it with -i.

-I|--monitor-mode

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.

-k <freq>,[<type>],[<center_freq1>],[<center_freq2>>

Set the channel on the interface; this is supported only on IEEE
802.11 Wi-Fi interfaces, and supported only on some operating
systems.

freq is the frequency of the channel. type is the type of the
channel, for 802.11n and 802.11ac. The values for type are

NOHT

Used for non-802.11n/non-802.1ac channels

HT20

20 MHz channel

HT40-

40 MHz primary channel and a lower secondary channel

HT40+

40 MHz primary channel and a higher secondary channel

HT80

80 MHz channel, with centerfreq1 as its center frequency

VHT80+80

two 80 MHz channels combined, with centerfreq1 and centerfreq2 as
the center frequencies of the two channels

VHT160

160 MHz channel, with centerfreq1 as its center frequency

-L|--list-data-link-types

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, -S or --list-time-stamp-types 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 pcapng. 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|--no-promiscuous-mode

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 pcapng. In situations
that require pcapng, 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|--snapshot-length <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 262144, 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|--version

Print the version and exit.

-w <outfile>

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

-y|--linktype <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, if supported by the
output file format.

This option is only available if we output the captured packets
to a single file.

This option may be specified multiple times. Note that Wireshark
currently only displays the first comment of a capture file.

--list-time-stamp-types

List time stamp types supported for the interface. If no time
stamp type can be set, no time stamp types are listed.

--time-stamp-type <type>

Change the interface's timestamp method.

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://gitlab.com/wireshark/wireshark/-/wikis/CaptureFilters>.

SEE ALSO

wireshark(1), tshark(1), editcap(1), mergecap(1), capinfos(1),
pcap(3), pcap-filter(7) or tcpdump(8)

NOTES

This is the manual page for Dumpcap 3.6.22. Dumpcap is part of the
Wireshark distribution. The latest version of Wireshark can be found
at <https://www.wireshark.org>.

HTML versions of the Wireshark project man pages are available at
<https://www.wireshark.org/docs/man-pages>.

AUTHORS

Dumpcap is derived from the Wireshark capturing engine code; see the
list of authors in the Wireshark man page for a list of authors of
that code.

2024-03-27 DUMPCAP(1)