-A Print each packet (minus its link level header) in ASCII.
Handy for capturing web pages.
-b Print the AS number in BGP packets in ASDOT notation
rather than ASPLAIN notation.
-B buffer_size
--buffer-size=buffer_size
Set the operating system capture buffer size to
buffer_size, in units of KiB (1024 bytes).
-c count
Exit after receiving count packets.
--count
Print only on stderr the packet count when reading capture
file(s) instead of parsing/printing the packets. If a
filter is specified on the command line, tcpdump counts
only packets that were matched by the filter expression.
-C file_size
Before writing a raw packet to a savefile, check whether
the file is currently larger than file_size and, if so,
close the current savefile and open a new one. Savefiles
after the first savefile will have the name specified with
the -w flag, with a number after it, starting at 1 and
continuing upward. The default unit of file_size is
millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
By adding a suffix of k/K, m/M or g/G to the value, the
unit can be changed to 1,024 (KiB), 1,048,576 (MiB), or
1,073,741,824 (GiB) respectively.
-d Dump the compiled packet-matching code in a human readable
form to standard output and stop.
Please mind that although code compilation is always DLT-
specific, typically it is impossible (and unnecessary) to
specify which DLT to use for the dump because tcpdump uses
either the DLT of the input pcap file specified with -r,
or the default DLT of the network interface specified with
-i, or the particular DLT of the network interface
specified with -y and -i respectively. In these cases the
dump shows the same exact code that would filter the input
file or the network interface without -d.
However, when neither -r nor -i is specified, specifying
-d prevents tcpdump from guessing a suitable network
interface (see -i). In this case the DLT defaults to
EN10MB and can be set to another valid value manually with
-y.
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (preceded
with a count).
-D
--list-interfaces
Print the list of the network interfaces available on the
system and on which tcpdump can capture packets. For each
network interface, a number and an interface name,
possibly followed by a text description of the interface,
are printed. The interface name or the number can be
supplied to the -i flag 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.
The -D flag will not be supported if tcpdump was built
with an older version of libpcap that lacks the
pcap_findalldevs(3PCAP) function.
-e Print the link-level header on each dump line. This can
be used, for example, to print MAC layer addresses for
protocols such as Ethernet and IEEE 802.11.
-E Use spi@ipaddr algo:secret for decrypting IPsec ESP
packets that are addressed to addr and contain Security
Parameter Index value spi. This combination may be
repeated with comma or newline separation.
Note that setting the secret for IPv4 ESP packets is
supported at this time.
Algorithms may be des-cbc, 3des-cbc, blowfish-cbc,
rc3-cbc, cast128-cbc, or none. The default is des-cbc.
The ability to decrypt packets is only present if tcpdump
was compiled with cryptography enabled.
secret is the ASCII text for ESP secret key. If preceded
by 0x, then a hex value will be read.
The option assumes RFC2406 ESP, not RFC1827 ESP. The
option is only for debugging purposes, and the use of this
option with a true `secret' key is discouraged. By
presenting IPsec secret key onto command line you make it
visible to others, via ps(1) and other occasions.
In addition to the above syntax, the syntax file name may
be used to have tcpdump read the provided file in. The
file is opened upon receiving the first ESP packet, so any
special permissions that tcpdump may have been given
should already have been given up.
-f Print `foreign' IPv4 addresses numerically rather than
symbolically (this option is intended to get around
serious brain damage in Sun's NIS server — usually it
hangs forever translating non-local internet numbers).
The test for `foreign' IPv4 addresses is done using the
IPv4 address and netmask of the interface on which capture
is being done. If that address or netmask are not
available, available, either because the interface on
which capture is being done has no address or netmask or
because the capture is being done on the Linux "any"
interface, which can capture on more than one interface,
this option will not work correctly.
-F file
Use file as input for the filter expression. An
additional expression given on the command line is
ignored.
-G rotate_seconds
If specified, rotates the dump file specified with the -w
option every rotate_seconds seconds. Savefiles will have
the name specified by -w which should include a time
format as defined by strftime(3). If no time format is
specified, each new file will overwrite the previous.
Whenever a generated filename is not unique, tcpdump will
overwrite the pre-existing data; providing a time
specification that is coarser than the capture period is
therefore not advised.
If used in conjunction with the -C option, filenames will
take the form of `file<count>'.
-h
--help Print the tcpdump and libpcap version strings, print a
usage message, and exit.
--version
Print the tcpdump and libpcap version strings and exit.
-H Attempt to detect 802.11s draft mesh headers.
-i interface
--interface=interface
Listen, report the list of link-layer types, report the
list of time stamp types, or report the results of
compiling a filter expression on interface. If
unspecified and if the -d flag is not given, tcpdump
searches the system interface list for the lowest
numbered, configured up interface (excluding loopback),
which may turn out to be, for example, ``eth0''.
On Linux systems with 2.2 or later kernels, an interface
argument of ``any'' can be used to capture packets from
all interfaces. Note that captures on the ``any'' device
will not be done in promiscuous mode.
If the -D flag is supported, an interface number as
printed by that flag can be used as the interface
argument, if no interface on the system has that number as
a name.
-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 flag will affect the output of the -L flag. If -I
isn't specified, only those link-layer types available
when not in monitor mode will be shown; if -I is
specified, only those link-layer types available when in
monitor mode will be shown.
--immediate-mode
Capture in "immediate mode". In this mode, packets are
delivered to tcpdump as soon as they arrive, rather than
being buffered for efficiency. This is the default when
printing packets rather than saving packets to a
``savefile'' if the packets are being printed to a
terminal rather than to a file or pipe.
-j tstamp_type
--time-stamp-type=tstamp_type
Set the time stamp type for the capture to tstamp_type.
The names to use for the time stamp types are given in
pcap-tstamp(@MAN_MISC_INFO@); not all the types listed
there will necessarily be valid for any given interface.
-J
--list-time-stamp-types
List the supported time stamp types for the interface and
exit. If the time stamp type cannot be set for the
interface, no time stamp types are listed.
--time-stamp-precision=tstamp_precision
When capturing, set the time stamp precision for the
capture to tstamp_precision. Note that availability of
high precision time stamps (nanoseconds) and their actual
accuracy is platform and hardware dependent. Also note
that when writing captures made with nanosecond accuracy
to a savefile, the time stamps are written with nanosecond
resolution, and the file is written with a different magic
number, to indicate that the time stamps are in seconds
and nanoseconds; not all programs that read pcap savefiles
will be able to read those captures.
When reading a savefile, convert time stamps to the
precision specified by timestamp_precision, and display
them with that resolution. If the precision specified is
less than the precision of time stamps in the file, the
conversion will lose precision.
The supported values for timestamp_precision are micro for
microsecond resolution and nano for nanosecond resolution.
The default is microsecond resolution.
--micro
--nano Shorthands for --time-stamp-precision=micro or
--time-stamp-precision=nano, adjusting the time stamp
precision accordingly. When reading packets from a
savefile, using --micro truncates time stamps if the
savefile was created with nanosecond precision. In
contrast, a savefile created with microsecond precision
will have trailing zeroes added to the time stamp when
--nano is used.
-K
--dont-verify-checksums
Don't attempt to verify IP, TCP, or UDP checksums. This
is useful for interfaces that perform some or all of those
checksum calculation in hardware; otherwise, all outgoing
TCP checksums will be flagged as bad.
-l Make stdout line buffered. Useful if you want to see the
data while capturing it. E.g.,
tcpdump -l | tee dat
or
tcpdump -l > dat & tail -f dat
Note that on Windows,``line buffered'' means
``unbuffered'', so that WinDump will write each character
individually if -l is specified.
-U is similar to -l in its behavior, but it will cause
output to be ``packet-buffered'', so that the output is
written to stdout at the end of each packet rather than at
the end of each line; this is buffered on all platforms,
including Windows.
-L
--list-data-link-types
List the known data link types for the interface, in the
specified mode, and exit. The list of known data link
types may be dependent on the specified mode; for example,
on some platforms, a Wi-Fi interface might support one set
of data link types when not in monitor mode (for example,
it might support only fake Ethernet headers, or might
support 802.11 headers but not support 802.11 headers with
radio information) and another set of data link types when
in monitor mode (for example, it might support 802.11
headers, or 802.11 headers with radio information, only in
monitor mode).
-m module
Load SMI MIB module definitions from file module. This
option can be used several times to load several MIB
modules into tcpdump.
-M secret
Use secret as a shared secret for validating the digests
found in TCP segments with the TCP-MD5 option (RFC 2385),
if present.
-n Don't convert addresses (i.e., host addresses, port
numbers, etc.) to names.
-N Don't print domain name qualification of host names.
E.g., if you give this flag then tcpdump will print
``nic'' instead of ``nic.ddn.mil''.
-#
--number
Print an optional packet number at the beginning of the
line.
-O
--no-optimize
Do not run the packet-matching code optimizer. This is
useful only if you suspect a bug in the optimizer.
-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 as an abbreviation for
`ether host {local-hw-addr} or ether broadcast'.
--print
Print parsed packet output, even if the raw packets are
being saved to a file with the -w flag.
-Q direction
--direction=direction
Choose send/receive direction direction for which packets
should be captured. Possible values are `in', `out' and
`inout'. Not available on all platforms.
-q Quick (quiet?) output. Print less protocol information so
output lines are shorter.
-r file
Read packets from file (which was created with the -w
option or by other tools that write pcap or pcapng files).
Standard input is used if file is ``-''.
-S
--absolute-tcp-sequence-numbers
Print absolute, rather than relative, TCP sequence
numbers.
-s snaplen
--snapshot-length=snaplen
Snarf snaplen bytes of data from each packet rather than
the default of 262144 bytes. Packets truncated because of
a limited snapshot are indicated in the output with
``[|proto]'', where proto is the name of the protocol
level at which the truncation has occurred.
Note that taking larger snapshots both increases the
amount of time it takes to process packets and,
effectively, decreases the amount of packet buffering.
This may cause packets to be lost. Note also that taking
smaller snapshots will discard data from protocols above
the transport layer, which loses information that may be
important. NFS and AFS requests and replies, for example,
are very large, and much of the detail won't be available
if a too-short snapshot length is selected.
If you need to reduce the snapshot size below the default,
you should limit snaplen to the smallest number that will
capture the protocol information you're interested in.
Setting snaplen to 0 sets it to the default of 262144, for
backwards compatibility with recent older versions of
tcpdump.
-T type
Force packets selected by "expression" to be interpreted
the specified type. Currently known types are aodv (Ad-
hoc On-demand Distance Vector protocol), carp (Common
Address Redundancy Protocol), cnfp (Cisco NetFlow
protocol), domain (Domain Name System), lmp (Link
Management Protocol), pgm (Pragmatic General Multicast),
pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), ptp (Precision Time
Protocol), radius (RADIUS), resp (REdis Serialization
Protocol), rpc (Remote Procedure Call), rtcp (Real-Time
Applications control protocol), rtp (Real-Time
Applications protocol), snmp (Simple Network Management
Protocol), someip (SOME/IP), tftp (Trivial File Transfer
Protocol), vat (Visual Audio Tool), vxlan (Virtual
eXtensible Local Area Network), wb (distributed White
Board) and zmtp1 (ZeroMQ Message Transport Protocol 1.0).
Note that the pgm type above affects UDP interpretation
only, the native PGM is always recognised as IP protocol
113 regardless. UDP-encapsulated PGM is often called
"EPGM" or "PGM/UDP".
Note that the pgm_zmtp1 type above affects interpretation
of both native PGM and UDP at once. During the native PGM
decoding the application data of an ODATA/RDATA packet
would be decoded as a ZeroMQ datagram with ZMTP/1.0
frames. During the UDP decoding in addition to that any
UDP packet would be treated as an encapsulated PGM packet.
-t Don't print a timestamp on each dump line.
-tt Print the timestamp, as seconds since January 1, 1970,
00:00:00, UTC, and fractions of a second since that time,
on each dump line.
-ttt Print a delta (microsecond or nanosecond resolution
depending on the --time-stamp-precision option) between
current and previous line on each dump line. The default
is microsecond resolution.
-tttt Print a timestamp, as hours, minutes, seconds, and
fractions of a second since midnight, preceded by the
date, on each dump line.
-ttttt Print a delta (microsecond or nanosecond resolution
depending on the --time-stamp-precision option) between
current and first line on each dump line. The default is
microsecond resolution.
-u Print undecoded NFS handles.
-U
--packet-buffered
If the -w option is not specified, or if it is specified
but the --print flag is also specified, make the printed
packet output ``packet-buffered''; i.e., as the
description of the contents of each packet is printed, it
will be written to the standard output, rather than, when
not writing to a terminal, being written only when the
output buffer fills.
If the -w option is specified, make the saved raw packet
output ``packet-buffered''; i.e., as each packet is saved,
it will be written to the output file, rather than being
written only when the output buffer fills.
The -U flag will not be supported if tcpdump was built
with an older version of libpcap that lacks the
pcap_dump_flush(3PCAP) function.
-v When parsing and printing, produce (slightly more) verbose
output. For example, the time to live, identification,
total length and options in an IP packet are printed.
Also enables additional packet integrity checks such as
verifying the IP and ICMP header checksum.
When writing to a file with the -w option and at the same
time not reading from a file with the -r option, report to
stderr, once per second, the number of packets captured.
In Solaris, FreeBSD and possibly other operating systems
this periodic update currently can cause loss of captured
packets on their way from the kernel to tcpdump.
-vv Even more verbose output. For example, additional fields
are printed from NFS reply packets, and SMB packets are
fully decoded.
-vvv Even more verbose output. For example, telnet SB ... SE
options are printed in full. With -X Telnet options are
printed in hex as well.
-V file
Read a list of filenames from file. Standard input is used
if file is ``-''.
-w file
Write the raw packets to file rather than parsing and
printing them out. They can later be printed with the -r
option. Standard output is used if file is ``-''.
This output will be buffered if written to a file or pipe,
so a program reading from the file or pipe may not see
packets for an arbitrary amount of time after they are
received. Use the -U flag to cause packets to be written
as soon as they are received.
The MIME type application/vnd.tcpdump.pcap has been
registered with IANA for pcap files. The filename
extension .pcap appears to be the most commonly used along
with .cap and .dmp. Tcpdump itself doesn't check the
extension when reading capture files and doesn't add an
extension when writing them (it uses magic numbers in the
file header instead). However, many operating systems and
applications will use the extension if it is present and
adding one (e.g. .pcap) is recommended.
See pcap-savefile(@MAN_FILE_FORMATS@) for a description of
the file format.
-W filecount
Used in conjunction with the -C option, this will limit
the number of files created to the specified number, and
begin overwriting files from the beginning, thus creating
a 'rotating' buffer. In addition, it will name the files
with enough leading 0s to support the maximum number of
files, allowing them to sort correctly.
Used in conjunction with the -G option, this will limit
the number of rotated dump files that get created, exiting
with status 0 when reaching the limit.
If used in conjunction with both -C and -G, the -W option
will currently be ignored, and will only affect the file
name.
-x When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet
(minus its link level header) in hex. The smaller of the
entire packet or snaplen bytes will be printed. Note that
this is the entire link-layer packet, so for link layers
that pad (e.g. Ethernet), the padding bytes will also be
printed when the higher layer packet is shorter than the
required padding. In the current implementation this flag
may have the same effect as -xx if the packet is
truncated.
-xx When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet,
including its link level header, in hex.
-X When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet
(minus its link level header) in hex and ASCII. This is
very handy for analysing new protocols. In the current
implementation this flag may have the same effect as -XX
if the packet is truncated.
-XX When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet,
including its link level header, in hex and ASCII.
-y datalinktype
--linktype=datalinktype
Set the data link type to use while capturing packets (see
-L) or just compiling and dumping packet-matching code
(see -d) to datalinktype.
-z postrotate-command
Used in conjunction with the -C or -G options, this will
make tcpdump run " postrotate-command file " where file is
the savefile being closed after each rotation. For
example, specifying -z gzip or -z bzip2 will compress each
savefile using gzip or bzip2.
Note that tcpdump will run the command in parallel to the
capture, using the lowest priority so that this doesn't
disturb the capture process.
And in case you would like to use a command that itself
takes flags or different arguments, you can always write a
shell script that will take the savefile name as the only
argument, make the flags & arguments arrangements and
execute the command that you want.
-Z user
--relinquish-privileges=user
If tcpdump is running as root, after opening the capture
device or input savefile, but before opening any savefiles
for output, change the user ID to user and the group ID to
the primary group of user.
This behavior can also be enabled by default at compile
time.
expression
selects which packets will be dumped. If no expression is
given, all packets on the net will be dumped. Otherwise,
only packets for which expression is `true' will be
dumped.
For the expression syntax, see pcap-
filter(@MAN_MISC_INFO@).
The expression argument can be passed to tcpdump as either
a single Shell argument, or as multiple Shell arguments,
whichever is more convenient. Generally, if the
expression contains Shell metacharacters, such as
backslashes used to escape protocol names, it is easier to
pass it as a single, quoted argument rather than to escape
the Shell metacharacters. Multiple arguments are
concatenated with spaces before being parsed.
