-A Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
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-b Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.
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-B Set the operating system capture buffer size to buffer_size.
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-c Exit after receiving count packets.
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-C 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 units of file_size are millions of bytes (1,000,000 bytes, not 1,048,576
bytes).
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-d Dump the compiled packet-matching code in a human readable form to standard output and stop.
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-dd Dump packet-matching code as a C program fragment.
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-ddd Dump packet-matching code as decimal numbers (preceded with a count).
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-D 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, is 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() function.
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-e Print the link-level header on each dump line.
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-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.
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-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.
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-F Use file as input for the filter expression. An additional expression given on the command line
is ignored.
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-G 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.
If used in conjunction with the -C option, filenames will take the form of `file<count>'.
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-h Print the tcpdump and libpcap version strings, print a usage message, and exit.
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-H Attempt to detect 802.11s draft mesh headers.
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-i Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest
numbered, configured up interface (excluding loopback). Ties are broken by choosing the earliest
match.
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.
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-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 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.
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-j 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-type(7); not all the types listed there will necessarily be valid for any
given interface.
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-J 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.
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-K 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.
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-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''.
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-L 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).
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-m Load SMI MIB module definitions from file module. This option can be used several times to load
several MIB modules into tcpdump.
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-M Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5
option (RFC 2385), if present.
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-n Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
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-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''.
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-O Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the
optimizer.
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-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 as an abbreviation for `ether host {local-
hw-addr} or ether broadcast'.
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-q Quick (quiet?) output. Print less protocol information so output lines are shorter.
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-R Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If specified,
tcpdump will not print replay prevention field. Since there is no protocol version field in
ESP/AH specification, tcpdump cannot deduce the version of ESP/AH protocol.
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-r Read packets from file (which was created with the -w option). Standard input is used if file is
``-''.
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-S Print absolute, rather than relative, TCP sequence numbers.
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-s Snarf snaplen bytes of data from each packet rather than the default of 65535 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. 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 65535, for backwards compatibility with recent
older versions of tcpdump.
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-T Force packets selected by "expression" to be interpreted the specified type. Currently known
types are aodv (Ad-hoc On-demand Distance Vector protocol), cnfp (Cisco NetFlow protocol), rpc
(Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications
control protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer
Protocol), vat (Visual Audio Tool), and wb (distributed White Board).
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-t Don't print a timestamp on each dump line.
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-tt Print an unformatted timestamp on each dump line.
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-ttt Print a delta (micro-second resolution) between current and previous line on each dump line.
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-tttt Print a timestamp in default format proceeded by date on each dump line.
-ttttt Print a delta (micro-second resolution) between current and first line on each dump line.
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-u Print undecoded NFS handles.
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-U Make output saved via the -w option ``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() function.
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-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, report, every 10 seconds, the number of packets
captured.
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-vv Even more verbose output. For example, additional fields are printed from NFS reply packets, and
SMB packets are fully decoded.
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-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.
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-w 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 ``-''. See pcap-savefile(5) for a
description of the file format.
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-W 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 with -C as well, the behavior will
result in cyclical files per timeslice.
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-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.
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-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.
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-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.
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-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.
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-y Set the data link type to use while capturing packets to datalinktype.
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-z Used in conjunction with the -C or -G options, this will make tcpdump run " 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.
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-Z 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.
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