tcpdump.1.in

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.TH TCPDUMP 1  "10 March 2025"
.SH NAME
tcpdump \- dump traffic on a network
.SH SYNOPSIS
.na
.B tcpdump
[
.B \-AbdDefhHIJKlLnNOpqStuUvxX#
] [
.B \-B
.I buffer_size
]
.ti +8
[
.B \-c
.I count
]
[
.B \-\-count
]
[
.B \-C
.I file_size
]
.ti +8
[
.B \-E
.I spi@ipaddr algo:secret,...
]
.ti +8
[
.B \-F
.I file
]
[
.B \-G
.I rotate_seconds
]
[
.B \-i
.I interface
]
.ti +8
[
.B \-\-immediate\-mode
]
[
.B \-j
.I tstamp_type
]
.ti +8
[
.BI \-\-lengths
]
[
.B \-m
.I module
]
.ti +8
[
.B \-M
.I secret
]
[
.B \-\-number
]
[
.B \-\-print
]
.ti +8
[
.B \-\-print\-sampling
.I nth
]
[
.B \-Q
.I in|out|inout
]
[
.B \-r
.I file
]
.ti +8
[
.B \-s
.I snaplen
]
[
.B \-\-skip
.I count
]
[
.B \-T
.I type
]
[
.B \-\-version
]
.ti +8
[
.B \-V
.I file
]
[
.B \-w
.I file
]
[
.B \-W
.I filecount
]
[
.B \-y
.I datalinktype
]
.ti +8
[
.B \-z
.I postrotate-command
]
[
.B \-Z
.I user
]
.ti +8
[
.BI \-\-time\-stamp\-precision= tstamp_precision
]
.ti +8
[
.BI \-\-micro
]
[
.BI \-\-nano
]
.ti +8
[
.I expression
]
.br
.ad
.SH DESCRIPTION
.LP
\fItcpdump\fP prints out a description of the contents of packets on a
network interface that match the Boolean \fIexpression\fP (see
.BR \%pcap-filter (@MAN_MISC_INFO@)
for the \fIexpression\fP syntax); the
description is preceded by a time stamp, printed, by default, as hours,
minutes, seconds, and fractions of a second since midnight.  It can also
be run with the
.B \-w
flag, which causes it to save the packet data to a file for later
analysis, and/or with the
.B \-r
flag, which causes it to read from a saved packet file rather than to
read packets from a network interface.  It can also be run with the
.B \-V
flag, which causes it to read a list of saved packet files. In all cases,
only packets that match
.I expression
will be processed by
.IR tcpdump .
.LP
.I tcpdump
will, if not run with the
.B \-c
flag, continue capturing packets until it is interrupted by a
.B SIGINT
signal (generated, for example, by typing your interrupt character,
typically control-C) or a
.B SIGTERM
signal (typically generated with the
.BR kill (1)
command); if run with the
.B \-c
flag, it will capture packets until it is interrupted by a
.B SIGINT
or
.B SIGTERM
signal or the specified number of packets have been processed.
.LP
When
.I tcpdump
finishes capturing packets, it will report counts of:
.IP
packets ``captured'' (this is the number of packets that
.I tcpdump
has received and processed);
.IP
packets ``received by filter'' (the meaning of this depends on the OS on
which you're running
.IR tcpdump ,
and possibly on the way the OS was configured - if a filter was
specified on the command line, on some OSes it counts packets regardless
of whether they were matched by the filter expression and, even if they
were matched by the filter expression, regardless of whether
.I tcpdump
has read and processed them yet, on other OSes it counts only packets that were
matched by the filter expression regardless of whether
.I tcpdump
has read and processed them yet, and on other OSes it counts only
packets that were matched by the filter expression and were processed by
.IR tcpdump );
.IP
packets ``dropped by kernel'' (this is the number of packets that were
dropped, due to a lack of buffer space, by the packet capture mechanism
in the OS on which
.I tcpdump
is running, if the OS reports that information to applications; if not,
it will be reported as 0).
.LP
On platforms that support the
.B SIGINFO
signal, such as most BSDs
(including macOS), it will report those counts
when it receives a
.B SIGINFO
signal (generated, for example, by typing
your ``status'' character, typically control-T, although on some
platforms, such as macOS, the ``status'' character is not set by
default, so you must set it with
.BR stty (1)
in order to use it) and will continue capturing packets. On platforms that
do not support the
.B SIGINFO
signal, the same can be achieved by using the
.B SIGUSR1
signal.
.LP
Using the
.B SIGUSR2
signal along with the
.B \-w
flag will forcibly flush the packet buffer into the output file.
.LP
Reading packets from a network interface may require that you have
special privileges; see the
.BR pcap (3PCAP)
man page for details.  Reading a saved packet file doesn't require
special privileges.
.SH OPTIONS
.TP
.B \-A
Print each packet (minus its link level header) in ASCII.  Handy for
capturing web pages.
No effect when
.B \-x[x]
or
.B \-X[X]
options are used.
.TP
.B \-b
Print the AS number in BGP packets using "asdot" rather than "asplain"
representation, in RFC 5396 terms.
.TP
.BI \-B " buffer_size"
.PD 0
.TP
.BI \-\-buffer\-size= buffer_size
.PD
Set the operating system capture buffer size to \fIbuffer_size\fP, in
units of KiB (1024 bytes).
.TP
.BI \-c " count"
Exit after receiving or reading \fIcount\fP packets.
If the
.B --skip
option is used, the \fIcount\fP starts after the skipped packets.
.TP
.BI \-\-count
Print only on stdout the packet count when reading capture file(s) instead
of parsing/printing the packets. If a filter is specified on the command
line, \fItcpdump\fP counts only packets that were matched by the filter
expression.
.TP
.BI \-C " file_size"
Before writing a raw packet to a savefile, check whether the file is
currently larger than \fIfile_size\fP and, if so, close the current
savefile and open a new one.  Savefiles after the first savefile will
have the name specified with the
.B \-w
flag, with a number after it, starting at 1 and continuing upward.
The default unit of \fIfile_size\fP is millions of bytes (1,000,000 bytes,
not 1,048,576 bytes).
.IP
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.
.TP
.B \-d
Dump the compiled packet-matching code in a human readable form to
standard output and stop.
.IP
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 \fItcpdump\fP uses either the DLT of the input pcap
file specified with
.BR -r ,
or the default DLT of the network interface specified with
.BR -i ,
or the particular DLT of the network interface specified with
.B -y
and
.B -i
respectively. In these cases the dump shows the same exact code that
would filter the input file or the network interface without
.BR -d .
.IP
However, when neither
.B -r
nor
.B -i
is specified, specifying
.B -d
prevents \fItcpdump\fP from guessing a suitable network interface (see
.BR -i ).
In this case the DLT defaults to EN10MB and can be set to another valid
value manually with
.BR -y .
.TP
.B \-dd
Dump packet-matching code as a C array of
.B "struct bpf_insn"
structures.
.TP
.B \-ddd
Dump packet-matching code as decimal numbers (preceded with a count).
.TP
.B \-D
.PD 0
.TP
.B \-\-list\-interfaces
.PD
Print the list of the network interfaces available on the system and on
which
.I 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
.B \-i
flag to specify an interface on which to capture.
.IP
This can be useful on systems that don't have a command to list them
(e.g., Windows systems, or UNIX systems lacking
.BR "ifconfig \-a" );
the number can be useful on Windows 2000 and later systems, where the
interface name is a somewhat complex string.
.TP
.B \-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.
.TP
.B \-E
Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
are addressed to \fIaddr\fP and contain Security Parameter Index value
\fIspi\fP. This combination may be repeated with comma or newline separation.
.IP
Note that setting the secret for IPv4 ESP packets is supported at this time.
.IP
Algorithms may be
\fBdes-cbc\fP,
\fB3des-cbc\fP,
\fBblowfish-cbc\fP,
\fBrc3-cbc\fP,
\fBcast128-cbc\fP,
or
\fBnone\fP.
The default is
.BR des-cbc .
The ability to decrypt packets is only present if \fItcpdump\fP was compiled
with cryptography enabled.
.IP
\fIsecret\fP is the ASCII text for ESP secret key.
If preceded by 0x, then a hex value will be read.
.IP
The option assumes RFC 2406 ESP, not RFC 1827 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
.BR ps (1)
and other occasions.
.IP
In addition to the above syntax, the syntax \fIfile name\fP 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.
.TP
.B \-f
Print `foreign' IPv4 addresses numerically rather than symbolically
(this option is intended to get around serious brain damage in
Sun's NIS server \(em usually it hangs forever translating non-local
internet numbers).
.IP
The test for `foreign' IPv4 addresses is done using the IPv4 address and
netmask of the interface on that capture is being done.  If the interface
has no IPv4 addresses (which by convention applies to the "any"
pseudo-interface (see the
.B \-i
flag below), the IPv4 netmask is assumed to be /0 and any IPv4 address is
considered non-foreign.  If the IPv4 netmask is /32, all IPv4 addresses
except the interface's own address are considered foreign.  If the
interface has more than one IPv4 address, it is not trivial to predict
which one will be used for the test.
.IP
Without the
.B \-f
flag, or when the netmask is assumed to be /0 (as discussed above), the
.B "ip broadcast"
primitive in the filter expression matches IPv4 packets that have either
0.0.0.0 or 255.255.255.255 as the destination address.  With the flag,
the primitive uses the same netmask (but not the network address) to test
the IPv4 destination address as the foreign address test.  One exception
is the netmask /32, in which case the primitive is considered invalid for
the interface.
.TP
.BI \-F " file"
Use \fIfile\fP as input for the filter expression.
An additional expression given on the command line is ignored.
.TP
.BI \-G " rotate_seconds"
If specified, rotates the dump file specified with the
.B \-w
option every \fIrotate_seconds\fP seconds.
Savefiles will have the name specified by
.B \-w
which should include a time format as defined by
.BR 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
preexisting data; providing a time specification that is coarser than the
capture period is therefore not advised.
.IP
If used in conjunction with the
.B \-C
option, filenames will take the form of `\fIfile\fP<count>'.
.TP
.B \-h
.PD 0
.TP
.B \-\-help
.PD
Print the tcpdump and libpcap version strings, print a usage message,
and exit.
.TP
.B \-\-version
.PD
Print the tcpdump and libpcap version strings and exit.
.TP
.B \-H
Attempt to detect 802.11s draft mesh headers.
.TP
.BI \-i " interface"
.PD 0
.TP
.BI \-\-interface= interface
.PD
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
\fIinterface\fP.  If unspecified and if the
.B -d
flag is not given, \fItcpdump\fP searches the system
interface list for the lowest numbered, configured up interface
(excluding loopback), which may turn out to be, for example, ``eth0''.
.IP
On all supported Linux systems, as well as on recent versions of macOS
and Solaris, an
.I interface
argument of ``any'' means a special pseudo-interface, which captures packets
from all regular network interfaces of the OS.  What is considered a regular
network interface is an implementation detail of the OS (for example, on
Linux this includes SocketCAN devices), so packets captured on the ``any''
pseudo-interface may represent more different network protocols than expected.
Also all regular network interfaces are a subset of all available capture
devices as printed by the
.B \-D
flag.  The latter may also include capture devices that libpcap implements by
means other than the OS network stack: Bluetooth, DAG, D-Bus, SNF and USB.
Note that captures on the ``any'' pseudo-interface will not be done in promiscuous
mode.
.IP
An interface number as printed by the
.B \-D
flag can be used as the
.I interface
argument, if no interface on the system has that number as a name.
.TP
.B \-I
.PD 0
.TP
.B \-\-monitor\-mode
.PD
Put the interface in "monitor mode"; this is supported only on IEEE
802.11 Wi-Fi interfaces, and supported only on some operating systems.
.IP
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.
.IP
This flag will affect the output of the
.B \-L
flag.  If
.B \-I
isn't specified, only those link-layer types available when not in
monitor mode will be shown; if
.B \-I
is specified, only those link-layer types available when in monitor mode
will be shown.
.TP
.BI \-\-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.
.TP
.BI \-j " tstamp_type"
.PD 0
.TP
.BI \-\-time\-stamp\-type= tstamp_type
.PD
Set the time stamp type for the capture to \fItstamp_type\fP.  The names
to use for the time stamp types are given in
.BR \%pcap-tstamp (@MAN_MISC_INFO@);
not all the types listed there will necessarily be valid for any given
interface.
.TP
.B \-J
.PD 0
.TP
.B \-\-list\-time\-stamp\-types
.PD
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.
.TP
.BI \-\-time\-stamp\-precision= tstamp_precision
When capturing, set the time stamp precision for the capture to
\fItstamp_precision\fP.  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.
.IP
When reading a savefile, convert time stamps to the precision specified
by \fItimestamp_precision\fP, 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.
.IP
The supported values for \fItimestamp_precision\fP are \fBmicro\fP for
microsecond resolution and \fBnano\fP for nanosecond resolution.  The
default is microsecond resolution.
.TP
.B \-\-micro
.PD 0
.TP
.B \-\-nano
.PD
Shorthands for
.B \-\-time\-stamp\-precision=micro
or
.BR \-\-time\-stamp\-precision=nano ,
adjusting the time stamp
precision accordingly.  When reading packets from a savefile, using
.B \-\-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
.B \-\-nano
is used.
.TP
.B \-K
.PD 0
.TP
.B \-\-dont\-verify\-checksums
.PD
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.
.TP
.B \-l
Make stdout line buffered.
Useful if you want to see the data
while capturing it.
E.g.,
.IP
.RS
.RS
.nf
\fBtcpdump \-l | tee dat\fP
.fi
.RE
.RE
.IP
or
.IP
.RS
.RS
.nf
\fBtcpdump \-l > dat & tail \-f dat\fP
.fi
.RE
.RE
.IP
Note that on Windows,``line buffered'' means ``unbuffered'', so that
tcpdump will write each character individually if
.B \-l
is specified.
.IP
.B \-U
is similar to
.B \-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.
.TP
.B \-L
.PD 0
.TP
.B \-\-list\-data\-link\-types
.PD
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).
.TP
.BI \-\-lengths
Print the captured and original packet lengths.
The lengths are printed at the beginning of the line or after the packet
number, if any.
\fIcaplen\fP is the captured packet length (see the
.B \-s
option).
\fIlen\fP is the original (on wire) packet length.
.TP
.BI \-m " module"
Load SMI MIB module definitions from file \fImodule\fR.
This option
can be used several times to load several MIB modules into \fItcpdump\fP.
.TP
.BI \-M " secret"
Use \fIsecret\fP as a shared secret for validating the digests found in
TCP segments with the TCP-MD5 option (RFC 2385), if present.
.TP
.B \-n
Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
.TP
.B \-N
Don't print domain name qualification of host names.
E.g.,
if you give this flag then \fItcpdump\fP will print ``nic''
instead of ``nic.ddn.mil''.
.TP
.B \-#
.PD 0
.TP
.B \-\-number
.PD
Print a packet number at the beginning of the line.
.TP
.B \-O
.PD 0
.TP
.B \-\-no\-optimize
.PD
Do not run the packet-matching code optimizer.
This is useful only
if you suspect a bug in the optimizer.
.TP
.B \-p
.PD 0
.TP
.B \-\-no\-promiscuous\-mode
.PD
\fIDon't\fP put the interface
into promiscuous mode.
Note that the interface might be in promiscuous
mode for some other reason; hence,
.B \-p
cannot be used as an abbreviation for
.BR "ether host {local-hw-addr} or ether broadcast" .
.TP
.BI \-\-print
Print parsed packet output, even if the raw packets are being saved to a
file with the
.B \-w
flag.
.TP
.BI \-\-print\-sampling= nth
.PD
Print every \fInth\fP packet. This option enables the
.B \-\-print
flag.
.IP
Unprinted packets are not parsed, which decreases processing time. Setting
\fInth\fP to \fB100\fP for example, will (counting from 1) parse and print the
100th packet, 200th packet, 300th packet, and so on.
.IP
This option also enables the
.B \-S
flag, as relative TCP sequence
numbers are not tracked for unprinted packets.
.TP
.BI \-Q " direction"
.PD 0
.TP
.BI \-\-direction= direction
.PD
Choose send/receive direction \fIdirection\fR for which packets should be
captured. Possible values are `in', `out' and `inout'. Not available
on all platforms.
.TP
.B \-q
Quick output.
Print less protocol information so output
lines are shorter.
.TP
.BI \-r " file"
Read packets from \fIfile\fR (which was created with the
.B \-w
option or by other tools that write pcap or pcapng files).
Standard input is used if \fIfile\fR is ``-''.
.TP
.B \-S
.PD 0
.TP
.B \-\-absolute\-tcp\-sequence\-numbers
.PD
Print absolute, rather than relative, TCP sequence numbers.
.TP
.BI \-s " snaplen"
.PD 0
.TP
.BI \-\-snapshot\-length= snaplen
.PD
Snarf \fIsnaplen\fP 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 ``[|\fIproto\fP]'', where \fIproto\fP
is the name of the protocol level at which the truncation has occurred.
.IP
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.
.IP
If you need to reduce the snapshot size below the default, you should
limit \fIsnaplen\fP to the smallest number that will capture the
protocol information you're interested in.  Setting
\fIsnaplen\fP to 0 sets it to the default of 262144,
for backwards compatibility with recent older versions of
.IR tcpdump .
.TP
.BI \-\-skip " count"
Skip \fIcount\fP packets before writing or printing.
\fIcount\fP with value 0 is allowed.
.TP
.BI \-T " type"
Force packets selected by "\fIexpression\fP" to be interpreted the
specified \fItype\fR.
Currently known types are
\fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
\fBcarp\fR (Common Address Redundancy Protocol),
\fBcnfp\fR (Cisco NetFlow protocol),
\fBdomain\fR (Domain Name System),
\fBlmp\fR (Link Management Protocol),
\fBpgm\fR (Pragmatic General Multicast),
\fBpgm_zmtp1\fR (ZMTP/1.0 inside PGM/EPGM),
\fBptp\fR (Precision Time Protocol),
\fBquic\fR (QUIC),
\fBradius\fR (RADIUS),
\fBresp\fR (REdis Serialization Protocol),
\fBrpc\fR (Remote Procedure Call),
\fBrtcp\fR (Real-Time Applications control protocol),
\fBrtp\fR (Real-Time Applications protocol),
\fBsnmp\fR (Simple Network Management Protocol),
\fBsomeip\fR (SOME/IP),
\fBtftp\fR (Trivial File Transfer Protocol),
\fBvat\fR (Visual Audio Tool),
\fBvxlan\fR (Virtual eXtensible Local Area Network),
\fBwb\fR (distributed White Board)
and
\fBzmtp1\fR (ZeroMQ Message Transport Protocol 1.0).
.IP
Note that the \fBpgm\fR 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".
.IP
Note that the \fBpgm_zmtp1\fR 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.
.TP
.B \-t
\fIDon't\fP print a timestamp on each dump line.
.TP
.B \-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.
.TP
.B \-ttt
Print a delta (microsecond or nanosecond resolution depending on the
.B \-\-time\-stamp-precision
option) between current and previous line on each dump line.
The default is microsecond resolution.
.TP
.B \-tttt
Print a timestamp, as hours, minutes, seconds, and fractions of a second
since midnight, preceded by the date, on each dump line.
.TP
.B \-ttttt
Print a delta (microsecond or nanosecond resolution depending on the
.B \-\-time\-stamp-precision
option) between current and first line on each dump line.
The default is microsecond resolution.
.TP
.B \-u
Print undecoded NFS handles.
.TP
.B \-U
.PD 0
.TP
.B \-\-packet\-buffered
.PD
If the
.B \-w
option is not specified, or if it is specified but the
.B \-\-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.
.IP
If the
.B \-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.
.TP
.B \-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.
.IP
When writing to a file with the
.B \-w
option and at the same time not reading from a file with the
.B \-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.
.TP
.B \-vv
Even more verbose output.
For example, additional fields are
printed from NFS reply packets, and SMB packets are fully decoded.
.TP
.B \-vvv
Even more verbose output.
For example,
telnet
.BR SB ... SE
options are printed in full.  With
.B \-X
telnet options are printed in hex as well.
.TP
.BI \-V " file"
Read a list of filenames from \fIfile\fR. Standard input is used
if \fIfile\fR is ``-''.
.TP
.BI \-w " file"
Write the raw packets to \fIfile\fR rather than parsing and printing
them out.
They can later be printed with the \-r option.
Standard output is used if \fIfile\fR is ``-''.
.IP
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
.B \-U
flag to cause packets to be written as soon as they are received.
.IP
The MIME type \fIapplication/vnd.tcpdump.pcap\fP has been registered
with IANA for \fIpcap\fP files. The filename extension \fI.pcap\fP
appears to be the most commonly used along with \fI.cap\fP and
\fI.dmp\fP. \fItcpdump\fP 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.
.IP
See
.BR \%pcap-savefile (@MAN_FILE_FORMATS@)
for a description of the file format.
.TP
.BI \-W " filecount"
Used in conjunction with the
.B \-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.
.IP
Used in conjunction with the
.B \-G
option, this will limit the number of rotated dump files that get
created, exiting with status 0 when reaching the limit.
.IP
If used in conjunction with both
.B \-C
and
.B \-G,
the
.B \-W
option will currently be ignored, and will only affect the file name.
.TP
.B \-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
.I 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
.B \-xx
if the packet is truncated.
No effect when
.B \-X[X]
option is used.
.TP
.B \-xx
When parsing and printing,
in addition to printing the headers of each packet, print the data of
each packet,
.I including
its link level header, in hex.
No effect when
.B \-X[X]
option is used.
.TP
.B \-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
.B \-XX
if the packet is truncated.
.TP
.B \-XX
When parsing and printing,
in addition to printing the headers of each packet, print the data of
each packet,
.I including
its link level header, in hex and ASCII.
.TP
.BI \-y " datalinktype"
.PD 0
.TP
.BI \-\-linktype= datalinktype
.PD
Set the data link type to use while capturing packets (see
.BR -L )
or just compiling and dumping packet-matching code (see
.BR -d )
to \fIdatalinktype\fP.
.TP
.BI \-z " postrotate-command"
Used in conjunction with the
.B -C
or
.B -G
options, this will make
.I tcpdump
run "
.I postrotate-command file
" where
.I file
is the savefile being closed after each rotation. For example, specifying
.B \-z gzip
or
.B \-z bzip2
will compress each savefile using gzip or bzip2.
.IP
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.
.IP
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.
.TP
.BI \-Z " user"
.PD 0
.TP
.BI \-\-relinquish\-privileges= user
.PD
If
.I 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
.I user
and the group ID to the primary group of
.IR user .
.IP
This behavior can also be enabled by default at compile time.
.IP "\fI expression\fP"
.RS
selects which packets will be dumped.
If no \fIexpression\fP
is given, all packets on the net will be dumped.
Otherwise,
only packets for that \fIexpression\fP is `true' will be dumped.
.LP
For the \fIexpression\fP syntax, see
.BR \%pcap-filter (@MAN_MISC_INFO@).
.LP
The \fIexpression\fP argument can be passed to \fItcpdump\fP 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.
.SH EXAMPLES
.LP
To print all packets arriving at or departing from \fIsundown\fP:
.RS
.nf
\fBtcpdump host sundown\fP
.fi
.RE
.LP
To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
.RS
.nf
\fBtcpdump host helios and \\( hot or ace \\)\fP
.fi
.RE
.LP
To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
.RS
.nf
\fBtcpdump ip host ace and not helios\fP
.fi
.RE
.LP
To print all traffic between local hosts and hosts at Berkeley:
.RS
.nf
.B
tcpdump net ucb-ether
.fi
.RE
.LP
To print all ftp traffic through internet gateway \fIsnup\fP:
(note that the expression is quoted to prevent the shell from
(mis-)interpreting the parentheses):
.RS
.nf
.B
tcpdump 'gateway snup and (port ftp or ftp-data)'
.fi
.RE
.LP
To print traffic neither sourced from nor destined for local hosts
(if you gateway to one other net, this stuff should never make it
onto your local net).
.RS
.nf
.B
tcpdump ip and not net \fIlocalnet\fP
.fi
.RE
.LP
To print the start and end packets (the SYN and FIN packets) of each
TCP conversation that involves a non-local host.
.RS
.nf
.B
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
.fi
.RE
.LP
To print the TCP packets with flags RST and ACK both set.
(i.e. select only the RST and ACK flags in the flags field, and if the result
is "RST and ACK both set", match)
.RS
.nf
.B
tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'
.fi
.RE
.LP
To print all IPv4 HTTP packets to and from port 80, i.e. print only
packets that contain data, not, for example, SYN and FIN packets and
ACK-only packets.  (IPv6 is left as an exercise for the reader.)
.RS
.nf
.B
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
.fi
.RE
.LP
To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
.RS
.nf
.B
tcpdump 'gateway snup and ip[2:2] > 576'
.fi
.RE
.LP
To print IP broadcast or multicast packets that were
.I not
sent via Ethernet broadcast or multicast:
.RS
.nf
.B
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
.fi
.RE
.LP
To print all ICMP packets that are not echo requests/replies (i.e., not
ping packets):
.RS
.nf
.B
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
.fi
.RE
.SH OUTPUT FORMAT
.LP
The output of \fItcpdump\fP is protocol dependent.
The following
gives a brief description and examples of most of the formats.
.de HD
.sp 1.5
.B
..
.SS Timestamps
.LP
By default, all output lines are preceded by a timestamp.
The timestamp
is the current clock time in the form
.RS
.nf
\fIhh:mm:ss.frac\fP
.fi
.RE
and is as accurate as the kernel's clock.
The timestamp reflects the time the kernel applied a time stamp to the packet.
No attempt is made to account for the time lag between when the network
interface finished receiving the packet from the network and when the
kernel applied a time stamp to the packet; that time lag could include a
delay between the time when the network interface finished receiving a
packet from the network and the time when an interrupt was delivered to
the kernel to get it to read the packet and a delay between the time
when the kernel serviced the `new packet' interrupt and the time when it
applied a time stamp to the packet.
.SS Interface
.LP
When the \fIany\fP interface is selected on capture or when a
.B LINKTYPE_LINUX_SLL2
capture file is read, the
interface name is printed after the timestamp. This is followed by the packet
type with \fIIn\fP and \fIOut\fP denoting a packet destined for this host or
originating from this host respectively. Other possible values are \fIB\fP
for broadcast packets, \fIM\fP for multicast packets, and \fIP\fP for packets
destined for other hosts.
.SS Link Level Headers
.LP
If the
.B \-e
option is given, the link level header is printed out.
On Ethernets, the source and destination addresses, protocol,
and packet length are printed.
.LP
On FDDI networks, the
.B \-e
option causes \fItcpdump\fP to print
the `frame control' field,  the source and destination addresses,
and the packet length.
(The `frame control' field governs the
interpretation of the rest of the packet.
Normal packets (such
as those containing IP datagrams) are `async' packets, with a priority
value between 0 and 7; for example, `\fBasync4\fR'.
Such packets
are assumed to contain an 802.2 Logical Link Control (LLC) packet;
the LLC header is printed if it is \fInot\fR an ISO datagram or a
so-called SNAP packet.
.LP
On Token Ring networks, the
.B \-e
option causes \fItcpdump\fP to print
the `access control' and `frame control' fields, the source and
destination addresses, and the packet length.
As on FDDI networks,
packets are assumed to contain an LLC packet.
Regardless of whether
the
.B \-e
option is specified or not, the source routing information is
printed for source-routed packets.
.LP
On 802.11 networks, the
.B \-e
option causes \fItcpdump\fP to print
the `frame control' fields, all of the addresses in the 802.11 header,
and the packet length.
As on FDDI networks,
packets are assumed to contain an LLC packet.
.LP
\fI(N.B.: The following description assumes familiarity with
the SLIP compression algorithm described in RFC 1144.)\fP
.LP
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
packet type, and compression information are printed out.
The packet type is printed first.
The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
No further link information is printed for \fIip\fR packets.
For TCP packets, the connection identifier is printed following the type.
If the packet is compressed, its encoded header is printed out.
The special cases are printed out as
\fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
the sequence number (or sequence number and ack) has changed.
If it is not a special case,
zero or more changes are printed.
A change is indicated by U (urgent pointer), W (window), A (ack),
S (sequence number), and I (packet ID), followed by a delta (+n or -n),
or a new value (=n).
Finally, the amount of data in the packet and compressed header length
are printed.
.LP
For example, the following line shows an outbound compressed TCP packet,
with an implicit connection identifier; the ack has changed by 6,
the sequence number by 49, and the packet ID by 6; there are 3 bytes of
data and 6 bytes of compressed header:
.RS
.nf
\fBO ctcp * A+6 S+49 I+6 3 (6)\fP
.fi
.RE
.SS ARP/RARP Packets
.LP
ARP/RARP output shows the type of request and its arguments.
The
format is intended to be self explanatory.
Here is a short sample taken from the start of an `rlogin' from
host \fIrtsg\fP to host \fIcsam\fP:
.RS
.nf
.sp .5
\f(CWarp who-has csam tell rtsg
arp reply csam is-at CSAM\fR
.sp .5
.fi
.RE
The first line says that rtsg sent an ARP packet asking
for the Ethernet address of internet host csam.
Csam
replies with its Ethernet address (in this example, Ethernet addresses
are in caps and internet addresses in lower case).
.LP
This would look less redundant if we had done
.BR "tcpdump \-n" :
.RS
.nf
.sp .5
\f(CWarp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
.fi
.RE
.LP
If we had done
.BR "tcpdump \-e" ,
the fact that the first packet is
broadcast and the second is point-to-point would be visible:
.RS
.nf
.sp .5
\f(CWRTSG Broadcast 0806  64: arp who-has csam tell rtsg
CSAM RTSG 0806  64: arp reply csam is-at CSAM\fR
.sp .5
.fi
.RE
For the first packet this says the Ethernet source address is RTSG, the
destination is the Ethernet broadcast address, the type field
contained hex 0806
.RB ( ETHERTYPE_ARP )
and the total length was 64 bytes.
.SS IPv4 Packets
.LP
If the link-layer header is not being printed, for IPv4 packets,
\fBIP\fP is printed after the time stamp.
.LP
If the
.B \-v
flag is specified, information from the IPv4 header is shown in
parentheses after the \fBIP\fP or the link-layer header.
The general format of this information is:
.RS
.nf
.sp .5
tos \fItos\fP, ttl \fIttl\fP, id \fIid\fP, offset \fIoffset\fP, flags [\fIflags\fP], proto \fIproto\fP, length \fIlength\fP, options (\fIoptions\fP)
.sp .5
.fi
.RE
\fItos\fP is the type of service field; if the ECN bits are non-zero,
those are reported as \fBECT(1)\fP, \fBECT(0)\fP, or \fBCE\fP.
\fIttl\fP is the time-to-live; it is not reported if it is zero.
\fIid\fP is the IP identification field.
\fIoffset\fP is the fragment offset field; it is printed whether this is
part of a fragmented datagram or not.
\fIflags\fP are the MF and DF flags; \fB+\fP is reported if MF is set,
and \fBDF\fP is reported if F is set.  If neither are set, \fB.\fP is
reported.
\fIproto\fP is the protocol ID field.
\fIlength\fP is the total length field; if the packet is a presumed TSO
(TCP Segmentation Offload) send, [was 0, presumed TSO] is reported.
\fIoptions\fP are the IP options, if any.
.LP
Next, for TCP and UDP packets, the source and destination IP addresses
and TCP or UDP ports, with a dot between each IP address and its
corresponding port, will be printed, with a > separating the source and
destination.  For other protocols, the addresses will be printed, with
a > separating the source and destination.  Higher level protocol
information, if any, will be printed after that.
.LP
For fragmented IP datagrams, the first fragment contains the higher
level protocol header; fragments after the first contain no higher level
protocol header.  Fragmentation information will be printed only with
the
.B \-v
flag, in the IP header information, as described above.
.SS TCP Packets
.LP
\fI(N.B.:The following description assumes familiarity with
the TCP protocol described in RFC 793.
If you are not familiar
with the protocol, this description will not
be of much use to you.)\fP
.LP
The general format of a TCP protocol line is:
.RS
.nf
.sp .5
\fIsrc\fP > \fIdst\fP: Flags [\fItcpflags\fP], seq \fIdata-seqno\fP, ack \fIackno\fP, win \fIwindow\fP, urg \fIurgent\fP, options [\fIopts\fP], length \fIlen\fP
.sp .5
.fi
.RE
\fISrc\fP and \fIdst\fP are the source and destination IP
addresses and ports.
\fITcpflags\fP are some combination of S (SYN), F (FIN),
P (PSH), R (RST), U (URG), W (CWR), E (ECE), e (AE) or
`.' (ACK), or `none' if no flags are set.
\fIData-seqno\fP describes the portion of sequence space covered
by the data in this packet (see example below).
\fIAckno\fP is sequence number of the next data expected the other
direction on this connection.
\fIWindow\fP is the number of bytes of receive buffer space available
the other direction on this connection.
\fIUrg\fP indicates there is `urgent' data in the packet.
\fIOpts\fP are TCP options (e.g., mss 1024).
\fILen\fP is the length of payload data.
.LP
\fIIptype\fR, \fISrc\fP, \fIdst\fP, and \fIflags\fP are always present.
The other fields
depend on the contents of the packet's TCP protocol header and
are output only if appropriate.
.LP
Here is the opening portion of an rlogin from host \fIrtsg\fP to
host \fIcsam\fP.
.RS
.nf
.sp .5
\f(CWIP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1\fR
.sp .5
.fi
.RE
The first line says that TCP port 1023 on rtsg sent a packet
to port \fIlogin\fP
on csam.
The \fBS\fP indicates that the \fISYN\fP flag was set.
The packet sequence number was 768512 and it contained no data.
(The notation is `first:last' which means `sequence
numbers \fIfirst\fP
up to but not including \fIlast\fP'.)
There was no piggy-backed ACK, the available receive window was 4096
bytes and there was a max-segment-size option requesting an MSS of
1024 bytes.
.LP
Csam replies with a similar packet except it includes a piggy-backed
ACK for rtsg's SYN.
Rtsg then ACKs csam's SYN.
The `.' means the ACK flag was set.
The packet contained no data so there is no data sequence number or length.
Note that the ACK sequence
number is a small integer (1).
The first time \fItcpdump\fP sees a
TCP `conversation', it prints the sequence number from the packet.
On subsequent packets of the conversation, the difference between
the current packet's sequence number and this initial sequence number
is printed.
This means that sequence numbers after the
first can be interpreted
as relative byte positions in the conversation's data stream (with the
first data byte each direction being `1').
.B \-S
will override this
feature, causing the original sequence numbers to be output.
.LP
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
in the rtsg \(-> csam side of the conversation).
The PSH flag is set in the packet.
On the 7th line, csam says it's received data sent by rtsg up to
but not including byte 21.
Most of this data is apparently sitting in the
socket buffer since csam's receive window has gotten 19 bytes smaller.
Csam also sends one byte of data to rtsg in this packet.
On the 8th and 9th lines,
csam sends two bytes of urgent, pushed data to rtsg.
.LP
If the snapshot was small enough that \fItcpdump\fP didn't capture
the full TCP header, it interprets as much of the header as it can
and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
be interpreted.
If the header contains a bogus option (one with a length
that's either too small or beyond the end of the header), \fItcpdump\fP
reports it as ``[\fIbad opt\fP]'' and does not interpret any further
options (since it's impossible to tell where they start).
If the header
length indicates options are present but the IP datagram length is not
long enough for the options to actually be there, \fItcpdump\fP reports
it as ``[\fIbad hdr length\fP]''.
.SS Particular TCP Flag Combinations (SYN-ACK, URG-ACK, etc.)
.PP
There are 9 bits in the control bits section of the TCP header:
.LP
\fIAE(e) CWR(W) ECE(E) URG(U) ACK(.) PSH(P) RST(R) SYN(S) FIN(F)\fP
.LP
Let's assume that we want to watch packets used in establishing
a TCP connection.
Recall that TCP uses a 3-way handshake protocol
when it initializes a new connection; the connection sequence with
regard to the TCP control bits is
.PP
.RS
1) Caller sends SYN
.RE
.RS
2) Recipient responds with SYN, ACK
.RE
.RS
3) Caller sends ACK
.RE
.PP
Now we're interested in capturing packets that have only the
SYN bit set (Step 1).
Note that we don't want packets from step 2
(SYN-ACK), just a plain initial SYN.
What we need is a correct filter
expression for \fItcpdump\fP.
.PP
Recall the structure of a TCP header without options:
.PP
.nf
 0                            15                              31
-----------------------------------------------------------------
|          source port          |       destination port        |
-----------------------------------------------------------------
|                        sequence number                        |
-----------------------------------------------------------------
|                     acknowledgment number                     |
-----------------------------------------------------------------
|header |re   |A|C|E|U|A|P|R|S|F|                               |
|length |serv |E|W|C|R|C|S|S|Y|I|        window size            |
|       |  ed | |R|E|G|K|H|T|N|N|                               |
-----------------------------------------------------------------
|         TCP checksum          |       urgent pointer          |
-----------------------------------------------------------------
.fi
.PP
A TCP header usually holds 20 octets of data, unless options are
present.
The first line of the graph contains octets 0 - 3, the
second line shows octets 4 - 7 etc.
.PP
Starting to count with 0, the relevant TCP control bits are contained
in octets 12 and 13:
.PP
.nf
 0             7|             15|             23|             31
----------------|---------------|---------------|----------------
|header |re   |A|C|E|U|A|P|R|S|F|                               |
|length |serv |E|W|C|R|C|S|S|Y|I|        window size            |
|       |  ed | |R|E|G|K|H|T|N|N|                               |
----------------|---------------|---------------|----------------
|               |  13th octet   |               |               |
.fi
.PP
Let's have a closer look at octet no. 13:
.PP
.nf
                |               |
                |---------------|
                |C|E|U|A|P|R|S|F|
                |W|C|R|C|S|S|Y|I|
                |R|E|G|K|H|T|N|N|
                |---------------|
                |7   5   3     0|
.fi
.PP
These are the TCP control bits we are interested
in.
We have numbered the bits in this octet from 0 to 7, right to
left, so the PSH bit is bit number 3, while the URG bit is number 5.
.PP
Recall that we want to capture packets with only SYN set.
Let's see what happens to octet 13 if a TCP datagram arrives
with the SYN bit set in its header:
.PP
.nf
                |C|E|U|A|P|R|S|F|
                |W|C|R|C|S|S|Y|I|
                |R|E|G|K|H|T|N|N|
                |---------------|
                |0 0 0 0 0 0 1 0|
                |---------------|
                |7 6 5 4 3 2 1 0|
.fi
.PP
Looking at the
control bits section we see that only bit number 1 (SYN) is set.
.PP
Assuming that octet number 13 is an 8-bit unsigned integer in
network byte order, the binary value of this octet is
.IP
00000010
.PP
and its decimal representation is
.PP
.nf
   7     6     5     4     3     2     1     0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2  =  2
.fi
.PP
We're almost done, because now we know that if only SYN is set,
the value of the 13th octet in the TCP header, when interpreted
as a 8-bit unsigned integer in network byte order, must be exactly 2.
.PP
This relationship can be expressed as
.RS
.B
tcp[13] == 2
.RE
.PP
We can use this expression as the filter for \fItcpdump\fP in order
to watch packets which have only SYN set:
.RS
.B
tcpdump -i xl0 'tcp[13] == 2'
.RE
.PP
The expression says "let the 13th octet of a TCP datagram have
the decimal value 2", which is exactly what we want.
.PP
Now, let's assume that we need to capture SYN packets, but we
don't care if ACK or any other TCP control bit is set at the
same time.
Let's see what happens to octet 13 when a TCP datagram
with SYN-ACK set arrives:
.PP
.nf
     |C|E|U|A|P|R|S|F|
     |W|C|R|C|S|S|Y|I|
     |R|E|G|K|H|T|N|N|
     |---------------|
     |0 0 0 1 0 0 1 0|
     |---------------|
     |7 6 5 4 3 2 1 0|
.fi
.PP
Now bits 1 and 4 are set in the 13th octet.
The binary value of
octet 13 is
.IP
     00010010
.PP
which translates to decimal
.PP
.nf
   7     6     5     4     3     2     1     0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2   = 18
.fi
.PP
Now we can't just use
.B "tcp[13] == 18"
in the \fItcpdump\fP filter
expression, because that would select only those packets that have
SYN-ACK set, but not those with only SYN set.
Remember that we don't care
if ACK or any other control bit is set as long as SYN is set.
.PP
In order to achieve our goal, we need to logically AND the
binary value of octet 13 with some other value to preserve
the SYN bit.
We know that we want SYN to be set in any case,
so we'll logically AND the value in the 13th octet with
the binary value of a SYN:
.PP
.nf

          00010010 SYN-ACK              00000010 SYN
     AND  00000010 (we want SYN)   AND  00000010 (we want SYN)
          --------                      --------
     =    00000010                 =    00000010
.fi
.PP
We see that this AND operation delivers the same result
regardless whether ACK or another TCP control bit is set.
The decimal representation of the AND value as well as
the result of this operation is 2 (binary 00000010),
so we know that for packets with SYN set the following
relation must hold true:
.IP
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
.PP
This points us to the \fItcpdump\fP filter expression
.RS
.B
tcpdump -i xl0 'tcp[13] & 2 == 2'
.RE
.PP
Some offsets and field values may be expressed as names
rather than as numeric values. For example,
.B tcp[13]
may be replaced with
.BR tcp[tcpflags] .
The following TCP flag
field values are also available: tcp-fin, tcp-syn, tcp-rst,
tcp-push, tcp-ack, tcp-urg, tcp-ece and tcp-cwr.
.PP
This can be demonstrated as:
.RS
.B
tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
.RE
.PP
Note that you should use single quotes or a backslash
in the expression to hide the AND ('&') special character
from the shell.
.SS UDP Packets
.LP
UDP format is illustrated by this rwho packet:
.RS
.nf
.sp .5
\f(CWactinide.who > broadcast.who: udp 84\fP
.sp .5
.fi
.RE
This says that port \fIwho\fP on host \fIactinide\fP sent a UDP
datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
broadcast address.
The packet contained 84 bytes of user data.
.LP
Some UDP services are recognized (from the source or destination
port number) and the higher level protocol information printed.
In particular, Domain Name service requests (RFC 1034/1035) and Sun
RPC calls (RFC 1050) to NFS.
.SS TCP or UDP Name Server Requests
.LP
\fI(N.B.:The following description assumes familiarity with
the Domain Service protocol described in RFC 1035.
If you are not familiar
with the protocol, the following description will appear to be written
in Greek.)\fP
.LP
Name server requests are formatted as
.RS
.nf
.sp .5
\fIsrc > dst: id op? flags qtype qclass name (len)\fP
.sp .5
\f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
.sp .5
.fi
.RE
Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
The query id was `3'.
The `+' indicates the \fIrecursion desired\fP flag
was set.
The query length was 37 bytes, excluding the TCP or UDP and
IP protocol headers.
The query operation was the normal one, \fIQuery\fP,
so the op field was omitted.
If the op had been anything else, it would
have been printed between the `3' and the `+'.
Similarly, the qclass was the normal one,
.BR C_IN ,
and omitted.
Any other qclass would have been printed
immediately after the `A'.
.LP
A few anomalies are checked and may result in extra fields enclosed in
square brackets:  If a query contains an answer, authority records or
additional records section,
.IR ancount ,
.IR nscount ,
or
.I arcount
are printed as `[\fIn\fPa]', `[\fIn\fPn]' or  `[\fIn\fPau]' where \fIn\fP
is the appropriate count.
If any of the response bits are set (AA, RA or rcode) or any of the
`must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
is printed, where \fIx\fP is the hex value of header bytes two and three.
.SS TCP or UDP Name Server Responses
.LP
Name server responses are formatted as
.RS
.nf
.sp .5
\fIsrc > dst:  id op rcode flags a/n/au type class data (len)\fP
.sp .5
\f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
.sp .5
.fi
.RE
In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
with 3 answer records, 3 name server records and 7 additional records.
The first answer record is type A (address) and its data is internet
address 128.32.137.3.
The total size of the response was 273 bytes,
excluding TCP or UDP and IP headers.
The op (Query) and response code
(NoError) were omitted, as was the class
.RB ( C_IN )
of the A record.
.LP
In the second example, \fIhelios\fP responds to query 2 with a
response code of nonexistent domain (NXDomain) with no answers,
one name server and no authority records.
The `*' indicates that
the \fIauthoritative answer\fP bit was set.
Since there were no
answers, no type, class or data were printed.
.LP
Other flag characters that might appear are `\-' (recursion available,
RA, \fInot\fP set) and `|' (truncated message, TC, set).
If the
`question' section doesn't contain exactly one entry, `[\fIn\fPq]'
is printed.
.SS SMB/CIFS Decoding
.LP
\fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
on UDP/137, UDP/138 and TCP/139.
Some primitive decoding of IPX and
NetBEUI SMB data is also done.
.LP
By default a fairly minimal decode is done, with a much more detailed
decode done if
.B \-v
is used.  Be warned that with
.B \-v
a single SMB packet may take up a page or more, so only use
.B \-v
if you really want all the gory details.
.LP
For information on SMB packet formats and what all the fields mean see
\%https://download.samba.org/pub/samba/specs/ and other online resources.
The SMB patches were written by Andrew Tridgell
(tridge@samba.org).
.SS NFS Requests and Replies
.LP
Network File System requests and replies are printed as:
.RS
.nf
.sp .5
\fIsrc.sport > dst.nfs: NFS request xid xid len op args\fP
\fIsrc.nfs > dst.dport: NFS reply xid xid reply stat len op results\fP
.sp .5
\f(CW
sushi.1023 > wrl.nfs: NFS request xid 26377
	112 readlink fh 21,24/10.73165
wrl.nfs > sushi.1023: NFS reply xid 26377
	reply ok 40 readlink "../var"
sushi.1022 > wrl.nfs: NFS request xid 8219
	144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.1022: NFS reply xid 8219
	reply ok 128 lookup fh 9,74/4134.3150
\fR
.sp .5
.fi
.RE
In the first line, host \fIsushi\fP sends a transaction with id \fI26377\fP
to \fIwrl\fP.
The request was 112 bytes,
excluding the UDP and IP headers.
The operation was a \fIreadlink\fP
(read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
(If one is lucky, as in this case, the file handle can be interpreted
as a major,minor device number pair, followed by the inode number and
generation number.) In the second line, \fIwrl\fP replies `ok' with
the same transaction id and the contents of the link.
.LP
In the third line, \fIsushi\fP asks (using a new transaction id) \fIwrl\fP
to lookup the name `\fIxcolors\fP' in directory file 9,74/4096.6878. In
the fourth line, \fIwrl\fP sends a reply with the respective transaction id.
.LP
Note that the data printed
depends on the operation type.
The format is intended to be self
explanatory if read in conjunction with
an NFS protocol spec.
Also note that older versions of tcpdump printed NFS packets in a
slightly different format: the transaction id (xid) would be printed
instead of the non-NFS port number of the packet.
.LP
If the
.B \-v
(verbose) flag is given, additional information is printed.
For example:
.RS
.nf
.sp .5
\f(CW
sushi.1023 > wrl.nfs: NFS request xid 79658
	148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1023: NFS reply xid 79658
	reply ok 1472 read REG 100664 ids 417/0 sz 29388
\fP
.sp .5
.fi
.RE
.RB ( \-v
also prints the IP header TTL, ID, length, and fragmentation fields,
which have been omitted from this example.)  In the first line,
\fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
at byte offset 24576.
\fIWrl\fP replies `ok'; the packet shown on the
second line is the first fragment of the reply, and hence is only 1472
bytes long (the other bytes will follow in subsequent fragments, but
these fragments do not have NFS or even UDP headers and so might not be
printed, depending on the filter expression used).
Because the \-v flag
is given, some of the file attributes (which are returned in addition
to the file data) are printed: the file type (``REG'', for regular file),
the file mode (in octal), the UID and GID, and the file size.
.LP
If the
.B \-v
flag is given more than once, even more details are printed.
.LP
NFS reply packets do not explicitly identify the RPC operation.
Instead,
\fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
replies using the transaction ID.
If a reply does not closely follow the
corresponding request, it might not be parsable.
.SS AFS Requests and Replies
.LP
Andrew File System requests and replies are printed
as:
.HD
.RS
.nf
.sp .5
\fIsrc.sport > dst.dport: rx packet-type\fP
\fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
\fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
.sp .5
\f(CW
elvis.7001 > pike.afsfs:
	rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
	new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename
\fR
.sp .5
.fi
.RE
In the first line, host elvis sends a RX packet to pike.
This was
a RX data packet to the fs (fileserver) service, and is the start of
an RPC call.
The RPC call was a rename, with the old directory file id
of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
file id of 536876964/1/1 and a new filename of `.newsrc'.
The host pike
responds with a RPC reply to the rename call (which was successful, because
it was a data packet and not an abort packet).
.LP
In general, all AFS RPCs are decoded at least by RPC call name.
Most
AFS RPCs have at least some of the arguments decoded (generally only
the `interesting' arguments, for some definition of interesting).
.LP
The format is intended to be self-describing, but it will probably
not be useful to people who are not familiar with the workings of
AFS and RX.
.LP
If the
.B \-v
(verbose) flag is given, acknowledgement packets and
additional header information is printed, such as the RX call ID,
call number, sequence number, serial number, and the RX packet flags.
.LP
If the
.B \-v
flag is given twice, additional information is printed,
such as the RX call ID, serial number, and the RX packet flags.
The MTU negotiation information is also printed from RX ack packets.
.LP
If the
.B \-v
flag is given three times, the security index and service id
are printed.
.LP
Error codes are printed for abort packets, with the exception of Ubik
beacon packets (because abort packets are used to signify a yes vote
for the Ubik protocol).
.LP
AFS reply packets do not explicitly identify the RPC operation.
Instead,
\fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
replies using the call number and service ID.
If a reply does not closely
follow the
corresponding request, it might not be parsable.

.SS KIP AppleTalk (DDP in UDP)
.LP
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
and dumped as DDP packets (i.e., all the UDP header information is
discarded).
The file
.I /etc/atalk.names
is used to translate AppleTalk net and node numbers to names.
Lines in this file have the form
.RS
.nf
.sp .5
\fInumber	name\fP

\f(CW1.254		ether
16.1		icsd-net
1.254.110	ace\fR
.sp .5
.fi
.RE
The first two lines give the names of AppleTalk networks.
The third
line gives the name of a particular host (a host is distinguished
from a net by the 3rd octet in the number \-
a net number \fImust\fP have two octets and a host number \fImust\fP
have three octets.)  The number and name should be separated by
whitespace (blanks or tabs).
The
.I /etc/atalk.names
file may contain blank lines or comment lines (lines starting with
a `#').
.LP
AppleTalk addresses are printed in the form
.RS
.nf
.sp .5
\fInet.host.port\fP

\f(CW144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2\fR
.sp .5
.fi
.RE
(If the
.I /etc/atalk.names
doesn't exist or doesn't contain an entry for some AppleTalk
host/net number, addresses are printed in numeric form.)
In the first example, NBP (DDP port 2) on net 144.1 node 209
is sending to whatever is listening on port 220 of net icsd node 112.
The second line is the same except the full name of the source node
is known (`office').
The third line is a send from port 235 on
net jssmag node 149 to broadcast on the icsd-net NBP port (note that
the broadcast address (255) is indicated by a net name with no host
number \- for this reason it's a good idea to keep node names and
net names distinct in /etc/atalk.names).
.LP
NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
packets have their contents interpreted.
Other protocols just dump
the protocol name (or number if no name is registered for the
protocol) and packet size.

.SS NBP Packets
NBP packets are formatted like the following examples:
.RS
.nf
.sp .5
\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR
.sp .5
.fi
.RE
The first line is a name lookup request for laserwriters sent by net icsd host
112 and broadcast on net jssmag.
The nbp id for the lookup is 190.
The second line shows a reply for this request (note that it has the
same id) from host jssmag.209 saying that it has a laserwriter
resource named "RM1140" registered on port 250.
The third line is
another reply to the same request saying host techpit has laserwriter
"techpit" registered on port 186.

.SS ATP Packets
ATP packet formatting is demonstrated by the following example:
.RS
.nf
.sp .5
\f(CWjssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR
.sp .5
.fi
.RE
Jssmag.209 initiates transaction id 12266 with host helios by requesting
up to 8 packets (the `<0-7>').
The hex number at the end of the line
is the value of the `userdata' field in the request.
.LP
Helios responds with 8 512-byte packets.
The `:digit' following the
transaction id gives the packet sequence number in the transaction
and the number in parens is the amount of data in the packet,
excluding the ATP header.
The `*' on packet 7 indicates that the
EOM bit was set.
.LP
Jssmag.209 then requests that packets 3 & 5 be retransmitted.
Helios
resends them then jssmag.209 releases the transaction.
Finally,
jssmag.209 initiates the next request.
The `*' on the request
indicates that XO (`exactly once') was \fInot\fP set.

.SH BACKWARD COMPATIBILITY
The TCP flag names
.B tcp-ece
and
.B tcp-cwr
became available when linking with libpcap 1.9.0 or later.
.LP
This version of tcpdump requires libpcap 1.0 or later.

.SH "SEE ALSO"
.BR stty (1),
.BR pcap (3PCAP),
.BR \%pcap-savefile (@MAN_FILE_FORMATS@),
.BR \%pcap-filter (@MAN_MISC_INFO@),
.BR \%pcap-tstamp (@MAN_MISC_INFO@)
.LP
.RS
.na
.I https://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
.ad
.RE
.LP
.SH AUTHORS
The original authors are:
.LP
Van Jacobson,
Craig Leres and
Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
.LP
It is currently maintained by The Tcpdump Group.
.LP
The current version is available via HTTPS:
.LP
.RS
.I https://www.tcpdump.org/
.RE
.LP
The original distribution is available via anonymous FTP:
.LP
.RS
.I ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z
.RE
.LP
IPv6/IPsec support is added by WIDE/KAME project.
This program uses OpenSSL/LibreSSL, under specific configurations.
.SH BUGS
To report a security issue please send an e-mail to \%security@tcpdump.org.
.LP
To report bugs and other problems, contribute patches, request a
feature, provide generic feedback etc. please see the file
.I CONTRIBUTING.md
in the tcpdump source tree root.
.LP
Some attempt should be made to reassemble IP fragments or, at least
to compute the right length for the higher level protocol.
.LP
Name server inverse queries are not dumped correctly: the (empty)
question section is printed rather than real query in the answer
section.
Some believe that inverse queries are themselves a bug and
prefer to fix the program generating them rather than \fItcpdump\fP.
.LP
A packet trace that crosses a daylight savings time change will give
skewed time stamps (the time change is ignored).