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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | OUTPUT FORMAT | BACKWARD COMPATIBILITY | SEE ALSO | AUTHORS | BUGS | COLOPHON |
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TCPDUMP(1) General Commands Manual TCPDUMP(1)
tcpdump - dump traffic on a network
tcpdump [ -AbdDefghHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]
[ -c count ] [ --count ] [ -C file_size ]
[ -E spi@ipaddr algo:secret,... ]
[ -F file ] [ -G rotate_seconds ] [ -i interface ]
[ --immediate-mode ] [ -j tstamp_type ] [ --lengths ] [ -m
module ]
[ -M secret ] [ --number ] [ --print ] [ --print-sampling
nth ]
[ -Q in|out|inout ] [ -r file ] [ -s snaplen ] [ --skip
count ]
[ -T type ] [ --version ] [ -V file ] [ -w file ] [ -W
filecount ]
[ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
[ --time-stamp-precision=tstamp_precision ] [ --micro ] [
--nano ]
[ expression ]
tcpdump prints out a description of the contents of packets on a
network interface that match the Boolean expression (see
pcap-filter(@MAN_MISC_INFO@) for the expression 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 -w flag, which causes it to save the
packet data to a file for later analysis, and/or with the -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 -V flag, which causes it to read a list of saved packet files.
In all cases, only packets that match expression will be processed
by tcpdump.
tcpdump will, if not run with the -c flag, continue capturing
packets until it is interrupted by a SIGINT signal (generated, for
example, by typing your interrupt character, typically control-C)
or a SIGTERM signal (typically generated with the kill(1)
command); if run with the -c flag, it will capture packets until
it is interrupted by a SIGINT or SIGTERM signal or the specified
number of packets have been processed.
When tcpdump finishes capturing packets, it will report counts of:
packets ``captured'' (this is the number of packets that
tcpdump has received and processed);
packets ``received by filter'' (the meaning of this depends
on the OS on which you're running 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 tcpdump has read and
processed them yet, on other OSes it counts only packets
that were matched by the filter expression regardless of
whether 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 tcpdump);
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 tcpdump
is running, if the OS reports that information to
applications; if not, it will be reported as 0).
On platforms that support the SIGINFO signal, such as most BSDs
(including macOS), it will report those counts when it receives a
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 stty(1) in order to use it) and will continue
capturing packets. On platforms that do not support the SIGINFO
signal, the same can be achieved by using the SIGUSR1 signal.
Using the SIGUSR2 signal along with the -w flag will forcibly
flush the packet buffer into the output file.
Reading packets from a network interface may require that you have
special privileges; see the pcap(3PCAP) man page for details.
Reading a saved packet file doesn't require special privileges.
-A Print each packet (minus its link level header) in ASCII.
Handy for capturing web pages. No effect when -x[x] or
-X[X] options are used.
-b Print the AS number in BGP packets using "asdot" rather
than "asplain" representation, in RFC 5396 terms.
-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 or reading count packets. If the
--skip option is used, the count starts after the skipped
packets.
--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, 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 array of struct bpf_insn
structures.
-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.
-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 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 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 that capture
is being done. If the interface has no IPv4 addresses
(which by convention applies to the "any" pseudo-interface
(see the -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.
Without the -f flag, or when the netmask is assumed to be
/0 (as discussed above), the 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.
-F file
Use file as input for the filter expression. An additional
expression given on the command line is ignored.
-g
--ip-oneline
Do not insert a line break after the IP header in verbose
mode.
-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 preexisting 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 all supported Linux systems, as well as on recent
versions of macOS and Solaris, an 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 -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.
An interface number as printed by the -D 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 tcpdump 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).
--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. caplen is the captured packet
length (see the -s option). len is the original (on wire)
packet length.
-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 a 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.
--print-sampling=nth
Print every nth packet. This option enables the --print
flag.
Unprinted packets are not parsed, which decreases
processing time. Setting nth to 100 for example, will
(counting from 1) parse and print the 100th packet, 200th
packet, 300th packet, and so on.
This option also enables the -S flag, as relative TCP
sequence numbers are not tracked for unprinted packets.
-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 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.
--skip count
Skip count packets before writing or printing. count with
value 0 is allowed.
-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), quic (QUIC),
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.
-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.
No effect when -X[X] option is used.
-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. No effect when
-X[X] option is used.
-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.
This option can only be used if fork subprocess is
implemented (e.g. not on Windows systems).
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 that 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.
To print all packets arriving at or departing from sundown:
tcpdump host sundown
To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup: (note that
the expression is quoted to prevent the shell from
(mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'
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).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN packets) of
each TCP conversation that involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
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)
tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'
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.)
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
To print IP packets longer than 576 bytes sent through gateway
snup:
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were not sent via
Ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies
(i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
The output of tcpdump is protocol dependent. The following gives
a brief description and examples of most of the formats.
Timestamps
By default, all output lines are preceded by a timestamp. The
timestamp is the current clock time in the form
hh:mm:ss.frac
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.
Interface
When the any interface is selected on capture or when a
LINKTYPE_LINUX_SLL2 capture file is read, the interface name is
printed after the timestamp. This is followed by the packet type
with In and Out denoting a packet destined for this host or
originating from this host respectively. Other possible values are
B for broadcast packets, M for multicast packets, and P for
packets destined for other hosts.
Link Level Headers
If the -e option is given, the link level header is printed out.
On Ethernets, the source and destination addresses, protocol, and
packet length are printed.
On FDDI networks, the -e option causes tcpdump 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, `async4'. Such
packets are assumed to contain an 802.2 Logical Link Control (LLC)
packet; the LLC header is printed if it is not an ISO datagram or
a so-called SNAP packet.
On Token Ring networks, the -e option causes tcpdump 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 -e option is specified or not, the
source routing information is printed for source-routed packets.
On 802.11 networks, the -e option causes tcpdump 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.
(N.B.: The following description assumes familiarity with the SLIP
compression algorithm described in RFC 1144.)
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 ip,
utcp, and ctcp. No further link information is printed for ip
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 *S+n
and *SA+n, where n 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.
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:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
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 rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
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).
This would look less redundant if we had done tcpdump -n:
arp 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
If we had done tcpdump -e, the fact that the first packet is
broadcast and the second is point-to-point would be visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
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 (ETHERTYPE_ARP) and the total length was
64 bytes.
IPv4 Packets
If the link-layer header is not being printed, for IPv4 packets,
IP is printed after the time stamp.
If the -v flag is specified, information from the IPv4 header is
shown in parentheses after the IP or the link-layer header. The
general format of this information is:
tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
tos is the type of service field; if the ECN bits are non-zero,
those are reported as ECT(1), ECT(0), or CE. ttl is the time-to-
live; it is not reported if it is zero. id is the IP
identification field. offset is the fragment offset field; it is
printed whether this is part of a fragmented datagram or not.
flags are the MF and DF flags; + is reported if MF is set, and DF
is reported if F is set. If neither are set, . is reported.
proto is the protocol ID field. length is the total length field;
if the packet is a presumed TSO (TCP Segmentation Offload) send,
[was 0, presumed TSO] is reported. options are the IP options, if
any.
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.
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 -v flag, in the IP header information, as
described above.
TCP Packets
(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.)
The general format of a TCP protocol line is:
src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
Src and dst are the source and destination IP addresses and ports.
Tcpflags 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. Data-seqno describes the portion of sequence
space covered by the data in this packet (see example below).
Ackno is sequence number of the next data expected the other
direction on this connection. Window is the number of bytes of
receive buffer space available the other direction on this
connection. Urg indicates there is `urgent' data in the packet.
Opts are TCP options (e.g., mss 1024). Len is the length of
payload data.
Iptype, Src, dst, and flags are always present. The other fields
depend on the contents of the packet's TCP protocol header and are
output only if appropriate.
Here is the opening portion of an rlogin from host rtsg to host
csam.
IP 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
The first line says that TCP port 1023 on rtsg sent a packet to
port login on csam. The S indicates that the SYN flag was set.
The packet sequence number was 768512 and it contained no data.
(The notation is `first:last' which means `sequence numbers first
up to but not including last'.) 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.
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 tcpdump 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').
-S will override this feature, causing the original sequence
numbers to be output.
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.
If the snapshot was small enough that tcpdump didn't capture the
full TCP header, it interprets as much of the header as it can and
then reports ``[|tcp]'' 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),
tcpdump reports it as ``[bad opt]'' 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, tcpdump reports it as ``[bad hdr length]''.
Particular TCP Flag Combinations (SYN-ACK, URG-ACK, etc.)
There are 9 bits in the control bits section of the TCP header:
AE(e) CWR(W) ECE(E) URG(U) ACK(.) PSH(P) RST(R) SYN(S) FIN(F)
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
1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK
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 tcpdump.
Recall the structure of a TCP header without options:
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 |
-----------------------------------------------------------------
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.
Starting to count with 0, the relevant TCP control bits are
contained in octets 12 and 13:
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 | | |
Let's have a closer look at octet no. 13:
| |
|---------------|
|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|
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.
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:
|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|
Looking at the control bits section we see that only bit number 1
(SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in
network byte order, the binary value of this octet is
00000010
and its decimal representation is
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
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.
This relationship can be expressed as
tcp[13] == 2
We can use this expression as the filter for tcpdump in order to
watch packets which have only SYN set:
tcpdump -i xl0 'tcp[13] == 2'
The expression says "let the 13th octet of a TCP datagram have the
decimal value 2", which is exactly what we want.
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:
|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|
Now bits 1 and 4 are set in the 13th octet. The binary value of
octet 13 is
00010010
which translates to decimal
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
Now we can't just use tcp[13] == 18 in the tcpdump 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.
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:
00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010
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:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'
Some offsets and field values may be expressed as names rather
than as numeric values. For example, tcp[13] may be replaced with
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.
This can be demonstrated as:
tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
Note that you should use single quotes or a backslash in the
expression to hide the AND ('&') special character from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a UDP datagram to
port who on host broadcast, the Internet broadcast address. The
packet contained 84 bytes of user data.
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.
TCP or UDP Name Server Requests
(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.)
Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an address
record (qtype=A) associated with the name ucbvax.berkeley.edu.
The query id was `3'. The `+' indicates the recursion desired
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, Query, 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, C_IN, and omitted.
Any other qclass would have been printed immediately after the
`A'.
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, ancount, nscount,
or arcount are printed as `[na]', `[nn]' or `[nau]' where n 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=x]' is printed, where x is the hex value of
header bytes two and three.
TCP or UDP Name Server Responses
Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo
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
(C_IN) of the A record.
In the second example, helios 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
authoritative answer bit was set. Since there were no answers, no
type, class or data were printed.
Other flag characters that might appear are `-' (recursion
available, RA, not set) and `|' (truncated message, TC, set). If
the `question' section doesn't contain exactly one entry, `[nq]'
is printed.
SMB/CIFS Decoding
tcpdump 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.
By default a fairly minimal decode is done, with a much more
detailed decode done if -v is used. Be warned that with -v a
single SMB packet may take up a page or more, so only use -v if
you really want all the gory details.
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).
NFS Requests and Replies
Network File System requests and replies are printed as:
src.sport > dst.nfs: NFS request xid xid len op args
src.nfs > dst.dport: NFS reply xid xid reply stat len op results
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
In the first line, host sushi sends a transaction with id 26377 to
wrl. The request was 112 bytes, excluding the UDP and IP headers.
The operation was a readlink (read symbolic link) on file handle
(fh) 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, wrl replies `ok' with the same transaction id and the
contents of the link.
In the third line, sushi asks (using a new transaction id) wrl to
lookup the name `xcolors' in directory file 9,74/4096.6878. In the
fourth line, wrl sends a reply with the respective transaction id.
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.
If the -v (verbose) flag is given, additional information is
printed. For example:
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
(-v also prints the IP header TTL, ID, length, and fragmentation
fields, which have been omitted from this example.) In the first
line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at
byte offset 24576. Wrl 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.
If the -v flag is given more than once, even more details are
printed.
NFS reply packets do not explicitly identify the RPC operation.
Instead, tcpdump 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.
AFS Requests and Replies
Andrew File System requests and replies are printed as:
src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args
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
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).
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).
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.
If the -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.
If the -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.
If the -v flag is given three times, the security index and
service id are printed.
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).
AFS reply packets do not explicitly identify the RPC operation.
Instead, tcpdump 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.
KIP AppleTalk (DDP in UDP)
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 /etc/atalk.names is used to
translate AppleTalk net and node numbers to names. Lines in this
file have the form
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
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 must have two octets and a host number must have three
octets.) The number and name should be separated by whitespace
(blanks or tabs). The /etc/atalk.names file may contain blank
lines or comment lines (lines starting with a `#').
AppleTalk addresses are printed in the form
net.host.port
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /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).
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.
NBP Packets
NBP packets are formatted like the following examples:
icsd-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
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.
ATP Packets
ATP packet formatting is demonstrated by the following example:
jssmag.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
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.
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.
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 not set.
The TCP flag names tcp-ece and tcp-cwr became available when
linking with libpcap 1.9.0 or later.
This version of tcpdump requires libpcap 1.0 or later.
stty(1), pcap(3PCAP), pcap-savefile(@MAN_FILE_FORMATS@),
pcap-filter(@MAN_MISC_INFO@), pcap-tstamp(@MAN_MISC_INFO@)
https://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
Berkeley National Laboratory, University of California, Berkeley,
CA.
It is currently maintained by The Tcpdump Group.
The current version is available via HTTPS:
https://www.tcpdump.org/
The original distribution is available via anonymous FTP:
ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z
IPv6/IPsec support is added by WIDE/KAME project. This program
uses OpenSSL/LibreSSL, under specific configurations.
To report a security issue please send an e-mail to
security@tcpdump.org.
To report bugs and other problems, contribute patches, request a
feature, provide generic feedback etc. please see the file
CONTRIBUTING.md in the tcpdump source tree root.
Some attempt should be made to reassemble IP fragments or, at
least to compute the right length for the higher level protocol.
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 tcpdump.
A packet trace that crosses a daylight savings time change will
give skewed time stamps (the time change is ignored).
This page is part of the tcpdump (a command-line network packet
analyzer) project. Information about the project can be found at
⟨http://www.tcpdump.org/⟩. If you have a bug report for this
manual page, see ⟨http://www.tcpdump.org/#patches⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/the-tcpdump-group/tcpdump⟩ on 2025-08-11. (At
that time, the date of the most recent commit that was found in
the repository was 2025-08-01.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
30 June 2025 TCPDUMP(1)
Pages that refer to this page: ovs-pcap(1), ovs-tcpundump(1), socket(7), csysdig(8), netsniff-ng(8), ovs-tcpdump(8), sysdig(8), tc-bpf(8), traceroute(8)
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