tcpdump
(1)
Name
tcpdump - dump traffic on a network
Synopsis
tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [
-c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -j tstamp_type ] [ -m module ] [
-M secret ]
[ -P in|out|inout ]
[ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [
-w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -z postrotate-command ] [ -Z
user ]
[ expression ]
Description
User Commands TCPDUMP(1)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [
-c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -j tstamp_type ] [ -m module ] [
-M secret ]
[ -P in|out|inout ]
[ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [
-w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -z postrotate-command ] [ -Z
user ]
[ expression ]
DESCRIPTION
Tcpdump prints out a description of the contents of packets
on a network interface that match the boolean expression.
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 captur-
ing packets until it is interrupted by a SIGINT signal (gen-
erated, for example, by typing your interrupt character,
typically control-C) or a SIGTERM signal (typically gener-
ated 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
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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 infor-
mation to applications; if not, it will be reported as
0).
On platforms that support the SIGINFO signal, such as most
BSDs (including Mac OS X) and Digital/Tru64 UNIX, it will
report those counts when it receives a SIGINFO signal (gen-
erated, for example, by typing your ``status'' character,
typically control-T, although on some platforms, such as Mac
OS X, the ``status'' character is not set by default, so you
must set it with stty(1) in order to use it) and will con-
tinue capturing packets.
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.
OPTIONS
-A Print each packet (minus its link level header) in
ASCII. Handy for capturing web pages.
-b Print the AS number in BGP packets in ASDOT notation
rather than ASPLAIN notation.
-B Set the operating system capture buffer size to
buffer_size, in units of KiB (1024 bytes).
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check
whether the file is currently larger than file_size
and, if so, close the current savefile and open a new
one. Savefiles after the first savefile will have the
name specified with the -w flag, with a number after
it, starting at 1 and continuing upward. The units of
file_size are millions of bytes (1,000,000 bytes, not
1,048,576 bytes).
-d Dump the compiled packet-matching code in a human read-
able form to standard output and stop.
-dd Dump packet-matching code as a C program fragment.
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-ddd Dump packet-matching code as decimal numbers (preceded
with a count).
-D Print the list of the network interfaces available on
the system and on which tcpdump can capture packets.
For each network interface, a number and an interface
name, possibly followed by a text description of the
interface, is printed. The interface name or the num-
ber can be supplied to the -i flag to specify an inter-
face 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 Win-
dows 2000 and later systems, where the interface name
is a somewhat complex string.
The -D flag will not be supported if tcpdump was built
with an older version of libpcap that lacks the
pcap_findalldevs() function.
-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 tcp-
dump was compiled with cryptography enabled.
secret is the ASCII text for ESP secret key. If pre-
ceded by 0x, then a hex value will be read.
The option assumes RFC2406 ESP, not RFC1827 ESP. The
option is only for debugging purposes, and the use of
this option with a true `secret' key is discouraged.
By presenting IPsec secret key onto command line you
make it visible to others, via ps(1) and other occa-
sions.
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
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given should already have been given up.
-f Print `foreign' IPv4 addresses numerically rather than
symbolically (this option is intended to get around
serious brain damage in Sun's NIS server -- usually it
hangs forever translating non-local internet numbers).
The test for `foreign' IPv4 addresses is done using the
IPv4 address and netmask of the interface on which cap-
ture is being done. If that address or netmask are not
available, available, either because the interface on
which capture is being done has no address or netmask
or because the capture is being done on the Linux "any"
interface, which can capture on more than one inter-
face, this option will not work correctly.
-F Use file as input for the filter expression. An addi-
tional expression given on the command line is ignored.
-G If specified, rotates the dump file specified with the
-w option every rotate_seconds seconds. Savefiles will
have the name specified by -w which should include a
time format as defined by strftime(3). If no time for-
mat is specified, each new file will overwrite the pre-
vious.
If used in conjunction with the -C option, filenames
will take the form of `file<count>'.
-h Print the tcpdump and libpcap version strings, print a
usage message, and exit.
-H Attempt to detect 802.11s draft mesh headers.
-i Listen on interface. If unspecified, tcpdump searches
the system interface list for the lowest numbered, con-
figured up interface (excluding loopback), which may
turn out to be, for example, ``eth0''.
On Linux systems with 2.2 or later kernels, an inter-
face argument of ``any'' can be used to capture packets
from all interfaces. Note that captures on the ``any''
device will not be done in promiscuous mode.
If the -D flag is supported, an interface number as
printed by that flag can be used as the interface argu-
ment.
-I Put the interface in "monitor mode"; this is supported
only on IEEE 802.11 Wi-Fi interfaces, and supported
only on some operating systems.
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Note that in monitor mode the adapter might disassoci-
ate 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 speci-
fied, only those link-layer types available when in
monitor mode will be shown.
-j Set the time stamp type for the capture to tstamp_type.
The names to use for the time stamp types are given in
pcap-tstamp-type(5); not all the types listed there
will necessarily be valid for any given interface.
-J List the supported time stamp types for the interface
and exit. If the time stamp type cannot be set for the
interface, no time stamp types are listed.
-K Don't attempt to verify IP, TCP, or UDP checksums.
This is useful for interfaces that perform some or all
of those checksum calculation in hardware; otherwise,
all outgoing TCP checksums will be flagged as bad.
-l Make stdout line buffered. Useful if you want to see
the data while capturing it. E.g.,
tcpdump -l | tee dat
or
tcpdump -l > dat & tail -f dat
Note that on Windows,``line buffered'' means
``unbuffered'', so that WinDump will write each charac-
ter 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 plat-
forms, including Windows.
-L List the known data link types for the interface, in
the specified mode, and exit. The list of known data
link types may be dependent on the specified mode; for
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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 sup-
port 802.11 headers with radio information) and another
set of data link types when in monitor mode (for exam-
ple, it might support 802.11 headers, or 802.11 headers
with radio information, only in monitor mode).
-m Load SMI MIB module definitions from file module. This
option can be used several times to load several MIB
modules into tcpdump.
-M 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''.
-O Do not run the packet-matching code optimizer. This is
useful only if you suspect a bug in the optimizer.
-p Don't put the interface into promiscuous mode. Note
that the interface might be in promiscuous mode for
some other reason; hence, `-p' cannot be used as an
abbreviation for `ether host {local-hw-addr} or ether
broadcast'.
-P Choose send/receive direction direction for which pack-
ets should be captured. Possible values are `in', `out'
and `inout'. Not available on all platforms.
-q Quick (quiet?) output. Print less protocol information
so output lines are shorter.
-R Assume ESP/AH packets to be based on old specification
(RFC1825 to RFC1829). If specified, tcpdump will not
print replay prevention field. Since there is no pro-
tocol version field in ESP/AH specification, tcpdump
cannot deduce the version of ESP/AH protocol.
-r Read packets from file (which was created with the -w
option). Standard input is used if file is ``-''.
-S Print absolute, rather than relative, TCP sequence num-
bers.
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-s Snarf snaplen bytes of data from each packet rather
than the default of 65535 bytes. Packets truncated
because of a limited snapshot are indicated in the out-
put 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, effec-
tively, decreases the amount of packet buffering. This
may cause packets to be lost. You should limit snaplen
to the smallest number that will capture the protocol
information you're interested in. Setting snaplen to 0
sets it to the default of 65535, for backwards compati-
bility with recent older versions of tcpdump.
-T Force packets selected by "expression" to be inter-
preted the specified type. Currently known types are
aodv (Ad-hoc On-demand Distance Vector protocol), carp
(Common Address Redundancy Protocol), cnfp (Cisco Net-
Flow protocol), lmp (Link Management Protocol), pgm
(Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0
inside PGM/EPGM), radius (RADIUS), rpc (Remote Proce-
dure Call), rtp (Real-Time Applications protocol), rtcp
(Real-Time Applications control protocol), snmp (Simple
Network Management Protocol), tftp (Trivial File Trans-
fer Protocol), vat (Visual Audio Tool), wb (distributed
White Board), zmtp1 (ZeroMQ Message Transport Protocol
1.0) and vxlan (Virtual eXtensible Local Area Network).
Note that the pgm type above affects UDP interpretation
only, the native PGM is always recognised as IP proto-
col 113 regardless. UDP-encapsulated PGM is often
called "EPGM" or "PGM/UDP".
Note that the pgm_zmtp1 type above affects interpreta-
tion 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 data-
gram 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 an unformatted timestamp on each dump line.
-ttt Print a delta (micro-second resolution) between current
and previous line on each dump line.
-tttt
Print a timestamp in default format proceeded by date
on each dump line.
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-ttttt
Print a delta (micro-second resolution) between current
and first line on each dump line.
-u Print undecoded NFS handles.
-U If the -w option is not specified, make the printed
packet output ``packet-buffered''; i.e., as the
description of the contents of each packet is printed,
it will be written to the standard output, rather than,
when not writing to a terminal, being written only when
the output buffer fills.
If the -w option is specified, make the saved raw
packet output ``packet-buffered''; i.e., as each packet
is saved, it will be written to the output file, rather
than being written only when the output buffer fills.
The -U flag will not be supported if tcpdump was built
with an older version of libpcap that lacks the
pcap_dump_flush() function.
-v When parsing and printing, produce (slightly more) ver-
bose output. For example, the time to live, identifi-
cation, total length and options in an IP packet are
printed. Also enables additional packet integrity
checks such as verifying the IP and ICMP header check-
sum.
When writing to a file with the -w option, report,
every 10 seconds, the number of packets captured.
-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 Read a list of filenames from file. Standard input is
used if file is ``-''.
-w Write the raw packets to file rather than parsing and
printing them out. They can later be printed with the
-r option. Standard output is used if file is ``-''.
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.
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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 reading capture files and doesn't
add an extension when writing them (it uses magic num-
bers in the file header instead). However, many operat-
ing systems and applications will use the extension if
it is present and adding one (e.g. .pcap) is recom-
mended.
See pcap-savefile(4) for a description of the file for-
mat.
-W Used in conjunction with the -C option, this will limit
the number of files created to the specified number,
and begin overwriting files from the beginning, thus
creating a 'rotating' buffer. In addition, it will
name the files with enough leading 0s to support the
maximum number of files, allowing them to sort cor-
rectly.
Used in conjunction with the -G option, this will limit
the number of rotated dump files that get created,
exiting with status 0 when reaching the limit. If used
with -C as well, the behavior will result in cyclical
files per timeslice.
-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.
-xx When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet,
including its link level header, in hex.
-X When parsing and printing, in addition to printing the
headers of each packet, print the data of each packet
(minus its link level header) in hex and ASCII. This
is very handy for analysing new protocols.
-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 Set the data link type to use while capturing packets
to datalinktype.
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-z Used in conjunction with the -C or -G options, this
will make tcpdump run " command file " where file is
the savefile being closed after each rotation. For
example, specifying -z gzip or -z bzip2 will compress
each savefile using gzip or bzip2.
Note that tcpdump will run the command in parallel to
the capture, using the lowest priority so that this
doesn't disturb the capture process.
And in case you would like to use a command that itself
takes flags or different arguments, you can always
write a shell script that will take the savefile name
as the only argument, make the flags & arguments
arrangements and execute the command that you want.
-Z If tcpdump is running as root, after opening the cap-
ture 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. Oth-
erwise, only packets for which expression is `true'
will be dumped.
For the expression syntax, see pcap-filter(5).
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.
EXAMPLES
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
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To print all traffic between local hosts and hosts at Berke-
ley:
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 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 gate-
way 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'
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The following
gives a brief description and examples of most of the for-
mats.
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
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`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 informa-
tion 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 num-
ber 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
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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 (type ETHER_ARP) and the
total length was 64 bytes.
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, neither this description nor tcpdump will
be of much use to you.)
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and
ports. Flags are some combination of S (SYN), F (FIN), P
(PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) 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). Ack is sequence number of the
next data expected the other direction on this connection.
Window is the number of bytes of receive buffer space avail-
able the other direction on this connection. Urg indicates
there is `urgent' data in the packet. Options are tcp
options enclosed in angle brackets (e.g., <mss 1024>).
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.
rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
rtsg.1023 > csam.login: . ack 1 win 4096
rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
csam.login > rtsg.1023: . ack 2 win 4096
rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
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csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 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 con-
tained no data. (The notation is `first:last(nbytes)' which
means `sequence numbers first up to but not including last
which is nbytes bytes of user data'.) 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. Note that the ack
sequence number is a small integer (1). The first time tcp-
dump sees a tcp `conversation', it prints the sequence num-
ber from the packet. On subsequent packets of the conversa-
tion, 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 inter-
preted 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 PUSH 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, tcp-
dump reports it as ``[bad hdr length]''.
Capturing TCP packets with particular flag
SunOS 5.11 Last change: 12 July 2012 14
User Commands TCPDUMP(1)
There are 8 bits in the control bits section of the TCP
header:
CWR | ECE | URG |
Let's assume that we want to watch packets used in estab-
lishing 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 |
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| 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 octet 13:
0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |
Let's have a closer look at octet no. 13:
| |
|---------------|
|C|E|U|A|P|R|S|F|
SunOS 5.11 Last change: 12 July 2012 15
User Commands TCPDUMP(1)
|---------------|
|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|
|---------------|
|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:
SunOS 5.11 Last change: 12 July 2012 16
User Commands TCPDUMP(1)
|C|E|U|A|P|R|S|F|
|---------------|
|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-act, tcp-urg.
SunOS 5.11 Last change: 12 July 2012 17
User Commands TCPDUMP(1)
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 desti-
nation port number) and the higher level protocol informa-
tion printed. In particular, Domain Name service requests
(RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS.
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 ucb-
vax.berkeley.edu. The query id was `3'. The `+' indicates
the recursion desired flag was set. The query length was 37
bytes, not including the 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.
UDP Name Server Responses
SunOS 5.11 Last change: 12 July 2012 18
User Commands TCPDUMP(1)
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 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 non-existent 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 www.cifs.org or the pub/samba/specs/ direc-
tory on your favorite samba.org mirror site. The SMB
patches were written by Andrew Tridgell (tridge@samba.org).
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are
printed as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
sushi.201b > wrl.nfs:
144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.201b:
reply ok 128 lookup fh 9,74/4134.3150
SunOS 5.11 Last change: 12 July 2012 19
User Commands TCPDUMP(1)
In the first line, host sushi sends a transaction with id
6709 to wrl (note that the number following the src host is
a transaction id, not the source port). 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 num-
ber pair, followed by the inode number and generation num-
ber.) Wrl replies `ok' with the contents of the link.
In the third line, sushi asks wrl to lookup the name `xcol-
ors' in directory file 9,74/4096.6878. 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.
If the -v (verbose) flag is given, additional information is
printed. For example:
sushi.1372a > wrl.nfs:
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
reply ok 1472 read REG 100664 ids 417/0 sz 29388
(-v also prints the IP header TTL, ID, length, and fragmen-
tation 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 frag-
ments 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.
Note that NFS requests are very large and much of the detail
won't be printed unless snaplen is increased. Try using `-s
192' to watch NFS traffic.
NFS reply packets do not explicitly identify the RPC opera-
tion. 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
SunOS 5.11 Last change: 12 July 2012 20
User Commands TCPDUMP(1)
Transarc AFS (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 twice, acknowledgement
packets and additional header information is printed, such
as the RX call ID, call number, sequence number, serial num-
ber, 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 excep-
tion of Ubik beacon packets (because abort packets are used
to signify a yes vote for the Ubik protocol).
Note that AFS requests are very large and many of the argu-
ments won't be printed unless snaplen is increased. Try
using `-s 256' to watch AFS traffic.
SunOS 5.11 Last change: 12 July 2012 21
User Commands TCPDUMP(1)
AFS reply packets do not explicitly identify the RPC opera-
tion. 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 corre-
sponding 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 lis-
tening 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 are formatted like the following examples:
SunOS 5.11 Last change: 12 July 2012 22
User Commands TCPDUMP(1)
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 laser-
writer "techpit" registered on port 186.
ATP packet formatting is demonstrated by the following exam-
ple:
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' fol-
lowing 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 retransmit-
ted. 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.
IP Fragmentation
Fragmented Internet datagrams are printed as
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates there are more fragments. The
SunOS 5.11 Last change: 12 July 2012 23
User Commands TCPDUMP(1)
second indicates this is the last fragment.)
Id is the fragment id. Size is the fragment size (in bytes)
excluding the IP header. Offset is this fragment's offset
(in bytes) in the original datagram.
The fragment information is output for each fragment. The
first fragment contains the higher level protocol header and
the frag info is printed after the protocol info. Fragments
after the first contain no higher level protocol header and
the frag info is printed after the source and destination
addresses. For example, here is part of an ftp from ari-
zona.edu to lbl-rtsg.arpa over a CSNET connection that
doesn't appear to handle 576 byte datagrams:
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses
in the 2nd line don't include port numbers. This is because
the TCP protocol information is all in the first fragment
and we have no idea what the port or sequence numbers are
when we print the later fragments. Second, the tcp sequence
information in the first line is printed as if there were
308 bytes of user data when, in fact, there are 512 bytes
(308 in the first frag and 204 in the second). If you are
looking for holes in the sequence space or trying to match
up acks with packets, this can fool you.
A packet with the IP don't fragment flag is marked with a
trailing (DF).
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 first saw the packet. No
attempt is made to account for the time lag between when the
Ethernet interface removed the packet from the wire and when
the kernel serviced the `new packet' interrupt.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
SunOS 5.11 Last change: 12 July 2012 24
User Commands TCPDUMP(1)
+---------------+--------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+--------------------+
|Availability | diagnostic/tcpdump |
+---------------+--------------------+
|Stability | Uncommitted |
+---------------+--------------------+
SEE ALSO
stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(4),
pcap-filter(5), pcap-tstamp-type(5)
http://www.iana.org/assignments/media-types/applica-
tion/vnd.tcpdump.pcap
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of Cali-
fornia, Berkeley, CA.
It is currently being maintained by tcpdump.org.
The current version is available via http:
http://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 pro-
gram uses Eric Young's SSLeay library, under specific con-
figurations.
BUGS
Please send problems, bugs, questions, desirable enhance-
ments, patches etc. to:
tcpdump-workers@lists.tcpdump.org
NIT doesn't let you watch your own outbound traffic, BPF
will. We recommend that you use the latter.
On Linux systems with 2.0[.x] kernels:
packets on the loopback device will be seen twice;
packet filtering cannot be done in the kernel, so that
all packets must be copied from the kernel in order to
be filtered in user mode;
SunOS 5.11 Last change: 12 July 2012 25
User Commands TCPDUMP(1)
all of a packet, not just the part that's within the
snapshot length, will be copied from the kernel (the
2.0[.x] packet capture mechanism, if asked to copy only
part of a packet to userland, will not report the true
length of the packet; this would cause most IP packets
to get an error from tcpdump);
capturing on some PPP devices won't work correctly.
We recommend that you upgrade to a 2.2 or later kernel.
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 generat-
ing them rather than tcpdump.
A packet trace that crosses a daylight savings time change
will give skewed time stamps (the time change is ignored).
Filter expressions on fields other than those in Token Ring
headers will not correctly handle source-routed Token Ring
packets.
Filter expressions on fields other than those in 802.11
headers will not correctly handle 802.11 data packets with
both To DS and From DS set.
ip6 proto should chase header chain, but at this moment it
does not. ip6 protochain is supplied for this behavior.
Arithmetic expression against transport layer headers, like
tcp[0], does not work against IPv6 packets. It only looks
at IPv4 packets.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from http://www.tcp-
dump.org/release/tcpdump-4.5.1.tar.gz
Further information about this software can be found on the
open source community website at http://www.tcpdump.org/.
SunOS 5.11 Last change: 12 July 2012 26