NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [
-AbdDefhHIJKlLnNOpqStuUvxX# ] [
-B
buffer_size ]
[
-c count ]
[
-C file_size ] [
-G rotate_seconds ] [
-F
file ]
[
-i interface ] [
-j tstamp_type ] [
-m
module ] [
-M secret ]
[
--number ] [
-Q 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 ] [
--time-stamp-precision=tstamp_precision ] [
--immediate-mode ] [
--version ] [
expression ]
DESCRIPTION
Tcpdump prints out a description of the contents of packets on a network
interface that match the boolean
expression; 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 Mac
OS X) and Digital/Tru64 UNIX, it will report those counts when it receives a
SIGINFO signal (generated, for example, by typing your ``status'' character,
typically control-T)
SIGUSR1 signal. and will continue capturing packets.
Reading packets from a network interface may require that you have special
privileges; see the
pcap (3) 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.
- -a
- Attempt to convert network and broadcast addresses to
names.
- -b
- Print the AS number in BGP packets in ASDOT notation rather
than ASPLAIN notation.
- -B buffer_size
- --buffer-size=buffer_size
- Set the operating system capture buffer size to
buffer_size, in units of KiB (1024 bytes).
- -c count
- Exit after receiving count packets.
- -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 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 readable
form to standard output and stop.
- -dd
- Dump packet-matching code as a C program
fragment.
- -ddd
- Dump packet-matching code as decimal numbers (preceded with
a count).
- -D
- --list-interfaces
- Print the list of the network interfaces available on the
system and on which tcpdump can capture packets. For each network
interface, a number and an interface name, possibly followed by a text
description of the interface, is printed. The interface name or the number
can be supplied to the -i flag to specify an interface on which to
capture.
- This can be useful on systems that don't have a command to
list them (e.g., Windows systems, or UNIX systems lacking ifconfig
-a); the number can be useful on Windows 2000 and later systems, where
the interface name is a somewhat complex string.
- The -D flag will not be supported if tcpdump
was built with an older version of libpcap that lacks the
pcap_findalldevs() function.
- -e
- Print the link-level header on each dump line. This can be
used, for example, to print MAC layer addresses for protocols such as
Ethernet and IEEE 802.11.
- -E
- Use spi@ipaddr algo:secret for decrypting IPsec ESP
packets that are addressed to addr and contain Security Parameter
Index value spi. This combination may be repeated with comma or
newline separation.
- Note that setting the secret for IPv4 ESP packets is
supported at this time.
- Algorithms may be des-cbc, 3des-cbc,
blowfish-cbc, rc3-cbc, cast128-cbc, or none.
The default is des-cbc. The ability to decrypt packets is only
present if tcpdump was compiled with cryptography enabled.
- secret is the ASCII text for ESP secret key. If
preceded by 0x, then a hex value will be read.
- The option assumes RFC2406 ESP, not RFC1827 ESP. The option
is only for debugging purposes, and the use of this option with a true
`secret' key is discouraged. By presenting IPsec secret key onto command
line you make it visible to others, via ps(1) and other
occasions.
- In addition to the above syntax, the syntax file
name may be used to have tcpdump read the provided file in. The file
is opened upon receiving the first ESP packet, so any special permissions
that tcpdump may have been given should already have been given up.
- -f
- Print `foreign' IPv4 addresses numerically rather than
symbolically (this option is intended to get around serious brain damage
in Sun's NIS server — usually it hangs forever translating non-local
internet numbers).
- The test for `foreign' IPv4 addresses is done using the
IPv4 address and netmask of the interface on which capture is being done.
If that address or netmask are not available, either because the interface
on which capture is being done has no address or netmask or because the
capture is being done on the Linux "any" interface, which can
capture on more than one interface, this option will not work
correctly.
- -F file
- Use file as input for the filter expression. An
additional expression given on the command line is ignored.
- -G rotate_seconds
- If specified, rotates the dump file specified with the
-w option every rotate_seconds seconds. Savefiles will have
the name specified by -w which should include a time format as
defined by strftime(3). If no time format is specified, each new
file will overwrite the previous.
- 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 on interface. If unspecified, tcpdump
searches the system interface list for the lowest numbered, configured up
interface (excluding loopback). Ties are broken by choosing the earliest
match.
- If the -D flag is supported, an interface number as
printed by that flag can be used as the interface argument, if no
interface on the system has that number as a name.
- -I
- --monitor-mode
- Put the interface in "monitor mode"; this is
supported only on IEEE 802.11 Wi-Fi interfaces, and supported only on some
operating systems.
- Note that in monitor mode the adapter might disassociate
from the network with which it's associated, so that you will not be able
to use any wireless networks with that adapter. This could prevent
accessing files on a network server, or resolving host names or network
addresses, if you are capturing in monitor mode and are not connected to
another network with another adapter.
- This flag will affect the output of the -L flag. If
-I isn't specified, only those link-layer types available when not
in monitor mode will be shown; if -I is specified, only those
link-layer types available when in monitor mode will be shown.
- --immediate-mode
- Capture in "immediate mode". In this mode,
packets are delivered to tcpdump as soon as they arrive, rather than being
buffered for efficiency. This is the default when printing packets rather
than saving packets to a ``savefile'' if the packets are being printed to
a terminal rather than to a file or pipe.
- -j tstamp_type
- --time-stamp-type=tstamp_type
- Set the time stamp type for the capture to
tstamp_type. The names to use for the time stamp types are given in
pcap-tstamp(@MAN_MISC_INFO@); not all the types listed there will
necessarily be valid for any given interface.
- -J
- --list-time-stamp-types
- List the supported time stamp types for the interface and
exit. If the time stamp type cannot be set for the interface, no time
stamp types are listed.
- --time-stamp-precision=tstamp_precision
- When capturing, set the time stamp precision for the
capture to tstamp_precision. Note that availability of high
precision time stamps (nanoseconds) and their actual accuracy is platform
and hardware dependent. Also note that when writing captures made with
nanosecond accuracy to a savefile, the time stamps are written with
nanosecond resolution, and the file is written with a different magic
number, to indicate that the time stamps are in seconds and nanoseconds;
not all programs that read pcap savefiles will be able to read those
captures.
When reading a savefile, convert time stamps to the precision specified by
timestamp_precision, and display them with that resolution. If the
precision specified is less than the precision of time stamps in the file, the
conversion will lose precision.
The supported values for
timestamp_precision are
micro for
microsecond resolution and
nano for nanosecond resolution. The default
is microsecond resolution.
- -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.,
- or
tcpdump -l > dat & tail -f dat
- Note that on Windows,``line buffered'' means
``unbuffered'', so that WinDump will write each character individually if
-l is specified.
- -U is similar to -l in its behavior, but it
will cause output to be ``packet-buffered'', so that the output is written
to stdout at the end of each packet rather than at the end of each line;
this is buffered on all platforms, including Windows.
- -L
- --list-data-link-types
- List the known data link types for the interface, in the
specified mode, and exit. The list of known data link types may be
dependent on the specified mode; for example, on some platforms, a Wi-Fi
interface might support one set of data link types when not in monitor
mode (for example, it might support only fake Ethernet headers, or might
support 802.11 headers but not support 802.11 headers with radio
information) and another set of data link types when in monitor mode (for
example, it might support 802.11 headers, or 802.11 headers with radio
information, only in monitor mode).
- -m module
- Load SMI MIB module definitions from file module.
This option can be used several times to load several MIB modules into
tcpdump.
- -M secret
- Use secret as a shared secret for validating the
digests found in TCP segments with the TCP-MD5 option (RFC 2385), if
present.
- -n
- Don't convert addresses (i.e., host addresses, port
numbers, etc.) to names.
- -N
- Don't print domain name qualification of host names. E.g.,
if you give this flag then tcpdump will print ``nic'' instead of
``nic.ddn.mil''.
- -#
- --number
- Print an optional packet number at the beginning of the
line.
- -O
- --no-optimize
- Do not run the packet-matching code optimizer. This is
useful only if you suspect a bug in the optimizer.
- -p
- --no-promiscuous-mode
- Don't put the interface into promiscuous mode. Note
that the interface might be in promiscuous mode for some other reason;
hence, `-p' cannot be used as an abbreviation for `ether host
{local-hw-addr} or ether broadcast'.
- -Q direction
- --direction=direction
- Choose send/receive direction direction for which
packets should be captured. Possible values are `in', `out' and `inout'.
Not available on all platforms.
- -q
- Quick (quiet?) output. Print less protocol information so
output lines are shorter.
- -r file
- Read packets from file (which was created with the
-w option or by other tools that write pcap or pcap-ng 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. You should limit
snaplen to the smallest number that will capture the protocol
information you're interested in. Setting snaplen to 0 sets it to
the default of 262144, for backwards compatibility with recent older
versions of tcpdump.
- -T type
- Force packets selected by "expression" to
be interpreted the specified type. Currently known types are
aodv (Ad-hoc On-demand Distance Vector protocol), carp
(Common Address Redundancy Protocol), cnfp (Cisco NetFlow
protocol), lmp (Link Management Protocol), pgm (Pragmatic
General Multicast), pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM),
resp (REdis Serialization Protocol), radius (RADIUS),
rpc (Remote Procedure Call), rtp (Real-Time Applications
protocol), rtcp (Real-Time Applications control protocol),
snmp (Simple Network Management Protocol), tftp (Trivial
File Transfer Protocol), vat (Visual Audio Tool), 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 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 (micro-second resolution) between current and
previous line on each dump line.
- -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 (micro-second resolution) between current and
first line on each dump line.
- -u
- Print undecoded NFS handles.
- -U
- --packet-buffered
- 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) verbose
output. For example, the time to live, identification, total length and
options in an IP packet are printed. Also enables additional packet
integrity checks such as verifying the IP and ICMP header checksum.
- When writing to a file with the -w option, report,
every 10 seconds, the number of packets captured.
- -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(5) for a description of the file
format.
- -W
- Used in conjunction with the -C option, this will
limit the number of files created to the specified number, and begin
overwriting files from the beginning, thus creating a 'rotating' buffer.
In addition, it will name the files with enough leading 0s to support the
maximum number of files, allowing them to sort correctly.
- Used in conjunction with the -G option, this will
limit the number of rotated dump files that get created, exiting with
status 0 when reaching the limit. If used with -C as well, the
behavior will result in cyclical files per timeslice.
- -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 datalinktype
- --linktype=datalinktype
- Set the data link type to use while capturing packets to
datalinktype. The available data link types may be found using the
-L option.
- -z postrotate-command
- Used in conjunction with the -C or -G
options, this will make tcpdump run " postrotate-command
file " where file is the savefile being closed after each
rotation. For example, specifying -z gzip or -z bzip2 will
compress each savefile using gzip or bzip2.
- Note that tcpdump will run the command in parallel to the
capture, using the lowest priority so that this doesn't disturb the
capture process.
- And in case you would like to use a command that itself
takes flags or different arguments, you can always write a shell script
that will take the savefile name as the only argument, make the flags
& arguments arrangements and execute the command that you want.
- -Z user
- --relinquish-privileges=user
- If tcpdump is running as root, after opening the
capture device or input savefile, but before opening any savefiles for
output, change the user ID to user and the group ID to the primary
group of user.
- This behavior is the default for NetBSD where
tcpdump runs as the user ``_tcpdump''.
- expression
selects which packets will be dumped. If no
expression is given, all packets on the net will be dumped. Otherwise,
only packets for which
expression is `true' will be dumped.
For the
expression syntax, see
pcap-filter(7).
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:
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:
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 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'
OUTPUT FORMAT
The output of
tcpdump is protocol dependent. The following gives a brief
description and examples of most of the formats.
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 (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 available 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
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 contained 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
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 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,
tcpdump reports it as ``[
bad hdr length]''.
Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK,
etc.)
There are 8 bits in the control bits section of the TCP header:
- CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
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 |
-----------------------------------------------------------------
| 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|
|---------------|
|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:
|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.
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.
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, 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
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/ directory 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.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.
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 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
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 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).
Note that AFS requests are very large and many of the arguments won't be printed
unless
snaplen is increased. Try using `
-s 256' to watch AFS
traffic.
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 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 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.
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 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 arizona.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
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.
SEE ALSO
stty(1), pcap(3), bpf(4), pcap-savefile(5), pcap-filter(7), pcap-tstamp(7)
http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
AUTHORS
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 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 program uses Eric Young's
SSLeay library, under specific configurations.
BUGS
Please send problems, bugs, questions, desirable enhancements, patches etc. to:
tcpdump-workers@lists.tcpdump.org
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).
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.