Provided by: tcpdump_4.5.1-2ubuntu1_amd64 
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
(please note tcpdump is protected via an enforcing apparmor(7) profile in Ubuntu which
limits the files tcpdump may access). 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,
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 continue 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 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 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, 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 Use file as input for the filter expression. An additional 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 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 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, configured up interface (excluding loopback), which may
turn out to be, for example, ``eth0''.
On Linux systems with 2.2 or later kernels, an interface argument of ``any'' can be
used to capture packets from all interfaces. Note that captures on the ``any''
device will not be done in promiscuous mode.
If the -D flag is supported, an interface number as printed by that flag can be
used as the interface argument.
-I Put the interface in "monitor mode"; this is supported only on IEEE 802.11 Wi-Fi
interfaces, and supported only on some operating systems.
Note that in monitor mode the adapter might disassociate from the network with
which it's associated, so that you will not be able to use any wireless networks
with that adapter. This could prevent accessing files on a network server, or
resolving host names or network addresses, if you are capturing in monitor mode and
are not connected to another network with another adapter.
This flag will affect the output of the -L flag. If -I isn't specified, only those
link-layer types available when not in monitor mode will be shown; if -I is
specified, only those link-layer types available when in monitor mode will be
shown.
-j Set the time stamp type for the capture to tstamp_type. The names to use for the
time stamp types are given in pcap-tstamp-type(7); not all the types listed there
will necessarily be valid for any given interface.
-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 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 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 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 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 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If
specified, tcpdump will not print replay prevention field. Since there is no
protocol version field in ESP/AH specification, tcpdump cannot deduce the version
of ESP/AH protocol.
-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 numbers.
-s Snarf snaplen bytes of data from each packet rather than the default of 65535
bytes. Packets truncated because of a limited snapshot are indicated in the output
with ``[|proto]'', where proto is the name of the protocol level at which the
truncation has occurred. Note that taking larger snapshots both increases the
amount of time it takes to process packets and, effectively, decreases the amount
of packet buffering. This may cause packets to be lost. You should limit snaplen
to the smallest number that will capture the protocol information you're interested
in. Setting snaplen to 0 sets it to the default of 65535, for backwards
compatibility with recent older versions of tcpdump.
-T 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), 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 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.
-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) 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 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.
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 Set the data link type to use while capturing packets to datalinktype.
-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 capture device or input savefile,
but before opening any savefiles for output, change the user ID to user and the
group ID to the primary group of user.
This behavior can also be enabled by default at compile time.
expression
selects which packets will be dumped. If no expression is given, all packets on
the net will be dumped. Otherwise, only packets for 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:
tcpdump host sundown
To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup: (note that the expression is
quoted to prevent the shell from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'
To print traffic neither sourced from nor destined for local hosts (if you gateway to one
other net, this stuff should never make it onto your local net).
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN packets) of each TCP conversation that
involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
To print 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.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
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 number pair, followed by the inode
number and generation number.) Wrl replies `ok' with the contents of the link.
In the third line, sushi asks wrl to lookup the name `xcolors' 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 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
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.
SEE ALSO
stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-tstamp-
type(7), apparmor(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
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;
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 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.
12 July 2012 TCPDUMP(8)