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TCPDUMP(1) TCPDUMP(1)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -AdDefgIKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -m module ] [ -M secret ]
[ -r 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. In all cases, only packets that match expres-sion expression
sion 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 cap-ture capture
ture 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 com-mand command
mand line, on some OSes it counts packets regardless of whether they were matched by the fil-ter filter
ter 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 Digi-tal/Tru64 Digital/Tru64
tal/Tru64 UNIX, it will report those counts when it receives a SIGINFO signal (generated, for exam-ple, example,
ple, 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 Set the operating system capture buffer size to buffer_size.
-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 cap-ture capture
ture 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 sys-tems, systems,
tems, 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.
-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 seperation.
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 cryp-tography cryptography
tography enabled.
secret is the ASCII text for ESP secret key. If preceeded 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 per-missions permissions
missions 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 inter-face interface
face 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 Do not insert line break after IP header in verbose mode for easier parsing.
-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>'.
-i Listen on interface. If unspecified, tcpdump searches the system interface list for the low-est lowest
est numbered, configured up interface (excluding loopback). Ties are broken by choosing the
earliest match.
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.
-K Don't attempt to verify TCP, UDP and IP checksums. This is useful for interfaces that perform
the checksum calculation in hardware; otherwise, all outgoing 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''.
-L List the known data link types for the interface and exit.
-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 TCPMD5
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 promiscu-ous promiscuous
ous mode for some other reason; hence, `-p' cannot be used as an abbreviation for `ether host
{local-hw-addr} or ether broadcast'.
-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 64K 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 tak-ing taking
ing larger snapshots both increases the amount of time it takes to process packets and, effec-tively, effectively,
tively, decreases the amount of packet buffering. This may cause packets to be lost. You
should limit snaplen to the smallest number that will capture the protocol information you're
interested in. Setting snaplen to 0 means use the required length to catch whole packets.
-T Force packets selected by "expression" to be interpreted the specified type. Currently known
types are aodv (Ad-hoc On-demand Distance Vector protocol), cnfp (Cisco NetFlow protocol), rpc
(Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications
control protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Pro-tocol), Protocol),
tocol), vat (Visual Audio Tool), and wb (distributed White Board).
-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 Make output saved via the -w option ``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.
-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 ``-''.
-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 Drops privileges (if root) and changes 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(4).
Expression arguments can be passed to tcpdump as either a single argument or as multiple argu-ments, arguments,
ments, whichever is more convenient. Generally, if the expression contains Shell metacharac-ters, metacharacters,
ters, it is easier to pass it as a single, quoted argument. Multiple arguments are concate-nated concatenated
nated 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 pre-vent prevent
vent 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 des-tination destination
tination 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' pack-ets, packets,
ets, with a priority value between 0 and 7; for example, `async4'. Such packets are assumed to con-tain contain
tain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is not an ISO data-gram datagram
gram or a so-called SNAP packet.
On Token Ring networks, the '-e' option causes tcpdump to print the `access control' and `frame con-trol' control'
trol' 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 com-pression compression
pression 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 connec-tion connection
tion 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 point-topoint
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), W (ECN CWR) or E (ECN-Echo), or a single `.' (no flags). Data-seqno Dataseqno
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 no flags were 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 ----------------------------------------------------------------A
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 con-trol control
trol bit is set at the same time. Let's see what happens to octet 13 when a TCP datagram with SYN-ACK SYNACK
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'
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-1_35. 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 opera-tion operation
tion 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 inter-net internet
net address 128.32.137.3. The total size of the response was 273 bytes, excluding UDP and IP head-ers. headers.
ers. 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 (NXDo-main) (NXDomain)
main) with no answers, one name server and no authority records. The `*' indicates that the authori-tative authoritative
tative 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 te 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 67_9 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 explana-tory explanatory
tory 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 (file-server) (fileserver)
server) 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 interest-ing). interesting).
ing).
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 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 the RX call ID, serial
number, and the RX packet flags. The MTU negotiation information is also printed from RX ack pack-ets. packets.
ets.
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 partic-ular particular
ular 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 num-ber, number,
ber, 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 con-tain contain
tain 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 con-nection connection
nection 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 num-bers. numbers.
bers. 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), pcap-filter(4), bpf(4), nit(4P)
AUTHORS
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, Uni-versity University
versity 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/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 user-land, userland,
land, 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 sourcerouted
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 pack-ets. packets.
ets. It only looks at IPv4 packets.
07 January 2008 TCPDUMP(1)
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