TCPDUMP(1)TCPDUMP(1)NAMEtcpdump - dump traffic on a network
SYNOPSIStcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
[ -C file_size ] [ -G rotate_seconds ] [ -F file ]
[ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
[ -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 net‐
work interface that match the boolean expression. It can also be run
with the -w flag, which causes it to save the packet data to a file for
later analysis, and/or with the -r flag, which causes it to read from a
saved packet file rather than to read packets from a network interface.
It can also be run with the -V flag, which causes it to read a list of
saved packet files. In all cases, only packets that match expression
will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue capturing packets
until it is interrupted by a SIGINT signal (generated, for example, by
typing your interrupt character, typically control-C) or a SIGTERM sig‐
nal (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 spe‐
cial 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() func‐
tion.
-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 sep‐
aration.
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 per‐
missions that tcpdump may have been given should already have
been given up.
-f Print `foreign' IPv4 addresses numerically rather than symboli‐
cally (this option is intended to get around serious brain dam‐
age 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 sys‐
tem interface list for the lowest numbered, configured up inter‐
face (excluding loopback). Ties are broken by choosing the ear‐
liest match.
On Linux systems with 2.2 or later kernels, an interface argu‐
ment of ``any'' can be used to capture packets from all inter‐
faces. 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 operat‐
ing 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 use‐
ful 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 speci‐
fied 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 sup‐
port 802.11 headers with radio information) and another set of
data link types when in monitor mode (for example, it might sup‐
port 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 tcp‐
dump.
-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'.
-q Quick (quiet?) output. Print less protocol information so out‐
put lines are shorter.
-R Assume ESP/AH packets to be based on old specification (RFC1825
to RFC1829). If specified, tcpdump will not print replay pre‐
vention 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 pack‐
ets 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 back‐
wards 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 Redun‐
dancy Protocol), cnfp (Cisco NetFlow protocol), radius (RADIUS),
rpc (Remote Procedure Call), rtp (Real-Time Applications proto‐
col), rtcp (Real-Time Applications control protocol), snmp (Sim‐
ple 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).
-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 pre‐
vious 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 out‐
put ``packet-buffered''; i.e., as the description of the con‐
tents of each packet is printed, it will be written to the stan‐
dard 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() func‐
tion.
-v When parsing and printing, produce (slightly more) verbose out‐
put. 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 sec‐
onds, 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. Stan‐
dard 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 num‐
ber of files created to the specified number, and begin over‐
writing 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 num‐
ber 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 cap‐
ture, 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).
Expression arguments can be passed to tcpdump as either a single
argument or as multiple 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 Eth‐
ernet 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 data‐
grams) are `async' packets, with a priority value between 0 and 7; for
example, `async4'. Such packets are assumed to contain an 802.2 Logi‐
cal 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 com‐
pression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out‐
bound), 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 pack‐
ets, 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 num‐
ber), 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 for‐
mat 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 Ether‐
net 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 proto‐
col 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 conver‐
sation, the difference between the current packet's sequence number and
this initial sequence number is printed. This means that sequence num‐
bers 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 sit‐
ting 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 dec‐
imal 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 representa‐
tion 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 follow‐
ing 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 con‐
tained 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 particu‐
lar, 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 head‐
ers. 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 (NoEr‐
ror) 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 Net‐
BEUI 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 off‐
set 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 success‐
ful, 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 dis‐
carded). 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 request‐
ing 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, jss‐
mag.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 indi‐
cates 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 frag‐
ment 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. Sec‐
ond, 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 time‐
stamp 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 ALSOstty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-fil‐
ter(7), pcap-tstamp-type(7)
http://www.iana.org/assignments/media-types/application/vnd.tcp‐
dump.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 rec‐
ommend 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 pack‐
ets 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 cap‐
ture mechanism, if asked to copy only part of a packet to user‐
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) ques‐
tion 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(1)
