Man page of TCPDUMP
TCPDUMP
Section: User Commands (1)
Updated: 05 March 2009
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NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump
[
-AbdDefIKlLnNOpqRStuUvxX
] [
-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
expression
will be processed by
tcpdump.
Tcpdump
will, if not run with the
-c
flag, continue capturing packets until it is interrupted by a SIGINT
signal (generated, for example, by typing your interrupt character,
typically control-C) or a SIGTERM signal (typically generated with the
kill(1)
command); if run with the
-c
flag, it will capture packets until it is interrupted by a SIGINT or
SIGTERM signal or the specified number of packets have been processed.
When
tcpdump
finishes capturing packets, it will report counts of:
-
packets ``captured'' (this is the number of packets that
tcpdump
has received and processed);
-
packets ``received by filter'' (the meaning of this depends on the OS on
which you're running
tcpdump,
and possibly on the way the OS was configured - if a filter was
specified on the command line, on some OSes it counts packets regardless
of whether they were matched by the filter expression and, even if they
were matched by the filter expression, regardless of whether
tcpdump
has read and processed them yet, on other OSes it counts only packets that were
matched by the filter expression regardless of whether
tcpdump
has read and processed them yet, and on other OSes it counts only
packets that were matched by the filter expression and were processed by
tcpdump);
-
packets ``dropped by kernel'' (this is the number of packets that were
dropped, due to a lack of buffer space, by the packet capture mechanism
in the OS on which
tcpdump
is running, if the OS reports that information to applications; if not,
it will be reported as 0).
On platforms that support the SIGINFO signal, such as most BSDs
(including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
when it receives a SIGINFO signal (generated, for example, by typing
your ``status'' character, typically control-T, although on some
platforms, such as Mac OS X, the ``status'' character is not set by
default, so you must set it with
stty(1)
in order to use it) and will continue capturing packets.
Reading packets from a network interface may require that you have
special privileges; see the
pcap (3PCAP)
man page for details. Reading a saved packet file doesn't require
special privileges.
OPTIONS
- -A
-
Print each packet (minus its link level header) in ASCII. Handy for
capturing web pages.
- -b
-
Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
notation.
- -B
-
Set the operating system capture buffer size to buffer_size.
- -c
-
Exit after receiving count packets.
- -C
-
Before writing a raw packet to a savefile, check whether the file is
currently larger than file_size and, if so, close the current
savefile and open a new one. Savefiles after the first savefile will
have the name specified with the
-w
flag, with a number after it, starting at 1 and continuing upward.
The units of file_size are millions of bytes (1,000,000 bytes,
not 1,048,576 bytes).
- -d
-
Dump the compiled packet-matching code in a human readable form to
standard output and stop.
- -dd
-
Dump packet-matching code as a
C
program fragment.
- -ddd
-
Dump packet-matching code as decimal numbers (preceded with a count).
- -D
-
Print the list of the network interfaces available on the system and on
which
tcpdump
can capture packets. For each network interface, a number and an
interface name, possibly followed by a text description of the
interface, is printed. The interface name or the number can be supplied
to the
-i
flag to specify an interface on which to capture.
-
This can be useful on systems that don't have a command to list them
(e.g., Windows systems, or UNIX systems lacking
ifconfig -a);
the number can be useful on Windows 2000 and later systems, where the
interface name is a somewhat complex string.
-
The
-D
flag will not be supported if
tcpdump
was built with an older version of
libpcap
that lacks the
pcap_findalldevs()
function.
- -e
-
Print the link-level header on each dump line.
- -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 cryptography 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 permissions that tcpdump
may have been given should already have been given up.
- -f
-
Print `foreign' IPv4 addresses numerically rather than symbolically
(this option is intended to get around serious brain damage in
Sun's NIS server --- usually it hangs forever translating non-local
internet numbers).
-
The test for `foreign' IPv4 addresses is done using the IPv4 address and
netmask of the interface on which capture is being done. If that
address or netmask are not available, available, either because the
interface on which capture is being done has no address or netmask or
because the capture is being done on the Linux "any" interface, which
can capture on more than one interface, this option will not work
correctly.
- -F
-
Use file as input for the filter expression.
An additional expression given on the command line is ignored.
- -G
-
If specified, rotates the dump file specified with the
-w
option every rotate_seconds seconds.
Savefiles will have the name specified by
-w
which should include a time format as defined by
strftime(3).
If no time format is specified, each new file will overwrite the previous.
-
If used in conjunction with the
-C
option, filenames will take the form of `file<count>'.
- -i
-
Listen on interface.
If unspecified, tcpdump searches the system interface list for the
lowest numbered, configured up interface (excluding loopback).
Ties are broken by choosing the earliest match.
-
On Linux systems with 2.2 or later kernels, an
interface
argument of ``any'' can be used to capture packets from all interfaces.
Note that captures on the ``any'' device will not be done in promiscuous
mode.
-
If the
-D
flag is supported, an interface number as printed by that flag can be
used as the
interface
argument.
- -I
-
Put the interface in "monitor mode"; this is supported only on IEEE
802.11 Wi-Fi interfaces, and supported only on some operating systems.
-
Note that in monitor mode the adapter might disassociate from the
network with which it's associated, so that you will not be able to use
any wireless networks with that adapter. This could prevent accessing
files on a network server, or resolving host names or network addresses,
if you are capturing in monitor mode and are not connected to another
network with another adapter.
-
This flag will affect the output of the
-L
flag. If
-I
isn't specified, only those link-layer types available when not in
monitor mode will be shown; if
-I
is specified, only those link-layer types available when in monitor mode
will be shown.
- -K
-
Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
interfaces that perform some or all of those checksum calculation in
hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
- -l
-
Make stdout line buffered.
Useful if you want to see the data
while capturing it.
E.g.,
``tcpdump -l | tee dat'' or
``tcpdump -l > dat & tail -f dat''.
- -L
-
List the known data link types for the interface, in the specified mode,
and exit. The list of known data link types may be dependent on the
specified mode; for example, on some platforms, a Wi-Fi interface might
support one set of data link types when not in monitor mode (for
example, it might support only fake Ethernet headers, or might support
802.11 headers but not support 802.11 headers with radio information)
and another set of data link types when in monitor mode (for example, it
might support 802.11 headers, or 802.11 headers with radio information,
only in monitor mode).
- -m
-
Load SMI MIB module definitions from file module.
This option
can be used several times to load several MIB modules into tcpdump.
- -M
-
Use secret as a shared secret for validating the digests found in
TCP segments with the TCP-MD5 option (RFC 2385), if present.
- -n
-
Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
- -N
-
Don't print domain name qualification of host names.
E.g.,
if you give this flag then tcpdump will print ``nic''
instead of ``nic.ddn.mil''.
- -O
-
Do not run the packet-matching code optimizer.
This is useful only
if you suspect a bug in the optimizer.
- -p
-
Don't put the interface
into promiscuous mode.
Note that the interface might be in promiscuous
mode for some other reason; hence, `-p' cannot be used as an abbreviation for
`ether host {local-hw-addr} or ether broadcast'.
- -q
-
Quick (quiet?) output.
Print less protocol information so output
lines are shorter.
- -R
-
Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
If specified, tcpdump will not print replay prevention field.
Since there is no protocol version field in ESP/AH specification,
tcpdump cannot deduce the version of ESP/AH protocol.
- -r
-
Read packets from file (which was created with the
-w
option).
Standard input is used if file is ``-''.
- -S
-
Print absolute, rather than relative, TCP sequence numbers.
- -s
-
Snarf snaplen bytes of data from each packet rather than the
default of 65535 bytes.
Packets truncated because of a limited snapshot
are indicated in the output with ``[|proto]'', where proto
is the name of the protocol level at which the truncation has occurred.
Note that taking larger snapshots both increases
the amount of time it takes to process packets and, effectively,
decreases the amount of packet buffering.
This may cause packets to be
lost.
You should limit snaplen to the smallest number that will
capture the protocol information you're interested in.
Setting
snaplen to 0 sets it to the default of 65535,
for backwards compatibility with recent older versions of
tcpdump.
- -T
-
Force packets selected by "expression" to be interpreted the
specified type.
Currently known types are
aodv (Ad-hoc On-demand Distance Vector protocol),
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 Protocol),
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 ``-''.
See
pcap-savefile(5)
for a description of the file format.
- -W
-
Used in conjunction with the
-C
option, this will limit the number
of files created to the specified number, and begin overwriting files
from the beginning, thus creating a 'rotating' buffer.
In addition, it will name
the files with enough leading 0s to support the maximum number of
files, allowing them to sort correctly.
-
Used in conjunction with the
-G
option, this will limit the number of rotated dump files that get
created, exiting with status 0 when reaching the limit. If used with
-C
as well, the behavior will result in cyclical files per timeslice.
- -x
-
When parsing and printing,
in addition to printing the headers of each packet, print the data of
each packet (minus its link level header) in hex.
The smaller of the entire packet or
snaplen
bytes will be printed. Note that this is the entire link-layer
packet, so for link layers that pad (e.g. Ethernet), the padding bytes
will also be printed when the higher layer packet is shorter than the
required padding.
- -xx
-
When parsing and printing,
in addition to printing the headers of each packet, print the data of
each packet,
including
its link level header, in hex.
- -X
-
When parsing and printing,
in addition to printing the headers of each packet, print the data of
each packet (minus its link level header) in hex and ASCII.
This is very handy for analysing new protocols.
- -XX
-
When parsing and printing,
in addition to printing the headers of each packet, print the data of
each packet,
including
its link level header, in hex and ASCII.
- -y
-
Set the data link type to use while capturing packets to datalinktype.
- -z
-
Used in conjunction with the
-C
or
-G
options, this will make
tcpdump
run "
command file
" where
file
is the savefile being closed after each rotation. For example, specifying
-z gzip
or
-z bzip2
will compress each savefile using gzip or bzip2.
-
Note that tcpdump will run the command in parallel to the capture, using
the lowest priority so that this doesn't disturb the capture process.
-
And in case you would like to use a command that itself takes flags or
different arguments, you can always write a shell script that will take the
savefile name as the only argument, make the flags & arguments arrangements
and execute the command that you want.
- -Z
-
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(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, it is
easier to pass it as a single, quoted argument.
Multiple arguments are concatenated with spaces before being parsed.
EXAMPLES
To print all packets arriving at or departing from sundown:
-
tcpdump host sundown
To print traffic between helios and either hot or ace:
-
tcpdump host helios and \( hot or ace \)
To print all IP packets between ace and any host except helios:
-
tcpdump ip host ace and not helios
To print all traffic between local hosts and hosts at Berkeley:
-
tcpdump net ucb-ether
To print all ftp traffic through internet gateway snup:
(note that the expression is quoted to prevent the shell from
(mis-)interpreting the parentheses):
-
tcpdump 'gateway snup and (port ftp or ftp-data)'
To print traffic neither sourced from nor destined for local hosts
(if you gateway to one other net, this stuff should never make it
onto your local net).
-
tcpdump ip and not net localnet
To print the start and end packets (the SYN and FIN packets) of each
TCP conversation that involves a non-local host.
-
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
To print all IPv4 HTTP packets to and from port 80, i.e. print only
packets that contain data, not, for example, SYN and FIN packets and
ACK-only packets. (IPv6 is left as an exercise for the reader.)
-
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
To print IP packets longer than 576 bytes sent through gateway snup:
-
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were
not
sent via Ethernet broadcast or multicast:
-
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies (i.e., not
ping packets):
-
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
OUTPUT FORMAT
The output of tcpdump is protocol dependent.
The following
gives a brief description and examples of most of the formats.
Link Level Headers
If the '-e' option is given, the link level header is printed out.
On Ethernets, the source and destination addresses, protocol,
and packet length are printed.
On FDDI networks, the '-e' option causes tcpdump to print
the `frame control' field, the source and destination addresses,
and the packet length.
(The `frame control' field governs the
interpretation of the rest of the packet.
Normal packets (such
as those containing IP datagrams) are `async' packets, with a priority
value between 0 and 7; for example, `async4'.
Such packets
are assumed to contain an 802.2 Logical Link Control (LLC) packet;
the LLC header is printed if it is not an ISO datagram or a
so-called SNAP packet.
On Token Ring networks, the '-e' option causes tcpdump to print
the `access control' and `frame control' fields, the source and
destination addresses, and the packet length.
As on FDDI networks,
packets are assumed to contain an LLC packet.
Regardless of whether
the '-e' option is specified or not, the source routing information is
printed for source-routed packets.
On 802.11 networks, the '-e' option causes tcpdump to print
the `frame control' fields, all of the addresses in the 802.11 header,
and the packet length.
As on FDDI networks,
packets are assumed to contain an LLC packet.
(N.B.: The following description assumes familiarity with
the SLIP compression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
packet type, and compression information are printed out.
The packet type is printed first.
The three types are ip, utcp, and ctcp.
No further link information is printed for ip packets.
For TCP packets, the connection identifier is printed following the type.
If the packet is compressed, its encoded header is printed out.
The special cases are printed out as
*S+n and *SA+n, where n is the amount by which
the sequence number (or sequence number and ack) has changed.
If it is not a special case,
zero or more changes are printed.
A change is indicated by U (urgent pointer), W (window), A (ack),
S (sequence number), and I (packet ID), followed by a delta (+n or -n),
or a new value (=n).
Finally, the amount of data in the packet and compressed header length
are printed.
For example, the following line shows an outbound compressed TCP packet,
with an implicit connection identifier; the ack has changed by 6,
the sequence number by 49, and the packet ID by 6; there are 3 bytes of
data and 6 bytes of compressed header:
-
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP Packets
Arp/rarp output shows the type of request and its arguments.
The
format is intended to be self explanatory.
Here is a short sample taken from the start of an `rlogin' from
host rtsg to host csam:
-
arp who-has csam tell rtsg
arp reply csam is-at CSAM
The first line says that rtsg sent an arp packet asking
for the Ethernet address of internet host csam.
Csam
replies with its Ethernet address (in this example, Ethernet addresses
are in caps and internet addresses in lower case).
This would look less redundant if we had done tcpdump -n:
-
arp who-has 128.3.254.6 tell 128.3.254.68
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet is
broadcast and the second is point-to-point would be visible:
-
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet this says the Ethernet source address is RTSG, the
destination is the Ethernet broadcast address, the type field
contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
TCP Packets
(N.B.:The following description assumes familiarity with
the TCP protocol described in RFC-793.
If you are not familiar
with the protocol, neither this description nor tcpdump will
be of much use to you.)
The general format of a tcp protocol line is:
-
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP
addresses and ports.
Flags are some combination of S (SYN),
F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
`.' (no flags).
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 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 TCP header usually holds 20 octets of data, unless options are
present.
The first line of the graph contains octets 0 - 3, the
second line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bits are contained
in octet 13:
0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |
Let's have a closer look at octet no. 13:
| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|
These are the TCP control bits we are interested
in.
We have numbered the bits in this octet from 0 to 7, right to
left, so the PSH bit is bit number 3, while the URG bit is number 5.
Recall that we want to capture packets with only SYN set.
Let's see what happens to octet 13 if a TCP datagram arrives
with the SYN bit set in its header:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Looking at the
control bits section we see that only bit number 1 (SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in
network byte order, the binary value of this octet is
-
00000010
and its decimal representation is
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
We're almost done, because now we know that if only SYN is set,
the value of the 13th octet in the TCP header, when interpreted
as a 8-bit unsigned integer in network byte order, must be exactly 2.
This relationship can be expressed as
-
tcp[13] == 2
We can use this expression as the filter for tcpdump in order
to watch packets which have only SYN set:
-
tcpdump -i xl0 tcp[13] == 2
The expression says "let the 13th octet of a TCP datagram have
the decimal value 2", which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but we
don't care if ACK or any other TCP control bit is set at the
same time.
Let's see what happens to octet 13 when a TCP datagram
with SYN-ACK set arrives:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Now bits 1 and 4 are set in the 13th octet.
The binary value of
octet 13 is
-
00010010
which translates to decimal
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
Now we can't just use 'tcp[13] == 18' in the tcpdump filter
expression, because that would select only those packets that have
SYN-ACK set, but not those with only SYN set.
Remember that we don't care
if ACK or any other control bit is set as long as SYN is set.
In order to achieve our goal, we need to logically AND the
binary value of octet 13 with some other value to preserve
the SYN bit.
We know that we want SYN to be set in any case,
so we'll logically AND the value in the 13th octet with
the binary value of a SYN:
00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010
We see that this AND operation delivers the same result
regardless whether ACK or another TCP control bit is set.
The decimal representation of the AND value as well as
the result of this operation is 2 (binary 00000010),
so we know that for packets with SYN set the following
relation must hold true:
-
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
-
tcpdump -i xl0 'tcp[13] & 2 == 2'
Some offsets and field values may be expressed as names
rather than as numeric values. For example tcp[13] may
be replaced with tcp[tcpflags]. The following TCP flag
field values are also available: tcp-fin, tcp-syn, tcp-rst,
tcp-push, tcp-act, tcp-urg.
This can be demonstrated as:
-
tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
Note that you should use single quotes or a backslash
in the expression to hide the AND ('&') special character
from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
-
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp
datagram to port who on host broadcast, the Internet
broadcast address.
The packet contained 84 bytes of user data.
Some UDP services are recognized (from the source or destination
port number) and the higher level protocol information printed.
In particular, Domain Name service requests (RFC-1034/1035) and Sun
RPC calls (RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with
the Domain Service protocol described in RFC-1035.
If you are not familiar
with the protocol, the following description will appear to be written
in greek.)
Name server requests are formatted as
-
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an
address record (qtype=A) associated with the name ucbvax.berkeley.edu.
The query id was `3'.
The `+' indicates the recursion desired flag
was set.
The query length was 37 bytes, not including the UDP and
IP protocol headers.
The query operation was the normal one, Query,
so the op field was omitted.
If the op had been anything else, it would
have been printed between the `3' and the `+'.
Similarly, the qclass was the normal one,
C_IN, and omitted.
Any other qclass would have been printed
immediately after the `A'.
A few anomalies are checked and may result in extra fields enclosed in
square brackets: If a query contains an answer, authority records or
additional records section,
ancount,
nscount,
or
arcount
are printed as `[na]', `[nn]' or `[nau]' where n
is the appropriate count.
If any of the response bits are set (AA, RA or rcode) or any of the
`must be zero' bits are set in bytes two and three, `[b2&3=x]'
is printed, where x is the hex value of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as
-
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo
with 3 answer records, 3 name server records and 7 additional records.
The first answer record is type A (address) and its data is internet
address 128.32.137.3.
The total size of the response was 273 bytes,
excluding UDP and IP headers.
The op (Query) and response code
(NoError) were omitted, as was the class (C_IN) of the A record.
In the second example, helios responds to query 2 with a
response code of non-existent domain (NXDomain) with no answers,
one name server and no authority records.
The `*' indicates that
the authoritative answer bit was set.
Since there were no
answers, no type, class or data were printed.
Other flag characters that might appear are `-' (recursion available,
RA, not set) and `|' (truncated message, TC, set).
If the
`question' section doesn't contain exactly one entry, `[nq]'
is printed.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
on UDP/137, UDP/138 and TCP/139.
Some primitive decoding of IPX and
NetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detailed
decode done if -v is used.
Be warned that with -v a single SMB packet
may take up a page or more, so only use -v if you really want all the
gory details.
For information on SMB packet formats and what all 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 6709
to wrl (note that the number following the src host is a
transaction id, not the source port).
The request was 112 bytes,
excluding the UDP and IP headers.
The operation was a readlink
(read symbolic link) on file handle (fh) 21,24/10.731657119.
(If one is lucky, as in this case, the file handle can be interpreted
as a major,minor device number pair, followed by the inode number and
generation number.)
Wrl replies `ok' with the contents of the link.
In the third line, sushi asks wrl to lookup the name
`xcolors' in directory file 9,74/4096.6878.
Note that the data printed
depends on the operation type.
The format is intended to be self
explanatory if read in conjunction with
an NFS protocol spec.
If the -v (verbose) flag is given, additional information is printed.
For example:
-
sushi.1372a > wrl.nfs:
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
reply ok 1472 read REG 100664 ids 417/0 sz 29388
(-v also prints the IP header TTL, ID, length, and fragmentation fields,
which have been omitted from this example.) In the first line,
sushi asks wrl to read 8192 bytes from file 21,11/12.195,
at byte offset 24576.
Wrl replies `ok'; the packet shown on the
second line is the first fragment of the reply, and hence is only 1472
bytes long (the other bytes will follow in subsequent fragments, but
these fragments do not have NFS or even UDP headers and so might not be
printed, depending on the filter expression used).
Because the -v flag
is given, some of the file attributes (which are returned in addition
to the file data) are printed: the file type (``REG'', for regular file),
the file mode (in octal), the uid and gid, and the file size.
If the -v flag is given more than once, even more details are printed.
Note that NFS requests are very large and much of the detail won't be printed
unless snaplen is increased.
Try using `-s 192' to watch
NFS traffic.
NFS reply packets do not explicitly identify the RPC operation.
Instead,
tcpdump keeps track of ``recent'' requests, and matches them to the
replies using the transaction ID.
If a reply does not closely follow the
corresponding request, it might not be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are printed
as:
-
src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args
elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike.
This was
a RX data packet to the fs (fileserver) service, and is the start of
an RPC call.
The RPC call was a rename, with the old directory file id
of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
file id of 536876964/1/1 and a new filename of `.newsrc'.
The host pike
responds with a RPC reply to the rename call (which was successful, because
it was a data packet and not an abort packet).
In general, all AFS RPCs are decoded at least by RPC call name.
Most
AFS RPCs have at least some of the arguments decoded (generally only
the `interesting' arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probably
not be useful to people who are not familiar with the workings of
AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets and
additional header information is printed, such as the 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 packets.
If the -v flag is given three times, the security index and service id
are printed.
Error codes are printed for abort packets, with the exception of Ubik
beacon packets (because abort packets are used to signify a yes vote
for the Ubik protocol).
Note that AFS requests are very large and many of the arguments won't
be printed unless snaplen is increased.
Try using `-s 256'
to watch AFS traffic.
AFS reply packets do not explicitly identify the RPC operation.
Instead,
tcpdump keeps track of ``recent'' requests, and matches them to the
replies using the call number and service ID.
If a reply does not closely
follow the
corresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
and dumped as DDP packets (i.e., all the UDP header information is
discarded).
The file
/etc/atalk.names
is used to translate AppleTalk net and node numbers to names.
Lines in this file have the form
-
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of AppleTalk networks.
The third
line gives the name of a particular host (a host is distinguished
from a net by the 3rd octet in the number -
a net number must have two octets and a host number must
have three octets.) The number and name should be separated by
whitespace (blanks or tabs).
The
/etc/atalk.names
file may contain blank lines or comment lines (lines starting with
a `#').
AppleTalk addresses are printed in the form
-
net.host.port
144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the
/etc/atalk.names
doesn't exist or doesn't contain an entry for some AppleTalk
host/net number, addresses are printed in numeric form.)
In the first example, NBP (DDP port 2) on net 144.1 node 209
is sending to whatever is listening on port 220 of net icsd node 112.
The second line is the same except the full name of the source node
is known (`office').
The third line is a send from port 235 on
net jssmag node 149 to broadcast on the icsd-net NBP port (note that
the broadcast address (255) is indicated by a net name with no host
number - for this reason it's a good idea to keep node names and
net names distinct in /etc/atalk.names).
NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
packets have their contents interpreted.
Other protocols just dump
the protocol name (or number if no name is registered for the
protocol) and packet size.
NBP packets are formatted like the following examples:
-
icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
The first line is a name lookup request for laserwriters sent by net icsd host
112 and broadcast on net jssmag.
The nbp id for the lookup is 190.
The second line shows a reply for this request (note that it has the
same id) from host jssmag.209 saying that it has a laserwriter
resource named "RM1140" registered on port 250.
The third line is
another reply to the same request saying host techpit has laserwriter
"techpit" registered on port 186.
ATP packet formatting is demonstrated by the following example:
-
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios by requesting
up to 8 packets (the `<0-7>').
The hex number at the end of the line
is the value of the `userdata' field in the request.
Helios responds with 8 512-byte packets.
The `:digit' following the
transaction id gives the packet sequence number in the transaction
and the number in parens is the amount of data in the packet,
excluding the atp header.
The `*' on packet 7 indicates that the
EOM bit was set.
Jssmag.209 then requests that packets 3 & 5 be retransmitted.
Helios
resends them then jssmag.209 releases the transaction.
Finally,
jssmag.209 initiates the next request.
The `*' on the request
indicates that XO (`exactly once') was not set.
IP Fragmentation
Fragmented Internet datagrams are printed as
-
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates there are more fragments.
The second
indicates this is the last fragment.)
Id is the fragment id.
Size is the fragment
size (in bytes) excluding the IP header.
Offset is this
fragment's offset (in bytes) in the original datagram.
The fragment information is output for each fragment.
The first
fragment contains the higher level protocol header and the frag
info is printed after the protocol info.
Fragments
after the first contain no higher level protocol header and the
frag info is printed after the source and destination addresses.
For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
over a CSNET connection that doesn't appear to handle 576 byte datagrams:
-
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses in the
2nd line don't include port numbers.
This is because the TCP
protocol information is all in the first fragment and we have no idea
what the port or sequence numbers are when we print the later fragments.
Second, the tcp sequence information in the first line is printed as if there
were 308 bytes of user data when, in fact, there are 512 bytes (308 in
the first frag and 204 in the second).
If you are looking for holes
in the sequence space or trying to match up acks
with packets, this can fool you.
A packet with the IP don't fragment flag is marked with a
trailing (DF).
Timestamps
By default, all output lines are preceded by a timestamp.
The timestamp
is the current clock time in the form
-
hh:mm:ss.frac
and is as accurate as the kernel's clock.
The timestamp reflects the time the kernel first saw the packet.
No attempt
is made to account for the time lag between when the
Ethernet interface removed the packet from the wire and when the kernel
serviced the `new packet' interrupt.
SEE ALSO
stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5),
pcap-filter(7)
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/tcpdump.tar.Z
IPv6/IPsec support is added by WIDE/KAME project.
This program uses Eric Young's SSLeay library, under specific configurations.
BUGS
Please send problems, bugs, questions, desirable enhancements, patches
etc. to:
-
tcpdump-workers@lists.tcpdump.org
NIT doesn't let you watch your own outbound traffic, BPF will.
We recommend that you use the latter.
On Linux systems with 2.0[.x] kernels:
-
packets on the loopback device will be seen twice;
-
packet filtering cannot be done in the kernel, so that all packets must
be copied from the kernel in order to be filtered in user mode;
-
all of a packet, not just the part that's within the snapshot length,
will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
asked to copy only part of a packet to userland, will not report the
true length of the packet; this would cause most IP packets to get an
error from
tcpdump);
-
capturing on some PPP devices won't work correctly.
We recommend that you upgrade to a 2.2 or later kernel.
Some attempt should be made to reassemble IP fragments or, at least
to compute the right length for the higher level protocol.
Name server inverse queries are not dumped correctly: the (empty)
question section is printed rather than real query in the answer
section.
Some believe that inverse queries are themselves a bug and
prefer to fix the program generating them rather than tcpdump.
A packet trace that crosses a daylight savings time change will give
skewed time stamps (the time change is ignored).
Filter expressions on fields other than those in Token Ring headers will
not correctly handle source-routed Token Ring packets.
Filter expressions on fields other than those in 802.11 headers will not
correctly handle 802.11 data packets with both To DS and From DS set.
ip6 proto
should chase header chain, but at this moment it does not.
ip6 protochain
is supplied for this behavior.
Arithmetic expression against transport layer headers, like tcp[0],
does not work against IPv6 packets.
It only looks at IPv4 packets.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- EXAMPLES
-
- OUTPUT FORMAT
-
- SEE ALSO
-
- AUTHORS
-
- BUGS
-
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Time: 07:34:21 GMT, September 13, 2011
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