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tcpdump – dump traffic on a network

SYNOPSIS

tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]

[ -c count ]

[ -C file_size ] [ -G rotate_seconds ] [ -F file ]

[ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]

[ –number ] [ -Q in|out|inout ]

[ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]

[ -W filecount ]

[ -E spi@ipaddr algo:secret,… ]

[ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]

[ –time-stamp-precision=tstamp_precision ]

[ –immediate-mode ] [ –version ]

[ expression ]

DESCRIPTION

Tcpdump prints out a description of the contents of packets on a network interface that match the boolean
expression; the description is preceded by a time stamp, printed, by default, as hours, minutes, seconds,
and fractions of a second since midnight. It can also be run with the -w flag, which causes it to save the
packet data to a file for later analysis, and/or with the -r flag, which causes it to read from a saved
packet file rather than to read packets from a network interface. It can also be run with the -V flag,
which causes it to read a list of saved packet files. In all cases, only packets that match expression will
be processed by tcpdump.

Tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT
signal (generated, for example, by typing your interrupt character, typically control-C) or a SIGTERM signal
(typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is
interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.

When tcpdump finishes capturing packets, it will report counts of:

packets “captured” (this is the number of packets that tcpdump has received and processed);

packets “received by filter” (the meaning of this depends on the OS on which you’re running tcp‐

dump, 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 pro‐

cessed 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 pack‐

ets 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 buf‐

fer 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 “sta‐
tus 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 pack‐
ets. On platforms that do not support the SIGINFO signal, the same can be achieved by using the SIGUSR1 sig‐
nal.

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 buffer_size
–buffer-size=buffer_size

Set the operating system capture buffer size to buffer_size, in units of KiB (1024 bytes).

-c count

Exit after receiving count packets.

-C file_size

Before writing a raw packet to a savefile, check whether the file is currently larger than file_size

and, if so, close the current savefile and open a new one. Savefiles after the first savefile will

have the name specified with the -w flag, with a number after it, starting at 1 and continuing

upward. The units of file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

-d Dump the compiled packet-matching code in a human readable form to standard output and stop.

-dd Dump packet-matching code as a C program fragment.

-ddd Dump packet-matching code as decimal numbers (preceded with a count).

-D
–list-interfaces

Print the list of the network interfaces available on the system and on which tcpdump can capture

packets. For each network interface, a number and an interface name, possibly followed by a text

description of the interface, is printed. The interface name or the number can be supplied to the -i

flag to specify an interface on which to capture.

This can be useful on systems that don’t have a command to list them (e.g., Windows systems, or UNIX

systems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems, where the

interface name is a somewhat complex string.

The -D flag will not be supported if tcpdump was built with an older version of libpcap that lacks

the pcap_findalldevs() function.

-e Print the link-level header on each dump line. This can be used, for example, to print MAC layer

addresses for protocols such as Ethernet and IEEE 802.11.

-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain

Security Parameter Index value spi. This combination may be repeated with comma or newline separa‐

tion.

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 com‐

mand 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 tcp‐

dump 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 file

Use file as input for the filter expression. An additional expression given on the command line is

ignored.

-G rotate_seconds

If specified, rotates the dump file specified with the -w option every rotate_seconds seconds. Save‐

files will have the name specified by -w which should include a time format as defined by strf‐

time(3). If no time format is specified, each new file will overwrite the previous.

If used in conjunction with the -C option, filenames will take the form of `file<count>’.

-h
–help Print the tcpdump and libpcap version strings, print a usage message, and exit.

–version

Print the tcpdump and libpcap version strings and exit.

-H Attempt to detect 802.11s draft mesh headers.

-i interface
–interface=interface
指定监听端口。
如果未指定,tcpdump会在系统接口列表中搜索最小编号的、已启动接口(但不包括lo),这可能是“eth0”。
在Linux 2.2+上,可以使用“any”来捕获所有接口的数据包。注意,“any”捕获不会在混杂模式下进行。
如果支持使用-D选项,那么由-D选项打印的接口编号可以用作interface参数,如果系统上的接口没有使用该数字作为接口名称。

-I
–monitor-mode

Put the interface in “monitor mode”; this is supported only on IEEE 802.11 Wi-Fi interfaces, and sup‐

ported 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 captur‐

ing in monitor mode and are not connected to another network with another adapter.

This flag will affect the output of the -L flag. If -I isn’t specified, only those link-layer types

available when not in monitor mode will be shown; if -I is specified, only those link-layer types

available when in monitor mode will be shown.

–immediate-mode

Capture in “immediate mode”. In this mode, packets are delivered to tcpdump as soon as they arrive,

rather than being buffered for efficiency. This is the default when printing packets rather than

saving packets to a “savefile” if the packets are being printed to a terminal rather than to a file

or pipe.

-j tstamp_type
–time-stamp-type=tstamp_type

Set the time stamp type for the capture to tstamp_type. The names to use for the time stamp types

are given in pcap-tstamp(7); not all the types listed there will necessarily be valid for any given

interface.

-J
–list-time-stamp-types

List the supported time stamp types for the interface and exit. If the time stamp type cannot be set

for the interface, no time stamp types are listed.

–time-stamp-precision=tstamp_precision

When capturing, set the time stamp precision for the capture to tstamp_precision. Note that avail‐

ability of high precision time stamps (nanoseconds) and their actual accuracy is platform and hard‐

ware dependent. Also note that when writing captures made with nanosecond accuracy to a savefile,

the time stamps are written with nanosecond resolution, and the file is written with a different

magic number, to indicate that the time stamps are in seconds and nanoseconds; not all programs that

read pcap savefiles will be able to read those captures.

When reading a savefile, convert time stamps to the precision specified by timestamp_precision, and display
them with that resolution. If the precision specified is less than the precision of time stamps in the
file, the conversion will lose precision.

The supported values for timestamp_precision are micro for microsecond resolution and nano for nanosecond
resolution. The default is microsecond resolution.

-K
–dont-verify-checksums

Don’t attempt to verify IP, TCP, or UDP checksums. This is useful for interfaces that perform some

or all of those checksum calculation in hardware; otherwise, all outgoing TCP checksums will be

flagged as bad.

-l Make stdout line buffered. Useful if you want to see the data while capturing it. E.g.,

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 charac‐

ter individually if -l is specified.

-U is similar to -l in its behavior, but it will cause output to be “packet-buffered”, so that the

output is written to stdout at the end of each packet rather than at the end of each line; this is

buffered on all platforms, including Windows.

-L
–list-data-link-types

List the known data link types for the interface, in the specified mode, and exit. The list of known

data link types may be dependent on the specified mode; for example, on some platforms, a Wi-Fi

interface might support one set of data link types when not in monitor mode (for example, it might

support only fake Ethernet headers, or might support 802.11 headers but not support 802.11 headers

with radio information) and another set of data link types when in monitor mode (for example, it

might support 802.11 headers, or 802.11 headers with radio information, only in monitor mode).

-m module

Load SMI MIB module definitions from file module. This option can be used several times to load sev‐

eral MIB modules into tcpdump.

-M secret

Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5

option (RFC 2385), if present.

-n
不转化地址,显示数据类型的IP地址、端口号。

-N Don’t print domain name qualification of host names. E.g., if you give this flag then tcpdump will

print “nic
instead of ``nic.ddn.mil.

-#
–number

Print an optional packet number at the beginning of the line.

-O
–no-optimize

Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the opti‐

mizer.

-p
–no-promiscuous-mode

Don’t put the interface into promiscuous mode. Note that the interface might be in promiscuous mode

for some other reason; hence, `-p’ cannot be used as an abbreviation for `ether host {local-hw-addr}

or ether broadcast’.

-Q direction
–direction=direction

Choose send/receive direction direction for which packets should be captured. Possible values are

`in’, `out’ and `inout’. Not available on all platforms.

-q Quick (quiet?) output. Print less protocol information so output lines are shorter.

-r file

Read packets from file (which was created with the -w option or by other tools that write pcap or

pcap-ng files). Standard input is used if file is “-”.

-S
–absolute-tcp-sequence-numbers

Print absolute, rather than relative, TCP sequence numbers.

-s snaplen
–snapshot-length=snaplen

Snarf snaplen bytes of data from each packet rather than the default of 262144 bytes. Packets trun‐

cated because of a limited snapshot are indicated in the output with “[|proto]”, where proto is the

name of the protocol level at which the truncation has occurred. Note that taking larger snapshots

both increases the amount of time it takes to process packets and, effectively, decreases the amount

of packet buffering. This may cause packets to be lost. You should limit snaplen to the smallest

number that will capture the protocol information you’re interested in. Setting snaplen to 0 sets it

to the default of 262144, for backwards compatibility with recent older versions of tcpdump.

-T type

Force packets selected by “expression” to be interpreted the specified type. Currently known types

are aodv (Ad-hoc On-demand Distance Vector protocol), carp (Common Address Redundancy Protocol), cnfp

(Cisco NetFlow protocol), lmp (Link Management Protocol), pgm (Pragmatic General Multicast),

pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), resp (REdis Serialization Protocol), radius (RADIUS), rpc

(Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications control

protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Protocol), vat

(Visual Audio Tool), wb (distributed White Board), zmtp1 (ZeroMQ Message Transport Protocol 1.0) and

vxlan (Virtual eXtensible Local Area Network).

Note that the pgm type above affects UDP interpretation only, the native PGM is always recognised as

IP protocol 113 regardless. UDP-encapsulated PGM is often called “EPGM” or “PGM/UDP”.

Note that the pgm_zmtp1 type above affects interpretation of both native PGM and UDP at once. During

the native PGM decoding the application data of an ODATA/RDATA packet would be decoded as a ZeroMQ

datagram with ZMTP/1.0 frames. During the UDP decoding in addition to that any UDP packet would be

treated as an encapsulated PGM packet.

-t Don’t print a timestamp on each dump line.

-tt Print the timestamp, as seconds since January 1, 1970, 00:00:00, UTC, and fractions of a second since

that time, on each dump line.

-ttt Print a delta (micro-second resolution) between current and previous line on each dump line.

-tttt Print a timestamp, as hours, minutes, seconds, and fractions of a second since midnight, preceded by

the date, on each dump line.

-ttttt Print a delta (micro-second resolution) between current and first line on each dump line.

-u Print undecoded NFS handles.

-U
–packet-buffered

If the -w option is not specified, make the printed packet output “packet-buffered”; i.e., as the

description of the contents of each packet is printed, it will be written to the standard output,

rather than, when not writing to a terminal, being written only when the output buffer fills.

If the -w option is specified, make the saved raw packet output “packet-buffered”; i.e., as each

packet is saved, it will be written to the output file, rather than being written only when the out‐

put buffer fills.

The -U flag will not be supported if tcpdump was built with an older version of libpcap that lacks

the pcap_dump_flush() function.

-v When parsing and printing, produce (slightly more) verbose output. For example, the time to live,

identification, total length and options in an IP packet are printed. Also enables additional packet

integrity checks such as verifying the IP and ICMP header checksum.

When writing to a file with the -w option, report, every 10 seconds, the number of packets captured.

-vv Even more verbose output. For example, additional fields are printed from NFS reply packets, and SMB

packets are fully decoded.

-vvv Even more verbose output. For example, telnet SB … SE options are printed in full. With -X Telnet

options are printed in hex as well.

-V file

Read a list of filenames from file. Standard input is used if file is “-”.

-w file

Write the raw packets to file rather than parsing and printing them out. They can later be printed

with the -r option. Standard output is used if file is “-”.

This output will be buffered if written to a file or pipe, so a program reading from the file or pipe

may not see packets for an arbitrary amount of time after they are received. Use the -U flag to

cause packets to be written as soon as they are received.

The MIME type application/vnd.tcpdump.pcap has been registered with IANA for pcap files. The filename

extension .pcap appears to be the most commonly used along with .cap and .dmp. Tcpdump itself doesn’t

check the extension when reading capture files and doesn’t add an extension when writing them (it

uses magic numbers in the file header instead). However, many operating systems and applications will

use the extension if it is present and adding one (e.g. .pcap) is recommended.

See pcap-savefile(5) for a description of the file format.

-W Used in conjunction with the -C option, this will limit the number of files created to the specified

number, and begin overwriting files from the beginning, thus creating a ‘rotating’ buffer. In addi‐

tion, 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. Ether‐

net), 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 proto‐

cols.

-XX When parsing and printing, in addition to printing the headers of each packet, print the data of each

packet, including its link level header, in hex and ASCII.

-y datalinktype
–linktype=datalinktype

Set the data link type to use while capturing packets to datalinktype.

-z postrotate-command

Used in conjunction with the -C or -G options, this will make tcpdump run ” postrotate-command file ”

where file is the savefile being closed after each rotation. For example, specifying -z gzip or -z

bzip2 will compress each savefile using gzip or bzip2.

Note that tcpdump will run the command in parallel to the capture, using the lowest priority so that

this doesn’t disturb the capture process.

And in case you would like to use a command that itself takes flags or different arguments, you can

always write a shell script that will take the savefile name as the only argument, make the flags &

arguments arrangements and execute the command that you want.

-Z user
–relinquish-privileges=user

If tcpdump is running as root, after opening the capture device or input savefile, but before opening

any savefiles for output, change the user ID to user and the group ID to the primary group of user.

This behavior can also be enabled by default at compile time.

expression

selects which packets will be dumped. If no expression is given, all packets on the net will be

dumped. Otherwise, only packets for which expression is `true’ will be dumped.

For the expression syntax, see pcap-filter(7).

The expression argument can be passed to tcpdump as either a single Shell argument, or as multiple

Shell arguments, whichever is more convenient. Generally, if the expression contains Shell metachar‐

acters, such as backslashes used to escape protocol names, it is easier to pass it as a single,

quoted argument rather than to escape the Shell metacharacters. Multiple arguments are concatenated

with spaces before being parsed.

EXAMPLES

To print all packets arriving at or departing from sundown:

tcpdump host sundown

To print traffic between helios and either hot or ace:

tcpdump host helios and \( hot or ace \)

To print all IP packets between ace and any host except helios:

tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at Berkeley:

tcpdump net ucb-ether

To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the
shell from (mis-)interpreting the parentheses):

tcpdump ‘gateway snup and (port ftp or ftp-data)’

To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this
stuff should never make it onto your local net).

tcpdump ip and not net localnet

To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-
local host.

tcpdump ‘tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet’

To print all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for
example, SYN and FIN packets and ACK-only packets. (IPv6 is left as an exercise for the reader.)

tcpdump ‘tcp port 80 and (((ip[2:2] – ((ip[0]&0xf)<<2)) – ((tcp[12]&0xf0)>>2)) != 0)’

To print IP packets longer than 576 bytes sent through gateway snup:

tcpdump ‘gateway snup and ip[2:2] > 576’

To print IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:

tcpdump ‘ether[0] & 1 = 0 and ip[16] >= 224’

To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):

tcpdump ‘icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply’

OUTPUT FORMAT

The output of tcpdump is protocol dependent. The following gives a brief description and examples of most
of the formats.

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 applied a time stamp

to the packet. No attempt is made to account for the time lag between when the network interface finished

receiving the packet from the network and when the kernel applied a time stamp to the packet; that time lag

could include a delay between the time when the network interface finished receiving a packet from the net‐

work and the time when an interrupt was delivered to the kernel to get it to read the packet and a delay

between the time when the kernel serviced the `new packet’ interrupt and the time when it applied a time

stamp to the packet.

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 desti‐
nation 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 identi‐
fier; 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 broad‐

cast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.

IPv4 Packets

If the link-layer header is not being printed, for IPv4 packets, IP is printed after the time stamp.

If the -v flag is specified, information from the IPv4 header is shown in parentheses after the IP or the
link-layer header. The general format of this information is:

tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)

tos is the type of service field; if the ECN bits are non-zero, those are reported as ECT(1), ECT(0), or CE.

ttl is the time-to-live; it is not reported if it is zero. id is the IP identification field. offset is

the fragment offset field; it is printed whether this is part of a fragmented datagram or not. flags are

the MF and DF flags; + is reported if MF is set, and DFP is reported if F is set. If neither are set, . is

reported. proto is the protocol ID field. length is the total length field. options are the IP options,

if any.

Next, for TCP and UDP packets, the source and destination IP addresses and TCP or UDP ports, with a dot
between each IP address and its corresponding port, will be printed, with a > separating the source and des‐
tination. For other protocols, the addresses will be printed, with a > separating the source and destina‐
tion. Higher level protocol information, if any, will be printed after that.

For fragmented IP datagrams, the first fragment contains the higher level protocol header; fragments after
the first contain no higher level protocol header. Fragmentation information will be printed only with the
-v flag, in the IP header information, as described above.

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, this description will not be of much use to you.)

The general format of a TCP protocol line is:

src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len

Src and dst are the source and destination IP addresses and ports. Tcpflags 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). Ackno 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 indi‐

cates there is `urgent’ data in the packet. Opts are TCP options (e.g., mss 1024). Len is the length of

payload data.

Iptype, 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.

IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]

IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]

IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096

IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1

IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096

IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19

IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1

IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1

IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 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’ which means `sequence numbers first up to but not including last.) 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 or length. 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 conversa‐
tion, 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 conver‐
sation). 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 con‐
tains 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 data‐
gram 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 char‐
acter 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 ucb‐

vax.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 mes‐
sage, TC, set). If the `question’ section doesn’t contain exactly one entry, `[nq]’ is printed.

SMB/CIFS decoding

tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139. Some
primitive decoding of IPX and NetBEUI SMB data is also done.

By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned
that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory
details.

For information on SMB packet formats and what all the fields mean see www.cifs.org or the pub/samba/specs/
directory on your favorite samba.org mirror site. The SMB patches were written by Andrew Tridgell
(tridge@samba.org).

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are printed as:

src.sport > dst.nfs: NFS request xid xid len op args

src.nfs > dst.dport: NFS reply xid xid reply stat len op results

sushi.1023 > wrl.nfs: NFS request xid 26377
112 readlink fh 21,24/10.73165
wrl.nfs > sushi.1023: NFS reply xid 26377
reply ok 40 readlink “../var”
sushi.1022 > wrl.nfs: NFS request xid 8219
144 lookup fh 9,74/4096.6878 “xcolors”
wrl.nfs > sushi.1022: NFS reply xid 8219
reply ok 128 lookup fh 9,74/4134.3150

In the first line, host sushi sends a transaction with id 26377 to wrl. The request was 112 bytes, exclud‐

ing the UDP and IP headers. The operation was a readlink (read symbolic link) on file handle (fh)

21,24/10.731657119. (If one is lucky, as in this case, the file handle can be interpreted as a major,minor

device number pair, followed by the inode number and generation number.) In the second line, wrl replies

`ok’ with the same transaction id and the contents of the link.

In the third line, sushi asks (using a new transaction id) wrl to lookup the name `xcolors’ in directory
file 9,74/4096.6878. In the fourth line, wrl sends a reply with the respective transaction id.

Note that the data printed depends on the operation type. The format is intended to be self explanatory if
read in conjunction with an NFS protocol spec. Also note that older versions of tcpdump printed NFS packets
in a slightly different format: the transaction id (xid) would be printed instead of the non-NFS port number
of the packet.

If the -v (verbose) flag is given, additional information is printed. For example:

sushi.1023 > wrl.nfs: NFS request xid 79658
148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1023: NFS reply xid 79658
reply ok 1472 read REG 100664 ids 417/0 sz 29388

(-v also prints the IP header TTL, ID, length, and fragmentation fields, which have been omitted from this

example.) In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset

  1. Wrl replies `ok’; the packet shown on the second line is the first fragment of the reply, and hence

is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not

have NFS or even UDP headers and so might not be printed, depending on the filter expression used). Because

the -v flag is given, some of the file attributes (which are returned in addition to the file data) are

printed: the file type (“REG”, for regular file), the file mode (in octal), the uid and gid, and the file

size.

If the -v flag is given more than once, even more details are printed.

Note that NFS requests are very large and much of the detail won’t be printed unless snaplen is increased.
Try using `-s 192′ to watch NFS traffic.

NFS reply packets do not explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent”
requests, and matches them to the replies using the transaction ID. If a reply does not closely follow the
corresponding request, it might not be parsable.

AFS Requests and Replies

Transarc AFS (Andrew File System) requests and replies are printed as:

src.sport > dst.dport: rx packet-type

src.sport > dst.dport: rx packet-type service call call-name args

src.sport > dst.dport: rx packet-type service reply call-name args

elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 “.newsrc.new”

new fid 536876964/1/1 “.newsrc”
pike.afsfs > elvis.7001: rx data fs reply rename

In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver)

service, and is the start of an RPC call. The RPC call was a rename, with the old directory file id of

536876964/1/1 and an old filename of `.newsrc.new’, and a new directory file id of 536876964/1/1 and a new

filename of `.newsrc’. The host pike responds with a RPC reply to the rename call (which was successful,

because it was a data packet and not an abort packet).

In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the
arguments decoded (generally only the `interesting’ arguments, for some definition of interesting).

The format is intended to be self-describing, but it will probably not be useful to people who are not
familiar with the workings of AFS and RX.

If the -v (verbose) flag is given twice, acknowledgement packets and additional header information is
printed, such as the RX call ID, call number, sequence number, serial number, and the RX packet flags.

If the -v flag is given twice, additional information is printed, such as the RX call ID, serial number, and
the RX packet flags. The MTU negotiation information is also printed from RX ack packets.

If the -v flag is given three times, the security index and service id are printed.

Error codes are printed for abort packets, with the exception of Ubik beacon packets (because abort packets
are used to signify a yes vote for the Ubik protocol).

Note that AFS requests are very large and many of the arguments won’t be printed unless snaplen is
increased. Try using `-s 256′ to watch AFS traffic.

AFS reply packets do not explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent”
requests, and matches them to the replies using the call number and service ID. If a reply does not closely
follow the corresponding request, it might not be parsable.

KIP AppleTalk (DDP in UDP)

AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all
the UDP header information is discarded). The file /etc/atalk.names is used to translate AppleTalk net and
node numbers to names. Lines in this file have the form

number name

1.254 ether

16.1 icsd-net

1.254.110 ace

The first two lines give the names of AppleTalk networks. The third line gives the name of a particular

host (a host is distinguished from a net by the 3rd octet in the number – a net number must have two octets

and a host number must have three octets.) The number and name should be separated by whitespace (blanks or

tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a `#’).

AppleTalk addresses are printed in the form

net.host.port

144.1.209.2 > icsd-net.112.220

office.2 > icsd-net.112.220

jssmag.149.235 > icsd-net.2

(If the /etc/atalk.names doesn’t exist or doesn’t contain an entry for some AppleTalk host/net number,

addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is

sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the

full name of the source node is known (`office’). The third line is a send from port 235 on net jssmag node

149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name

with no host number – for this reason it’s a good idea to keep node names and net names distinct in

/etc/atalk.names).

NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents inter‐
preted. 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 jss‐

mag. 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.

SEE ALSO

stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-tstamp(7)

BUGS

To report a security issue please send an e-mail to security@tcpdump.org.

To report bugs and other problems, contribute patches, request a feature, provide generic feedback etc
please see the file CONTRIBUTING in the tcpdump source tree root.

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 pack‐
ets 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.

参考文献

  • man 8 tcpdump, version 4.9.2