Manual Pages for UNIX Darwin command on man tcpdump

TCPDUMP(1) TCPDUMP(1)

NAME

tcpdump - dump traffic on a network

SYNOPSIS

ttccppdduummpp [ -AAddDDeeffllLLnnNNOOppqqRRSSttuuUUvvxxXX ] [ -cc count ]

[ -CC filesize ] [ -FF file ]

[ -ii interface ] [ -mm module ] [ -MM secret ]

[ -rr file ] [ -ss snaplen ] [ -TT type ] [ -ww file ]

[ -WW filecount ]

[ -EE spi@ipaddr algo:secret,... ]

[ -yy datalinktype ] [ -ZZ user ]

[ expression ]

DESCRIPTION

Tcpdump prints out the headers of packets on a network interface that

match the boolean expression. It can also be run with the -ww flag,

which causes it to save the packet data to a file for later analysis,

and/or with the -rr 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 -cc flag, continue capturing packets

until it is interrupted by a SIGINT signal (generated, for example, by

typing your interrupt character, typically control-C) or a SIGTERM sig-

nal (typically generated with the kkiillll(1) command); if run with the -cc

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 ssttttyy(1) in order to use it) and will continue capturing packets.

Reading packets from a network interface may require that you have spe-

cial privileges: UUnnddeerr SSuunnOOSS 33..xx oorr 44..xx wwiitthh NNIITT oorr BBPPFF:: You must have read access to /dev/nit or /dev/bpf*. UUnnddeerr SSoollaarriiss wwiitthh DDLLPPII:: You must have read/write access to the network pseudo device, e.g. /dev/le. On at least some versions of Solaris, however,

this is not sufficient to allow tcpdump to capture in promiscu-

ous mode; on those versions of Solaris, you must be root, or

tcpdump must be installed setuid to root, in order to capture in

promiscuous mode. Note that, on many (perhaps all) interfaces, if you don't capture in promiscuous mode, you will not see any outgoing packets, so a capture not done in promiscuous mode may not be very useful.

UUnnddeerr HHPP-UUXX wwiitthh DDLLPPII::

You must be root or tcpdump must be installed setuid to root.

UUnnddeerr IIRRIIXX wwiitthh ssnnoooopp::

You must be root or tcpdump must be installed setuid to root.

UUnnddeerr LLiinnuuxx::

You must be root or tcpdump must be installed setuid to root

(unless your distribution has a kernel that supports capability bits such as CAPNETRAW and code to allow those capability bits to be given to particular accounts and to cause those bits to be set on a user's initial processes when they log in, in which case you must have CAPNETRAW in order to capture and CAPNETADMIN to enumerate network devices with, for example,

the -DD flag).

UUnnddeerr UULLTTRRIIXX aanndd DDiiggiittaall UUNNIIXX//TTrruu6644 UUNNIIXX::

Any user may capture network traffic with tcpdump. However, no

user (not even the super-user) can capture in promiscuous mode

on an interface unless the super-user has enabled promiscuous-

mode operation on that interface using pfconfig(8), and no user

(not even the super-user) can capture unicast traffic received

by or sent by the machine on an interface unless the super-user

has enabled copy-all-mode operation on that interface using

pfconfig, so useful packet capture on an interface probably

requires that either promiscuous-mode or copy-all-mode opera-

tion, or both modes of operation, be enabled on that interface. UUnnddeerr BBSSDD ((tthhiiss iinncclluuddeess MMaacc OOSS XX)):: You must have read access to /dev/bpf*. On BSDs with a devfs

(this includes Mac OS X), this might involve more than just hav-

ing somebody with super-user access setting the ownership or

permissions on the BPF devices - it might involve configuring

devfs to set the ownership or permissions every time the system

is booted, if the system even supports that; if it doesn't sup-

port that, you might have to find some other way to make that happen at boot time. Reading a saved packet file doesn't require special privileges. OOPPTTIIOONNSS

-AA Print each packet (minus its link level header) in ASCII. Handy

for capturing web pages.

-cc Exit after receiving count packets.

-CC Before writing a raw packet to a savefile, check whether the

file is currently larger than filesize and, if so, close the current savefile and open a new one. Savefiles after the first

savefile will have the name specified with the -ww flag, with a

number after it, starting at 1 and continuing upward. The units of filesize are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

-dd Dump the compiled packet-matching code in a human readable form

to standard output and stop.

-dddd Dump packet-matching code as a CC program fragment.

-dddddd Dump packet-matching code as decimal numbers (preceded with a

count).

-DD 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 -ii 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 iiffccoonnffiigg

-aa); the number can be useful on Windows 2000 and later systems,

where the interface name is a somewhat complex string.

The -DD flag will not be supported if tcpdump was built with an

older version of libpcap that lacks the ppccaappffiinnddaallllddeevvss(()) func-

tion.

-ee Print the link-level header on each dump line.

-EE 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 ddeess-ccbbcc, 33ddeess-ccbbcc, bblloowwffiisshh-ccbbcc, rrcc33-ccbbcc,

ccaasstt112288-ccbbcc, or nnoonnee. The default is ddeess-ccbbcc. 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 per-

missions that tcpdump may have been given should already have

been given up.

-ff Print `foreign' IPv4 addresses numerically rather than symboli-

cally (this option is intended to get around serious brain dam-

age in Sun's NIS server - usually it hangs forever translating

non-local internet numbers).

The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of the interface on which capture is being done. If that address or netmask are not available, available, either because the interface on which capture is being done has no address or netmask or because the capture is being done on the Linux "any" interface, which can capture on more than one interface, this option will not work correctly.

-FF Use file as input for the filter expression. An additional

expression given on the command line is ignored.

-ii Listen on interface. If unspecified, tcpdump searches the sys-

tem interface list for the lowest numbered, configured up inter-

face (excluding loopback). Ties are broken by choosing the ear-

liest match.

On Linux systems with 2.2 or later kernels, an interface argu-

ment of ``any'' can be used to capture packets from all inter-

faces. Note that captures on the ``any'' device will not be done in promiscuous mode.

If the -DD flag is supported, an interface number as printed by

that flag can be used as the interface argument.

-ll 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''.

-LL List the known data link types for the interface and exit.

-mm Load SMI MIB module definitions from file module. This option

can be used several times to load several MIB modules into tcp-

dump.

-MM Use secret as a shared secret for validating the digests found

in TCP segments with the TCP-MD5 option (RFC 2385), if present.

-nn Don't convert addresses (i.e., host addresses, port numbers,

etc.) to names.

-NN 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''.

-OO Do not run the packet-matching code optimizer. This is useful

only if you suspect a bug in the optimizer.

-pp 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'.

-qq Quick (quiet?) output. Print less protocol information so out-

put lines are shorter.

-RR Assume ESP/AH packets to be based on old specification (RFC1825

to RFC1829). If specified, tcpdump will not print replay pre-

vention field. Since there is no protocol version field in

ESP/AH specification, tcpdump cannot deduce the version of

ESP/AH protocol.

-rr Read packets from file (which was created with the -ww option).

Standard input is used if file is ``-''.

-SS Print absolute, rather than relative, TCP sequence numbers.

-ss Snarf snaplen bytes of data from each packet rather than the

default of 68 (with SunOS's NIT, the minimum is actually 96). 68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate protocol information from name server and NFS packets (see below). 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 cap-

ture the protocol information you're interested in. Setting

snaplen to 0 means use the required length to catch whole pack-

ets.

-TT Force packets selected by "expression" to be interpreted the

specified type. Currently known types are aaooddvv (Ad-hoc On-

demand Distance Vector protocol), ccnnffpp (Cisco NetFlow protocol),

rrppcc (Remote Procedure Call), rrttpp (Real-Time Applications proto-

col), rrttccpp (Real-Time Applications control protocol), ssnnmmpp (Sim-

ple Network Management Protocol), ttffttpp (Trivial File Transfer Protocol), vvaatt (Visual Audio Tool), and wwbb (distributed White Board).

-tt Don't print a timestamp on each dump line.

-tttt Print an unformatted timestamp on each dump line.

-tttttt Print a delta (in micro-seconds) between current and previous

line on each dump line.

-tttttttt Print a timestamp in default format proceeded by date on each

dump line.

-uu Print undecoded NFS handles.

-UU Make output saved via the -ww 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 -UU flag will not be supported if tcpdump was built with an

older version of libpcap that lacks the ppccaappdduummppfflluusshh(()) func-

tion.

-vv When parsing and printing, produce (slightly more) verbose out-

put. For example, the time to live, identification, total length and options in an IP packet are printed. Also enables additional packet integrity checks such as verifying the IP and ICMP header checksum.

When writing to a file with the -ww option, report, every 10 sec-

onds, the number of packets captured.

-vvvv Even more verbose output. For example, additional fields are

printed from NFS reply packets, and SMB packets are fully decoded.

-vvvvvv Even more verbose output. For example, telnet SSBB ... SSEE options

are printed in full. With -XX Telnet options are printed in hex

as well.

-ww Write the raw packets to file rather than parsing and printing

them out. They can later be printed with the -r option. Stan-

dard output is used if file is ``-''.

-WW Used in conjunction with the -C option, this will limit the num-

ber of files created to the specified number, and begin over-

writing files from the beginning, thus creating a 'rotating' buffer. In addition, it will name the files with enough leading 0s to support the maximum number of files, allowing them to sort correctly.

-xx Print 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 lay-

ers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer packet is shorter than the required padding.

-xxxx Print each packet, including its link level header, in hex.

-XX Print each packet (minus its link level header) in hex and

ASCII. This is very handy for analysing new protocols.

-XXXX Print each packet, including its link level header, in hex and

ASCII.

-yy Set the data link type to use while capturing packets to

datalinktype.

-ZZ 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. The expression consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifier: type qualifiers say what kind of thing the id name or number

refers to. Possible types are hhoosstt, nneett ,, ppoorrtt and ppoorr-

ttrraannggee. E.g., `host foo', `net 128.3', `port 20', `por-

trange 6000-6008'. If there is no type qualifier, hhoosstt

is assumed. dir qualifiers specify a particular transfer direction to and/or from id. Possible directions are ssrrcc, ddsstt, ssrrcc oorr ddsstt and ssrrcc aanndd ddsstt. E.g., `src foo', `dst net 128.3',

`src or dst port ftp-data'. If there is no dir quali-

fier, ssrrcc oorr ddsstt is assumed. For some link layers, such as SLIP and the ``cooked'' Linux capture mode used for the ``any'' device and for some other device types, the iinnbboouunndd and oouuttbboouunndd qualifiers can be used to specify a desired direction. proto qualifiers restrict the match to a particular protocol. Possible protos are: eetthheerr, ffddddii, ttrr, wwllaann, iipp, iipp66, aarrpp, rraarrpp, ddeeccnneett, ttccpp and uuddpp. E.g., `ether src foo', `arp

net 128.3', `tcp port 21', `udp portrange 7000-7009'. If

there is no proto qualifier, all protocols consistent with the type are assumed. E.g., `src foo' means `(ip or arp or rarp) src foo' (except the latter is not legal syntax), `net bar' means `(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'. [`fddi' is actually an alias for `ether'; the parser treats them

identically as meaning ``the data link level used on the speci-

fied network interface.'' FDDI headers contain Ethernet-like

source and destination addresses, and often contain Ethernet-

like packet types, so you can filter on these FDDI fields just

as with the analogous Ethernet fields. FDDI headers also con-

tain other fields, but you cannot name them explicitly in a fil-

ter expression. Similarly, `tr' and `wlan' are aliases for `ether'; the previous paragraph's statements about FDDI headers also apply to Token Ring and 802.11 wireless LAN headers. For 802.11 headers, the destination address is the DA field and the source address is the SA field; the BSSID, RA, and TA fields aren't tested.] In addition to the above, there are some special `primitive' keywords that don't follow the pattern: ggaatteewwaayy, bbrrooaaddccaasstt, lleessss, ggrreeaatteerr and arithmetic expressions. All of these are described below. More complex filter expressions are built up by using the words aanndd, oorr and nnoott to combine primitives. E.g., `host foo and not

port ftp and not port ftp-data'. To save typing, identical

qualifier lists can be omitted. E.g., `tcp dst port ftp or ftp-

data or domain' is exactly the same as `tcp dst port ftp or tcp

dst port ftp-data or tcp dst port domain'.

Allowable primitives are: ddsstt hhoosstt host True if the IPv4/v6 destination field of the packet is host, which may be either an address or a name. ssrrcc hhoosstt host True if the IPv4/v6 source field of the packet is host. hhoosstt host True if either the IPv4/v6 source or destination of the packet is host. Any of the above host expressions can be prepended with the keywords, iipp, aarrpp, rraarrpp, or iipp66 as in: iipp hhoosstt host which is equivalent to: eetthheerr pprroottoo \ip aanndd hhoosstt host If host is a name with multiple IP addresses, each address will be checked for a match. eetthheerr ddsstt ehost True if the Ethernet destination address is ehost. Ehost may be either a name from /etc/ethers or a number (see ethers(3N) for numeric format). eetthheerr ssrrcc ehost True if the Ethernet source address is ehost. eetthheerr hhoosstt ehost True if either the Ethernet source or destination address is ehost. ggaatteewwaayy host True if the packet used host as a gateway. I.e., the

Ethernet source or destination address was host but nei-

ther the IP source nor the IP destination was host. Host must be a name and must be found both by the machine's

host-name-to-IP-address resolution mechanisms (host name

file, DNS, NIS, etc.) and by the machine's host-name-to-

Ethernet-address resolution mechanism (/etc/ethers,

etc.). (An equivalent expression is eetthheerr hhoosstt ehost aanndd nnoott hhoosstt host which can be used with either names or numbers for host /

ehost.) This syntax does not work in IPv6-enabled con-

figuration at this moment. ddsstt nneett net True if the IPv4/v6 destination address of the packet has a network number of net. Net may be either a name from /etc/networks or a network number (see networks(4) for details). ssrrcc nneett net True if the IPv4/v6 source address of the packet has a network number of net. nneett net True if either the IPv4/v6 source or destination address of the packet has a network number of net. nneett net mmaasskk netmask True if the IPv4 address matches net with the specific netmask. May be qualified with ssrrcc or ddsstt. Note that this syntax is not valid for IPv6 net. nneett net/len True if the IPv4/v6 address matches net with a netmask len bits wide. May be qualified with ssrrcc or ddsstt. ddsstt ppoorrtt port True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a destination port value of port. The port can be a number or a name used in /etc/services (see tcp(4P) and udp(4P)). If a name is used, both the port number and protocol are checked. If a number or ambiguous name is used, only the port number is checked (e.g., ddsstt ppoorrtt

551133 will print both tcp/login traffic and udp/who traf-

fic, and ppoorrtt ddoommaaiinn will print both tcp/domain and udp/domain traffic). ssrrcc ppoorrtt port True if the packet has a source port value of port. ppoorrtt port True if either the source or destination port of the packet is port.

ddsstt ppoorrttrraannggee port1-port2

True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a destination port value between port1 and port2. port1 and port2 are interpreted in the same fashion as the port parameter for ppoorrtt.

ssrrcc ppoorrttrraannggee port1-port2

True if the packet has a source port value between port1 and port2.

ppoorrttrraannggee port1-port2

True if either the source or destination port of the packet is between port1 and port2. Any of the above port or port range expressions can be prepended with the keywords, ttccpp or uuddpp, as in: ttccpp ssrrcc ppoorrtt port which matches only tcp packets whose source port is port. lleessss length True if the packet has a length less than or equal to length. This is equivalent to: lleenn <<== length.. ggrreeaatteerr length True if the packet has a length greater than or equal to length. This is equivalent to: lleenn >>== length.. iipp pprroottoo protocol

True if the packet is an IPv4 packet (see ip(4P)) of pro-

tocol type protocol. Protocol can be a number or one of the names iiccmmpp, iiccmmpp66, iiggmmpp, iiggrrpp, ppiimm, aahh, eesspp, vvrrrrpp, uuddpp, or ttccpp. Note that the identifiers ttccpp, uuddpp, and iiccmmpp are also keywords and must be escaped via backslash

(\), which is \\ in the C-shell. Note that this primi-

tive does not chase the protocol header chain. iipp66 pprroottoo protocol True if the packet is an IPv6 packet of protocol type protocol. Note that this primitive does not chase the protocol header chain. iipp66 pprroottoocchhaaiinn protocol True if the packet is IPv6 packet, and contains protocol header with type protocol in its protocol header chain. For example, iipp66 pprroottoocchhaaiinn 66 matches any IPv6 packet with TCP protocol header in the

protocol header chain. The packet may contain, for exam-

ple, authentication header, routing header, or hop-by-hop

option header, between IPv6 header and TCP header. The BPF code emitted by this primitive is complex and cannot

be optimized by BPF optimizer code in tcpdump, so this

can be somewhat slow. iipp pprroottoocchhaaiinn protocol Equivalent to iipp66 pprroottoocchhaaiinn protocol, but this is for IPv4. eetthheerr bbrrooaaddccaasstt True if the packet is an Ethernet broadcast packet. The ether keyword is optional. iipp bbrrooaaddccaasstt True if the packet is an IPv4 broadcast packet. It

checks for both the all-zeroes and all-ones broadcast

conventions, and looks up the subnet mask on the inter-

face on which the capture is being done. If the subnet mask of the interface on which the capture

is being done is not available, either because the inter-

face on which capture is being done has no netmask or because the capture is being done on the Linux "any" interface, which can capture on more than one interface, this check will not work correctly. eetthheerr mmuullttiiccaasstt True if the packet is an Ethernet multicast packet. The eetthheerr keyword is optional. This is shorthand for `eetthheerr[[00]] && 11 !!== 00'. iipp mmuullttiiccaasstt True if the packet is an IPv4 multicast packet. iipp66 mmuullttiiccaasstt True if the packet is an IPv6 multicast packet. eetthheerr pprroottoo protocol True if the packet is of ether type protocol. Protocol can be a number or one of the names iipp, iipp66, aarrpp, rraarrpp, aattaallkk, aaaarrpp, ddeeccnneett, ssccaa, llaatt, mmooppddll, mmoopprrcc, iissoo, ssttpp,

iippxx, or nneettbbeeuuii. Note these identifiers are also key-

words and must be escaped via backslash (\). [In the case of FDDI (e.g., `ffddddii pprroottooccooll aarrpp'), Token Ring (e.g., `ttrr pprroottooccooll aarrpp'), and IEEE 802.11 wireless

LANS (e.g., `wwllaann pprroottooccooll aarrpp'), for most of those pro-

tocols, the protocol identification comes from the 802.2

Logical Link Control (LLC) header, which is usually lay-

ered on top of the FDDI, Token Ring, or 802.11 header. When filtering for most protocol identifiers on FDDI,

Token Ring, or 802.11, tcpdump checks only the protocol

ID field of an LLC header in so-called SNAP format with

an Organizational Unit Identifier (OUI) of 0x000000, for encapsulated Ethernet; it doesn't check whether the packet is in SNAP format with an OUI of 0x000000. The exceptions are:

iissoo tcpdump checks the DSAP (Destination Service

Access Point) and SSAP (Source Service Access Point) fields of the LLC header; ssttpp and nneettbbeeuuii

tcpdump checks the DSAP of the LLC header;

aattaallkk tcpdump checks for a SNAP-format packet with an

OUI of 0x080007 and the AppleTalk etype.

In the case of Ethernet, tcpdump checks the Ethernet type

field for most of those protocols. The exceptions are: iissoo, ssttpp, and nneettbbeeuuii

tcpdump checks for an 802.3 frame and then checks

the LLC header as it does for FDDI, Token Ring, and 802.11;

aattaallkk tcpdump checks both for the AppleTalk etype in an

Ethernet frame and for a SNAP-format packet as it

does for FDDI, Token Ring, and 802.11;

aaaarrpp tcpdump checks for the AppleTalk ARP etype in

either an Ethernet frame or an 802.2 SNAP frame with an OUI of 0x000000;

iippxx tcpdump checks for the IPX etype in an Ethernet

frame, the IPX DSAP in the LLC header, the

802.3-with-no-LLC-header encapsulation of IPX, and

the IPX etype in a SNAP frame. ddeeccnneett ssrrcc host True if the DECNET source address is host, which may be an address of the form ``10.123'', or a DECNET host name. [DECNET host name support is only available on ULTRIX systems that are configured to run DECNET.] ddeeccnneett ddsstt host True if the DECNET destination address is host. ddeeccnneett hhoosstt host True if either the DECNET source or destination address is host. iiffnnaammee interface

True if the packet was logged as coming from the speci-

fied interface (applies only to packets logged by OpenBSD's ppff(4)). oonn interface Synonymous with the iiffnnaammee modifier. rrnnrr num True if the packet was logged as matching the specified PF rule number (applies only to packets logged by OpenBSD's ppff(4)). rruulleennuumm num Synonomous with the rrnnrr modifier. rreeaassoonn code

True if the packet was logged with the specified PF rea-

son code. The known codes are: mmaattcchh, bbaadd-ooffffsseett, ffrraagg-

mmeenntt, sshhoorrtt, nnoorrmmaalliizzee, and mmeemmoorryy (applies only to pack-

ets logged by OpenBSD's ppff(4)). rrsseett name True if the packet was logged as matching the specified PF ruleset name of an anchored ruleset (applies only to packets logged by ppff(4)). rruulleesseett name Synonomous with the rrsseett modifier. ssrrnnrr num True if the packet was logged as matching the specified PF rule number of an anchored ruleset (applies only to packets logged by ppff(4)). ssuubbrruulleennuumm num Synonomous with the ssrrnnrr modifier. aaccttiioonn act True if PF took the specified action when the packet was logged. Known actions are: ppaassss and bblloocckk (applies only to packets logged by OpenBSD's ppff(4)). iipp, iipp66, aarrpp, rraarrpp, aattaallkk, aaaarrpp, ddeeccnneett, iissoo, ssttpp, iippxx, netbeui Abbreviations for: eetthheerr pprroottoo p where p is one of the above protocols. llaatt, mmoopprrcc, mmooppddll Abbreviations for: eetthheerr pprroottoo p

where p is one of the above protocols. Note that tcpdump

does not currently know how to parse these protocols. vvllaann [vlanid] True if the packet is an IEEE 802.1Q VLAN packet. If [vlanid] is specified, only true if the packet has the specified vlanid. Note that the first vvllaann keyword encountered in expression changes the decoding offsets for the remainder of expression on the assumption that

the packet is a VLAN packet. The vvllaann [vlanid] expres-

sion may be used more than once, to filter on VLAN hier-

archies. Each use of that expression increments the fil-

ter offsets by 4. For example: vvllaann 110000 &&&& vvllaann 220000 filters on VLAN 200 encapsulated within VLAN 100, and vvllaann &&&& vvllaann 330000 &&&& iipp

filters IPv4 protocols encapsulated in VLAN 300 encapsu-

lated within any higher order VLAN. mmppllss [labelnum] True if the packet is an MPLS packet. If [labelnum] is specified, only true is the packet has the specified labelnum. Note that the first mmppllss keyword encountered in expression changes the decoding offsets for the remainder of expression on the assumption that the packet

is a MPLS-encapsulated IP packet. The mmppllss [labelnum]

expression may be used more than once, to filter on MPLS hierarchies. Each use of that expression increments the filter offsets by 4. For example: mmppllss 110000000000 &&&& mmppllss 11002244 filters packets with an outer label of 100000 and an inner label of 1024, and mmppllss &&&& mmppllss 11002244 &&&& hhoosstt 119922..99..220000..11 filters packets to or from 192.9.200.1 with an inner label of 1024 and any outer label.

ppppppooeedd True if the packet is a PPP-over-Ethernet Discovery

packet (Ethernet type 0x8863).

ppppppooeess True if the packet is a PPP-over-Ethernet Session packet

(Ethernet type 0x8864). Note that the first ppppppooeess key-

word encountered in expression changes the decoding off-

sets for the remainder of expression on the assumption that the packet is a PPPoE session packet. For example: ppppppooeess &&&& iipp filters IPv4 protocols encapsulated in PPPoE. ttccpp, uuddpp, iiccmmpp Abbreviations for: iipp pprroottoo p oorr iipp66 pprroottoo p where p is one of the above protocols. iissoo pprroottoo protocol

True if the packet is an OSI packet of protocol type pro-

tocol. Protocol can be a number or one of the names ccllnnpp, eessiiss, or iissiiss. ccllnnpp, eessiiss, iissiiss Abbreviations for: iissoo pprroottoo p where p is one of the above protocols. ll11, ll22, iiiihh, llsspp, ssnnpp, ccssnnpp, ppssnnpp

Abbreviations for IS-IS PDU types.

vvppii n True if the packet is an ATM packet, for SunATM on Solaris, with a virtual path identifier of n. vvccii n True if the packet is an ATM packet, for SunATM on Solaris, with a virtual channel identifier of n. llaannee True if the packet is an ATM packet, for SunATM on Solaris, and is an ATM LANE packet. Note that the first llaannee keyword encountered in expression changes the tests done in the remainder of expression on the assumption that the packet is either a LANE emulated Ethernet packet or a LANE LE Control packet. If llaannee isn't specified, the tests are done under the assumption that the packet

is an LLC-encapsulated packet.

llllcc True if the packet is an ATM packet, for SunATM on

Solaris, and is an LLC-encapsulated packet.

ooaammff44ss True if the packet is an ATM packet, for SunATM on Solaris, and is a segment OAM F4 flow cell (VPI=0 & VCI=3). ooaammff44ee True if the packet is an ATM packet, for SunATM on

Solaris, and is an end-to-end OAM F4 flow cell (VPI=0 &

VCI=4). ooaammff44 True if the packet is an ATM packet, for SunATM on

Solaris, and is a segment or end-to-end OAM F4 flow cell

(VPI=0 & (VCI=3 | VCI=4)). ooaamm True if the packet is an ATM packet, for SunATM on

Solaris, and is a segment or end-to-end OAM F4 flow cell

(VPI=0 & (VCI=3 | VCI=4)). mmeettaacc True if the packet is an ATM packet, for SunATM on Solaris, and is on a meta signaling circuit (VPI=0 & VCI=1). bbcccc True if the packet is an ATM packet, for SunATM on Solaris, and is on a broadcast signaling circuit (VPI=0 & VCI=2). sscc True if the packet is an ATM packet, for SunATM on Solaris, and is on a signaling circuit (VPI=0 & VCI=5). iillmmiicc True if the packet is an ATM packet, for SunATM on Solaris, and is on an ILMI circuit (VPI=0 & VCI=16). ccoonnnneeccttmmssgg True if the packet is an ATM packet, for SunATM on Solaris, and is on a signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect, Connect Ack, Release, or Release Done message. mmeettaaccoonnnneecctt True if the packet is an ATM packet, for SunATM on Solaris, and is on a meta signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect, Release, or Release Done message. expr relop expr True if the relation holds, where relop is one of >, <,

>=, <=, =, !=, and expr is an arithmetic expression com-

posed of integer constants (expressed in standard C syn-

tax), the normal binary operators [+, -, *, /, &, |, <<,

>>], a length operator, and special packet data acces-

sors. Note that all comparisons are unsigned, so that, for example, 0x80000000 and 0xffffffff are > 0. To access data inside the packet, use the following syntax: proto [[ expr :: size ]] Proto is one of eetthheerr,, ffddddii,, ttrr,, wwllaann,, pppppp,, sslliipp,, lliinnkk,,

iipp,, aarrpp,, rraarrpp,, ttccpp,, uuddpp,, iiccmmpp,, iipp66 or rraaddiioo, and indi-

cates the protocol layer for the index operation. (eetthheerr,, ffddddii,, wwllaann,, ttrr,, pppppp,, sslliipp and lliinnkk all refer to the link layer. rraaddiioo refers to the "radio header" added to some 802.11 captures.) Note that tcp, udp and other

upper-layer protocol types only apply to IPv4, not IPv6

(this will be fixed in the future). The byte offset, relative to the indicated protocol layer, is given by expr. Size is optional and indicates the number of bytes in the field of interest; it can be either one, two, or

four, and defaults to one. The length operator, indi-

cated by the keyword lleenn, gives the length of the packet. For example, `eetthheerr[[00]] && 11 !!== 00' catches all multicast traffic. The expression `iipp[[00]] && 00xxff !!== 55' catches all IPv4 packets with options. The expression `iipp[[66::22]] && 00xx11ffffff == 00' catches only unfragmented IPv4 datagrams and frag zero of fragmented IPv4 datagrams. This check is implicitly applied to the ttccpp and uuddpp index operations. For instance, ttccpp[[00]] always means the first byte of the

TCP header, and never means the first byte of an inter-

vening fragment. Some offsets and field values may be expressed as names rather than as numeric values. The following protocol header field offsets are available: iiccmmppttyyppee (ICMP type field), iiccmmppccooddee (ICMP code field), and ttccppffllaaggss (TCP flags field).

The following ICMP type field values are available: iiccmmpp-

eecchhoorreeppllyy, iiccmmpp-uunnrreeaacchh, iiccmmpp-ssoouurrcceeqquueenncchh, iiccmmpp-rreeddii-

rreecctt, iiccmmpp-eecchhoo, iiccmmpp-rroouutteerraaddvveerrtt, iiccmmpp-rroouutteerrssoolliicciitt,

iiccmmpp-ttiimmxxcceeeedd, iiccmmpp-ppaarraammpprroobb, iiccmmpp-ttssttaammpp, iiccmmpp-ttssttaamm-

pprreeppllyy, iiccmmpp-iirreeqq, iiccmmpp-iirreeqqrreeppllyy, iiccmmpp-mmaasskkrreeqq, iiccmmpp-

mmaasskkrreeppllyy.

The following TCP flags field values are available: ttccpp-

ffiinn, ttccpp-ssyynn, ttccpp-rrsstt, ttccpp-ppuusshh, ttccpp-aacckk, ttccpp-uurrgg.

Primitives may be combined using:

A parenthesized group of primitives and operators (paren-

theses are special to the Shell and must be escaped). Negation (`!!' or `nnoott'). Concatenation (`&&&&' or `aanndd'). Alternation (`||||' or `oorr'). Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit aanndd tokens, not juxtaposition, are now required for concatenation. If an identifier is given without a keyword, the most recent keyword is assumed. For example, nnoott hhoosstt vvss aanndd aaccee is short for nnoott hhoosstt vvss aanndd hhoosstt aaccee which should not be confused with nnoott (( hhoosstt vvss oorr aaccee ))

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. EEXXAAMMPPLLEESS To print all packets arriving at or departing from sundown: ttccppdduummpp hhoosstt ssuunnddoowwnn To print traffic between helios and either hot or ace: ttccppdduummpp hhoosstt hheelliiooss aanndd \\(( hhoott oorr aaccee \\)) To print all IP packets between ace and any host except helios: ttccppdduummpp iipp hhoosstt aaccee aanndd nnoott hheelliiooss To print all traffic between local hosts and hosts at Berkeley:

ttccppdduummpp nneett uuccbb-eetthheerr

To print all ftp traffic through internet gateway snup: (note that the

expression is quoted to prevent the shell from (mis-)interpreting the

parentheses):

ttccppdduummpp ''ggaatteewwaayy ssnnuupp aanndd ((ppoorrtt ffttpp oorr ffttpp-ddaattaa))''

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). ttccppdduummpp iipp aanndd nnoott nneett localnet To print the start and end packets (the SYN and FIN packets) of each

TCP conversation that involves a non-local host.

ttccppdduummpp ''ttccpp[[ttccppffllaaggss]] && ((ttccpp-ssyynn||ttccpp-ffiinn)) !!== 00 aanndd nnoott ssrrcc aanndd ddsstt nneett 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.)

ttccppdduummpp ''ttccpp ppoorrtt 8800 aanndd ((((((iipp[[22::22]] - ((((iipp[[00]]&&00xxff))<<<<22)))) - ((((ttccpp[[1122]]&&00xxff00))>>>>22)))) !!== 00))''

To print IP packets longer than 576 bytes sent through gateway snup: ttccppdduummpp ''ggaatteewwaayy ssnnuupp aanndd iipp[[22::22]] >> 557766''

To print IP broadcast or multicast packets that were not sent via Eth-

ernet broadcast or multicast: ttccppdduummpp ''eetthheerr[[00]] && 11 == 00 aanndd iipp[[1166]] >>== 222244'' To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):

ttccppdduummpp ''iiccmmpp[[iiccmmppttyyppee]] !!== iiccmmpp-eecchhoo aanndd iiccmmpp[[iiccmmppttyyppee]] !!== iiccmmpp-eecchhoorreeppllyy''

OOUUTTPPUUTT FFOORRMMAATT

The output of tcpdump is protocol dependent. The following gives a

brief description and examples of most of the formats. LLiinnkk LLeevveell HHeeaaddeerrss

If the '-e' option is given, the link level header is printed out. On

Ethernets, the source and destination addresses, protocol, and packet length are printed.

On FDDI networks, the '-e' option causes tcpdump to print the `frame

control' field, the source and destination addresses, and the packet length. (The `frame control' field governs the interpretation of the

rest of the packet. Normal packets (such as those containing IP data-

grams) are `async' packets, with a priority value between 0 and 7; for

example, `aassyynncc44'. Such packets are assumed to contain an 802.2 Logi-

cal Link Control (LLC) packet; the LLC header is printed if it is not

an ISO datagram or a so-called SNAP packet.

On Token Ring networks, the '-e' option causes tcpdump to print the

`access control' and `frame control' fields, the source and destination addresses, and the packet length. As on FDDI networks, packets are

assumed to contain an LLC packet. Regardless of whether the '-e'

option is specified or not, the source routing information is printed

for source-routed packets.

On 802.11 networks, the '-e' option causes tcpdump to print the `frame

control' fields, all of the addresses in the 802.11 header, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet.

(N.B.: The following description assumes familiarity with the SLIP com-

pression algorithm described in RFC-1144.)

On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out-

bound), packet type, and compression information are printed out. The packet type is printed first. The three types are ip, utcp, and ctcp.

No further link information is printed for ip packets. For TCP pack-

ets, the connection identifier is printed following the type. If the packet is compressed, its encoded header is printed out. The special cases are printed out as **SS++n and **SSAA++n, where n is the amount by which the sequence number (or sequence number and ack) has changed. If it is not a special case, zero or more changes are printed. A change is

indicated by U (urgent pointer), W (window), A (ack), S (sequence num-

ber), and I (packet ID), followed by a delta (+n or -n), or a new value

(=n). Finally, the amount of data in the packet and compressed header length are printed. For example, the following line shows an outbound compressed TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header: OO ccttccpp ** AA++66 SS++4499 II++66 33 ((66)) AARRPP//RRAARRPP PPaacckkeettss

Arp/rarp output shows the type of request and its arguments. The for-

mat is intended to be self explanatory. Here is a short sample taken from the start of an `rlogin' from host rtsg to host csam:

arp who-has csam tell rtsg

arp reply csam is-at CSAM

The first line says that rtsg sent an arp packet asking for the Ether-

net address of internet host csam. Csam replies with its Ethernet address (in this example, Ethernet addresses are in caps and internet addresses in lower case).

This would look less redundant if we had done tcpdump -n:

arp who-has 128.3.254.6 tell 128.3.254.68

arp reply 128.3.254.6 is-at 02:07:01:00:01:c4

If we had done tcpdump -e, the fact that the first packet is broadcast

and the second is point-to-point would be visible:

RTSG Broadcast 0806 64: arp who-has csam tell rtsg

CSAM RTSG 0806 64: arp reply csam is-at CSAM

For the first packet this says the Ethernet source address is RTSG, the destination is the Ethernet broadcast address, the type field contained hex 0806 (type ETHERARP) and the total length was 64 bytes. TTCCPP PPaacckkeettss

(N.B.:The following description assumes familiarity with the TCP proto-

col described in RFC-793. If you are not familiar with the protocol,

neither this description nor tcpdump will be of much use to you.)

The general format of a tcp protocol line is:

src > dst: flags data-seqno ack window urgent options

Src and dst are the source and destination IP addresses and ports. Flags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), 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., ). 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 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 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 SS 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 ini-

tial 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 sit-

ting in the socket buffer since csam's receive window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg.

If the snapshot was small enough that tcpdump didn't capture the full

TCP header, it interprets as much of the header as it can and then reports ``[|tcp]'' to indicate the remainder could not be interpreted. If the header contains a bogus option (one with a length that's either

too small or beyond the end of the header), tcpdump reports it as

``[bad opt]'' and does not interpret any further options (since it's impossible to tell where they start). If the header length indicates options are present but the IP datagram length is not long enough for

the options to actually be there, tcpdump reports it as ``[bad hdr

length]''.

CCaappttuurriinngg TTCCPP ppaacckkeettss wwiitthh ppaarrttiiccuullaarr ffllaagg ccoommbbiinnaattiioonnss ((SSYYNN-AACCKK,, UURRGG-

AACCKK,, eettcc..)) 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 |

--------|--------|--------|--------

|--------|

|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 ttccpp[[1133]] ==== 22

We can use this expression as the filter for tcpdump in order to watch

packets which have only SYN set:

ttccppdduummpp -ii xxll00 ttccpp[[1133]] ==== 22

The expression says "let the 13th octet of a TCP datagram have the dec-

imal value 2", which is exactly what we want. Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time.

Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set

arrives: |C|E|U|A|P|R|S|F|

|--------|

|0 0 0 1 0 0 1 0|

|--------|

|7 6 5 4 3 2 1 0| Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is 00010010 which translates to decimal 7 6 5 4 3 2 1 0 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18

Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression,

because that would select only those packets that have SYN-ACK set, but

not those with only SYN set. Remember that we don't care if ACK or any other control bit is set as long as SYN is set. In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know that we want SYN to be set in any case, so we'll logically AND the value in the 13th octet with the binary value of a SYN:

00010010 SYN-ACK 00000010 SYN

AND 00000010 (we want SYN) AND 00000010 (we want SYN)

---- ----

= 00000010 = 00000010 We see that this AND operation delivers the same result regardless

whether ACK or another TCP control bit is set. The decimal representa-

tion of the AND value as well as the result of this operation is 2

(binary 00000010), so we know that for packets with SYN set the follow-

ing relation must hold true: ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression

ttccppdduummpp -ii xxll00 ''ttccpp[[1133]] && 22 ==== 22''

Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell. UUDDPP PPaacckkeettss UDP format is illustrated by this rwho packet: actinide.who > broadcast.who: udp 84 This says that port who on host actinide sent a udp datagram to port

who on host broadcast, the Internet broadcast address. The packet con-

tained 84 bytes of user data. Some UDP services are recognized (from the source or destination port

number) and the higher level protocol information printed. In particu-

lar, Domain Name service requests (RFC-1034/1035) and Sun RPC calls

(RFC-1050) to NFS.

UUDDPP NNaammee SSeerrvveerr RReeqquueessttss (N.B.:The following description assumes familiarity with the Domain

Service protocol described in RFC-1035. If you are not familiar with

the protocol, the following description will appear to be written in greek.) Name server requests are formatted as src > dst: id op? flags qtype qclass name (len) h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37) Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucbvax.berkeley.edu. The query id was `3'. The `+' indicates the recursion desired flag was set. The

query length was 37 bytes, not including the UDP and IP protocol head-

ers. The query operation was the normal one, Query, so the op field was omitted. If the op had been anything else, it would have been printed between the `3' and the `+'. Similarly, the qclass was the normal one, CIN, 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. UUDDPP NNaammee SSeerrvveerr RReessppoonnsseess Name server responses are formatted as src > dst: id op rcode flags a/n/au type class data (len) helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273) helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97) In the first example, helios responds to query id 3 from h2opolo with 3 answer records, 3 name server records and 7 additional records. The first answer record is type A (address) and its data is internet address 128.32.137.3. The total size of the response was 273 bytes,

excluding UDP and IP headers. The op (Query) and response code (NoEr-

ror) were omitted, as was the class (CIN) 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. Note that name server requests and responses tend to be large and the default snaplen of 68 bytes may not capture enough of the packet to

print. Use the -ss flag to increase the snaplen if you need to seri-

ously investigate name server traffic. `-ss 112288' has worked well for

me. SSMMBB//CCIIFFSS ddeeccooddiinngg

tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on

UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and Net-

BEUI SMB data is also done. By default a fairly minimal decode is done, with a much more detailed

decode done if -v is used. Be warned that with -v a single SMB packet

may take up a page or more, so only use -v if you really want all the

gory details. For information on SMB packet formats and what all 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). NNFFSS RReeqquueessttss aanndd RReepplliieess Sun NFS (Network File System) requests and replies are printed as: src.xid > dst.nfs: len op args src.nfs > dst.xid: reply stat len op results sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165 wrl.nfs > sushi.6709: reply ok 40 readlink "../var" sushi.201b > wrl.nfs: 144 lookup fh 9,74/4096.6878 "xcolors" wrl.nfs > sushi.201b: reply ok 128 lookup fh 9,74/4134.3150 In the first line, host sushi sends a transaction with id 6709 to wrl (note that the number following the src host is a transaction id, not the source port). The request was 112 bytes, excluding the UDP and IP headers. The operation was a readlink (read symbolic link) on file handle (fh) 21,24/10.731657119. (If one is lucky, as in this case, the file handle can be interpreted as a major,minor device number pair, followed by the inode number and generation number.) Wrl replies `ok' with the contents of the link. In the third line, sushi asks wrl to lookup the name `xcolors' in directory file 9,74/4096.6878. Note that the data printed depends on the operation type. The format is intended to be self explanatory if read in conjunction with an NFS protocol spec.

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

For example: sushi.1372a > wrl.nfs: 148 read fh 21,11/12.195 8192 bytes @ 24576 wrl.nfs > sushi.1372a: reply ok 1472 read REG 100664 ids 417/0 sz 29388

(-v also prints the IP header TTL, ID, length, and fragmentation

fields, which have been omitted from this example.) In the first line,

sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte off-

set 24576. Wrl replies `ok'; the packet shown on the second line is the first fragment of the reply, and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not have NFS or even UDP headers and so might not be printed, depending

on the filter expression used). Because the -v flag is given, some of

the file attributes (which are returned in addition to the file data) are printed: the file type (``REG'', for regular file), the file mode (in octal), the uid and gid, and the file size.

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

Note that NFS requests are very large and much of the detail won't be

printed unless snaplen is increased. Try using `-ss 119922' 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. AAFFSS RReeqquueessttss aanndd RReepplliieess Transarc AFS (Andrew File System) requests and replies are printed as:

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

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

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

elvis.7001 > pike.afsfs: rx data fs call rename old fid 536876964/1/1 ".newsrc.new" new fid 536876964/1/1 ".newsrc" pike.afsfs > elvis.7001: rx data fs reply rename In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call. The RPC call was a rename, with the old directory file id of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory file id of 536876964/1/1 and a new filename of `.newsrc'. The host

pike responds with a RPC reply to the rename call (which was success-

ful, because it was a data packet and not an abort packet). In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the arguments decoded (generally only the `interesting' arguments, for some definition of interesting).

The format is intended to be self-describing, but it will probably not

be useful to people who are not familiar with the workings of AFS and RX.

If the -v (verbose) flag is given twice, acknowledgement packets and

additional header information is printed, such as the 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 `-ss 225566' 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. KKIIPP AApppplleeTTaallkk ((DDDDPP iinn UUDDPP))

AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated

and dumped as DDP packets (i.e., all the UDP header information is dis-

carded). The file /etc/atalk.names is used to translate AppleTalk net and node numbers to names. Lines in this file have the form number name 1.254 ether

16.1 icsd-net

1.254.110 ace The first two lines give the names of AppleTalk networks. The third line gives the name of a particular host (a host is distinguished from

a net by the 3rd octet in the number - a net number must have two

octets and a host number must have three octets.) The number and name should be separated by whitespace (blanks or tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines

starting with a `#').

AppleTalk addresses are printed in the form net.host.port

144.1.209.2 > icsd-net.112.220

office.2 > icsd-net.112.220

jssmag.149.235 > icsd-net.2

(If the /etc/atalk.names doesn't exist or doesn't contain an entry for some AppleTalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (`office'). The third line is a send from port 235 on net jssmag node

149 to broadcast on the icsd-net NBP port (note that the broadcast

address (255) is indicated by a net name with no host number - for this

reason it's a good idea to keep node names and net names distinct in /etc/atalk.names). NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size. NNBBPP ppaacckkeettss 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. AATTPP ppaacckkeett formatting is demonstrated by the following example:

jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001

helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000

jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001

helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000

helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000

jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001

jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002

Jssmag.209 initiates transaction id 12266 with host helios by request-

ing up to 8 packets (the `<0-7>'). The hex number at the end of the

line is the value of the `userdata' field in the request.

Helios responds with 8 512-byte packets. The `:digit' following the

transaction id gives the packet sequence number in the transaction and the number in parens is the amount of data in the packet, excluding the atp header. The `*' on packet 7 indicates that the EOM bit was set. Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios

resends them then jssmag.209 releases the transaction. Finally, jss-

mag.209 initiates the next request. The `*' on the request indicates that XO (`exactly once') was not set. IIPP FFrraaggmmeennttaattiioonn Fragmented Internet datagrams are printed as ((ffrraagg id::size@@offset++)) ((ffrraagg id::size@@offset))

(The first form indicates there are more fragments. The second indi-

cates this is the last fragment.) Id is the fragment id. Size is the fragment size (in bytes) excluding the IP header. Offset is this fragment's offset (in bytes) in the original datagram.

The fragment information is output for each fragment. The first frag-

ment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp

from arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't

appear to handle 576 byte datagrams:

arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)

arizona > rtsg: (frag 595a:204@328)

rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560

There are a couple of things to note here: First, addresses in the 2nd line don't include port numbers. This is because the TCP protocol information is all in the first fragment and we have no idea what the

port or sequence numbers are when we print the later fragments. Sec-

ond, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the second). If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you. A packet with the IP don't fragment flag is marked with a trailing ((DDFF)). TTiimmeessttaammppss

By default, all output lines are preceded by a timestamp. The time-

stamp is the current clock time in the form hh:mm:ss.frac and is as accurate as the kernel's clock. The timestamp reflects the time the kernel first saw the packet. No attempt is made to account for the time lag between when the Ethernet interface removed the packet from the wire and when the kernel serviced the `new packet' interrupt.

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 configuration.

BUGS

Please send problems, bugs, questions, desirable enhancements, etc. to:

tcpdump-workers@tcpdump.org

Please send source code contributions, etc. to:

patches@tcpdump.org

NIT doesn't let you watch your own outbound traffic, BPF will. We rec-

ommend that you use the latter. On Linux systems with 2.0[.x] kernels: packets on the loopback device will be seen twice;

packet filtering cannot be done in the kernel, so that all pack-

ets must be copied from the kernel in order to be filtered in user mode; all of a packet, not just the part that's within the snapshot

length, will be copied from the kernel (the 2.0[.x] packet cap-

ture mechanism, if asked to copy only part of a packet to user-

land, will not report the true length of the packet; this would cause most IP packets to get an error from ttccppdduummpp); capturing on some PPP devices won't work correctly. We recommend that you upgrade to a 2.2 or later kernel. Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher level protocol.

Name server inverse queries are not dumped correctly: the (empty) ques-

tion section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to

fix the program generating them rather than tcpdump.

A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored). Filter expressions on fields other than those in Token Ring headers

will not correctly handle source-routed Token Ring packets.

Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data packets with both To DS and From DS set. iipp66 pprroottoo should chase header chain, but at this moment it does not. iipp66 pprroottoocchhaaiinn is supplied for this behavior. Arithmetic expression against transport layer headers, like ttccpp[[00]], does not work against IPv6 packets. It only looks at IPv4 packets. 18 April 2005 TCPDUMP(1)