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TCPDUMP(1)		    General Commands Manual		    TCPDUMP(1)

       tcpdump - dump traffic on a network

       tcpdump [ -adeflLnNOpqRStuvxX ] [ -c count ]
	       [ -C file_size ]	[ -F file ]
	       [ -i interface ]	[ -m module ] [	-r file	]
	       [ -s snaplen ] [	-T type	] [ -w file ]
	       [ -E algo:secret	] [ -y datalinktype ]
	       [ expression ]

       Tcpdump	prints	out the	headers	of packets on a	network	interface that
       match the boolean expression.  It can also be run  with	the  -w	 flag,
       which  causes  it to save the packet data to a file for later analysis,
       and/or with the -r flag,	which causes it	to read	from  a	 saved	packet
       file  rather  than  to  read  packets from a network interface.	In all
       cases, only packets that	match expression will be processed by tcpdump.

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

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

	      packets  ``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 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, it
       will report those counts	when it	receives a SIGINFO signal  (generated,
       for  example, by	typing your ``status'' character, typically control-T)
       and will	continue capturing packets.

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

       Under SunOS 3.x or 4.x with NIT or BPF:
	      You must have read access	to /dev/nit or /dev/bpf*.

       Under Solaris with DLPI:
	      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.

       Under HP-UX with	DLPI:
	      You must be root or tcpdump must be installed setuid to root.

       Under IRIX with snoop:
	      You must be root or tcpdump must be installed setuid to root.

       Under Linux:
	      You must be root or tcpdump must be installed setuid to root.

       Under Ultrix and	Digital	UNIX:
	      Once the super-user has enabled promiscuous-mode operation using
	      pfconfig(8), any user may	capture	network	traffic	with tcpdump.

       Under BSD:
	      You must have read access	to /dev/bpf*.

       Reading a saved packet file doesn't require special privileges.

       -a     Attempt to convert network and broadcast addresses to names.

       -c     Exit after receiving count packets.

       -C     Before  writing  a  raw  packet to a savefile, check whether the
	      file is currently	larger than file_size and, if  so,  close  the
	      current  savefile	and open a new one.  Savefiles after the first
	      savefile will have the name specified with the -w	flag,  with  a
	      number after it, starting	at 2 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

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

       -E     Use  algo:secret	for  decrypting	IPsec ESP packets.  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  en-
	      abled.   secret  the  ascii  text	for ESP	secret key.  We	cannot
	      take arbitrary binary value at this moment.  The option  assumes
	      RFC2406  ESP, not	RFC1827	ESP.  The option is only for debugging
	      purposes,	and the	use of this option with	truly `secret' key  is
	      discouraged.   By	 presenting IPsec secret key onto command line
	      you make it visible to others, via ps(1) and other occasions.

       -f     Print `foreign' IPv4 addresses numerically rather	than  symboli-
	      cally  (this option is intended to get around serious brain dam-
	      age in Sun's NIS server -- usually it hangs forever  translating
	      non-local	internet numbers).

       -F     Use  file	as input for the filter	expression.  An	additional ex-
	      pression given on	the command line is ignored.

       -i     Listen on	interface.  If unspecified, tcpdump searches the  sys-
	      tem interface list for the lowest	numbered, configured up	inter-
	      face (excluding loopback).  Ties are broken by choosing the ear-
	      liest match.

	      On  Linux	 systems with 2.2 or later kernels, an interface argu-
	      ment of ``any'' can be used to capture packets from  all	inter-
	      faces.   Note  that  captures  on	the ``any'' device will	not be
	      done in promiscuous mode.

       -l     Make stdout line buffered.  Useful if you	want to	see  the  data
	      while capturing it.  E.g.,
	      ``tcpdump	 -l  |	tee	dat''	  or	 ``tcpdump  -l	     >
	      dat  &  tail  -f	dat''.

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

       -m     Load SMI MIB module definitions from file	module.	  This	option
	      can  be used several times to load several MIB modules into tcp-

       -n     Don't convert addresses (i.e.,  host  addresses,	port  numbers,
	      etc.) to names.

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

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

       -p     Don't put	the interface into promiscuous mode.   Note  that  the
	      interface	 might	be  in promiscuous mode	for some other reason;
	      hence, `-p' cannot be used as an abbreviation  for  `ether  host
	      {local-hw-addr} or ether broadcast'.

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

       -R     Assume ESP/AH packets to be based	on old specification  (RFC1825
	      to  RFC1829).   If specified, tcpdump will not print replay pre-
	      vention field.  Since there is  no  protocol  version  field  in
	      ESP/AH  specification,  tcpdump  cannot  deduce  the  version of
	      ESP/AH protocol.

       -r     Read packets from	file (which was	created	with the  -w  option).
	      Standard input is	used if	file is	``-''.

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

       -s     Snarf snaplen bytes of data from each packet rather than the de-
	      fault 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  be-
	      low).  Packets truncated because of a limited snapshot are indi-
	      cated 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-

       -T     Force packets selected by	"expression"  to  be  interpreted  the
	      specified	 type.	 Currently known types are cnfp	(Cisco NetFlow
	      protocol), rpc (Remote Procedure Call), rtp (Real-Time  Applica-
	      tions protocol), rtcp (Real-Time Applications control protocol),
	      snmp (Simple Network Management  Protocol),  vat	(Visual	 Audio
	      Tool), and wb (distributed White Board).

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

       -tt    Print an unformatted timestamp on	each dump line.

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

       -tttt  Print a timestamp	in default format proceeded by	date  on  each
	      dump line.

       -u     Print undecoded NFS handles.

       -v     (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.

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

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

       -w     Write the	raw packets to file rather than	parsing	 and  printing
	      them  out.  They can later be printed with the -r	option.	 Stan-
	      dard output is used if file is ``-''.

       -x     Print each packet	(minus its link	level  header)	in  hex.   The
	      smaller of the entire packet or snaplen bytes will be printed.

       -X     When printing hex, print ascii too.  Thus	if -x is also set, the
	      packet  is  printed  in  hex/ascii.   This  is  very  handy  for
	      analysing	new protocols.	Even if	-x is not also set, some parts
	      of some packets may be printed in	hex/ascii.

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

	      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 host, net and port.	 E.g.,
		     `host  foo', `net 128.3', `port 20'.  If there is no type
		     qualifier,	host is	assumed.

	      dir    qualifiers	specify	a  particular  transfer	 direction  to
		     and/or from id.  Possible directions are src, dst,	src or
		     dst and src and dst.  E.g., `src foo', `dst  net  128.3',
		     `src  or  dst  port ftp-data'.  If	there is no dir	quali-
		     fier, src or dst is  assumed.   For  `null'  link	layers
		     (i.e.  point to point protocols such as slip) the inbound
		     and outbound qualifiers can be used to specify a  desired

	      proto  qualifiers	 restrict  the match to	a particular protocol.
		     Possible protos are: ether, fddi, tr, ip, ip6, arp, rarp,
		     decnet,  lat,  sca,  moprc, mopdl,	iso, esis, isis, icmp,
		     icmp6, tcp	and udp.  E.g.,	 `ether	 src  foo',  `arp  net
		     128.3',  `tcp  port 21'.  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' (ex-
		     cept 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'	is  an	alias  for `ether'; the	previous para-
	      graph's statements about FDDI headers also apply to  Token  Ring

	      In  addition  to	the  above, there are some special `primitive'
	      keywords that don't  follow  the	pattern:  gateway,  broadcast,
	      less,  greater and arithmetic expressions.  All of these are de-
	      scribed below.

	      More complex filter expressions are built	up by using the	 words
	      and,  or and not 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:

	      dst host host
		     True  if  the  IPv4/v6 destination	field of the packet is
		     host, which may be	either an address or a name.

	      src host host
		     True if the IPv4/v6 source	field of the packet is host.

	      host 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, ip, arp, rarp, or ip6	as in:
			  ip host host
		     which is equivalent to:
			  ether	proto \ip and host host
		     If	host is	a name with multiple IP	 addresses,  each  ad-
		     dress will	be checked for a match.

	      ether dst	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).

	      ether src	ehost
		     True if the ethernet source address is ehost.

	      ether host ehost
		     True if either the	ethernet source	or destination address
		     is	ehost.

	      gateway 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
			  ether	host ehost and not host	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.

	      dst net 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

	      src net net
		     True if the IPv4/v6 source	address	of the	packet	has  a
		     network number of net.

	      net net
		     True  if either the IPv4/v6 source	or destination address
		     of	the packet has a network number	of net.

	      net net mask netmask
		     True if the IP address matches net	with the specific net-
		     mask.   May be qualified with src or dst.	Note that this
		     syntax is not valid for IPv6 net.

	      net net/len
		     True if the IPv4/v6 address matches net  with  a  netmask
		     len bits wide.  May be qualified with src or dst.

	      dst port 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., dst  port
		     513  will	print both tcp/login traffic and udp/who traf-
		     fic, and port  domain  will  print	 both  tcp/domain  and
		     udp/domain	traffic).

	      src port port
		     True if the packet	has a source port value	of port.

	      port port
		     True  if  either  the  source  or destination port	of the
		     packet is port.  Any of the above port expressions	can be
		     prepended with the	keywords, tcp or udp, as in:
			  tcp src port port
		     which matches only	tcp packets whose source port is port.

	      less length
		     True  if  the  packet  has	a length less than or equal to
		     length.  This is equivalent to:
			  len <= length.

	      greater length
		     True if the packet	has a length greater than or equal  to
		     length.  This is equivalent to:
			  len >= length.

	      ip proto protocol
		     True if the packet	is an IP packet	(see ip(4P)) of	proto-
		     col type protocol.	 Protocol can be a number  or  one  of
		     the  names	 icmp,	icmp6, igmp, igrp, pim,	ah, esp, vrrp,
		     udp, or tcp.  Note	that the  identifiers  tcp,  udp,  and
		     icmp  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.

	      ip6 proto	protocol
		     True  if  the  packet  is an IPv6 packet of protocol type
		     protocol.	Note that this primitive does  not  chase  the
		     protocol header chain.

	      ip6 protochain protocol
		     True  if the packet is IPv6 packet, and contains protocol
		     header with type protocol in its protocol	header	chain.
		     For example,
			  ip6 protochain 6
		     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.

	      ip protochain protocol
		     Equivalent	 to  ip6  protochain protocol, but this	is for

	      ether broadcast
		     True if the packet	is an ethernet broadcast packet.   The
		     ether keyword is optional.

	      ip broadcast
		     True  if the packet is an IP broadcast packet.  It	checks
		     for both the all-zeroes and  all-ones  broadcast  conven-
		     tions, and	looks up the local subnet mask.

	      ether multicast
		     True  if the packet is an ethernet	multicast packet.  The
		     ether  keyword  is	 optional.   This  is  shorthand   for
		     `ether[0] & 1 != 0'.

	      ip multicast
		     True if the packet	is an IP multicast packet.

	      ip6 multicast
		     True if the packet	is an IPv6 multicast packet.

	      ether proto protocol
		     True  if  the packet is of	ether type protocol.  Protocol
		     can be a number or	one of the names ip, ip6,  arp,	 rarp,
		     atalk,  aarp,  decnet,  sca, lat, mopdl, moprc, iso, stp,
		     ipx, or netbeui.  Note these identifiers  are  also  key-
		     words and must be escaped via backslash (\).

		     [In  the case of FDDI (e.g., `fddi	protocol arp') and To-
		     ken Ring (e.g., `tr protocol arp'),  for  most  of	 those
		     protocols,	 the  protocol	identification	comes from the
		     802.2 Logical Link	Control	(LLC) header, which is usually
		     layered on	top of the FDDI	or Token Ring header.

		     When  filtering  for most protocol	identifiers on FDDI or
		     Token Ring, tcpdump checks	only the protocol ID field  of
		     an	 LLC header in so-called SNAP format with an Organiza-
		     tional Unit Identifier (OUI) of  0x000000,	 for  encapsu-
		     lated Ethernet; it	doesn't	check whether the packet is in
		     SNAP format with an OUI of	0x000000.

		     The exceptions are	iso, for  which	 it  checks  the  DSAP
		     (Destination  Service Access Point) and SSAP (Source Ser-
		     vice Access Point)	fields of the LLC header, stp and net-
		     beui,  where  it  checks  the DSAP	of the LLC header, and
		     atalk, where it 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
		     iso,  sap,	 and netbeui, for which	it checks for an 802.3
		     frame and then checks the LLC header as it	does for  FDDI
		     and  Token	 Ring, atalk, where it checks both for the Ap-
		     pletalk etype in an Ethernet frame	and for	a  SNAP-format
		     packet as it does for FDDI	and Token Ring,	aarp, where it
		     checks for	the Appletalk ARP etype	in either an  Ethernet
		     frame or an 802.2 SNAP frame with an OUI of 0x000000, and
		     ipx, where	it 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.]

	      decnet src 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.]

	      decnet dst host
		     True if the DECNET	destination address is host.

	      decnet host host
		     True if either the	DECNET source or  destination  address
		     is	host.

	      ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
		     Abbreviations for:
			  ether	proto p
		     where p is	one of the above protocols.

	      lat, moprc, mopdl
		     Abbreviations for:
			  ether	proto p
		     where p is	one of the above protocols.  Note that tcpdump
		     does not currently	know how to parse these	protocols.

	      vlan [vlan_id]
		     True if the packet	is an IEEE  802.1Q  VLAN  packet.   If
		     [vlan_id]	is  specified, only true is the	packet has the
		     specified vlan_id.	 Note that the first vlan keyword  en-
		     countered	in expression changes the decoding offsets for
		     the remainder of expression on the	 assumption  that  the
		     packet is a VLAN packet.

	      tcp, udp,	icmp
		     Abbreviations for:
			  ip proto p or	ip6 proto p
		     where p is	one of the above protocols.

	      iso proto	protocol
		     True if the packet	is an OSI packet of protocol type pro-
		     tocol.  Protocol can be a number  or  one	of  the	 names
		     clnp, esis, or isis.

	      clnp, esis, isis
		     Abbreviations for:
			  iso proto p
		     where p is	one of the above protocols.  Note that tcpdump
		     does an incomplete	job of parsing these protocols.

	      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  accessors.   To
		     access data inside	the packet, use	the following syntax:
			  proto	[ expr : size ]
		     Proto is one of ether, fddi, tr, ip, arp, rarp, tcp, udp,
		     icmp or ip6, and indicates	the protocol layer for the in-
		     dex  operation.  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  op-
		     tional  and indicates the number of bytes in the field of
		     interest; it can be either	one, two,  or  four,  and  de-
		     faults  to	 one.	The  length operator, indicated	by the
		     keyword len, gives	the length of the packet.

		     For example, `ether[0] & 1	!= 0'  catches	all  multicast
		     traffic.	The  expression	`ip[0] & 0xf !=	5' catches all
		     IP	packets	 with  options.	  The  expression  `ip[6:2]  &
		     0x1fff  = 0' catches only unfragmented datagrams and frag
		     zero of fragmented	datagrams.  This check	is  implicitly
		     applied  to  the  tcp  and	udp index operations.  For in-
		     stance, tcp[0] always means the first  byte  of  the  TCP
		     header,  and never	means the first	byte of	an intervening

		     Some offsets and field values may be expressed  as	 names
		     rather  than  as  numeric values.	The following protocol
		     header field offsets are available: icmptype  (ICMP  type
		     field),  icmpcode	(ICMP  code  field), and tcpflags (TCP
		     flags field).

		     The following ICMP	type field values are available: icmp-
		     echoreply,	 icmp-unreach,	icmp-sourcequench,  icmp-redi-
		     rect, icmp-echo,  icmp-routeradvert,  icmp-routersolicit,
		     icmp-timxceed,  icmp-paramprob,  icmp-tstamp, icmp-tstam-
		     preply, icmp-ireq,	 icmp-ireqreply,  icmp-maskreq,	 icmp-

		     The  following TCP	flags field values are available: tcp-
		     fin, tcp-syn, tcp-rst, tcp-push, tcp-push,	tcp-ack,  tcp-

	      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 `not').

		     Concatenation (`&&' or `and').

		     Alternation (`||' or `or').

	      Negation has highest precedence.	Alternation and	 concatenation
	      have  equal  precedence  and associate left to right.  Note that
	      explicit and tokens, not juxtaposition,  are  now	 required  for

	      If  an  identifier  is  given without a keyword, the most	recent
	      keyword is assumed.  For example,
		   not host vs and ace
	      is short for
		   not host vs and host	ace
	      which should not be confused with
		   not ( host vs or ace	)

	      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 ar-
	      guments are concatenated with spaces before being	parsed.

       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
	      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	IP packets longer than 576 bytes sent through gateway snup:
	      tcpdump 'gateway snup and	ip[2:2]	> 576'

       To  print IP broadcast or multicast packets that	were not sent via eth-
       ernet broadcast or multicast:
	      tcpdump 'ether[0]	& 1 = 0	and ip[16] >= 224'

       To print	all ICMP packets that are not echo requests/replies (i.e., not
       ping packets):
	      tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

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

       Link Level Headers

       If the '-e' option is given, the	link level header is printed out.   On
       ethernets,  the	source and destination addresses, protocol, and	packet
       length are printed.

       On FDDI networks, the  '-e' option causes tcpdump to print  the	`frame
       control'	 field,	  the source and destination addresses,	and the	packet
       length.	(The `frame control' field governs the interpretation  of  the
       rest  of	the packet.  Normal packets (such as those containing IP data-
       grams) are `async' packets, with	a priority value between 0 and 7;  for
       example,	 `async4'.  Such packets are assumed to	contain	an 802.2 Logi-
       cal Link	Control	(LLC) packet; the LLC header is	printed	if it  is  not
       an ISO datagram or a so-called SNAP packet.

       On  Token  Ring	networks,  the '-e' option causes tcpdump to print the
       `access control'	and `frame control' fields, the	source and destination
       addresses, and the packet length.  As on	FDDI networks, packets are as-
       sumed 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.

       (N.B.: The following description	assumes	familiarity with the SLIP com-
       pression	algorithm described in RFC-1144.)

       On SLIP links, a	direction indicator (``I'' for inbound,	``O'' for out-
       bound), packet type, and	compression information	are printed out.   The
       packet  type is printed first.  The three types are ip, utcp, and ctcp.
       No further link information is printed for ip packets.  For  TCP	 pack-
       ets,  the  connection identifier	is printed following the type.	If the
       packet is compressed, its encoded header	is printed out.	  The  special
       cases are printed out as	*S+n and *SA+n,	where n	is the amount by which
       the sequence number (or sequence	number and ack)	has changed.  If it is
       not  a special case, zero or more changes are printed.  A change	is in-
       dicated by U (urgent pointer), W	(window), A (ack),  S  (sequence  num-
       ber), and I (packet ID),	followed by a delta (+n	or -n),	or a new value
       (=n).  Finally, the amount of data in the packet	and compressed	header
       length are printed.

       For  example,  the  following  line  shows  an  outbound	compressed TCP
       packet, with an implicit	connection identifier; the ack has changed  by
       6, the sequence number by 49, and the packet ID by 6; there are 3 bytes
       of data and 6 bytes of compressed header:
	      O	ctcp * A+6 S+49	I+6 3 (6)

       ARP/RARP	Packets

       Arp/rarp	output shows the type of request and its arguments.  The  for-
       mat  is	intended to be self explanatory.  Here is a short sample taken
       from the	start of an `rlogin' from host rtsg to host csam:
	      arp who-has csam tell rtsg
	      arp reply	csam is-at CSAM
       The first line says that	rtsg sent an arp packet	asking for the	ether-
       net  address of internet	host csam.  Csam replies with its ethernet ad-
       dress (in this example, ethernet	addresses are in caps and internet ad-
       dresses in lower	case).

       This would look less redundant if we had	done tcpdump -n:
	      arp who-has tell
	      arp reply is-at 02:07:01:00:01:c4

       If  we had done tcpdump -e, the fact that the first packet is broadcast
       and the second is point-to-point	would be visible:
	      RTSG Broadcast 0806  64: arp who-has csam	tell rtsg
	      CSAM RTSG	0806  64: arp reply csam is-at CSAM
       For the first packet this says the ethernet source address is RTSG, the
       destination is the ethernet broadcast address, the type field contained
       hex 0806	(type ETHER_ARP) and the total length was 64 bytes.

       TCP Packets

       (N.B.:The following description assumes familiarity with	the TCP	proto-
       col  described  in RFC-793.  If you are not familiar with the protocol,
       neither this description	nor tcpdump will be of much use	to you.)

       The general format of a tcp protocol line is:
	      src _ dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP	addresses  and	ports.
       Flags  are some combination of S	(SYN), F (FIN),	P (PUSH) or R (RST) or
       a single	`.' (no	flags).	 Data-seqno describes the portion of  sequence
       space  covered  by the data in this packet (see example below).	Ack is
       sequence	number of the next data	expected the other direction  on  this
       connection.   Window  is	 the  number  of bytes of receive buffer space
       available the other direction on	this connection.  Urg indicates	 there
       is  `urgent'  data  in the packet.  Options are tcp options enclosed in
       angle brackets (e.g., <mss 1024>).

       Src, dst	and flags are always present.  The other fields	depend on  the
       contents	of the packet's	tcp protocol header and	are output only	if ap-

       Here is the opening portion of an rlogin	from host rtsg to host csam.
	      rtsg.1023	> csam.login: S	768512:768512(0) win 4096 <mss 1024>
	      csam.login > rtsg.1023: S	947648:947648(0) ack 768513 win	4096 <mss 1024>
	      rtsg.1023	> csam.login: .	ack 1 win 4096
	      rtsg.1023	> csam.login: P	1:2(1) ack 1 win 4096
	      csam.login > rtsg.1023: .	ack 2 win 4096
	      rtsg.1023	> csam.login: P	2:21(19) ack 1 win 4096
	      csam.login > rtsg.1023: P	1:2(1) ack 21 win 4077
	      csam.login > rtsg.1023: P	2:3(1) ack 21 win 4077 urg 1
	      csam.login > rtsg.1023: P	3:4(1) ack 21 win 4077 urg 1
       The first line says that	tcp port 1023 on rtsg sent a  packet  to  port
       login  on csam.	The S indicates	that the SYN flag was set.  The	packet
       sequence	number was 768512 and it contained no data.  (The notation  is
       `first:last(nbytes)'  which means `sequence numbers first up to but not
       including last which is nbytes bytes of	user  data'.)	There  was  no
       piggy-backed ack, the available receive window was 4096 bytes and there
       was a max-segment-size option requesting	an mss of 1024 bytes.

       Csam replies with a similar packet except it  includes  a  piggy-backed
       ack for rtsg's SYN.  Rtsg then acks csam's SYN.	The `.'	means no flags
       were set.  The packet contained no data so there	is  no	data  sequence
       number.	Note that the ack sequence number is a small integer (1).  The
       first time tcpdump sees a tcp `conversation', it	 prints	 the  sequence
       number from the packet.	On subsequent packets of the conversation, the
       difference between the current packet's sequence	number and  this  ini-
       tial  sequence number is	printed.  This means that sequence numbers af-
       ter the first can be interpreted	as relative byte positions in the con-
       versation'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 re-
       ports ``[|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

       Capturing  TCP packets with particular flag combinations	(SYN-ACK, URG-
       ACK, etc.)

       There are 8 bits	in the control bits section of the TCP header:

	      CWR | ECE	| URG |	ACK | PSH | RST	| SYN |	FIN

       Let's assume that we want to watch packets used in establishing	a  TCP
       connection.   Recall  that  TCP uses a 3-way handshake protocol when it
       initializes a new connection; the connection sequence  with  regard  to
       the TCP control bits is

	      1) Caller	sends SYN
	      2) Recipient responds with SYN, ACK
	      3) Caller	sends ACK

       Now  we're  interested  in capturing packets that have only the SYN bit
       set (Step 1).  Note that	we don't want packets from step	 2  (SYN-ACK),
       just  a plain initial SYN.  What	we need	is a correct filter expression
       for tcpdump.

       Recall the structure of a TCP header without options:

	0			     15				     31
       |	  source port	       |       destination port	       |
       |			sequence number			       |
       |		     acknowledgment number		       |
       |  HL   | rsvd  |C|E|U|A|P|R|S|F|	window size	       |
       |	 TCP checksum	       |       urgent pointer	       |

       A TCP header usually holds  20  octets  of  data,  unless  options  are
       present.	 The first line	of the graph contains octets 0 - 3, the	second
       line shows octets 4 - 7 etc.

       Starting	to count with 0, the relevant TCP control bits	are  contained
       in octet	13:

	0	      7|	     15|	     23|	     31
       |  HL   | rsvd  |C|E|U|A|P|R|S|F|	window size	       |
       |	       |  13th octet   |	       |	       |

       Let's have a closer look	at octet no. 13:

		       |	       |
		       |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:

		       |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


       and its decimal representation is

	  7	6     5	    4	  3	2     1	    0
       0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2  =	 2

       We're  almost  done,  because  now we know that if only SYN is set, the
       value of	the 13th octet in the TCP header, when interpreted as a	 8-bit
       unsigned	integer	in network byte	order, must be exactly 2.

       This relationship can be	expressed as
	      tcp[13] == 2

       We  can use this	expression as the filter for tcpdump in	order to watch
       packets which have only SYN set:
	      tcpdump -i xl0 tcp[13] ==	2

       The expression says "let	the 13th octet of a TCP	datagram have the dec-
       imal value 2", which is exactly what we want.

       Now,  let's  assume  that  we need to capture SYN packets, but we don't
       care if ACK or any other	TCP control bit	 is  set  at  the  same	 time.
       Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set

	    |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


       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 (bi-
       nary 00000010), so we know that for packets with	SYN set	the  following
       relation	must hold true:

	      (	( value	of octet 13 ) AND ( 2 )	) == ( 2 )

       This points us to the tcpdump filter expression
		   tcpdump -i xl0 'tcp[13] & 2 == 2'

       Note that you should use	single quotes or a backslash in	the expression
       to hide the AND ('&') special character from the	shell.

       UDP Packets

       UDP format is illustrated by this rwho packet:
	      actinide.who > broadcast.who: udp	84
       This says that port who on host actinide	sent a udp  datagram  to  port
       who on host broadcast, the Internet broadcast address.  The packet con-
       tained 84 bytes of user data.

       Some UDP	services are recognized	(from the source or  destination  port
       number) and the higher level protocol information printed.  In particu-
       lar, Domain Name	service	requests (RFC-1034/1035)  and  Sun  RPC	 calls
       (RFC-1050) to NFS.

       UDP Name	Server Requests

       (N.B.:The  following  description  assumes  familiarity with the	Domain
       Service protocol	described in RFC-1035.	If you are not	familiar  with
       the  protocol,  the  following description will appear to be written in

       Name server requests are	formatted as
	      src _ dst: id op?	flags qtype qclass name	(len)
	      h2opolo.1538 > helios.domain: 3+ A? (37)
       Host h2opolo asked the domain server on helios for  an  address	record
       (qtype=A)  associated  with the name  The query id
       was `3'.	 The `+' indicates the recursion desired flag  was  set.   The
       query  length was 37 bytes, not including the UDP and IP	protocol head-
       ers.  The query operation was the normal	one, Query, so	the  op	 field
       was  omitted.   If  the	op  had	been anything else, it would have been
       printed between the `3' and the `+'.  Similarly,	 the  qclass  was  the
       normal  one,  C_IN,  and	 omitted.   Any	 other	qclass would have been
       printed immediately after the `A'.

       A few anomalies are checked and may result in extra fields enclosed  in
       square  brackets:   If a	query contains an answer, authority records or
       additional records section, ancount, nscount, or	arcount	are printed as
       `[na]', `[nn]' or  `[nau]' where	n is the appropriate count.  If	any of
       the response bits are set (AA, RA or rcode) or  any  of	the  `must  be
       zero' bits are set in bytes two and three, `[b2&3=x]' is	printed, where
       x is the	hex value of header bytes two and three.

       UDP Name	Server Responses

       Name server responses are formatted as
	      src _ dst:  id op	rcode flags a/n/au type	class data (len)
	      helios.domain > h2opolo.1538: 3 3/3/7 A (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  ad-
       dress  The total size of the response was	273 bytes, ex-
       cluding 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

       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  -s flag to increase the snaplen if you need to seri-
       ously investigate name server traffic.  `-s 128'	has  worked  well  for

       SMB/CIFS	decoding

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

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

       If you are decoding SMB sessions	containing unicode  strings  then  you
       may  wish to set	the environment	variable USE_UNICODE to	1.  A patch to
       auto-detect unicode srings would	be welcome.

       For information on SMB packet formats and what all te fields  mean  see  or	 the  pub/samba/specs/	directory  on  your  favourite mirror	site.  The SMB patches were written by Andrew Tridgell

       NFS Requests and	Replies

       Sun NFS (Network	File System) requests and replies are printed as:
	      src.xid _	dst.nfs: len op	args
	      src.nfs _	dst.xid: reply stat len	op results
	      sushi.6709 > wrl.nfs: 112	readlink fh 21,24/10.73165
	      wrl.nfs >	sushi.6709: reply ok 40	readlink "../var"
	      sushi.201b > wrl.nfs:
		   144 lookup fh 9,74/4096.6878	"xcolors"
	      wrl.nfs >	sushi.201b:
		   reply ok 128	lookup fh 9,74/4134.3150
       In  the	first line, host sushi sends a transaction with	id 6709	to wrl
       (note that the number following the src host is a transaction  id,  not
       the  source port).  The request was 112 bytes, excluding	the UDP	and IP
       headers.	 The operation was a readlink (read  symbolic  link)  on  file
       handle (fh) 21,24/10.731657119.	(If one	is lucky, as in	this case, the
       file handle can be interpreted as a  major,minor	 device	 number	 pair,
       followed	 by the	inode number and generation number.)  Wrl replies `ok'
       with the	contents of the	link.

       In the third line, sushi	asks wrl to lookup the name `xcolors'  in  di-
       rectory file 9,74/4096.6878.  Note that the data	printed	depends	on the
       operation type.	The format is intended to be self explanatory if  read
       in conjunction with an NFS protocol spec.

       If  the	-v (verbose) flag is given, additional information is printed.
       For example:
	      sushi.1372a > wrl.nfs:
		   148 read fh 21,11/12.195 8192 bytes @ 24576
	      wrl.nfs >	sushi.1372a:
		   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v also	prints the  IP	header	TTL,  ID,  length,  and	 fragmentation
       fields, which have been omitted from this example.)  In the first line,
       sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte  off-
       set  24576.   Wrl  replies `ok';	the packet shown on the	second line is
       the first fragment of the reply,	and hence is only 1472 bytes long (the
       other bytes will	follow in subsequent fragments,	but these fragments do
       not have	NFS or even UDP	headers	and so might not be printed, depending
       on  the filter expression used).	 Because the -v	flag is	given, some of
       the file	attributes (which are returned in addition to the  file	 data)
       are  printed:  the file type (``REG'', for regular file), the file mode
       (in octal), the uid and gid, and	the file size.

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

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

       NFS reply packets do not	explicitly identify the	 RPC  operation.   In-
       stead,  tcpdump keeps track of ``recent'' requests, and matches them to
       the replies using the transaction ID.  If a reply does not closely fol-
       low the corresponding request, it might not be parsable.

       AFS Requests and	Replies

       Transarc	AFS (Andrew File System) requests and replies are printed as:	_ dst.dport: rx	packet-type	_ dst.dport: rx	packet-type service call call-name args	_ dst.dport: rx	packet-type service reply call-name args
	      elvis.7001 > pike.afsfs:
		   rx data fs call rename old fid 536876964/1/1	""
		   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 `', 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

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

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

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

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

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

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

       KIP Appletalk (DDP in UDP)

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

	      1.254	     ether
	      16.1	icsd-net
	      1.254.110	ace
       The first two lines give	the names of appletalk	networks.   The	 third
       line  gives the name of a particular host (a host is distinguished from
       a net by	the 3rd	octet in the number -  a  net  number  must  have  two
       octets  and a host number must have three octets.)  The number and name
       should  be   separated	by   whitespace	  (blanks   or	 tabs).	   The
       /etc/atalk.names	 file  may contain blank lines or comment lines	(lines
       starting	with a `#').

       Appletalk addresses are printed in the form > 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 (`of-
       fice').	 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

       NBP  (name  binding  protocol) and ATP (Appletalk transaction protocol)
       packets have their contents interpreted.	 Other protocols just dump the
       protocol	name (or number	if no name is registered for the protocol) and
       packet size.

       NBP packets are formatted like the following examples:
	      icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
	      jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
	      techpit.2	> icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
       The first line is a name	lookup request for laserwriters	 sent  by  net
       icsd  host  112 and broadcast on	net jssmag.  The nbp id	for the	lookup
       is 190.	The second line	shows a	reply for this request (note  that  it
       has  the	same id) from host jssmag.209 saying that it has a laserwriter
       resource	named "RM1140" registered on port 250.	The third line is  an-
       other  reply  to	 the  same request saying host techpit has laserwriter
       "techpit" registered on port 186.

       ATP packet formatting is	demonstrated by	the following example:
	      jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
	      helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
	      jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
	      helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
	      jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
	      jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
       Jssmag.209 initiates transaction	id 12266 with host helios by  request-
       ing  up	to  8 packets (the `<0-7>').  The hex number at	the end	of the
       line is the value of the	`userdata' field in the	request.

       Helios responds with 8 512-byte packets.	 The  `:digit'	following  the
       transaction  id gives the packet	sequence number	in the transaction and
       the number in parens is the amount of data in the packet, excluding the
       atp header.  The	`*' on packet 7	indicates that the EOM bit was set.

       Jssmag.209  then	 requests that packets 3 & 5 be	retransmitted.	Helios
       resends them then jssmag.209 releases the transaction.	Finally,  jss-
       mag.209	initiates  the next request.  The `*' on the request indicates
       that XO (`exactly once')	was not	set.

       IP Fragmentation

       Fragmented Internet datagrams are printed as
	      (frag id:size@offset+)
	      (frag id:size@offset)
       (The first form indicates there are more	fragments.  The	 second	 indi-
       cates this is the last fragment.)

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

       The  fragment information is output for each fragment.  The first frag-
       ment contains the higher	level protocol header and  the	frag  info  is
       printed	after the protocol info.  Fragments after the first contain no
       higher level protocol header and	the frag info  is  printed  after  the
       source  and destination addresses.  For example,	here is	part of	an ftp
       from	to 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  in-
       formation  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


       By default, all output lines are	preceded by a  timestamp.   The	 time-
       stamp is	the current clock time in the form
       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.

       bpf(4), pcap(3)

       The original authors are:

       Van Jacobson, Craig Leres and  Steven  McCanne,	all  of	 the  Lawrence
       Berkeley	National Laboratory, University	of California, Berkeley, CA.

       It is currently being maintained	by

       The current version is available	via http:

       The original distribution is available via anonymous ftp:

       IPv6/IPsec  support  is	added by WIDE/KAME project.  This program uses
       Eric Young's SSLeay library, under specific configuration.

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

       Please send source code contributions, etc. to:

       NIT doesn't let you watch your own outbound traffic, BPF	will.  We rec-
       ommend that you use the latter.

       On Linux	systems	with 2.0[.x] kernels:

	      packets on the loopback device will be seen twice;

	      packet filtering cannot be done in the kernel, so	that all pack-
	      ets must be copied from the kernel in order to  be  filtered  in
	      user mode;

	      all  of  a  packet, not just the part that's within the snapshot
	      length, will be copied from the kernel (the 2.0[.x] packet  cap-
	      ture  mechanism, if asked	to copy	only part of a packet to user-
	      land, will not report the	true length of the packet; this	 would
	      cause most IP packets to get an error from tcpdump).

       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 that manipulate FDDI or Token	 Ring  headers	assume
       that  all  FDDI	and  Token Ring	packets	are SNAP-encapsulated Ethernet
       packets.	 This is true for IP, ARP, and DECNET Phase  IV,  but  is  not
       true  for  protocols such as ISO	CLNS.  Therefore, the filter may inad-
       vertently accept	certain	packets	that do	not properly match the	filter

       Filter  expressions  on	fields	other than those that manipulate Token
       Ring headers will not correctly handle source-routed Token  Ring	 pack-

       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.

				3 January 2001			    TCPDUMP(1)


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