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TCPDUMP(1)		    General Commands Manual		    TCPDUMP(1)

       tcpdump - dump traffic on a network

       tcpdump [ -adeflnNOpqRStuvxX ] [	-c count ]
	       [ -C file_size ]	[ -F file ]
	       [ -i interface ]	[ -m module ] [	-r file	]
	       [ -s snaplen ] [	-T type	] [ -w file ]
	       [ -E algo:secret	] [ 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 -b 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' internet addresses numerically rather than  sym-
	      bolically	 (this	option is intended to get around serious brain
	      damage in	Sun's yp server	-- usually it hangs forever  translat-
	      ing 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''.

       -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.

	      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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