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TCPDUMP(1)		    General Commands Manual		    TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AdDeflLnNOpqRStuUvxX ] [ -c count ]
	       [ -C file_size ]	[ -F file ]
	       [ -i interface ]	[ -m module ] [	-M secret ]
	       [ -r file ] [ -s	snaplen	] [ -T type ] [	-w file	]
	       [ -W filecount ]
	       [ -E spi@ipaddr algo:secret,...	]
	       [ -y datalinktype ] [ -Z	user ]
	       [ -y datalinktype ]
	       [ expression ]

DESCRIPTION
       Tcpdump	prints	out the	headers	of packets on a	network	interface that
       match the boolean expression.  It can also be run  with	the  -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 ``captured'' (this is the	number of packets that tcpdump
	      has received and processed);

	      packets ``received by filter'' (the meaning of this  depends  on
	      the  OS on which you're running tcpdump, and possibly on the way
	      the OS was configured - if a filter was specified	on the command
	      line,  on	some OSes it counts packets regardless of whether they
	      were matched by the filter expression and,  even	if  they  were
	      matched  by the filter expression, regardless of whether tcpdump
	      has read and processed them yet, on other	OSes  it  counts  only
	      packets that were	matched	by the filter expression regardless of
	      whether tcpdump has read and processed them yet,	and  on	 other
	      OSes  it counts only packets that	were matched by	the filter ex-
	      pression 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  (in-
       cluding	Mac  OS	X) and Digital/Tru64 UNIX, it will report those	counts
       when it receives	a SIGINFO signal (generated, for  example,  by	typing
       your  ``status''	character, typically control-T,	although on some plat-
       forms, such as Mac OS X,	the ``status'' character is  not  set  by  de-
       fault,  so  you	must  set it with stty(1) in order to use it) and will
       continue	capturing packets.

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

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

       Under ULTRIX and	Digital	UNIX/Tru64 UNIX:
	      Any user may capture network traffic with	tcpdump.  However,  no
	      user  (not  even the super-user) can capture in promiscuous mode
	      on an interface unless the super-user has	 enabled  promiscuous-
	      mode  operation on that interface	using pfconfig(8), and no user
	      (not even	the super-user)	can capture unicast  traffic  received
	      by  or sent by the machine on an interface unless	the super-user
	      has enabled copy-all-mode	operation on that interface using  pf-
	      config,  so  useful  packet capture on an	interface probably re-
	      quires that either promiscuous-mode or copy-all-mode  operation,
	      or both modes of operation, be enabled on	that interface.

       Under BSD (this includes	Mac OS X):
	      You  must	 have  read access to /dev/bpf*.  On BSDs with a devfs
	      (this includes Mac OS X),	this might involve more	than just hav-
	      ing  somebody  with  super-user  access setting the ownership or
	      permissions on the BPF devices - it  might  involve  configuring
	      devfs  to	set the	ownership or permissions every time the	system
	      is booted, if the	system even supports that; if it doesn't  sup-
	      port  that,  you	might have to find some	other way to make that
	      happen at	boot time.

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

OPTIONS
       -A     Print each packet	(minus its link	level header) in ASCII.	 Handy
	      for capturing web	pages.

       -c     Exit after receiving count packets.

       -C     Before  writing  a  raw  packet to a savefile, check whether the
	      file is currently	larger than file_size and, if  so,  close  the
	      current  savefile	and open a new one.  Savefiles after the first
	      savefile will have the name specified with the -w	flag,  with  a
	      number after it, starting	at 1 and continuing upward.  The units
	      of  file_size  are  millions  of	bytes  (1,000,000  bytes,  not
	      1,048,576	bytes).

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

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

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

       -D     Print the	list of	the network interfaces available on the	system
	      and on which tcpdump can capture packets.	 For each network  in-
	      terface,	a number and an	interface name,	possibly followed by a
	      text description of the interface, is  printed.	The  interface
	      name  or the number can be supplied to the -i flag to specify an
	      interface	on which to capture.

	      This can be useful on systems that don't have a command to  list
	      them  (e.g.,  Windows  systems, or UNIX systems lacking ifconfig
	      -a); the number can be useful on Windows 2000 and	later systems,
	      where the	interface name is a somewhat complex string.

	      The  -D  flag will not be	supported if tcpdump was built with an
	      older version of libpcap that lacks the pcap_findalldevs() func-
	      tion.

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

       -E     Use spi@ipaddr algo:secret for decrypting	IPsec ESP packets that
	      are addressed to addr and	contain	Security Parameter Index value
	      spi.  This  combination  may  be	repeated with comma or newline
	      seperation.

	      Note that	setting	the secret for IPv4 ESP	packets	 is  supported
	      at this time.

	      Algorithms  may  be  des-cbc,  3des-cbc,	blowfish-cbc, rc3-cbc,
	      cast128-cbc, or none.  The default is des-cbc.  The  ability  to
	      decrypt  packets	is  only  present if tcpdump was compiled with
	      cryptography enabled.

	      secret is	the ASCII text for ESP secret key.   If	 preceeded  by
	      0x, then a hex value will	be read.

	      The  option assumes RFC2406 ESP, not RFC1827 ESP.	 The option is
	      only for debugging purposes, and the use of this option  with  a
	      true  `secret'  key  is discouraged.  By presenting IPsec	secret
	      key onto command line you	make it	visible	to others,  via	 ps(1)
	      and other	occasions.

	      In  addition  to	the  above syntax, the syntax file name	may be
	      used to have tcpdump read	the provided  file  in.	 The  file  is
	      opened  upon receiving the first ESP packet, so any special per-
	      missions that tcpdump may	have been given	 should	 already  have
	      been given up.

       -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).

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

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

	      If  the  -D flag is supported, an	interface number as printed by
	      that flag	can be used as the interface argument.

       -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-
	      dump.

       -M     Use secret as a shared secret for	validating the	digests	 found
	      in TCP segments with the TCP-MD5 option (RFC 2385), if present.

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

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

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

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

       -q     Quick (quiet?) output.  Print less protocol information so  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-
	      ets.

       -T     Force  packets  selected	by  "expression" to be interpreted the
	      specified	type.  Currently known types are aodv  (Ad-hoc	On-de-
	      mand  Distance  Vector protocol),	cnfp (Cisco NetFlow protocol),
	      rpc (Remote Procedure Call), rtp (Real-Time Applications	proto-
	      col), rtcp (Real-Time Applications control protocol), snmp (Sim-
	      ple Network Management Protocol),	tftp  (Trivial	File  Transfer
	      Protocol),  vat  (Visual	Audio Tool), and wb (distributed White
	      Board).

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

       -tt    Print an unformatted timestamp on	each dump line.

       -ttt   Print a delta (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.

       -U     Make output saved	via the	-w option  ``packet-buffered'';	 i.e.,
	      as  each packet is saved,	it will	be written to the output file,
	      rather than being	written	only when the output buffer fills.

	      The -U flag will not be supported	if tcpdump was built  with  an
	      older  version of	libpcap	that lacks the pcap_dump_flush() func-
	      tion.

       -v     When parsing and printing, produce (slightly more) verbose  out-
	      put.   For  example,  the	 time  to  live, identification, total
	      length and options in an IP packet are  printed.	 Also  enables
	      additional  packet integrity checks such as verifying the	IP and
	      ICMP header checksum.

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

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

       -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 ``-''.

       -W     Used in conjunction with the -C option, this will	limit the num-
	      ber of files created to the specified number,  and  begin	 over-
	      writing  files  from  the	 beginning, thus creating a 'rotating'
	      buffer.  In addition, it will name the files with	enough leading
	      0s to support the	maximum	number of files, allowing them to sort
	      correctly.

       -x     Print each packet	(minus its link	level  header)	in  hex.   The
	      smaller  of  the entire packet or	snaplen	bytes will be printed.
	      Note that	this is	the entire link-layer packet, so for link lay-
	      ers  that	 pad  (e.g.  Ethernet),	the padding bytes will also be
	      printed when the higher layer packet is  shorter	than  the  re-
	      quired padding.

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

       -X     Print  each  packet  (minus  its	link  level header) in hex and
	      ASCII.  This is very handy for analysing new protocols.

       -XX    Print each packet, including its link level header, in  hex  and
	      ASCII.

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

       -Z     Drops privileges (if root) and changes user ID to	user  and  the
	      group ID to the primary group of user.

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

	expression
	      selects  which  packets  will  be	 dumped.   If no expression is
	      given, all packets on the	net will be dumped.   Otherwise,  only
	      packets for which	expression is `true' will be dumped.

	      The  expression  consists	of one or more primitives.  Primitives
	      usually consist of an id (name or	number)	 preceded  by  one  or
	      more qualifiers.	There are three	different kinds	of qualifier:

	      type   qualifiers	 say  what kind	of thing the id	name or	number
		     refers to.	 Possible types	are host, net ,	port and  por-
		     trange.   E.g., `host foo', `net 128.3', `port 20', `por-
		     trange 6000-6008'.	 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	some link layers, such
		     as	SLIP and the ``cooked''	Linux capture  mode  used  for
		     the  ``any''  device and for some other device types, the
		     inbound and outbound qualifiers can be used to specify  a
		     desired direction.

	      proto  qualifiers	 restrict  the match to	a particular protocol.
		     Possible protos are: ether, fddi, tr, wlan, 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',	`udp portrange 7000-7009'.  If
		     there is no proto	qualifier,  all	 protocols  consistent
		     with the type are assumed.	 E.g., `src foo' means `(ip or
		     arp or rarp) src foo' (except the	latter	is  not	 legal
		     syntax),  `net  bar'  means `(ip or arp or	rarp) net bar'
		     and `port 53' means `(tcp or udp) port 53'.

	      [`fddi' is actually an alias for `ether';	the parser treats them
	      identically  as meaning ``the data link level used on the	speci-
	      fied network interface.''	 FDDI  headers	contain	 Ethernet-like
	      source  and  destination	addresses, and often contain Ethernet-
	      like packet types, so you	can filter on these FDDI  fields  just
	      as  with	the analogous Ethernet fields.	FDDI headers also con-
	      tain other fields, but you cannot	name them explicitly in	a fil-
	      ter expression.

	      Similarly, `tr' and `wlan' are aliases for `ether'; the previous
	      paragraph's statements about FDDI	headers	also  apply  to	 Token
	      Ring  and	 802.11	wireless LAN headers.  For 802.11 headers, the
	      destination address is the DA field and the  source  address  is
	      the SA field; the	BSSID, RA, and TA fields aren't	tested.]

	      In  addition  to	the  above, there are some special `primitive'
	      keywords that don't  follow  the	pattern:  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
		     details).

	      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 IPv4 address matches net  with	 the  specific
		     netmask.	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.

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

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

	      portrange	port1-port2
		     True if either the	source	or  destination	 port  of  the
		     packet is between port1 and port2.

		     Any  of  the  above port or port range expressions	can be
		     prepended with the	keywords, 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 IPv4 packet (see ip(4P)) of pro-
		     tocol  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
		     IPv4.

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

	      ip broadcast
		     True if the packet	 is  an	 IPv4  broadcast  packet.   It
		     checks  for  both	the  all-zeroes	and all-ones broadcast
		     conventions, and looks up the subnet mask on  the	inter-
		     face on which the capture is being	done.

		     If	 the subnet mask of the	interface on which the capture
		     is	being done is not available, either because the	inter-
		     face on which capture is being done has no	netmask	or be-
		     cause the capture is being	done on	the Linux "any"	inter-
		     face,  which can capture on more than one interface, this
		     check will	not work correctly.

	      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 IPv4 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'),	 Token
		     Ring  (e.g., `tr protocol arp'), and IEEE 802.11 wireless
		     LANS (e.g., `wlan protocol	arp'), for most	of those  pro-
		     tocols,  the protocol identification comes	from the 802.2
		     Logical Link Control (LLC)	header,	which is usually  lay-
		     ered on top of the	FDDI, Token Ring, or 802.11 header.

		     When filtering for	most protocol identifiers on FDDI, To-
		     ken Ring, or 802.11, tcpdump checks only the protocol  ID
		     field  of	an LLC header in so-called SNAP	format with an
		     Organizational Unit Identifier (OUI) of 0x000000, for en-
		     capsulated	 Ethernet; it doesn't check whether the	packet
		     is	in SNAP	format with an OUI of  0x000000.   The	excep-
		     tions are:

		     iso    tcpdump  checks  the DSAP (Destination Service Ac-
			    cess Point)	and SSAP (Source Service Access	Point)
			    fields of the LLC header;

		     stp and netbeui
			    tcpdump checks the DSAP of the LLC header;

		     atalk  tcpdump  checks  for  a SNAP-format	packet with an
			    OUI	of 0x080007 and	the AppleTalk etype.

		     In	the case of Ethernet, tcpdump checks the Ethernet type
		     field for most of those protocols.	 The exceptions	are:

		     iso, stp, and netbeui
			    tcpdump  checks for	an 802.3 frame and then	checks
			    the	LLC header as it does for  FDDI,  Token	 Ring,
			    and	802.11;

		     atalk  tcpdump  checks both for the AppleTalk etype in an
			    Ethernet frame and for a SNAP-format packet	as  it
			    does for FDDI, Token Ring, and 802.11;

		     aarp   tcpdump  checks for	the AppleTalk ARP etype	in ei-
			    ther an Ethernet frame or an 802.2 SNAP frame with
			    an OUI of 0x000000;

		     ipx    tcpdump  checks  for  the IPX etype	in an Ethernet
			    frame,  the	 IPX  DSAP  in	the  LLC  header,  the
			    802.3-with-no-LLC-header encapsulation of IPX, and
			    the	IPX etype in a SNAP frame.

	      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.

	      ifname interface
		     True if the packet	was logged as coming from  the	speci-
		     fied  interface  (applies only to packets logged by Open-
		     BSD's pf(4)).

	      on interface
		     Synonymous	with the ifname	modifier.

	      rnr num
		     True if the packet	was logged as matching	the  specified
		     PF	 rule  number (applies only to packets logged by Open-
		     BSD's pf(4)).

	      rulenum num
		     Synonomous	with the rnr modifier.

	      reason code
		     True if the packet	was logged with	the specified PF  rea-
		     son  code.	 The known codes are: match, bad-offset, frag-
		     ment, short, normalize, and memory	(applies only to pack-
		     ets logged	by OpenBSD's pf(4)).

	      rset name
		     True  if  the packet was logged as	matching the specified
		     PF	ruleset	name of	an anchored ruleset (applies  only  to
		     packets logged by pf(4)).

	      ruleset name
		     Synonomous	with the rset modifier.

	      srnr num
		     True  if  the packet was logged as	matching the specified
		     PF	rule number of an anchored ruleset  (applies  only  to
		     packets logged by pf(4)).

	      subrulenum num
		     Synonomous	with the srnr modifier.

	      action act
		     True  if PF took the specified action when	the packet was
		     logged.  Known actions are: pass and block	(applies  only
		     to	packets	logged by OpenBSD's pf(4)).

	      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.	the [vlan_id] statement	may be
		     used more than once, to filter on vlan hierarchies.  each
		     use  of  the  [vlan_id]  expression increments the	filter
		     offsets by	4.
		     example(s):
		     "vlan 100 && vlan 200" filters on vlan  200  encapsulated
		     within vlan 100
		     "vlan  && vlan 300	&& ip" filters IPv4 protocols encapsu-
		     lated in vlan 300 encapsulated within  any	 higher	 order
		     vlan

	      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.

	      l1, l2, iih, lsp,	snp, csnp, psnp
		     Abbreviations for IS-IS PDU types.

	      vpi n  True  if  the  packet is an ATM packet, for SunATM	on So-
		     laris, with a virtual path	identifier of n.

	      vci n  True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris, with a virtual channel identifier of n.

	      lane   True  if  the  packet is an ATM packet, for SunATM	on So-
		     laris, and	is an ATM LANE packet.	Note  that  the	 first
		     lane  keyword encountered in expression changes the tests
		     done in the remainder of  expression  on  the  assumption
		     that the packet is	either a LANE emulated Ethernet	packet
		     or	a LANE LE Control packet.  If  lane  isn't  specified,
		     the  tests	 are done under	the assumption that the	packet
		     is	an LLC-encapsulated packet.

	      llc    True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris, and	is an LLC-encapsulated packet.

	      oamf4s True  if  the  packet is an ATM packet, for SunATM	on So-
		     laris, and	is a segment OAM F4 flow cell (VPI=0 & VCI=3).

	      oamf4e True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris,  and  is  an  end-to-end OAM F4 flow cell (VPI=0 &
		     VCI=4).

	      oamf4  True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris,  and  is  a	segment	or end-to-end OAM F4 flow cell
		     (VPI=0 & (VCI=3 | VCI=4)).

	      oam    True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris,  and  is  a	segment	or end-to-end OAM F4 flow cell
		     (VPI=0 & (VCI=3 | VCI=4)).

	      metac  True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris,  and  is  on  a  meta  signaling  circuit (VPI=0 &
		     VCI=1).

	      bcc    True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris,  and  is on	a broadcast signaling circuit (VPI=0 &
		     VCI=2).

	      sc     True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris, and	is on a	signaling circuit (VPI=0 & VCI=5).

	      ilmic  True  if  the  packet is an ATM packet, for SunATM	on So-
		     laris, and	is on an ILMI circuit (VPI=0 & VCI=16).

	      connectmsg
		     True if the packet	is an ATM packet, for  SunATM  on  So-
		     laris,  and  is  on  a  signaling circuit and is a	Q.2931
		     Setup, Call Proceeding, Connect, Connect Ack, Release, or
		     Release Done message.

	      metaconnect
		     True  if  the  packet is an ATM packet, for SunATM	on So-
		     laris, and	is on a	meta signaling circuit and is a	Q.2931
		     Setup, Call Proceeding, Connect, Release, or Release Done
		     message.

	      expr relop expr
		     True if the relation holds, where relop is	one of	>,  <,
		     >=,  <=, =, !=, and expr is an arithmetic expression com-
		     posed of integer constants	(expressed in standard C  syn-
		     tax),  the	normal binary operators	[+, -, *, /, &,	|, <<,
		     >>], a length operator, and special  packet  data	acces-
		     sors.   Note  that	all comparisons	are unsigned, so that,
		     for example, 0x80000000 and 0xffffffff are	> 0.   To  ac-
		     cess data inside the packet, use the following syntax:
			  proto	[ expr : size ]
		     Proto  is	one of ether, fddi, tr,	wlan, ppp, slip, link,
		     ip, arp, rarp, tcp, udp, icmp, ip6	or  radio,  and	 indi-
		     cates   the  protocol  layer  for	the  index  operation.
		     (ether, fddi, wlan, tr, ppp, slip and link	all  refer  to
		     the  link layer. radio refers to the "radio header" added
		     to	some 802.11 captures.)	Note that tcp, udp  and	 other
		     upper-layer  protocol  types only apply to	IPv4, not IPv6
		     (this will	be fixed in the	 future).   The	 byte  offset,
		     relative  to  the	indicated  protocol layer, is given by
		     expr.  Size is optional and indicates the number of bytes
		     in	 the  field of interest; it can	be either one, two, or
		     four, and defaults	to one.	 The  length  operator,	 indi-
		     cated by the keyword 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
		     IPv4  packets  with  options.   The expression `ip[6:2] &
		     0x1fff = 0' catches only unfragmented IPv4	datagrams  and
		     frag  zero	 of  fragmented	IPv4 datagrams.	 This check is
		     implicitly	applied	to the tcp and udp  index  operations.
		     For  instance,  tcp[0] always means the first byte	of the
		     TCP header, and never means the first byte	of  an	inter-
		     vening fragment.

		     Some  offsets  and	field values may be expressed as names
		     rather than as numeric values.   The  following  protocol
		     header  field  offsets are	available: 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-
		     maskreply.

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

	      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
	      concatenation.

	      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.

EXAMPLES
       To print	all packets arriving at	or departing from sundown:
	      tcpdump host sundown

       To print	traffic	between	helios and either hot or ace:
	      tcpdump host helios and \( hot or	ace \)

       To print	all IP packets between ace and any host	except helios:
	      tcpdump ip host ace and not helios

       To print	all traffic between local hosts	and hosts at Berkeley:
	      tcpdump net ucb-ether

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

       To print	traffic	neither	sourced	from nor destined for local hosts  (if
       you gateway to one other	net, this stuff	should never make it onto your
       local net).
	      tcpdump ip and not net localnet

       To print	the start and end packets (the SYN and FIN  packets)  of  each
       TCP conversation	that involves a	non-local host.
	      tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

       To  print  all  IPv4  HTTP packets to and from port 80, i.e. print only
       packets that contain data, not, for example, SYN	and  FIN  packets  and
       ACK-only	packets.  (IPv6	is left	as an exercise for the reader.)
	      tcpdump 'tcp port	80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) -	((tcp[12]&0xf0)>>2)) !=	0)'

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

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

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

       Link Level Headers

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

       On FDDI networks, the  '-e' option causes tcpdump to print  the	`frame
       control'	 field,	  the source and destination addresses,	and the	packet
       length.	(The `frame control' field governs the interpretation  of  the
       rest  of	the packet.  Normal packets (such as those containing IP 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.

       On 802.11 networks, the '-e' option causes tcpdump to print the	`frame
       control'	 fields,  all  of  the addresses in the	802.11 header, and the
       packet length.  As on FDDI networks, packets are	assumed	to contain  an
       LLC packet.

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

       On SLIP links, a	direction indicator (``I'' for inbound,	``O'' for out-
       bound),	packet type, and compression information are printed out.  The
       packet type is printed first.  The three	types are ip, utcp, and	 ctcp.
       No  further  link information is	printed	for ip packets.	 For TCP pack-
       ets, the	connection identifier is printed following the type.   If  the
       packet  is  compressed, its encoded header is printed out.  The special
       cases are printed out as	*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 128.3.254.6 tell 128.3.254.68
	      arp reply	128.3.254.6 is-at 02:07:01:00:01:c4

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

       TCP Packets

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

       The general format of a tcp protocol line is:
	      src _ dst: flags data-seqno ack window urgent options
       Src  and	 dst  are  the	source and destination IP addresses and	ports.
       Flags are some combination of S (SYN), F	(FIN), P (PUSH),  R  (RST),  W
       (ECN  CWR) or E (ECN-Echo), or a	single `.' (no flags).	Data-seqno de-
       scribes 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.   Op-
       tions 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-
       propriate.

       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
       length]''.

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

		       |	       |
		       |---------------|
		       |C|E|U|A|P|R|S|F|
		       |---------------|
		       |7   5	3     0|

       These are the TCP control bits we are interested	in.  We	have  numbered
       the  bits  in  this octet from 0	to 7, right to left, so	the PSH	bit is
       bit number 3, while the URG bit is number 5.

       Recall that we want to capture packets with only	SYN  set.   Let's  see
       what happens to octet 13	if a TCP datagram arrives with the SYN bit set
       in its header:

		       |C|E|U|A|P|R|S|F|
		       |---------------|
		       |0 0 0 0	0 0 1 0|
		       |---------------|
		       |7 6 5 4	3 2 1 0|

       Looking at the control bits section we see that only bit	number 1 (SYN)
       is set.

       Assuming	 that  octet number 13 is an 8-bit unsigned integer in network
       byte order, the binary value of this octet is

	      00000010

       and its decimal representation is

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

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

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

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

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

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

	    |C|E|U|A|P|R|S|F|
	    |---------------|
	    |0 0 0 1 0 0 1 0|
	    |---------------|
	    |7 6 5 4 3 2 1 0|

       Now  bits 1 and 4 are set in the	13th octet.  The binary	value of octet
       13 is

		   00010010

       which translates	to decimal

	  7	6     5	    4	  3	2     1	    0
       0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2   = 18

       Now we can't just use 'tcp[13] == 18' in	the tcpdump filter expression,
       because that would select only those packets that have SYN-ACK set, but
       not those with only SYN set.  Remember that we don't care if ACK	or any
       other control bit is set	as long	as SYN is set.

       In order	to achieve our goal, we	need to	logically AND the binary value
       of octet	13 with	some other value to preserve the  SYN  bit.   We  know
       that  we	 want  SYN  to	be set in any case, so we'll logically AND the
       value in	the 13th octet with the	binary value of	a SYN:

		 00010010 SYN-ACK	       00000010	SYN
	    AND	 00000010 (we want SYN)	  AND  00000010	(we want SYN)
		 --------		       --------
	    =	 00000010		  =    00000010

       We see that this	AND operation  delivers	 the  same  result  regardless
       whether ACK or another TCP control bit is set.  The decimal representa-
       tion of the AND value as	well as	the result of this operation is	2 (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
       greek.)

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

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

       UDP Name	Server Responses

       Name server responses are formatted as
	      src _ dst:  id op	rcode flags a/n/au type	class data (len)
	      helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
	      helios.domain > h2opolo.1537: 2 NXDomain*	0/1/0 (97)
       In the first example, helios responds to	query id 3 from	h2opolo	with 3
       answer records, 3 name server records and 7  additional	records.   The
       first  answer  record  is type A	(address) and its data is internet ad-
       dress 128.32.137.3.  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
       printed.

       Other flag characters that might	appear are `-'	(recursion  available,
       RA,  not	 set) and `|' (truncated message, TC, set).  If	the `question'
       section doesn't contain exactly one entry, `[nq]' is printed.

       Note that name server requests and responses tend to be large  and  the
       default	snaplen	 of  68	 bytes may not capture enough of the packet to
       print.  Use the -s flag to increase the snaplen if you  need  to	 seri-
       ously  investigate  name	 server	traffic.  `-s 128' has worked well for
       me.

       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.

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

       NFS Requests and	Replies

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

       In  the	third line, sushi asks wrl to lookup the name `xcolors'	in 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
       traffic.

       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:

	      src.sport	_ dst.dport: rx	packet-type
	      src.sport	_ dst.dport: rx	packet-type service call call-name args
	      src.sport	_ dst.dport: rx	packet-type service reply call-name args
	      elvis.7001 > pike.afsfs:
		   rx data fs call rename old fid 536876964/1/1	".newsrc.new"
		   new fid 536876964/1/1 ".newsrc"
	      pike.afsfs > elvis.7001: rx data fs reply	rename
       In the first line, host elvis sends a RX	packet to pike.	 This was a RX
       data  packet to the fs (fileserver) service, and	is the start of	an RPC
       call.  The RPC call was a rename, with the old  directory  file	id  of
       536876964/1/1 and an old	filename of `.newsrc.new', and a new directory
       file id of 536876964/1/1	and a new filename  of	`.newsrc'.   The  host
       pike  responds  with a RPC reply	to the rename call (which was success-
       ful, because it was a data packet and not an abort packet).

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

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

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

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

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

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

       Note  that  AFS requests	are very large and many	of the arguments won't
       be printed unless snaplen is increased.	Try using `-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
	      net.host.port

	      144.1.209.2 > icsd-net.112.220
	      office.2 > icsd-net.112.220
	      jssmag.149.235 > icsd-net.2
       (If  the	/etc/atalk.names doesn't exist or doesn't contain an entry for
       some AppleTalk host/net number, addresses are printed in	numeric	form.)
       In the first example, NBP (DDP port 2) on net 144.1 node	209 is sending
       to whatever is listening	on port	220 of net icsd	node 112.  The	second
       line is the same	except the full	name of	the source node	is known (`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
       /etc/atalk.names).

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

       NBP packets are formatted like the following examples:
	      icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
	      jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
	      techpit.2	> icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
       The  first  line	 is a name lookup request for laserwriters sent	by net
       icsd host 112 and broadcast on net jssmag.  The nbp id for  the	lookup
       is  190.	  The second line shows	a reply	for this request (note that it
       has the same id)	from host jssmag.209 saying that it has	a  laserwriter
       resource	 named "RM1140"	registered on port 250.	 The third line	is 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  arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't
       appear to handle	576 byte datagrams:
	      arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
	      arizona >	rtsg: (frag 595a:204@328)
	      rtsg.1170	> arizona.ftp-data: . ack 1536 win 2560
       There are a couple of things to note here:  First, addresses in the 2nd
       line  don't include port	numbers.  This is because the TCP protocol 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
       (DF).

       Timestamps

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

SEE ALSO
       bpf(4), pcap(3)

AUTHORS
       The original authors are:

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

       It is currently being maintained	by tcpdump.org.

       The current version is available	via http:

	      http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

	      ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

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

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

	      tcpdump-workers@tcpdump.org

       Please send source code contributions, etc. to:

	      patches@tcpdump.org

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

       On Linux	systems	with 2.0[.x] kernels:

	      packets on the loopback device will be seen twice;

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

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

	      capturing	on some	PPP devices won't work correctly.

       We recommend that you upgrade to	a 2.2 or later kernel.

       Some attempt should be made to reassemble IP fragments or, at least  to
       compute the right length	for the	higher level protocol.

       Name server inverse queries are not dumped correctly: the (empty) ques-
       tion section is printed rather than real	query in the  answer  section.
       Some  believe  that  inverse queries are	themselves a bug and prefer to
       fix the program generating them rather than tcpdump.

       A packet	trace that crosses a daylight savings time  change  will  give
       skewed time stamps (the time change is ignored).

       Filter  expressions  on	fields	other than those in Token Ring headers
       will not	correctly handle source-routed Token Ring packets.

       Filter expressions on fields other than those in	 802.11	 headers  will
       not  correctly  handle  802.11 data packets with	both To	DS and From DS
       set.

       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.

				 18 April 2005			    TCPDUMP(1)

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