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TCPDUMP(1)							    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
	      expression and were processed by tcpdump);

	      packets ``dropped	by kernel'' (this is  the  number  of  packets
	      that  were dropped, due to a lack	of buffer space, by the	packet
	      capture mechanism	in the OS on which tcpdump is running, if  the
	      OS  reports that information to applications; if not, it will be
	      reported as 0).

       On platforms that  support  the	SIGINFO	 signal,  such	as  most  BSDs
       (including  Mac	OS  X)	and  Digital/Tru64  UNIX, it will report those
       counts when it receives a SIGINFO signal	(generated,  for  example,  by
       typing your ``status'' character, typically control-T, although on some
       platforms, such as Mac OS X, the	``status'' character  is  not  set  by
       default,	 so  you must set it with 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
	      pfconfig,	 so  useful  packet  capture  on an interface probably
	      requires that either promiscuous-mode  or	 copy-all-mode	opera-
	      tion,  or	both modes of operation, be enabled on that interface.

       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
	      interface, a number and an interface name, possibly followed  by
	      a	 text description of the interface, is printed.	 The interface
	      name or the number can be	supplied to the	-i flag	to specify  an
	      interface	on which to capture.

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

	      The -D flag will not be supported	if tcpdump was built  with  an
	      older version of libpcap that lacks the pcap_findalldevs() 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
	      address and netmask of the interface on which capture  is	 being
	      done.   If that address or netmask are not available, available,
	      either because the interface on which capture is being done  has
	      no  address  or  netmask or because the capture is being done on
	      the Linux	"any" interface, which can capture on  more  than  one
	      interface, this option will not work correctly.

       -F     Use  file	 as  input  for	 the filter expression.	 An additional
	      expression 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
	      default  of  68  (with SunOS's NIT, the minimum is actually 96).
	      68 bytes is adequate for IP, ICMP, TCP and UDP but may  truncate
	      protocol	information  from  name	 server	 and  NFS packets (see
	      below).  Packets truncated because of  a	limited	 snapshot  are
	      indicated	 in  the  output with ``[|proto]'', where proto	is the
	      name of the protocol level at which the truncation has occurred.
	      Note  that  taking larger	snapshots both increases the amount of
	      time it takes to process packets and, effectively, decreases the
	      amount  of packet	buffering.  This may cause packets to be lost.
	      You should limit snaplen to the smallest number that  will  cap-
	      ture  the	 protocol  information	you're interested in.  Setting
	      snaplen to 0 means use the required length to catch whole	 pack-
	      ets.

       -T     Force  packets  selected	by  "expression" to be interpreted the
	      specified	type.  Currently known	types  are  aodv  (Ad-hoc  On-
	      demand 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
	      decoded.

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

       -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
	      required 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
	      described	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
		     address 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
		     because  the  capture  is	being  done on the Linux "any"
		     interface,	which can capture on more than one  interface,
		     this check	will not work correctly.

	      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,
		     Token  Ring,  or 802.11, tcpdump checks only the protocol
		     ID	field of an LLC	header in so-called SNAP  format  with
		     an	 Organizational	Unit Identifier	(OUI) of 0x000000, for
		     encapsulated  Ethernet;  it  doesn't  check  whether  the
		     packet  is	 in  SNAP format with an OUI of	0x000000.  The
		     exceptions	are:

		     iso    tcpdump  checks  the  DSAP	(Destination   Service
			    Access  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
			    either  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
		     OpenBSD'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
		     OpenBSD'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
		     encountered 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  fil-
		     ter 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
		     Solaris, with a virtual path identifier of	n.

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

	      lane   True if the packet	 is  an	 ATM  packet,  for  SunATM  on
		     Solaris,  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
		     Solaris, and is an	LLC-encapsulated packet.

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

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

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

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

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

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

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

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

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

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

	      expr relop expr
		     True if the relation holds, where relop is	one of	>,  <,
		     >=,  <=, =, !=, and expr is an arithmetic expression com-
		     posed of integer constants	(expressed in standard C  syn-
		     tax),  the	normal binary operators	[+, -, *, /, &,	|, <<,
		     >>], a length operator, and special  packet  data	acces-
		     sors.   Note  that	all comparisons	are unsigned, so that,
		     for example, 0x80000000  and  0xffffffff  are  >  0.   To
		     access data inside	the packet, use	the following syntax:
			  proto	[ expr : size ]
		     Proto  is	one of 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
	      arguments	are concatenated with spaces before being parsed.

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

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

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

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

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

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

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

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

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

       To print	IP broadcast or	multicast packets that were not	sent via  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
       assumed	to  contain  an	 LLC  packet.	Regardless of whether the '-e'
       option is specified or not, the source routing information  is  printed
       for source-routed packets.

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

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

       On SLIP links, a	direction indicator (``I'' for inbound,	``O'' for out-
       bound), packet type, and	compression information	are printed out.   The
       packet  type is printed first.  The three types are ip, utcp, and ctcp.
       No further link information is printed for ip packets.  For  TCP	 pack-
       ets,  the  connection identifier	is printed following the type.	If the
       packet is compressed, its encoded header	is printed out.	  The  special
       cases are printed out as	*S+n and *SA+n,	where n	is the amount by which
       the sequence number (or sequence	number and ack)	has changed.  If it is
       not  a  special	case,  zero  or	more changes are printed.  A change is
       indicated by U (urgent pointer),	W (window), A (ack), S (sequence  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
       address (in this	example, Ethernet addresses are	in caps	 and  internet
       addresses in lower case).

       This would look less redundant if we had	done tcpdump -n:
	      arp who-has 128.3.254.6 tell 128.3.254.68
	      arp reply	128.3.254.6 is-at 02:07:01:00:01:c4

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

       Here is the opening portion of an rlogin	from host rtsg to host csam.
	      rtsg.1023	> csam.login: S	768512:768512(0) win 4096 <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
       after  the  first  can be interpreted as	relative byte positions	in the
       conversation's data stream (with	the first  data	 byte  each  direction
       being  `1').   `-S'  will  override  this feature, causing the original
       sequence	numbers	to be output.

       On the 6th line,	rtsg sends csam	19 bytes of data (bytes	2  through  20
       in the rtsg -> csam side	of the conversation).  The PUSH	flag is	set in
       the packet.  On the 7th line, csam says it's received data sent by rtsg
       up  to but not including	byte 21.  Most of this data is apparently sit-
       ting in the socket buffer since csam's receive  window  has  gotten  19
       bytes  smaller.	 Csam  also  sends  one	 byte  of data to rtsg in this
       packet.	On the 8th and 9th lines, csam	sends  two  bytes  of  urgent,
       pushed data to rtsg.

       If  the	snapshot was small enough that tcpdump didn't capture the full
       TCP header, it interprets as much of the	header	as  it	can  and  then
       reports	``[|tcp]'' to indicate the remainder could not be interpreted.
       If the header contains a	bogus option (one with a length	that's	either
       too  small  or  beyond  the  end	 of the	header), tcpdump reports it as
       ``[bad opt]'' and does not interpret any	further	 options  (since  it's
       impossible  to  tell where they start).	If the header length indicates
       options are present but the IP datagram length is not long  enough  for
       the  options  to	 actually  be  there, tcpdump reports it as ``[bad hdr
       length]''.

       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
       (binary 00000010), so we	know that for packets with SYN set the follow-
       ing relation must hold true:

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

       This points us to the tcpdump filter expression
		   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
       address 128.32.137.3.  The total	size of	the response  was  273	bytes,
       excluding  UDP and IP headers.  The op (Query) and response code	(NoEr-
       ror) were omitted, as was the class (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
       directory file 9,74/4096.6878.  Note that the data printed  depends  on
       the  operation  type.  The format is intended to	be self	explanatory if
       read in conjunction with	an NFS protocol	spec.

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

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

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

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

       AFS Requests and	Replies

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

	      src.sport	_ dst.dport: rx	packet-type
	      src.sport	_ dst.dport: rx	packet-type service call call-name args
	      src.sport	_ dst.dport: rx	packet-type service reply call-name args
	      elvis.7001 > pike.afsfs:
		   rx data fs call rename old fid 536876964/1/1	".newsrc.new"
		   new fid 536876964/1/1 ".newsrc"
	      pike.afsfs > elvis.7001: rx data fs reply	rename
       In the first line, host elvis sends a RX	packet to pike.	 This was a RX
       data packet to the fs (fileserver) service, and is the start of an  RPC
       call.   The  RPC	 call  was a rename, with the old directory file id of
       536876964/1/1 and an old	filename of `.newsrc.new', and a new directory
       file  id	 of  536876964/1/1  and	a new filename of `.newsrc'.  The host
       pike responds with a RPC	reply to the rename call (which	 was  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.
       Instead,	 tcpdump  keeps	track of ``recent'' requests, and matches them
       to the replies using the	call number and	service	ID.  If	a  reply  does
       not closely follow the corresponding request, it	might not be parsable.

       KIP AppleTalk (DDP in UDP)

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

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

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

	      144.1.209.2 > icsd-net.112.220
	      office.2 > icsd-net.112.220
	      jssmag.149.235 > icsd-net.2
       (If  the	/etc/atalk.names doesn't exist or doesn't contain an entry for
       some AppleTalk host/net number, addresses are printed in	numeric	form.)
       In the first example, NBP (DDP port 2) on net 144.1 node	209 is sending
       to whatever is listening	on port	220 of net icsd	node 112.  The	second
       line  is	 the  same  except  the	 full name of the source node is known
       (`office').  The	third line is a	send from port 235 on net jssmag  node
       149  to	broadcast  on  the  icsd-net NBP port (note that the broadcast
       address (255) is	indicated by a net name	with no	host number - for this
       reason  it's  a	good idea to keep node names and net names distinct in
       /etc/atalk.names).

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

       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
       another reply to	the same request saying	host techpit  has  laserwriter
       "techpit" registered on port 186.

       ATP packet formatting is	demonstrated by	the following example:
	      jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
	      helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
	      jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
	      helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
	      jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
	      jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
       Jssmag.209  initiates transaction id 12266 with host helios by 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
       information is all in the first fragment	and we have no idea  what  the
       port  or	 sequence numbers are when we print the	later fragments.  Sec-
       ond, the	tcp sequence information in the	first line is  printed	as  if
       there  were  308	 bytes of user data when, in fact, there are 512 bytes
       (308 in the first frag and 204 in the second).  If you are looking  for
       holes  in  the  sequence	space or trying	to match up acks with packets,
       this can	fool you.

       A packet	with the IP don't fragment flag	 is  marked  with  a  trailing
       (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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