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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 if 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 [vlan_id]  expres-
		     sion  may be used more than once, to filter on VLAN hier-
		     archies.  Each use	of that	expression increments the fil-
		     ter offsets by 4.

		     For example:
			  vlan 100 && vlan 200
		     filters on	VLAN 200 encapsulated within VLAN 100, and
			  vlan && vlan 300 && ip
		     filters  IPv4 protocols encapsulated in VLAN 300 encapsu-
		     lated within any higher order VLAN.

	      mpls [label_num]
		     True if the packet	is an MPLS packet.  If [label_num]  is
		     specified,	 only  true  is	 the  packet has the specified
		     label_num.	 Note that the first mpls keyword  encountered
		     in	 expression  changes  the  decoding  offsets  for  the
		     remainder of expression on	the assumption that the	packet
		     is	 a  MPLS-encapsulated IP packet.  The mpls [label_num]
		     expression	may be used more than once, to filter on  MPLS
		     hierarchies.   Each use of	that expression	increments the
		     filter offsets by 4.

		     For example:
			  mpls 100000 && mpls 1024
		     filters packets with an outer  label  of  100000  and  an
		     inner label of 1024, and
			  mpls && mpls 1024 && host 192.9.200.1
		     filters  packets  to  or  from  192.9.200.1 with an inner
		     label of 1024 and any outer label.

	      pppoed True if  the  packet  is  a  PPP-over-Ethernet  Discovery
		     packet (Ethernet type 0x8863).

	      pppoes True  if the packet is a PPP-over-Ethernet	Session	packet
		     (Ethernet type 0x8864).  Note that	the first pppoes  key-
		     word  encountered in expression changes the decoding off-
		     sets for the remainder of expression  on  the  assumption
		     that the packet is	a PPPoE	session	packet.

		     For example:
			  pppoes && ip
		     filters IPv4 protocols encapsulated in PPPoE.

	      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.

       When running tcpdump with the -v	option on a network interface support-
       ing checksum off-loading, IP packets sourced  from  this	 machine  will
       have many false 'bad cksum 0' errors.

       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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