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TCPDUMP(1)		    General Commands Manual		    TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size	] [ -c count ]
	       [ -C file_size ]	[ -G rotate_seconds ] [	-F file	]
	       [ -i interface ]	[ -j tstamp_type ] [ -m	module ] [ -M secret ]
	       [ -r file ] [ -V	file ] [ -s snaplen ] [	-T type	] [ -w file ]
	       [ -W filecount ]
	       [ -E spi@ipaddr algo:secret,...	]
	       [ -y datalinktype ] [ -z	postrotate-command ] [ -Z user ]
	       [ expression ]

DESCRIPTION
       Tcpdump	prints	out a description of the contents of packets on	a net-
       work 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.
       It can also be run with the -V flag, which causes it to read a list  of
       saved  packet  files.  In all cases, only packets that match expression
       will be processed by tcpdump.

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

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

	      packets ``captured'' (this is the	number of packets that tcpdump
	      has received and processed);

	      packets ``received by filter'' (the meaning of this  depends  on
	      the  OS on which you're running tcpdump, and possibly on the way
	      the OS was configured - if a filter was specified	on the command
	      line,  on	some OSes it counts packets regardless of whether they
	      were matched by the filter expression and,  even	if  they  were
	      matched  by the filter expression, regardless of whether tcpdump
	      has read and processed them yet, on other	OSes  it  counts  only
	      packets that were	matched	by the filter expression regardless of
	      whether tcpdump has read and processed them yet,	and  on	 other
	      OSes  it counts only packets that	were matched by	the filter ex-
	      pression and were	processed by tcpdump);

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

       On platforms that support the SIGINFO signal, such as  most  BSDs  (in-
       cluding	Mac  OS	X) and Digital/Tru64 UNIX, it will report those	counts
       when it receives	a SIGINFO signal (generated, for  example,  by	typing
       your  ``status''	character, typically control-T,	although on some plat-
       forms, such as Mac OS X,	the ``status'' character is  not  set  by  de-
       fault,  so  you	must  set it with stty(1) in order to use it) and will
       continue	capturing packets.

       Reading packets from a network interface	may require that you have spe-
       cial  privileges; see the pcap (3PCAP) man page for details.  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.

       -b     Print the	AS number in BGP packets in ASDOT notation rather than
	      ASPLAIN notation.

       -B     Set the operating	system capture buffer size to buffer_size,  in
	      units of KiB (1024 bytes).

       -c     Exit after receiving count packets.

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

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

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

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

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

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

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

       -e     Print  the  link-level  header  on  each dump line.  This	can be
	      used, for	example, to print MAC layer  addresses	for  protocols
	      such as Ethernet and IEEE	802.11.

       -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 sep-
	      aration.

	      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 preceded	by 0x,
	      then a hex value will be read.

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

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

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

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

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

       -G     If specified, rotates the	dump file specified with the -w	option
	      every rotate_seconds seconds.   Savefiles	 will  have  the  name
	      specified	by -w which should include a time format as defined by
	      strftime(3).  If no time format is specified, each new file will
	      overwrite	the previous.

	      If  used	in conjunction with the	-C option, filenames will take
	      the form of `file<count>'.

       -h     Print the	tcpdump	and libpcap version  strings,  print  a	 usage
	      message, and exit.

       -H     Attempt to detect	802.11s	draft mesh headers.

       -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.

       -I     Put the interface	in "monitor mode"; this	is supported  only  on
	      IEEE 802.11 Wi-Fi	interfaces, and	supported only on some operat-
	      ing systems.

	      Note that	in monitor mode	the adapter  might  disassociate  from
	      the  network with	which it's associated, so that you will	not be
	      able to use any wireless networks	with that adapter.  This could
	      prevent  accessing  files	on a network server, or	resolving host
	      names or network addresses, if you are capturing in monitor mode
	      and are not connected to another network with another adapter.

	      This  flag  will	affect the output of the -L flag.  If -I isn't
	      specified, only those link-layer types  available	 when  not  in
	      monitor mode will	be shown; if -I	is specified, only those link-
	      layer types available when in monitor mode will be shown.

       -j     Set the time stamp type for the  capture	to  tstamp_type.   The
	      names  to	use for	the time stamp types are given in pcap-tstamp-
	      type(7); not all the types  listed  there	 will  necessarily  be
	      valid for	any given interface.

       -J     List  the	supported time stamp types for the interface and exit.
	      If the time stamp	type cannot be set for the interface, no  time
	      stamp types are listed.

       -K     Don't attempt to verify IP, TCP, or UDP checksums.  This is use-
	      ful for interfaces that perform some or all  of  those  checksum
	      calculation  in  hardware; otherwise, all	outgoing TCP checksums
	      will be flagged as bad.

       -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

	      Note  that on Windows,``line buffered'' means ``unbuffered'', so
	      that WinDump will	write each character  individually  if	-l  is
	      specified.

	      -U is similar to -l in its behavior, but it will cause output to
	      be ``packet-buffered'', so that the output is written to	stdout
	      at  the  end of each packet rather than at the end of each line;
	      this is buffered on all platforms, including Windows.

       -L     List the known data link types for the interface,	in the	speci-
	      fied  mode,  and exit.  The list of known	data link types	may be
	      dependent	on the specified mode; for example, on some platforms,
	      a	 Wi-Fi interface might support one set of data link types when
	      not in monitor mode (for example,	it  might  support  only  fake
	      Ethernet	headers,  or might support 802.11 headers but not sup-
	      port 802.11 headers with radio information) and another  set  of
	      data link	types when in monitor mode (for	example, it might sup-
	      port 802.11 headers, or 802.11 headers with  radio  information,
	      only in monitor mode).

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

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

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

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

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

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

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

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

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

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

       -s     Snarf snaplen bytes of data from each packet rather than the de-
	      fault of 65535 bytes.  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 pack-
	      ets to be	lost.  You should limit	snaplen	to the smallest	number
	      that will	capture	the protocol information you're	interested in.
	      Setting snaplen to 0 sets	it to the default of 65535, for	 back-
	      wards compatibility with recent older versions of	tcpdump.

       -T     Force  packets  selected	by  "expression" to be interpreted the
	      specified	type.  Currently known types are aodv  (Ad-hoc	On-de-
	      mand  Distance Vector protocol), carp (Common Address Redundancy
	      Protocol), cnfp (Cisco NetFlow protocol),	radius	(RADIUS),  rpc
	      (Remote  Procedure Call),	rtp (Real-Time Applications protocol),
	      rtcp (Real-Time Applications  control  protocol),	 snmp  (Simple
	      Network Management Protocol), tftp (Trivial File Transfer	Proto-
	      col), vat	(Visual	Audio Tool),  wb  (distributed	White  Board),
	      zmtp1 (ZeroMQ Message Transport Protocol 1.0) and	vxlan (Virtual
	      eXtensible Local Area Network).

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

       -tt    Print an unformatted timestamp on	each dump line.

       -ttt   Print a delta (micro-second resolution) between current and pre-
	      vious line on each dump line.

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

       -ttttt Print a delta  (micro-second  resolution)	 between  current  and
	      first line on each dump line.

       -u     Print undecoded NFS handles.

       -U     If  the -w option	is not specified, make the printed packet out-
	      put ``packet-buffered''; i.e., as	the description	 of  the  con-
	      tents of each packet is printed, it will be written to the stan-
	      dard output, rather than,	when not writing to a terminal,	 being
	      written only when	the output buffer fills.

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

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

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

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

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

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

       -V     Read  a  list  of	filenames from file. Standard input is used if
	      file is ``-''.

       -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 ``-''.

	      This output will be buffered if written to a file	or pipe, so  a
	      program reading from the file or pipe may	not see	packets	for an
	      arbitrary	amount of time after they are received.	  Use  the  -U
	      flag  to	cause  packets	to  be written as soon as they are re-
	      ceived.

	      The MIME type application/vnd.tcpdump.pcap has  been  registered
	      with  IANA  for pcap files. The filename extension .pcap appears
	      to be the	most commonly used along with .cap and	.dmp.  Tcpdump
	      itself  doesn't  check  the extension when reading capture files
	      and doesn't add an extension when	writing	them  (it  uses	 magic
	      numbers  in  the	file  header instead). However,	many operating
	      systems and applications will use	the extension if it is present
	      and adding one (e.g. .pcap) is recommended.

	      See pcap-savefile(5) for a description of	the file format.

       -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.

	      Used in conjunction with the -G option, this will	limit the num-
	      ber  of rotated dump files that get created, exiting with	status
	      0	when reaching the limit. If used with -C as well, the behavior
	      will result in cyclical files per	timeslice.

       -x     When  parsing  and printing, in addition to printing the headers
	      of each packet, print the	data of	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 layers that pad (e.g. Ethernet),
	      the padding bytes	will also be printed  when  the	 higher	 layer
	      packet is	shorter	than the required padding.

       -xx    When  parsing  and printing, in addition to printing the headers
	      of each packet, print the	data of	 each  packet,	including  its
	      link level header, in hex.

       -X     When  parsing  and printing, in addition to printing the headers
	      of each packet, print the	data of	each packet  (minus  its  link
	      level  header)  in  hex  and  ASCII.   This  is  very  handy for
	      analysing	new protocols.

       -XX    When parsing and printing, in addition to	printing  the  headers
	      of  each	packet,	 print	the data of 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     Used  in	conjunction  with the -C or -G options,	this will make
	      tcpdump run " command file " where file is  the  savefile	 being
	      closed  after  each rotation. For	example, specifying -z gzip or
	      -z bzip2 will compress each savefile using gzip or bzip2.

	      Note that	tcpdump	will run the command in	parallel to  the  cap-
	      ture, using the lowest priority so that this doesn't disturb the
	      capture process.

	      And in case you would like to use	a command  that	 itself	 takes
	      flags  or	 different  arguments,	you  can  always write a shell
	      script that will take the	savefile name as  the  only  argument,
	      make  the	flags &	arguments arrangements and execute the command
	      that you want.

       -Z     If tcpdump is running as root, after opening the capture	device
	      or  input	savefile, but before opening any savefiles for output,
	      change the 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.

	      For the expression syntax, see pcap-filter(7).

	      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, such
	      as backslashes used to escape protocol names, it	is  easier  to
	      pass  it	as a single, quoted argument rather than to escape the
	      Shell metacharacters.  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 as-
       sumed  to contain an LLC	packet.	 Regardless of whether the '-e'	option
       is specified or not, the	source	routing	 information  is  printed  for
       source-routed packets.

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

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

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

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

       ARP/RARP	Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
	      src _ dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP	addresses  and	ports.
       Flags  are  some	 combination of	S (SYN), F (FIN), P (PUSH), R (RST), U
       (URG), W	(ECN CWR), E (ECN-Echo)	or `.' (ACK), or `none'	 if  no	 flags
       are set.	 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  di-
       rection	on  this  connection.  Urg indicates there is `urgent' data in
       the packet.  Options are	tcp options enclosed in	angle brackets	(e.g.,
       <mss 1024>).

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

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

       Csam  replies  with  a similar packet except it includes	a piggy-backed
       ack for rtsg's SYN.  Rtsg then acks csam's SYN.	The `.'	means the  ACK
       flag  was  set.	 The  packet contained no data so there	is no data se-
       quence 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  conver-
       sation, the difference between the current packet's sequence number and
       this initial sequence number is printed.	 This means that sequence num-
       bers  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 se-
       quence numbers to be output.

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

	      00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

		   00010010

       which translates	to decimal

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

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

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

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

       We see that this	AND operation  delivers	 the  same  result  regardless
       whether ACK or another TCP control bit is set.  The decimal representa-
       tion of the AND value as	well as	the result of this operation is	2 (bi-
       nary  00000010),	so we know that	for packets with SYN set the following
       relation	must hold true:

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

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

       Some offsets and	field values may be expressed as names rather than  as
       numeric values. For example tcp[13] may be replaced with	tcp[tcpflags].
       The following TCP flag field values are also available:	tcp-fin,  tcp-
       syn, tcp-rst, tcp-push, tcp-act,	tcp-urg.

       This can	be demonstrated	as:
		   tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'

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

       UDP Packets

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

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

       UDP Name	Server Requests

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

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

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

       UDP Name	Server Responses

       Name server responses are formatted as
	      src _ dst:  id op	rcode flags a/n/au type	class data (len)
	      helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
	      helios.domain > h2opolo.1537: 2 NXDomain*	0/1/0 (97)
       In the first example, helios responds to	query id 3 from	h2opolo	with 3
       answer  records,	 3  name server	records	and 7 additional records.  The
       first answer record is type A (address) and its data  is	 internet  ad-
       dress  128.32.137.3.  The total size of the response was	273 bytes, ex-
       cluding UDP and IP headers.  The	op (Query) and response	code (NoError)
       were omitted, as	was the	class (C_IN) of	the A record.

       In  the second example, helios responds to query	2 with a response code
       of non-existent domain (NXDomain) with no answers, one name server  and
       no  authority records.  The `*' indicates that the authoritative	answer
       bit was set.  Since there were no answers, no type, class or data  were
       printed.

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

       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 the fields mean  see
       www.cifs.org   or  the  pub/samba/specs/	 directory  on	your  favorite
       samba.org mirror	site.  The SMB patches were written by Andrew Tridgell
       (tridge@samba.org).

       NFS Requests and	Replies

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

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

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

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

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

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

       AFS Requests and	Replies

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

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

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

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

       If  the	-v  (verbose) flag is given twice, acknowledgement packets and
       additional header information is	printed, such as the 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 RX call ID, serial number, and the RX packet flags.  The MTU ne-
       gotiation information is	also printed from RX ack packets.

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

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

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

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

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

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

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

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

       IP Fragmentation

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

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

       The  fragment information is output for each fragment.  The first frag-
       ment contains the higher	level protocol header and  the	frag  info  is
       printed	after the protocol info.  Fragments after the first contain no
       higher level protocol header and	the frag info  is  printed  after  the
       source  and destination addresses.  For example,	here is	part of	an ftp
       from arizona.edu	to lbl-rtsg.arpa over a	CSNET connection that  doesn't
       appear to handle	576 byte datagrams:
	      arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
	      arizona >	rtsg: (frag 595a:204@328)
	      rtsg.1170	> arizona.ftp-data: . ack 1536 win 2560
       There are a couple of things to note here:  First, addresses in the 2nd
       line don't include port numbers.	 This is because the TCP protocol  in-
       formation  is  all  in  the first fragment and we have no idea what the
       port or sequence	numbers	are when we print the later  fragments.	  Sec-
       ond,  the  tcp  sequence	information in the first line is printed as if
       there were 308 bytes of user data when, in fact,	there  are  512	 bytes
       (308  in	the first frag and 204 in the second).	If you are looking for
       holes in	the sequence space or trying to	match up  acks	with  packets,
       this can	fool you.

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

       Timestamps

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

SEE ALSO
       stty(1),	 pcap(3PCAP),  bpf(4),	nit(4P),  pcap-savefile(5),  pcap-fil-
       ter(7), pcap-tstamp-type(7)

	      http://www.iana.org/assignments/media-types/application/vnd.tcp-
	      dump.pcap

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/old/tcpdump.tar.Z

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

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

	      tcpdump-workers@lists.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.

				 12 July 2012			    TCPDUMP(1)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | OUTPUT FORMAT | SEE ALSO | AUTHORS | BUGS

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