Command Section
TCPDUMP(1)							    TCPDUMP(1)

       tcpdump - dump traffic on a network

       tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX# ]	[ -B buffer_size ]
	       [ -c count ]
	       [ -C file_size ]	[ -G rotate_seconds ] [	-F file	]
	       [ -i interface ]	[ -j tstamp_type ] [ -m	module ] [ -M secret ]
	       [ --number ] [ -Q in|out|inout ]
	       [ -r file ] [ -V	file ] [ -s snaplen ] [	-T type	] [ -w file ]
	       [ -W filecount ]
	       [ -E [email protected] algo:secret,...	]
	       [ -y datalinktype ] [ -z	postrotate-command ] [ -Z user ]
	       [ --time-stamp-precision=tstamp_precision ]
	       [ --immediate-mode ] [ --version	]
	       [ expression ]

       Tcpdump prints out a description	of the contents	of packets on  a  net-
       work  interface	that  match the	boolean	expression; the	description is
       preceded	by a time stamp, printed, by default, as hours,	minutes,  sec-
       onds,  and  fractions  of  a second since midnight.  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
	      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. On platforms	that do	not support  the  SIG-
       INFO signal, the	same can be achieved by	using the SIGUSR1 signal.

       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.

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

       -c count
	      Exit after receiving count packets.

       -C file_size
	      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

	      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-

       -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 [email protected] 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-

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

       -G rotate_seconds
	      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>'.

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

	      Print the	tcpdump	and libpcap version strings and	exit.

       -H     Attempt to detect	802.11s	draft mesh headers.

       -i interface
	      Listen on	interface.  If unspecified, tcpdump searches the  sys-
	      tem interface list for the lowest	numbered, configured up	inter-
	      face (excluding loopback), which may turn	out to be,  for	 exam-
	      ple, ``eth0''.

	      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.

	      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.

	      Capture  in  "immediate mode".  In this mode, packets are	deliv-
	      ered to tcpdump as  soon	as  they  arrive,  rather  than	 being
	      buffered	for  efficiency.   This	 is  the default when printing
	      packets rather than saving packets  to  a	 ``savefile''  if  the
	      packets are being	printed	to a terminal rather than to a file or

       -j tstamp_type
	      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(7); not all the types listed there	 will  necessarily  be
	      valid for	any given interface.

	      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.

	      When  capturing, set the time stamp precision for	the capture to
	      tstamp_precision.	 Note that availability	of high	precision time
	      stamps  (nanoseconds)  and their actual accuracy is platform and
	      hardware dependent.  Also	note that when writing	captures  made
	      with  nanosecond	accuracy  to  a	 savefile, the time stamps are
	      written with nanosecond resolution, and the file is written with
	      a	 different  magic number, to indicate that the time stamps are
	      in seconds and nanoseconds; not  all  programs  that  read  pcap
	      savefiles	will be	able to	read those captures.

       When reading a savefile,	convert	time stamps to the precision specified
       by timestamp_precision, and display them	with that resolution.  If  the
       precision  specified  is	 less than the precision of time stamps	in the
       file, the conversion will lose precision.

       The supported values for	timestamp_precision are	micro for  microsecond
       resolution   and	 nano  for  nanosecond	resolution.   The  default  is
       microsecond resolution.

	      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


		     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

	      -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.

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

       -M secret
	      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

	      Print an optional	packet number at the beginning of the line.

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

	      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 direction
	      Choose send/receive direction direction for which	packets	should
	      be captured. Possible values are `in', `out'  and	 `inout'.  Not
	      available	on all platforms.

       -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 file
	      Read packets from	file (which was	created	with the -w option  or
	      by  other	 tools	that  write  pcap or pcap-ng files).  Standard
	      input is used if file is ``-''.

	      Print absolute, rather than relative, TCP	sequence numbers.

       -s snaplen
	      Snarf snaplen bytes of data from each  packet  rather  than  the
	      default  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 type
	      Force packets selected by	"expression"  to  be  interpreted  the
	      specified	 type.	 Currently  known  types  are aodv (Ad-hoc On-
	      demand Distance Vector protocol),	carp  (Common  Address	Redun-
	      dancy  Protocol),	 cnfp (Cisco NetFlow protocol),	lmp (Link Man-
	      agement Protocol), pgm (Pragmatic	General	Multicast),  pgm_zmtp1
	      (ZMTP/1.0	 inside	PGM/EPGM), radius (RADIUS), rpc	(Remote	Proce-
	      dure Call), rtp (Real-Time Applications protocol),  rtcp	(Real-
	      Time  Applications  control protocol), snmp (Simple Network Man-
	      agement Protocol), tftp (Trivial File  Transfer  Protocol),  vat
	      (Visual Audio Tool), wb (distributed White Board), zmtp1 (ZeroMQ
	      Message Transport	Protocol 1.0) and  vxlan  (Virtual  eXtensible
	      Local Area Network).

	      Note  that  the  pgm type	above affects UDP interpretation only,
	      the native PGM is	always recognised as IP	protocol  113  regard-
	      less.  UDP-encapsulated PGM is often called "EPGM" or "PGM/UDP".

	      Note that	the pgm_zmtp1 type  above  affects  interpretation  of
	      both  native PGM and UDP at once.	During the native PGM decoding
	      the application data of an ODATA/RDATA packet would  be  decoded
	      as  a  ZeroMQ  datagram  with  ZMTP/1.0  frames.	During the UDP
	      decoding in addition to that any UDP packet would	be treated  as
	      an encapsulated PGM packet.

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

       -tt    Print the	timestamp, as seconds since January 1, 1970, 00:00:00,
	      UTC, and fractions of a second since that	 time,	on  each  dump

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

       -tttt  Print a timestamp, as hours, minutes, seconds, and fractions  of
	      a	 second	 since	midnight,  preceded  by	the date, on each dump

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

       -u     Print undecoded NFS handles.

	      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-

       -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

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

       -w file
	      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

	      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

	      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 datalinktype
	      Set the data  link  type	to  use	 while	capturing  packets  to

       -z postrotate-command
	      Used  in	conjunction  with the -C or -G options,	this will make
	      tcpdump run " postrotate-command file " where file is the	 save-
	      file  being  closed after	each rotation. For example, specifying
	      -z gzip or -z bzip2 will compress	each savefile  using  gzip  or

	      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 user
	      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.

	      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).

	      The expression argument can be passed to	tcpdump	 as  either  a
	      single Shell argument, or	as multiple Shell 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 argu-
	      ments are	concatenated with spaces before	being parsed.

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

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

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

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

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

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

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

       To  print  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'

       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 tell
	      arp reply is-at 02:07:01:00:01:c4

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

       TCP Packets

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

       The general format of a tcp protocol line is:
	      src > dst: flags data-seqno ack window urgent options
       Src  and	 dst  are  the	source and destination IP addresses and	ports.
       Flags are some combination of S (SYN), F	(FIN), P (PUSH),  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
       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

       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
       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 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
       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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

		       |	       |
		       |7   5	3     0|

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

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

		       |0 0 0 0	0 0 1 0|
		       |7 6 5 4	3 2 1 0|

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

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


       and its decimal representation is

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

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

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

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

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

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

	    |0 0 0 1 0 0 1 0|
	    |7 6 5 4 3 2 1 0|

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


       which translates	to decimal

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

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

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

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

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

       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

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

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

       UDP Name	Server Responses

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

       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  or	 the  pub/samba/specs/	directory  on  your   favorite mirror	site.  The SMB patches were written by Andrew Tridgell
       ([email protected]).

       NFS Requests and	Replies

       Sun NFS (Network	File System) requests and replies are printed as:	> dst.nfs: NFS request xid xid len op args
	      src.nfs >	dst.dport: NFS reply xid xid reply stat	len op results
	      sushi.1023 > wrl.nfs: NFS	request	xid 26377
		   112 readlink	fh 21,24/10.73165
	      wrl.nfs >	sushi.1023: NFS	reply xid 26377
		   reply ok 40 readlink	"../var"
	      sushi.1022 > wrl.nfs: NFS	request	xid 8219
		   144 lookup fh 9,74/4096.6878	"xcolors"
	      wrl.nfs >	sushi.1022: NFS	reply xid 8219
		   reply ok 128	lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id 26377 to wrl.
       The request was 112 bytes, excluding the	UDP and	IP headers.  The oper-
       ation  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.) In the second line, wrl replies
       `ok' with the same transaction id and the contents of the link.

       In the third line, sushi	asks (using  a	new  transaction  id)  wrl  to
       lookup  the  name  `xcolors'  in	 directory file	9,74/4096.6878.	In the
       fourth line, wrl	sends a	reply with the respective transaction id.

       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.  Also note that older versions of	 tcpdump  printed  NFS
       packets	in a slightly different	format:	the transaction	id (xid) would
       be printed instead of the non-NFS port number of	the packet.

       If the -v (verbose) flag	is given, additional information  is  printed.
       For example:
	      sushi.1023 > wrl.nfs: NFS	request	xid 79658
		   148 read fh 21,11/12.195 8192 bytes @ 24576
	      wrl.nfs >	sushi.1023: NFS	reply xid 79658
		   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v  also  prints  the  IP  header  TTL,	 ID, length, and fragmentation
       fields, which have been omitted from this example.)  In the first line,
       sushi  asks wrl to read 8192 bytes from file 21,11/12.195, at byte off-
       set 24576.  Wrl replies `ok'; the packet	shown on the  second  line  is
       the first fragment of the reply,	and hence is only 1472 bytes long (the
       other bytes will	follow in subsequent fragments,	but these fragments do
       not have	NFS or even UDP	headers	and so might not be printed, depending
       on the filter expression	used).	Because	the -v flag is given, some  of
       the  file  attributes (which are	returned in addition to	the file data)
       are printed: the	file type (``REG'', for	regular	file), the  file  mode
       (in octal), the uid and gid, and	the file size.

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

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

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

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

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

       If  the	-v  (verbose) flag is given twice, acknowledgement packets and
       additional header information is	printed, such as the 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
       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 > 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

       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: "=:[email protected]*"
	      jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:[email protected]*" 250
	      techpit.2	> icsd-net.112.220: nbp-reply 190: "techpit:[email protected]*" 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 to over a CSNET connection that doesn't
       appear to handle	576 byte datagrams:
	      arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:[email protected]+)
	      arizona >	rtsg: (frag 595a:[email protected])
	      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


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

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

       The original authors are:

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

       It is currently being maintained	by

       The current version is available	via http:

       The original distribution is available via anonymous ftp:

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

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

	      [email protected]

       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

       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.

				 11 July 2014			    TCPDUMP(1)
Command Section