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TCPDUMP(1)		    General Commands Manual		    TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

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

       -c     Exit after receiving count packets.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -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>'.

       -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.

       -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''.

       -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),	cnfp (Cisco NetFlow protocol),
	      rpc (Remote Procedure Call), rtp (Real-Time Applications	proto-
	      col), rtcp (Real-Time Applications control protocol), snmp (Sim-
	      ple Network Management Protocol),	tftp  (Trivial	File  Transfer
	      Protocol),  vat  (Visual	Audio Tool), and wb (distributed White
	      Board).

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

       -tt    Print an unformatted timestamp on	each dump line.

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

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

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

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

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

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

       -w     Write the	raw packets to file rather than	parsing	 and  printing
	      them  out.  They can later be printed with the -r	option.	 Stan-
	      dard output is used if file is ``-''.  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     Drops privileges (if root) and changes user ID to	user  and  the
	      group ID to the primary group of user.

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

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

	      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, it
	      is easier	to pass	it as a	single,	quoted argument.  Multiple ar-
	      guments are concatenated with spaces before being	parsed.

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

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

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

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

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

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

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

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

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

       To print	IP broadcast or	multicast packets that were not	sent via  Eth-
       ernet broadcast or multicast:
	      tcpdump 'ether[0]	& 1 = 0	and ip[16] >= 224'

       To print	all ICMP packets that are not echo requests/replies (i.e., not
       ping packets):
	      tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

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

       Link Level Headers

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

       On  FDDI	 networks, the	'-e' option causes tcpdump to print the	`frame
       control'	field,	the source and destination addresses, and  the	packet
       length.	 (The  `frame control' field governs the interpretation	of the
       rest of the packet.  Normal packets (such as those containing IP	 data-
       grams)  are `async' packets, with a priority value between 0 and	7; for
       example,	`async4'.  Such	packets	are assumed to contain an 802.2	 Logi-
       cal  Link  Control (LLC)	packet;	the LLC	header is printed if it	is not
       an ISO datagram or a so-called SNAP packet.

       On Token	Ring networks, the '-e'	option causes  tcpdump	to  print  the
       `access control'	and `frame control' fields, the	source and destination
       addresses, and the packet length.  As on	FDDI networks, packets are as-
       sumed  to contain an LLC	packet.	 Regardless of whether the '-e'	option
       is specified or not, the	source	routing	 information  is  printed  for
       source-routed packets.

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

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

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

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

       ARP/RARP	Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
	      src _ dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP	addresses  and	ports.
       Flags  are  some	 combination of	S (SYN), F (FIN), P (PUSH), R (RST), W
       (ECN CWR) or E (ECN-Echo), or a single `.' (no flags).  Data-seqno  de-
       scribes	the  portion  of  sequence  space  covered by the data in this
       packet (see example below).  Ack	is sequence number of  the  next  data
       expected	 the other direction on	this connection.  Window is the	number
       of bytes	of receive buffer space	available the other direction on  this
       connection.   Urg  indicates there is `urgent' data in the packet.  Op-
       tions are tcp options enclosed in angle brackets	(e.g., <mss 1024>).

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

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

       Csam replies with a similar packet except it  includes  a  piggy-backed
       ack for rtsg's SYN.  Rtsg then acks csam's SYN.	The `.'	means no flags
       were set.  The packet contained no data so there	is  no	data  sequence
       number.	Note that the ack sequence number is a small integer (1).  The
       first time tcpdump sees a tcp `conversation', it	 prints	 the  sequence
       number from the packet.	On subsequent packets of the conversation, the
       difference between the current packet's sequence	number and  this  ini-
       tial  sequence number is	printed.  This means that sequence numbers af-
       ter the first can be interpreted	as relative byte positions in the con-
       versation's  data stream	(with the first	data byte each direction being
       `1').  `-S' will	override this feature, causing the  original  sequence
       numbers to be output.

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

	      00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

		   00010010

       which translates	to decimal

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

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

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

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

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

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

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

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

       NFS Requests and	Replies

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

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

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

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

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

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

       AFS Requests and	Replies

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

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

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

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

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

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

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

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

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

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

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

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

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

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

       IP Fragmentation

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

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

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

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

       Timestamps

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

SEE ALSO
       stty(1),	pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5),	pcap-filter(7)

AUTHORS
       The original authors are:

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

       It is currently being maintained	by tcpdump.org.

       The current version is available	via http:

	      http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

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

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

				 05 March 2009			    TCPDUMP(1)

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

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