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TCPDUMP(1)							    TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AdDefIKlLnNOpqRStuUvxX ] [ -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 ]
	       [ -y datalinktype ]
	       [ 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
	      expression and were processed by tcpdump);

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

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

       Reading packets from a network interface	may require that you have spe-
       cial  privileges; 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     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
	      interface,  a number and an interface name, possibly followed by
	      a	text description of the	interface, is printed.	The  interface
	      name  or the number can be supplied to the -i flag to specify an
	      interface	on which to capture.

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

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

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

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

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

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

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

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

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

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

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

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

       -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.

       -K     Don't attempt to verify  TCP  checksums.	 This  is  useful  for
	      interfaces  that	perform	 the TCP checksum calculation in hard-
	      ware; 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 and exit.

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

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

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

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

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

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

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

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

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

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

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

       -T     Force  packets  selected	by  "expression" to be interpreted the
	      specified	type.  Currently known	types  are  aodv  (Ad-hoc  On-
	      demand Distance Vector protocol),	cnfp (Cisco NetFlow protocol),
	      rpc (Remote Procedure Call), rtp (Real-Time Applications	proto-
	      col), rtcp (Real-Time Applications control protocol), snmp (Sim-
	      ple Network Management Protocol),	tftp  (Trivial	File  Transfer
	      Protocol),  vat  (Visual	Audio Tool), and wb (distributed White
	      Board).

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

       -tt    Print an unformatted timestamp on	each dump line.

       -ttt   Print a delta (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
	      decoded.

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

       -w     Write the	raw packets to file rather than	parsing	 and  printing
	      them  out.  They can later be printed with the -r	option.	 Stan-
	      dard output is used if file is ``-''.

       -W     Used in conjunction with the -C option, this will	limit the num-
	      ber  of  files  created to the specified number, and begin over-
	      writing files from the beginning,	 thus  creating	 a  'rotating'
	      buffer.  In addition, it will name the files with	enough leading
	      0s to support the	maximum	number of files, allowing them to sort
	      correctly.

	      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(4).

	      Expression arguments can be passed to tcpdump as either a	single
	      argument or as multiple arguments, whichever is more convenient.
	      Generally, if the	expression contains Shell  metacharacters,  it
	      is  easier  to  pass  it as a single, quoted argument.  Multiple
	      arguments	are concatenated with spaces before being parsed.

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

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

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

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

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

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

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

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

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

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

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

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

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP	Packets

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

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

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

       TCP Packets

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

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

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

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

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

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

	      00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

		   00010010

       which translates	to decimal

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

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

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

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

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

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

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

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

       UDP Packets

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

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

       UDP Name	Server Requests

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

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

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

       UDP Name	Server Responses

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

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

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

       Note  that  name	server requests	and responses tend to be large and the
       default snaplen of 68 bytes may not capture enough  of  the  packet  to
       print.	Use  the  -s flag to increase the snaplen if you need to seri-
       ously investigate name server traffic.  `-s 128'	has  worked  well  for
       me.

       SMB/CIFS	decoding

       tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on
       UDP/137,	UDP/138	and TCP/139.  Some primitive decoding of IPX and  Net-
       BEUI SMB	data is	also done.

       By  default  a fairly minimal decode is done, with a much more detailed
       decode done if -v is used.  Be warned that with -v a single SMB	packet
       may  take  up a page or more, so	only use -v if you really want all the
       gory details.

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

       NFS Requests and	Replies

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

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

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

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

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

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

       AFS Requests and	Replies

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

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

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

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

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

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

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

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

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

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

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

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

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

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

       IP Fragmentation

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

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

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

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

       Timestamps

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

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

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.

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

       On Linux	systems	with 2.0[.x] kernels:

	      packets on the loopback device will be seen twice;

	      packet filtering cannot be done in the kernel, so	that all pack-
	      ets must be copied from the kernel in order to  be  filtered  in
	      user mode;

	      all  of  a  packet, not just the part that's within the snapshot
	      length, will be copied from the kernel (the 2.0[.x] packet  cap-
	      ture  mechanism, if asked	to copy	only part of a packet to user-
	      land, will not report the	true length of the packet; this	 would
	      cause most IP packets to get an error from tcpdump);

	      capturing	on some	PPP devices won't work correctly.

       We recommend that you upgrade to	a 2.2 or later kernel.

       Some  attempt should be made to reassemble IP fragments or, at least to
       compute the right length	for the	higher level protocol.

       Name server inverse queries are not dumped correctly: the (empty) ques-
       tion  section  is printed rather	than real query	in the answer section.
       Some believe that inverse queries are themselves	a bug  and  prefer  to
       fix the program generating them rather than tcpdump.

       A  packet  trace	 that crosses a	daylight savings time change will give
       skewed time stamps (the time change is ignored).

       Filter expressions on fields other than those  in  Token	 Ring  headers
       will not	correctly handle source-routed Token Ring packets.

       Filter  expressions  on	fields other than those	in 802.11 headers will
       not correctly handle 802.11 data	packets	with both To DS	 and  From  DS
       set.

       ip6  proto  should  chase header	chain, but at this moment it does not.
       ip6 protochain is supplied for this behavior.

       Arithmetic expression against transport	layer  headers,	 like  tcp[0],
       does not	work against IPv6 packets.  It only looks at IPv4 packets.

				07 January 2008			    TCPDUMP(1)

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

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