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TCPDUMP(8)		    System Manager's Manual		    TCPDUMP(8)



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 ]
	       [ 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).

	      Note that when used with -Z option (enabled by default),	privi-
	      leges are dropped before opening first savefile.

       -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.

	      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 host addresses to names.   This  can  be  used  to
	      avoid DNS lookups.

       -nn    Don't convert protocol and port numbers etc. to names either.

       -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 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-
	      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 ``-''.  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  is  enabled  by default (-Z tcpdump), and can be
	      disabled by -Z root.


	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
	      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.


       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), 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(8)