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

       tcpdump - dump traffic on a network

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

       Tcpdump	prints	out a description of the contents of packets on a net-
       work interface that match the boolean expression;  the  description  is
       preceded	 by a time stamp, printed, by default, as hours, minutes, sec-
       onds, and fractions of a second since midnight.	It  can	 also  be  run
       with the -w flag, which causes it to save the packet data to a file for
       later analysis, and/or with the -r flag, which causes it to read from a
       saved packet file rather than to read packets from a network interface.
       It can also be run with the -V flag, which causes it to read a list  of
       saved  packet  files.  In all cases, only packets that match expression
       will be processed by tcpdump.

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

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

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

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

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

       On platforms that  support  the	SIGINFO	 signal,  such	as  most  BSDs
       (including  Mac	OS  X)	and  Digital/Tru64  UNIX, it will report those
       counts when it receives a SIGINFO signal (generated,  for  example,  by
       typing your ``status'' character, typically control-T, although on some
       platforms, such as Mac OS X, the ``status'' character  is  not  set  by
       default,	 so  you must set it with stty(1) in order to use it) and will
       continue capturing packets. On platforms that do not support  the  SIG-
       INFO signal, the same can be achieved by using the SIGUSR1 signal.

       Reading packets from a network interface may require that you have spe-
       cial privileges; see the pcap (3PCAP) man page for details.  Reading  a
       saved packet file doesn't require special privileges.

       -A     Print each packet (minus its link level header) in ASCII.	 Handy
	      for capturing web pages.

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

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

       -c count
	      Exit after receiving count packets.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	      Print the tcpdump and libpcap version strings and exit.

       -H     Attempt to detect 802.11s draft mesh headers.

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

	      On  Linux	 systems with 2.2 or later kernels, an interface argu-
	      ment of ``any'' can be used to capture packets from  all	inter-
	      faces.   Note  that  captures  on the ``any'' device will not be
	      done in promiscuous mode.

	      If the -D flag is supported, an interface number as  printed  by
	      that flag can be used as the interface argument, if no interface
	      on the system has that number as a name.

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

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

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

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

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

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

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

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

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

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

       -l     Make stdout line buffered.  Useful if you want to see  the  data
	      while capturing it.  E.g.,

		     tcpdump -l | tee dat


		     tcpdump -l > dat & tail -f dat

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

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

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

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

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

       -n     Don't convert 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

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

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

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

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

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

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

	      Print absolute, rather than relative, TCP sequence numbers.

       -s snaplen
	      Snarf snaplen bytes of data from each  packet  rather  than  the
	      default of 262144 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 262144, for back-
	      wards compatibility with recent older versions of tcpdump.

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

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

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

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

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

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

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

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

       -u     Print undecoded NFS handles.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -x     When parsing and printing, in addition to printing  the  headers
	      of  each	packet,	 print the data of each packet (minus its link
	      level header) in hex.  The  smaller  of  the  entire  packet  or
	      snaplen  bytes  will  be	printed.  Note that this is the entire
	      link-layer packet, so for link layers that pad (e.g.  Ethernet),
	      the  padding  bytes  will	 also be printed when the higher layer
	      packet is shorter than the required padding.

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

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

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

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

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

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

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

       -Z user
	      If tcpdump is running as root, after opening the capture	device
	      or  input savefile, but before opening any savefiles for output,
	      change the user ID to user and the group ID to the primary group
	      of user.

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

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

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

	      The  expression  argument	 can  be passed to tcpdump as either a
	      single Shell argument, or as multiple Shell arguments, whichever
	      is more convenient.  Generally, if the expression contains Shell
	      metacharacters, such as  backslashes  used  to  escape  protocol
	      names,  it  is  easier  to  pass it as a single, quoted argument
	      rather than to escape the Shell metacharacters.  Multiple	 argu-
	      ments are concatenated with spaces before being parsed.

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

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

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

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

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

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

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

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

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

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

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

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


       By  default,  all  output lines are preceded by a timestamp.  The time-
       stamp is the current clock time in the form
       and is as accurate as the kernel's clock.  The timestamp	 reflects  the
       time the kernel applied a time stamp to the packet.  No attempt is made
       to account for the time lag between when the network interface finished
       receiving  the  packet  from  the network and when the kernel applied a
       time stamp to the packet; that time lag could include a	delay  between
       the  time  when	the network interface finished receiving a packet from
       the network and the time when an interrupt was delivered to the	kernel
       to get it to read the packet and a delay between the time when the ker-
       nel serviced the `new packet' interrupt and the time when it applied  a
       time stamp to the packet.

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP Packets

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

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

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

       IPv4 Packets

       If the link-layer header is not being printed, for IPv4 packets, IP  is
       printed after the time stamp.

       If  the -v flag is specified, information from the IPv4 header is shown
       in parentheses after the IP or the link-layer header.  The general for-
       mat of this information is:
	      tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
       tos  is	the type of service field; if the ECN bits are non-zero, those
       are reported as ECT(1), ECT(0), or CE.  ttl is the time-to-live; it  is
       not reported if it is zero.  id is the IP identification field.	offset
       is the fragment offset field; it is printed whether this is part	 of  a
       fragmented  datagram  or	 not.	flags  are  the	 MF and DF flags; + is
       reported if MF is set, and DFP is reported if F is set.	If neither are
       set,  .	is  reported.	proto is the protocol ID field.	 length is the
       total length field.  options are the IP options, if any.

       Next, for TCP and UDP packets, the source and destination IP  addresses
       and TCP or UDP ports, with a dot between each IP address and its corre-
       sponding port, will be printed, with a > separating the source and des-
       tination.  For other protocols, the addresses will be printed, with a >
       separating the source and destination.  Higher level protocol  informa-
       tion, if any, will be printed after that.

       For  fragmented	IP  datagrams,	the first fragment contains the higher
       level protocol header; fragments after  the  first  contain  no	higher
       level  protocol header.	Fragmentation information will be printed only
       with the -v flag, in the IP header information, as described above.

       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,
       this description will not be of much use to you.)

       The general format of a TCP protocol line is:
	      src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
       Src and dst are the source and  destination  IP	addresses  and	ports.
       Tcpflags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U
       (URG), W (ECN CWR), E (ECN-Echo) or `.' (ACK), or `none'	 if  no	 flags
       are set.	 Data-seqno describes the portion of sequence space covered by
       the data in this packet (see example below).  Ackno is sequence	number
       of the next data expected the other direction on this connection.  Win-
       dow 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.  Opts are TCP options (e.g., mss 1024).  Len is the	length
       of payload data.

       Iptype,	Src,  dst,  and	 flags	are  always present.  The other fields
       depend on the contents of the packet's TCP protocol header and are out-
       put only if appropriate.

       Here is the opening portion of an rlogin from host rtsg to host csam.
	      IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
	      IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
	      IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
	      IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
	      IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
	      IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
	      IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
	      IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
	      IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 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' which means `sequence numbers first up to but not  includ-
       ing last.)  There was no piggy-backed ack, the available receive window
       was 4096 bytes and there was a max-segment-size	option	requesting  an
       mss of 1024 bytes.

       Csam  replies  with  a similar packet except it includes a piggy-backed
       ack for rtsg's SYN.  Rtsg then acks csam's SYN.	The `.' means the  ACK
       flag  was  set.	 The  packet  contained	 no  data  so there is no data
       sequence number or length.  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 initial	 sequence  number  is  printed.	  This
       means that sequence numbers after the first can be interpreted as rela-
       tive byte positions in the conversation's data stream (with  the	 first
       data  byte each direction being `1').  `-S' will override this feature,
       causing the original sequence numbers to be output.

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

		       |	       |
		       |7   5	3     0|

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

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

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

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

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


       and its decimal representation is

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

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

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

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

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

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

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

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


       which translates to decimal

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

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

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

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

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

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

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

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

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

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

       UDP Packets

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

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

       UDP Name Server Requests

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

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

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

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

       SMB/CIFS decoding

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

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

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

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
	      src.sport > dst.nfs: NFS request xid xid len op args
	      src.nfs > dst.dport: NFS reply xid xid reply stat len op results
	      sushi.1023 > wrl.nfs: NFS request xid 26377
		   112 readlink fh 21,24/10.73165
	      wrl.nfs > sushi.1023: NFS reply xid 26377
		   reply ok 40 readlink "../var"
	      sushi.1022 > wrl.nfs: NFS request xid 8219
		   144 lookup fh 9,74/4096.6878 "xcolors"
	      wrl.nfs > sushi.1022: NFS reply xid 8219
		   reply ok 128 lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id 26377 to wrl.
       The request was 112 bytes, excluding the UDP and IP headers.  The oper-
       ation  was  a  readlink	(read  symbolic	 link)	on  file  handle  (fh)
       21,24/10.731657119.  (If one is lucky, as in this case, the file handle
       can be interpreted as a major,minor device number pair, followed by the
       inode number and generation number.) In the second  line,  wrl  replies
       `ok' with the same transaction id and the contents of the link.

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

       Note  that  the data printed depends on the operation type.  The format
       is intended to be self explanatory if read in conjunction with  an  NFS
       protocol	 spec.	 Also  note that older versions of tcpdump printed NFS
       packets in a slightly different format: the transaction id (xid)	 would
       be printed instead of the non-NFS port number of the packet.

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

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

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

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

       AFS Requests and Replies

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

	      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

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

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

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

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

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

       NBP packets are formatted like the following examples:
	      icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=: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.

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


       The original authors are:

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

       It is currently being maintained by tcpdump.org.

       The current version is available via http:


       The original distribution is available via anonymous ftp:


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

       To   report   a	 security   issue   please   send   an	  e-mail    to

       To  report  bugs and other problems, contribute patches, request a fea-
       ture, provide generic feedback etc please see the file CONTRIBUTING  in
       the tcpdump source tree root.

       NIT doesn't let you watch your own outbound traffic, BPF will.  We rec-
       ommend that you use the latter.

       On Linux systems with 2.0[.x] kernels:

	      packets on the loopback device will be seen twice;

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

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

	      capturing on some PPP devices won't work correctly.

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

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

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

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

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

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

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

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

				2 February 2017			    TCPDUMP(8)