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NAME

       tcpdump - dump traffic on a network

SYNOPSIS

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

DESCRIPTION

       Tcpdump  prints  out  a  description  of  the  contents of packets on a
       network 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  (please  note  tcpdump  is  protected   via   an   enforcing
       apparmor(7)  profile  in  Ubuntu  which  limits  the  files tcpdump may
       access).  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
       signal  (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
       special privileges; see the pcap (3PCAP) man page for details.  Reading
       a saved packet file doesn't require special privileges.

OPTIONS

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

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

       -B buffer_size
       --buffer-size=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
              count).

       -D
       --list-interfaces
              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()
              function.

       -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
              separation.

              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
              permissions that tcpdump may have been given should already have
              been given up.

       -f     Print   `foreign'   IPv4   addresses   numerically  rather  than
              symbolically (this option is  intended  to  get  around  serious
              brain  damage  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>'.

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

       --version
              Print the tcpdump and libpcap version strings and exit.

       -H     Attempt to detect 802.11s draft mesh headers.

       -i interface
       --interface=interface
              Listen  on  interface.   If  unspecified,  tcpdump  searches the
              system interface list for the  lowest  numbered,  configured  up
              interface  (excluding  loopback),  which may turn out to be, for
              example, ``eth0''.

              On Linux  systems  with  2.2  or  later  kernels,  an  interface
              argument  of  ``any''  can  be  used to capture packets from all
              interfaces.  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
       --monitor-mode
              Put the interface in "monitor mode"; this is supported  only  on
              IEEE  802.11  Wi-Fi  interfaces,  and  supported  only  on  some
              operating 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.

       -j tstamp_type
       --time-stamp-type=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.

       -J
       --list-time-stamp-types
              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.

       --time-stamp-precision=tstamp_precision
              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.

       -K
       --dont-verify-checksums
              Don't attempt to verify IP, TCP,  or  UDP  checksums.   This  is
              useful 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

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

              -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.

       -L
       --list-data-link-types
              List  the  known  data  link  types  for  the  interface, in the
              specified 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  support  802.11 headers with radio information) and another
              set of data link types when in monitor  mode  (for  example,  it
              might  support  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
              tcpdump.

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

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

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

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

       -O
       --no-optimize
              Do not run the packet-matching code optimizer.  This  is  useful
              only if you suspect a bug in the optimizer.

       -p
       --no-promiscuous-mode
              Don't  put  the  interface into promiscuous mode.  Note that the
              interface might be in promiscuous mode for  some  other  reason;
              hence,  `-p'  cannot  be used as an abbreviation for `ether host
              {local-hw-addr} or ether broadcast'.

       -Q direction
       --direction=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
              output lines are shorter.

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

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

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

       -s snaplen
       --snapshot-length=snaplen
              Snarf  snaplen  bytes  of  data from each packet rather than the
              default of 65535 bytes.  Packets truncated because of a  limited
              snapshot  are  indicated  in the output with ``[|proto]'', where
              proto is the name of the protocol level at which the  truncation
              has  occurred.  Note that taking larger snapshots both increases
              the amount of time it takes to process packets and, effectively,
              decreases  the  amount  of  packet  buffering.   This  may cause
              packets 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 backwards 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
              Redundancy  Protocol),  cnfp (Cisco NetFlow protocol), lmp (Link
              Management  Protocol),  pgm   (Pragmatic   General   Multicast),
              pgm_zmtp1  (ZMTP/1.0  inside  PGM/EPGM),  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 Protocol 1.0) and  vxlan
              (Virtual eXtensible Local Area Network).

              Note  that  the  pgm type above affects UDP interpretation only,
              the  native  PGM  is  always  recognised  as  IP  protocol   113
              regardless.  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 an unformatted timestamp on each dump line.

       -ttt   Print  a  delta  (micro-second  resolution)  between current and
              previous 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
       --packet-buffered
              If the -w option is  not  specified,  make  the  printed  packet
              output  ``packet-buffered'';  i.e.,  as  the  description of the
              contents of each packet is printed, it will be  written  to  the
              standard  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()
              function.

       -v     When  parsing  and  printing,  produce  (slightly  more) verbose
              output.  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
              seconds, 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.

       -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.
              Standard 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
              received.

              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
              number  of  files  created  to  the  specified number, and begin
              overwriting 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
              number 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
       --linktype=datalinktype
              Set  the  data  link  type  to  use  while  capturing packets to
              datalinktype.

       -z postrotate-command
              Used in conjunction with the -C or -G options,  this  will  make
              tcpdump  run  "  postrotate-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
              capture,  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
       --relinquish-privileges=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.

        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).

              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
              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
       Ethernet 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
       datagrams)  are `async' packets, with a priority value between 0 and 7;
       for example, `async4'.  Such packets are assumed to  contain  an  802.2
       Logical  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
       compression algorithm described in RFC-1144.)

       On  SLIP  links,  a  direction  indicator (``I'' for inbound, ``O'' for
       outbound), 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
       packets,  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  number),  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
       format  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
       Ethernet 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
       protocol  described  in  RFC-793.   If  you  are  not familiar with the
       protocol, neither this description nor tcpdump will be of much  use  to
       you.)

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src  and  dst  are  the  source and destination IP addresses and ports.
       Flags are some combination of S (SYN), F (FIN), P (PUSH),  R  (RST),  U
       (URG),  W  (ECN  CWR), E (ECN-Echo) or `.' (ACK), or `none' if no flags
       are set.  Data-seqno describes the portion of sequence space covered by
       the data in this packet (see example below).  Ack is sequence number of
       the next data expected the other direction on this connection.   Window
       is  the  number  of  bytes  of receive buffer space available the other
       direction on this connection.  Urg indicates there is `urgent' data  in
       the  packet.  Options are tcp options enclosed in angle brackets (e.g.,
       <mss 1024>).

       Src, dst and flags are always present.  The other fields depend on  the
       contents  of  the  packet's  tcp protocol header and are output only if
       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 the ACK
       flag was set.  The packet  contained  no  data  so  there  is  no  data
       sequence  number.  Note that the ack sequence number is a small integer
       (1).  The first time tcpdump sees a tcp `conversation', it  prints  the
       sequence  number  from  the  packet.   On  subsequent  packets  of  the
       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  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
       sitting  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
       decimal 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
       representation 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
       following relation must hold true:

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

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

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

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

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

       UDP Packets

       UDP format is illustrated by this rwho packet:
              actinide.who > broadcast.who: udp 84
       This  says  that  port who on host actinide sent a udp datagram to port
       who on host broadcast, the  Internet  broadcast  address.   The  packet
       contained 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
       particular,  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
       headers.   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
       (NoError) were omitted, as was the class (C_IN) of the A record.

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

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

       SMB/CIFS decoding

       tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on
       UDP/137,  UDP/138  and  TCP/139.   Some  primitive  decoding of IPX and
       NetBEUI 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
       (tridge@samba.org).

       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
       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.) 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
       offset 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
       successful, 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 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
       discarded).  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
       requesting 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,
       jssmag.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
       indicates 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
       fragment 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.
       Second, 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
       timestamp 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), pcap-tstamp(7), apparmor(7)

              http://www.iana.org/assignments/media-
              types/application/vnd.tcpdump.pcap

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/old/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
       recommend 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
              packets 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
              capture  mechanism,  if  asked  to copy only part of a packet to
              userland, 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)
       question  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.

                                 11 July 2014                       TCPDUMP(8)



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