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 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 ex-
              pression and were processed by tcpdump);

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

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

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

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

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

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

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

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

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

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

       -F file
              Use file as input for the filter expression.  An additional  ex-
              pression 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 mi-
       crosecond 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  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

              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 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  in-
              put 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 de-
              fault 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-de-
              mand  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), resp (REdis Serialization Protocol),
              radius (RADIUS), rpc (Remote Procedure Call), rtp (Real-Time Ap-
              plications protocol), rtcp (Real-Time Applications control  pro-
              tocol), snmp (Simple Network Management Protocol), tftp (Trivial
              File Transfer Protocol), vat (Visual Audio Tool),  wb  (distrib-
              uted 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 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 de-
              coding 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  de-

       -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 re-

              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, change the user ID to user and the  group  ID
              to the primary group of user.

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

              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 as-
       sumed to contain an LLC packet.  Regardless of whether the '-e'  option
       is  specified  or  not,  the  source routing information is printed for
       source-routed packets.

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

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

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

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

       ARP/RARP Packets

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

       This would look less redundant if we had done tcpdump -n:
              arp who-has 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  re-
       ported  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 to-
       tal 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  de-
       pend 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.
              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  se-
       quence  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  relative  byte
       positions  in  the conversation's data stream (with the first data byte
       each direction being `1').  `-S' will override  this  feature,  causing
       the original sequence numbers to be output.

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

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

       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 (bi-
       nary 00000010), so we know that for packets with SYN set the  following
       relation must hold true:

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

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

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

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

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

       UDP Packets

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

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

       UDP Name Server Requests

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

       Name server requests are formatted as
              src > dst: id op? flags qtype qclass name (len)
              h2opolo.1538 > helios.domain: 3+ A? (37)
       Host  h2opolo  asked  the domain server on helios for an address record
       (qtype=A) associated with the name  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 ad-
       dress  The total size of the response was 273 bytes,  ex-
       cluding UDP and IP headers.  The op (Query) and response code (NoError)
       were omitted, as was the class (C_IN) of the A record.

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

       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  or  the  pub/samba/specs/  directory  on  your   favorite 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:
     > 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.  In-
       stead, tcpdump keeps track of ``recent'' requests, and matches them  to
       the replies using the transaction ID.  If a reply does not closely fol-
       low the corresponding request, it might not be parsable.

       AFS Requests and Replies

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

     > dst.dport: rx packet-type
     > dst.dport: rx packet-type service call call-name args
     > dst.dport: rx packet-type service reply call-name args
              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ""
                   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 `', 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 ne-
       gotiation information is also printed from RX ack packets.

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

       AppleTalk addresses are printed in the form

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

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

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

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

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

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

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

       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)
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