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tcpdump(1)                                                          tcpdump(1)




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 ]
               [ -P in|out|inout ]
               [ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ]
               [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
               [ expression ]


DESCRIPTION

       Tcpdump  prints  out a description of the contents of packets on a net-
       work interface that match the boolean expression.  It can also  be  run
       with the -w flag, which causes it to save the packet data to a file for
       later analysis, and/or with the -r flag, which causes it to read from a
       saved packet file rather than to read packets from a network interface.
       It can also be run with the -V flag, which causes it to read a list  of
       saved  packet  files.  In all cases, only packets that match expression
       will be processed by tcpdump.

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

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

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

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

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

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

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


OPTIONS

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

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

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

       -c     Exit after receiving count packets.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -H     Attempt to detect 802.11s draft mesh headers.

       -i     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.

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

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

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

       -j     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-
              type(7);  not  all  the  types  listed there will necessarily be
              valid for any given interface.

       -J     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.

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

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

                     tcpdump -l | tee dat

              or

                     tcpdump -l > dat & tail -f dat

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

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

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

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

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

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

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

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

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

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

       -s     Snarf  snaplen  bytes  of  data from each packet rather than the
              default of 65535 bytes.  Packets truncated because of a  limited
              snapshot  are  indicated  in the output with ``[|proto]'', where
              proto is the name of the protocol level at which the  truncation
              has  occurred.  Note that taking larger snapshots both increases
              the amount of time it takes to process packets and, effectively,
              decreases  the amount of packet buffering.  This may cause pack-
              ets to be lost.  You should limit snaplen to the smallest number
              that will capture the protocol information you're interested in.
              Setting snaplen to 0 sets it to the default of 65535, for  back-
              wards compatibility with recent older versions of tcpdump.

       -T     Force  packets  selected  by  "expression" to be interpreted the
              specified type.  Currently known  types  are  aodv  (Ad-hoc  On-
              demand  Distance  Vector  protocol), carp (Common Address Redun-
              dancy Protocol), cnfp (Cisco NetFlow protocol), lmp  (Link  Man-
              agement  Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1
              (ZMTP/1.0 inside PGM/EPGM), radius (RADIUS), rpc (Remote  Proce-
              dure  Call),  rtp (Real-Time Applications protocol), rtcp (Real-
              Time Applications control protocol), snmp (Simple  Network  Man-
              agement  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 regard-
              less. UDP-encapsulated PGM is often called "EPGM" or  "PGM/UDP".

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

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

       -tt    Print an unformatted timestamp on each dump line.

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

       -tttt  Print  a  timestamp  in default format proceeded by date on each
              dump line.

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

       -u     Print undecoded NFS handles.

       -U     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-
              tion.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -Z     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  argu-
              ments are concatenated with spaces before being parsed.


EXAMPLES

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

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

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

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

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

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

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

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

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

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

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


OUTPUT FORMAT

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

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP  addresses  and  ports.
       Flags  are  some  combination of S (SYN), F (FIN), P (PUSH), R (RST), 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  conver-
       sation, the difference between the current packet's sequence number and
       this initial sequence number is printed.  This means that sequence num-
       bers  after  the first can be interpreted as relative byte positions in
       the conversation's data stream (with the first data byte each direction
       being  `1').   `-S'  will  override  this feature, causing the original
       sequence numbers to be output.

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

                   00010010

       which translates to decimal

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

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

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


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

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

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

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

       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
       greek.)

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

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

       UDP Name Server Responses

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

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

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

       SMB/CIFS decoding

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

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

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

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

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

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

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

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

       AFS Requests and Replies

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

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

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

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

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

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

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

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

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

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


       KIP AppleTalk (DDP in UDP)

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

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

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

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

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

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

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

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

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


       IP Fragmentation

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

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

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

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

       Timestamps

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


SEE ALSO

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

              http://www.iana.org/assignments/media-types/application/vnd.tcp-
              dump.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 rec-
       ommend that you use the latter.

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

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

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

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

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

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

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

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

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



                                 12 July 2012                       tcpdump(1)

tcpdump 4.5.1 - Generated Sun Dec 1 07:42:15 CST 2013