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man tcpdump(1)
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tcpdump(1)                                                          tcpdump(1)


       tcpdump - dump traffic on a network


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


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

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

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

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

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

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

       On  platforms  that  support  the  SIGINFO  signal,  such  as most BSDs
       (including macOS) 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 macOS, the ``status'' character is not set by default,
       so you must set it with stty(1) in order to use it) and  will  continue
       capturing packets. On platforms that do not support the SIGINFO signal,
       the same can be achieved by using the SIGUSR1 signal.

       Using the SIGUSR2 signal along with the -w flag will forcibly flush the
       packet buffer into the output file.

       Reading packets from a network interface may require that you have spe-
       cial privileges; see the pcap(3) 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.

              Print only on stderr  the  packet  count  when  reading  capture
              file(s)  instead of parsing/printing the packets. If a filter is
              specified on the command line, tcpdump counts only packets  that
              were matched by the filter expression.

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

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

              Please mind that although code compilation  is  always  DLT-spe-
              cific,  typically  it is impossible (and unnecessary) to specify
              which DLT to use for the dump because tcpdump  uses  either  the
              DLT of the input pcap file specified with -r, or the default DLT
              of the network interface specified with -i,  or  the  particular
              DLT  of  the  network interface specified with -y and -i respec-
              tively. In these cases the dump shows the same exact  code  that
              would filter the input file or the network interface without -d.

              However, when neither -r nor -i is specified, specifying -d pre-
              vents  tcpdump  from  guessing a suitable network interface (see
              -i).  In this case the DLT defaults to EN10MB and can be set  to
              another valid value manually with -y.

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

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

              Print the list of the network interfaces available on the system
              and  on  which  tcpdump  can  capture packets.  For each network
              interface, a number and an interface name, possibly followed  by
              a text description of the interface, are 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(3PCAP)

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

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

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

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

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

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

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

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

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

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

       -G rotate_seconds
              If specified, rotates the dump file specified with the -w option
              every  rotate_seconds  seconds.   Savefiles  will  have the name
              specified by -w which should include a time format as defined by
              strftime(3).  If no time format is specified, each new file will
              overwrite the previous.  Whenever a generated  filename  is  not
              unique,  tcpdump will overwrite the pre-existing data; providing
              a time specification that is coarser than the capture period  is
              therefore not advised.

              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,  report the list of link-layer types, report the list of
              time stamp types, or report the results of  compiling  a  filter
              expression  on  interface.  If unspecified and if the -d flag is
              not given, tcpdump searches the system interface  list  for  the
              lowest  numbered,  configured up interface (excluding loopback),
              which may turn out to be, for example, ``eth0''.

              On Linux systems with 2.2 or later kernels, an  interface  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 preci-
              sion of time stamps in the file, the conversion will lose preci-

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

       --nano Shorthands for --time-stamp-precision=micro or --time-stamp-pre-
              cision=nano, adjusting the  time  stamp  precision  accordingly.
              When  reading  packets  from a savefile, using --micro truncates
              time stamps if the savefile was created with  nanosecond  preci-
              sion.   In  contrast, a savefile created with microsecond preci-
              sion will have trailing zeroes added  to  the  time  stamp  when
              --nano is used.

              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'.

              Print parsed packet output, even if the raw  packets  are  being
              saved to a file with the -w flag.

       -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 pcapng files).  Standard input
              is used if file is ``-''.

              Print absolute, rather than relative, TCP sequence numbers.

       -s snaplen
              Snarf snaplen bytes of data from each  packet  rather  than  the
              default of 262144 bytes.  Packets truncated because of a limited
              snapshot are indicated in the output  with  ``[|proto]'',  where
              proto  is the name of the protocol level at which the truncation
              has occurred.

              Note that taking larger snapshots both increases the  amount  of
              time it takes to process packets and, effectively, decreases the
              amount of packet buffering.  This may cause packets to be  lost.
              Note  also  that taking smaller snapshots will discard data from
              protocols above the transport  layer,  which  loses  information
              that  may  be  important.  NFS and AFS requests and replies, for
              example, are very large, and much of the detail won't be  avail-
              able if a too-short snapshot length is selected.

              If  you  need to reduce the snapshot size below the default, 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 backwards compatibil-
              ity with recent older versions of tcpdump.

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

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

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

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

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

       -ttt   Print a delta (microsecond or nanosecond resolution depending on
              the  --time-stamp-precision option) between current and previous
              line on each dump line.  The default is microsecond  resolution.

       -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 (microsecond or nanosecond resolution depending on
              the --time-stamp-precision option)  between  current  and  first
              line  on each dump line.  The default is microsecond resolution.

       -u     Print undecoded NFS handles.

              If the -w option is not specified, or if it is specified but the
              --print  flag  is also specified, make the printed packet output
              ``packet-buffered''; i.e., as the description of the contents of
              each  packet is printed, it will be written to the standard out-
              put, 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(3PCAP)

       -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 and at the  same  time
              not  reading  from  a file with the -r option, report to stderr,
              once per second, the number of  packets  captured.  In  Solaris,
              FreeBSD  and  possibly  other  operating  systems  this periodic
              update currently can cause loss of captured packets on their way
              from the kernel to tcpdump.

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

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

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

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

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

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

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

       -W filecount
              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 in conjunction with both -C and -G, the -W  option  will
              currently be ignored, and will only affect the file name.

       -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.  In the current
              implementation this flag may have the same effect as -xx if  the
              packet is truncated.

       -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.  In  the  current  implementation  this
              flag may have the same effect as -XX if the packet is truncated.

       -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  (see  -L)
              or  just  compiling and dumping packet-matching code (see -d) to

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

       To print the TCP packets with flags RST and ACK both set.  (i.e. select
       only  the  RST  and  ACK flags in the flags field, and if the result is
       "RST and ACK both set", match)
              tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'

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

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

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

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


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


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

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP Packets

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

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

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

       IPv4 Packets

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

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

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

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

       TCP Packets

       (N.B.:The following description assumes familiarity with the TCP proto-
       col  described  in RFC-793.  If you are not familiar with the protocol,
       this description will not be of much use to you.)

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

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

       Here is the opening portion of an rlogin from host rtsg to host csam.
              IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
              IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
              IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
              IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
              IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
              IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
              IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
              IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
              IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1
       The  first  line  says that TCP port 1023 on rtsg sent a packet to port
       login on csam.  The S indicates that the SYN flag was set.  The  packet
       sequence  number was 768512 and it contained no data.  (The notation is
       `first:last' which means `sequence numbers first up to but not  includ-
       ing  last'.)  There was no piggy-backed ACK, the available receive win-
       dow was 4096 bytes and there was a max-segment-size  option  requesting
       an MSS of 1024 bytes.

       Csam  replies  with  a similar packet except it includes a piggy-backed
       ACK for rtsg's SYN.  Rtsg then ACKs csam's SYN.  The `.' means the  ACK
       flag  was  set.   The  packet  contained  no  data  so there is no data
       sequence number or length.  Note that the  ACK  sequence  number  is  a
       small  integer  (1).  The first time tcpdump sees a TCP `conversation',
       it prints the sequence number from the packet.  On  subsequent  packets
       of  the  conversation,  the  difference  between  the  current packet's
       sequence number and this initial  sequence  number  is  printed.   This
       means that sequence numbers after the first can be interpreted as rela-
       tive byte positions in the conversation's data stream (with  the  first
       data  byte each direction being `1').  `-S' will override this feature,
       causing the original sequence numbers to be output.

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

                       |               |
                       |7   5   3     0|

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

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

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

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

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


       and its decimal representation is

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

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

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

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

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

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

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

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


       which translates to decimal

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

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

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

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

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

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

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

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

       TCP or 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, excluding the TCP or UDP and IP protocol
       headers.  The query operation was the normal  one,  Query,  so  the  op
       field  was  omitted.   If  the op had been anything else, it would have
       been printed between the `3' and the `+'.  Similarly,  the  qclass  was
       the  normal  one,  C_IN, and omitted.  Any other qclass would have been
       printed immediately after the `A'.

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

       TCP or UDP Name Server Responses

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

       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:

     > 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
       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).

       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

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

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

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

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

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

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


       stty(1),     pcap(3),     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 HTTPS:


       The original distribution is available via anonymous ftp:


       IPv6/IPsec support is added by WIDE/KAME project.   This  program  uses
       OpenSSL/LibreSSL, 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
              userspace, 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.

                               21 December 2020                     tcpdump(1)

tcpdump 4.99.4 - Generated Fri Jun 11 14:55:38 CDT 2021
© 2000-2021
Individual documents may contain additional copyright information.