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TCPDUMP(1)                                                                                        TCPDUMP(1)



NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AbdDefhgHIJKlLnNOpPqRStuUvxX ] [ -B buffer_size ] [ -c count ]
               [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
               [ -i interface ] [ -j tstamp_type ] [ -k (metadata_arg) ]
               [ -m module ] [ -M secret ]
               [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ]
               [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
               [ -Q packet-metadata-filter ]
               [ expression ]

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

       Tcpdump  will,  if  not run with the -c flag, continue capturing packets until it is interrupted by a
       SIGINT signal (generated, for example, by typing your interrupt character, typically control-C) or  a
       SIGTERM  signal (typically generated with the kill(1) command); if run with the -c flag, it will cap-ture capture
       ture 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 com-mand command
              mand  line, on some OSes it counts packets regardless of whether they were matched by the fil-ter filter
              ter 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  Digi-tal/Tru64 Digital/Tru64
       tal/Tru64  UNIX,  it will report those counts when it receives a SIGINFO signal (generated, for exam-ple, example,
       ple, by typing your ``status'' character, typically control-T, although on some  platforms,  such  as
       Mac OS X, the ``status'' character is not set by default, so you must set it with stty(1) in order to
       use it) and will continue capturing packets.

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

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

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

       -B     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 cap-ture capture
              ture 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 sys-tems, systems,
              tems, where the interface name is a somewhat complex string.

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

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

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

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

              Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none.  The default
              is des-cbc.  The ability to decrypt packets is only present if tcpdump was compiled with cryp-tography cryptography
              tography 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 permissions
              missions that tcpdump may have been given should already have been given up.

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

              The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of the inter-face interface
              face 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     Do not insert line break after IP header in verbose mode for easier parsing.

       -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  the  -D  flag is supported, an interface number as printed by that flag can be used as the
              interface argument.

              On Darwin systems version 13 or later, when the interface is unspecified, tcpdump will  use  a
              pseudo  interface to capture packets on a set of interfaces determined by the kernel (excludes
              by default loopback and tunnel interfaces).

              Alternatively, to capture on more than one interface at a time, one may  use  "pktap"  as  the
              interface  parameter  followed  by  an  optional  list  of  comma separated interface names to
              include.  For example, to capture on the loopback and en0 interface:

                     tcpdump -i pktap,lo0,en0

              An interface argument of "all" or "pktap,all" can be used to capture packets from  all  inter-faces, interfaces,
              faces, including loopback and tunnel interfaces.

              A  pktap  pseudo interface provides for packet metadata using the default PKTAP data link type
              and files are written in the Pcap-ng file format.  The RAW data link  type  must  be  used  to
              force to use the legacy pcap-savefile(5) file format with a ptkap pseudo interface.  Note that
              captures on a ptkap pseudo interface will not be done in promiscuous mode.

              An interface argument of "iptap" can be used to capture packets from at the  IP  layer.   This
              capture  packets as they are passed to the input and output routines of the IPv4 and IPv6 pro-tocol protocol
              tocol handlers of the networking stack.  Note that captures will not be  done  in  promiscuous
              mode.

              On  other OSes, if unspecified, tcpdump searches the system interface list for the lowest num-bered, numbered,
              bered, configured up interface (excluding loopback).  Ties are broken by choosing the earliest
              match.

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

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

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

              This flag will affect the output of the -L flag.  If -I isn't specified, only those link-layer
              types  available  when not in monitor mode will be shown; if -I is specified, only those link-layer linklayer
              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     Control  the  display of packet metadata via an optional metadata_arg argument. This is useful
              when displaying packet saved in the pcap-ng file format or with interfaces  that  support  the
              PKTAP data link type.

              By  default,  when  the  metadata_arg optional argument is not specified, any available packet
              metadata information is printed out.

              The metadata_arg argument controls the display of specific packet metadata information using a
              flag word, where each character corresponds to a type of packet metadata as follows:

                     I     interface name (or interface ID)
                     N     process name
                     P     process ID
                     S     service class
                     D     direction
                     C     comment

              This is an Apple modification.

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

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

                     tcpdump -l | tee dat

              or

                     tcpdump -l > dat & tail -f dat

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

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

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

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

       -M     Use  secret  as a shared secret for validating the digests found in TCP segments with the TCP-MD5 TCPMD5
              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     Use the pcap-ng file format when saving files.  Apple modification.

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

       -Q     To specify a filter expression based on packet metadata information like interface or  process
              name.  Packet with matching packet metadata will be displayed or saved to a file.  See section
              PACKET METADATA FILTER for the syntax of packet metadata filter expressions.  This is an Apple
              addition.

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

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

       -r     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  tak-ing taking
              ing larger snapshots both increases the amount of time it takes to process packets and, effec-tively, effectively,
              tively, decreases the amount of packet buffering.  This may cause packets  to  be  lost.   You
              should  limit snaplen to the smallest number that will capture the protocol information you're
              interested in.  Setting snaplen to 0 sets it to the default of 65535, for  backwards  compati-bility compatibility
              bility 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), cnfp (Cisco NetFlow protocol), rpc
              (Remote  Procedure  Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications
              control protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Pro-tocol), Protocol),
              tocol), vat (Visual Audio Tool), and wb (distributed White Board).

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

       -tt    Print an unformatted timestamp on each dump line.

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

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

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

       -t n   An  alternate  form for specifying the kind of timestamp display where n is a number between 0
              and 5 with the following meaning:

                     0     time
                     1     no time
                     2     unformatted timestamp
                     3     microseconds since previous line
                     4     date and time
                     5     microseconds since first line

              This option may be specified more than once to display more than one kind of timestamp on each
              dump line.

       -u     Print undecoded NFS handles.

       -U     If  the  -w option is not specified, make the printed packet output ``packet-buffered''; i.e.,
              as the description of the contents of each packet is printed, it will be written to the  stan-dard standard
              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() function.

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

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

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

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

       -V     Print the packet number at the beginning of each dump line.

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

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

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

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

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

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

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

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

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

       -y     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 capture, using the  lowest  priority
              so that this doesn't disturb the capture process.

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

       -Z     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).

              Expression arguments can be passed to tcpdump as either a single argument or as multiple argu-ments, arguments,
              ments,  whichever is more convenient.  Generally, if the expression contains Shell metacharac-ters, metacharacters,
              ters, it is easier to pass it as a single, quoted argument.  Multiple arguments  are  concate-nated concatenated
              nated 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 pre-vent prevent
       vent the shell from (mis-)interpreting the parentheses):
              tcpdump 'gateway snup and (port ftp or ftp-data)'

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

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

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

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

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

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

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

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP Packets

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

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

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

       TCP Packets

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

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

       Src, dst and flags are always present.  The other fields depend on the contents of the  packet's  tcp
       protocol header and are output only if appropriate.

       Here is the opening portion of an rlogin from host rtsg to host csam.
              rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
              csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
              rtsg.1023 > csam.login: . ack 1 win 4096
              rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
              csam.login > rtsg.1023: . ack 2 win 4096
              rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
              csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
              csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
              csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
       The  first line says that tcp port 1023 on rtsg sent a packet to port login on csam.  The S indicates
       that the SYN flag was set.  The packet sequence number was 768512 and it  contained  no  data.   (The
       notation  is  `first:last(nbytes)'  which  means `sequence numbers first up to but not including last
       which is nbytes bytes of user data'.)  There was no piggy-backed ack, the  available  receive  window
       was 4096 bytes and there was a max-segment-size option requesting an mss of 1024 bytes.

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

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

                   00010010

       which translates to decimal

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

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

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


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

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

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

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

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

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

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

       UDP Packets

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

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

       UDP Name Server Requests

       (N.B.:The following description assumes familiarity with the Domain  Service  protocol  described  in
       RFC-1_35.   If  you  are  not familiar with the protocol, the following description will appear to be
       written in greek.)

       Name server requests are formatted as
              src > dst: id op? flags qtype qclass name (len)
              h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
       Host h2opolo asked the domain server on helios for an address record (qtype=A)  associated  with  the
       name  ucbvax.berkeley.edu.   The  query id was `3'.  The `+' indicates the recursion desired flag was
       set.  The query length was 37 bytes, not including the UDP and IP protocol headers.  The query opera-tion operation
       tion  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 inter-net internet
       net  address  128.32.137.3.  The total size of the response was 273 bytes, excluding UDP and IP head-ers. headers.
       ers.  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 (NXDo-main) (NXDomain)
       main) with no answers, one name server and no authority records.  The `*' indicates that the authori-tative authoritative
       tative answer bit was set.  Since there were no answers, no type, class or data were printed.

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

       SMB/CIFS decoding

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

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

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

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
              src.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 67_9 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 explana-tory explanatory
       tory 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
       offset 24576.  Wrl replies `ok'; the packet shown on the second line is the  first  fragment  of  the
       reply,  and  hence  is only 1472 bytes long (the other bytes will follow in subsequent fragments, but
       these fragments do not have NFS or even UDP headers and so might not be  printed,  depending  on  the
       filter  expression  used).   Because  the  -v  flag  is given, some of the file attributes (which are
       returned in addition to the file data) are printed: the file type (``REG'', for  regular  file),  the
       file mode (in octal), the uid and gid, and the file size.

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

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

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

       AFS Requests and Replies

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

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

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

       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 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 the RX call ID, serial
       number, and the RX packet flags.  The MTU negotiation information is also printed from RX  ack  pack-ets. packets.
       ets.

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

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

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

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


       KIP AppleTalk (DDP in UDP)

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

              1.254          ether
              16.1      icsd-net
              1.254.110 ace
       The first two lines give the names of AppleTalk networks.  The third line gives the name of a partic-ular particular
       ular 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 num-ber, number,
       ber, addresses are printed in numeric form.)  In the first example, NBP (DDP port  2)  on  net  144.1
       node  209  is  sending to whatever is listening on port 220 of net icsd node 112.  The second line is
       the same except the full name of the source node is known (`office').  The third line is a send  from
       port  235  on  net  jssmag  node  149  to broadcast on the icsd-net NBP port (note that the broadcast
       address (255) is indicated by a net name with no host number - for this reason it's a  good  idea  to
       keep node names and net names distinct in /etc/atalk.names).

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

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

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

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

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


       IP Fragmentation

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

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

       The  fragment  information is output for each fragment.  The first fragment contains the higher level
       protocol header and the frag info is printed after the protocol info.  Fragments after the first con-tain contain
       tain  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  con-nection connection
       nection 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 num-bers. numbers.
       bers.  This is because the TCP protocol information is all in the first fragment and we have no  idea
       what  the  port  or sequence numbers are when we print the later fragments.  Second, the tcp sequence
       information in the first line is printed as if there were 308 bytes of user data when, in fact, there
       are  512  bytes  (308  in the first frag and 204 in the second).  If you are looking for holes in the
       sequence space or trying to match up acks with packets, this can fool you.

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

       Timestamps

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

PACKET METADATA FILTER
       Use  packet  metadata  filter  expression  to match packets against descriptive information about the
       packet: interface, process, service type or direction.

       Note this is meaningful only with capture files in the Pcap-ng file format or for interfaces support-ing supporting
       ing the PKTAP data link type.

       The syntax supports the following operators:

              or     logical or
              and    logical and
              not    negation
              (...)  to group sub-expressions
              =      is equal
              !=     is not equal
              ||     logical or (alternate)
              &&     logical and (alternate)
              !      negation (alternate)

       The  syntax  support the following keywords to denote which of packet metadata contents is to be com-pared: compared:
       pared:

              if     interface name
              proc   process name
              pid    process ID
              svc    service class
              dir    direction
              eproc  effective process name
              epid   effective process ID

       For example to filter packets from interface en0 "sent" by the process named "nc" or incoming  packet
       not on interface en0:

              -Q "( if=en0 and proc =nc ) || (if != en0 and dir=in)"

       Note  that  a  complex  packet  metadata filter expression needs to be put in quotes as the option -Q
       takes a single string parameter.

SEE ALSO
       stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-filter(7), pcap-tstamp-type(7)

AUTHORS
       The original authors are:

       Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory,  Uni-versity University
       versity 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/tcpdump.tar.Z

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

BUGS
       Please send problems, bugs, questions, desirable enhancements, patches etc. to:

              tcpdump-workers@lists.tcpdump.org

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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

              all of a packet, not just the part that's within the snapshot length, will be copied from  the
              kernel  (the 2.0[.x] packet capture mechanism, if asked to copy only part of a packet to user-land, userland,
              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) question section is printed  rather
       than  real  query  in the answer section.  Some believe that inverse queries are themselves a bug and
       prefer to fix the program generating them rather than tcpdump.

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

       Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed sourcerouted
       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  pack-ets. packets.
       ets.  It only looks at IPv4 packets.



                                                05 March 2009                                     TCPDUMP(1)

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