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[править] NAME

      tcpdump - dump traffic on a network

[править] SYNOPSIS

      tcpdump [ -AbdDefIKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
              [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
              [ -i interface ] [ -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 ]
              [ expression ]


      Tcpdump  prints  out a description of the contents of packets on a net-
      work interface that match the boolean expression.  It can also  be  run
      with the -w flag, which causes it to save the packet data to a file for
      later analysis, and/or with the -r flag, which causes it to read from a
      saved packet file rather than to read packets from a network interface.
      In all cases, only packets that match expression will be  processed  by
      Tcpdump  will,  if not run with the -c flag, continue capturing packets
      until it is interrupted by a SIGINT signal (generated, for example,  by
      typing your interrupt character, typically control-C) or a SIGTERM sig-
      nal (typically generated with the kill(1) command); if run with the  -c
      flag,  it  will  capture packets until it is interrupted by a SIGINT or
      SIGTERM signal or the specified number of packets have been  processed.
      When tcpdump finishes capturing packets, it will report counts of:
             packets ``captured (this is the number of packets that tcpdump
             has received and processed);
             packets ``received by filter (the meaning of this  depends  on
             the  OS on which you're running tcpdump, and possibly on the way
             the OS was configured - if a filter was specified on the command
             line,  on some OSes it counts packets regardless of whether they
             were matched by the filter expression and,  even  if  they  were
             matched  by the filter expression, regardless of whether tcpdump
             has read and processed them yet, on other OSes  it  counts  only
             packets that were matched by the filter expression regardless of
             whether tcpdump has read and processed them yet,  and  on  other
             OSes  it  counts  only  packets  that were matched by the filter
             expression and were processed by tcpdump);
             packets ``dropped by kernel (this is  the  number  of  packets
             that  were dropped, due to a lack of buffer space, by the packet
             capture mechanism in the OS on which tcpdump is running, if  the
             OS  reports that information to applications; if not, it will be
             reported as 0).
      On platforms that  support  the  SIGINFO  signal,  such  as  most  BSDs
      (including  Mac  OS  X)  and  Digital/Tru64  UNIX, it will report those
      counts when it receives a SIGINFO signal (generated,  for  example,  by
      typing your ``status character, typically control-T, although on some
      platforms, such as Mac OS X, the ``status character  is  not  set  by
      default,  so  you must set it with stty(1) in order to use it) and will
      continue capturing packets.
      Reading packets from a network interface may require that you have spe-
      cial  privileges; see the pcap (3PCAP) man page for details.  Reading a
      saved packet file doesn't require special privileges.

[править] 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.
      -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
      -D     Print the list of the network interfaces available on the system
             and  on  which  tcpdump  can  capture packets.  For each network
             interface, a number and an interface name, possibly followed  by
             a  text description of the interface, is printed.  The interface
             name or the number can be supplied to the -i flag to specify  an
             interface on which to capture.
             This  can be useful on systems that don't have a command to list
             them (e.g., Windows systems, or UNIX  systems  lacking  ifconfig
             -a); the number can be useful on Windows 2000 and later systems,
             where the interface name is a somewhat complex string.
             The -D flag will not be supported if tcpdump was built  with  an
             older version of libpcap that lacks the pcap_findalldevs() func-
      -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
             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 preceeded by
             0x, then a hex value will be read.
             The option assumes RFC2406 ESP, not RFC1827 ESP.  The option  is
             only  for  debugging purposes, and the use of this option with a
             true `secret' key is discouraged.  By  presenting  IPsec  secret
             key  onto  command line you make it visible to others, via ps(1)
             and other occasions.
             In addition to the above syntax, the syntax  file  name  may  be
             used  to  have  tcpdump  read  the provided file in. The file is
             opened upon receiving the first ESP packet, so any special  per-
             missions  that  tcpdump  may have been given should already have
             been given up.
      -f     Print `foreign' IPv4 addresses numerically rather than  symboli-
             cally  (this option is intended to get around serious brain dam-
             age in Sun's NIS server -- usually it hangs forever  translating
             non-local internet numbers).
             The  test  for  `foreign'  IPv4 addresses is done using the IPv4
             address and netmask of the interface on which capture  is  being
             done.   If that address or netmask are not available, available,
             either because the interface on which capture is being done  has
             no  address  or  netmask or because the capture is being done on
             the Linux "any" interface, which can capture on  more  than  one
             interface, this option will not work correctly.
      -F     Use  file  as  input  for  the filter expression.  An additional
             expression given on the command line is ignored.
      -G     If specified, rotates the dump file specified with the -w option
             every  rotate_seconds  seconds.   Savefiles  will  have the name
             specified by -w which should include a time format as defined by
             strftime(3).  If no time format is specified, each new file will
             overwrite the previous.
             If used in conjunction with the -C option, filenames  will  take
             the form of `file<count>'.
      -i     Listen  on interface.  If unspecified, tcpdump searches the sys-
             tem interface list for the lowest numbered, configured up inter-
             face (excluding loopback).  Ties are broken by choosing the ear-
             liest match.
             On Linux systems with 2.2 or later kernels, an  interface  argu-
             ment  of  ``any can be used to capture packets from all inter-
             faces.  Note that captures on the ``any  device  will  not  be
             done in promiscuous mode.
             If  the  -D flag is supported, an interface number as printed by
             that flag can be used as the interface argument.
      -I     Put the interface in "monitor mode"; this is supported  only  on
             IEEE 802.11 Wi-Fi interfaces, and supported only on some operat-
             ing systems.
             Note that in monitor mode the adapter  might  disassociate  from
             the  network with which it's associated, so that you will not be
             able to use any wireless networks with that adapter.  This could
             prevent  accessing  files on a network server, or resolving host
             names or network addresses, if you are capturing in monitor mode
             and are not connected to another network with another adapter.
             This  flag  will  affect the output of the -L flag.  If -I isn't
             specified, only those link-layer types  available  when  not  in
             monitor mode will be shown; if -I is specified, only those link-
             layer types available when in monitor mode will be shown.
      -K     Don't attempt to verify IP, TCP, or UDP checksums.  This is use-
             ful  for  interfaces  that perform some or all of those checksum
             calculation in hardware; otherwise, all outgoing  TCP  checksums
             will be flagged as bad.
      -l     Make  stdout  line buffered.  Useful if you want to see the data
             while capturing it.  E.g.,
             ``tcpdump  -l  |  tee     dat     or     ``tcpdump  -l       >
             dat  &  tail  -f  dat.
      -L     List  the known data link types for the interface, in the speci-
             fied mode, and exit.  The list of known data link types  may  be
             dependent on the specified mode; for example, on some platforms,
             a Wi-Fi interface might support one set of data link types  when
             not  in  monitor  mode  (for example, it might support only fake
             Ethernet headers, or might support 802.11 headers but  not  sup-
             port  802.11  headers with radio information) and another set of
             data link types when in monitor mode (for example, it might sup-
             port  802.11  headers, or 802.11 headers with radio information,
             only in monitor mode).
      -m     Load SMI MIB module definitions from file module.   This  option
             can  be used several times to load several MIB modules into tcp-
      -M     Use secret as a shared secret for validating the  digests  found
             in  TCP segments with the TCP-MD5 option (RFC 2385), if present.
      -n     Don't convert addresses (i.e.,  host  addresses,  port  numbers,
             etc.) to names.
      -N     Don't  print  domain name qualification of host names.  E.g., if
             you give this flag then tcpdump will print  ``nic  instead  of
      -O     Do  not  run the packet-matching code optimizer.  This is useful
             only if you suspect a bug in the optimizer.
      -p     Don't put the interface into promiscuous mode.   Note  that  the
             interface  might  be  in promiscuous mode for some other reason;
             hence, `-p' cannot be used as an abbreviation  for  `ether  host
             {local-hw-addr} or ether broadcast'.
      -q     Quick  (quiet?) output.  Print less protocol information so out-
             put lines are shorter.
      -R     Assume ESP/AH packets to be based on old specification  (RFC1825
             to  RFC1829).   If specified, tcpdump will not print replay pre-
             vention field.  Since there is  no  protocol  version  field  in
             ESP/AH  specification,  tcpdump  cannot  deduce  the  version of
             ESP/AH protocol.
      -r     Read packets from file (which was created with the  -w  option).
             Standard input is used if file is ``-.
      -S     Print absolute, rather than relative, TCP sequence numbers.
      -s     Snarf  snaplen  bytes  of  data from each packet rather than the
             default of 65535 bytes.  Packets truncated because of a  limited
             snapshot  are  indicated  in the output with ``[|proto], where
             proto is the name of the protocol level at which the  truncation
             has  occurred.  Note that taking larger snapshots both increases
             the amount of time it takes to process packets and, effectively,
             decreases  the amount of packet buffering.  This may cause pack-
             ets to be lost.  You should limit snaplen to the smallest number
             that will capture the protocol information you're interested in.
             Setting snaplen to 0 sets it to the default of 65535, for  back-
             wards compatibility with recent older versions of tcpdump.
      -T     Force  packets  selected  by  "expression" to be interpreted the
             specified type.  Currently known  types  are  aodv  (Ad-hoc  On-
             demand Distance Vector protocol), cnfp (Cisco NetFlow protocol),
             rpc (Remote Procedure Call), rtp (Real-Time Applications  proto-
             col), rtcp (Real-Time Applications control protocol), snmp (Sim-
             ple Network Management Protocol), tftp  (Trivial  File  Transfer
             Protocol),  vat  (Visual  Audio Tool), and wb (distributed White
      -t     Don't print a timestamp on each dump line.
      -tt    Print an unformatted timestamp on each dump line.
      -ttt   Print a delta (micro-second resolution) between current and pre-
             vious line on each dump line.
      -tttt  Print  a  timestamp  in default format proceeded by date on each
             dump line.
      -ttttt Print a delta  (micro-second  resolution)  between  current  and
             first line on each dump line.
      -u     Print undecoded NFS handles.
      -U     Make  output  saved via the -w option ``packet-buffered; i.e.,
             as each packet is saved, it will be written to the output  file,
             rather than being written only when the output buffer fills.
             The  -U  flag will not be supported if tcpdump was built with an
             older version of libpcap that lacks the pcap_dump_flush()  func-
      -v     When  parsing and printing, produce (slightly more) verbose out-
             put.  For example,  the  time  to  live,  identification,  total
             length  and  options  in an IP packet are printed.  Also enables
             additional packet integrity checks such as verifying the IP  and
             ICMP header checksum.
             When writing to a file with the -w option, report, every 10 sec-
             onds, the number of packets captured.
      -vv    Even more verbose output.  For example,  additional  fields  are
             printed  from  NFS  reply  packets,  and  SMB  packets are fully
      -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.
      -w     Write the raw packets to file rather than parsing  and  printing
             them  out.  They can later be printed with the -r option.  Stan-
             dard output is used if file is ``-.  See pcap-savefile(5)  for
             a description of the file format.
      -W     Used in conjunction with the -C option, this will limit the num-
             ber of files created to the specified number,  and  begin  over-
             writing  files  from  the  beginning, thus creating a 'rotating'
             buffer.  In addition, it will name the files with enough leading
             0s to support the maximum number of files, allowing them to sort
             Used in conjunction with the -G option, this will limit the num-
             ber  of rotated dump files that get created, exiting with status
             0 when reaching the limit. If used with -C as well, the behavior
             will result in cyclical files per timeslice.
      -x     When  parsing  and printing, in addition to printing the headers
             of each packet, print the data of each packet  (minus  its  link
             level  header)  in  hex.   The  smaller  of the entire packet or
             snaplen bytes will be printed.  Note that  this  is  the  entire
             link-layer  packet, so for link layers that pad (e.g. Ethernet),
             the padding bytes will also be printed  when  the  higher  layer
             packet is shorter than the required padding.
      -xx    When  parsing  and printing, in addition to printing the headers
             of each packet, print the data of  each  packet,  including  its
             link level header, in hex.
      -X     When  parsing  and printing, in addition to printing the headers
             of each packet, print the data of each packet  (minus  its  link
             level  header)  in  hex  and  ASCII.   This  is  very  handy for
             analysing new protocols.
      -XX    When parsing and printing, in addition to printing  the  headers
             of  each  packet,  print  the data of each packet, including its
             link level header, in hex and ASCII.
      -y     Set the data  link  type  to  use  while  capturing  packets  to
      -z     Used  in  conjunction  with the -C or -G options, this will make
             tcpdump run " command file " where file is  the  savefile  being
             closed  after  each rotation. For example, specifying -z gzip or
             -z bzip2 will compress each savefile using gzip or bzip2.
             Note that tcpdump will run the command in parallel to  the  cap-
             ture, using the lowest priority so that this doesn't disturb the
             capture process.
             And in case you would like to use a command  that  itself  takes
             flags  or  different  arguments,  you  can  always write a shell
             script that will take the savefile name as  the  only  argument,
             make  the flags & arguments arrangements and execute the command
             that you want.
      -Z     Drops privileges (if root) and changes 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).
             Expression arguments can be passed to tcpdump as either a single
             argument or as multiple arguments, whichever is more convenient.
             Generally,  if  the expression contains Shell metacharacters, it
             is easier to pass it as a  single,  quoted  argument.   Multiple
             arguments are concatenated with spaces before being parsed.

[править] EXAMPLES

      To print all packets arriving at or departing from sundown:
             tcpdump host sundown
      To print traffic between helios and either hot or ace:
             tcpdump host helios and \( hot or ace \)
      To print all IP packets between ace and any host except helios:
             tcpdump ip host ace and not helios
      To print all traffic between local hosts and hosts at Berkeley:
             tcpdump net ucb-ether
      To  print all ftp traffic through internet gateway snup: (note that the
      expression is quoted to prevent the shell from  (mis-)interpreting  the
             tcpdump 'gateway snup and (port ftp or ftp-data)'
      To  print traffic neither sourced from nor destined for local hosts (if
      you gateway to one other net, this stuff should never make it onto your
      local net).
             tcpdump ip and not net localnet
      To  print  the  start and end packets (the SYN and FIN packets) of each
      TCP conversation that involves a non-local host.
             tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
      To print all IPv4 HTTP packets to and from port  80,  i.e.  print  only
      packets  that  contain  data, not, for example, SYN and FIN packets and
      ACK-only packets.  (IPv6 is left as an exercise for the reader.)
             tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
      To print IP packets longer than 576 bytes sent through gateway snup:
             tcpdump 'gateway snup and ip[2:2] > 576'
      To print IP broadcast or multicast packets that were not sent via  Eth-
      ernet broadcast or multicast:
             tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
      To print all ICMP packets that are not echo requests/replies (i.e., not
      ping packets):
             tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'


      The output of tcpdump is protocol dependent.   The  following  gives  a
      brief description and examples of most of the formats.
      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.
      TCP Packets
      (N.B.:The following description assumes familiarity with the TCP proto-
      col  described  in RFC-793.  If you are not familiar with the protocol,
      neither this description nor tcpdump will be of much use to you.)
      The general format of a tcp protocol line is:
             src > dst: flags data-seqno ack window urgent options
      Src and dst are the source and  destination  IP  addresses  and  ports.
      Flags  are  some  combination of S (SYN), F (FIN), P (PUSH), R (RST), W
      (ECN CWR) or E (ECN-Echo), or a  single  `.'  (no  flags).   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
      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 no flags
      were  set.   The  packet contained no data so there is no data sequence
      number.  Note that the ack sequence number is a small integer (1).  The
      first  time  tcpdump  sees a tcp `conversation', it prints the sequence
      number from the packet.  On subsequent packets of the conversation, the
      difference  between  the current packet's sequence number and this ini-
      tial sequence number is printed.   This  means  that  sequence  numbers
      after  the  first  can be interpreted as relative byte positions in the
      conversation's data stream (with the first  data  byte  each  direction
      being  `1').   `-S'  will  override  this feature, causing the original
      sequence numbers to be output.
      On the 6th line, rtsg sends csam 19 bytes of data (bytes 2  through  20
      in the rtsg -> csam side of the conversation).  The PUSH flag is set in
      the packet.  On the 7th line, csam says it's received data sent by rtsg
      up  to but not including byte 21.  Most of this data is apparently sit-
      ting in the socket buffer since csam's receive  window  has  gotten  19
      bytes  smaller.   Csam  also  sends  one  byte  of data to rtsg in this
      packet.  On the 8th and 9th lines, csam  sends  two  bytes  of  urgent,
      pushed data to rtsg.
      If  the  snapshot was small enough that tcpdump didn't capture the full
      TCP header, it interprets as much of the header  as  it  can  and  then
      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-act, tcp-urg.
      This can be demonstrated as:
                  tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
      Note that you should use single quotes or a backslash in the expression
      to hide the AND ('&') special character from the shell.
      UDP Packets
      UDP format is illustrated by this rwho packet:
             actinide.who > broadcast.who: udp 84
      This says that port who on host actinide sent a udp  datagram  to  port
      who on host broadcast, the Internet broadcast address.  The packet con-
      tained 84 bytes of user data.
      Some UDP services are recognized (from the source or  destination  port
      number) and the higher level protocol information printed.  In particu-
      lar, Domain Name service requests (RFC-1034/1035)  and  Sun  RPC  calls
      (RFC-1050) to NFS.
      UDP Name Server Requests
      (N.B.:The  following  description  assumes  familiarity with the Domain
      Service protocol described in RFC-1035.  If you are not  familiar  with
      the  protocol,  the  following description will appear to be written in
      Name server requests are formatted as
             src > dst: id op? flags qtype qclass name (len)
             h2opolo.1538 > helios.domain: 3+ A? (37)
      Host h2opolo asked the domain server on helios for  an  address  record
      (qtype=A)  associated  with the name  The query id
      was `3'.  The `+' indicates the recursion desired flag  was  set.   The
      query  length was 37 bytes, not including the UDP and IP protocol head-
      ers.  The query operation was the normal one, Query, so  the  op  field
      was  omitted.   If  the  op  had been anything else, it would have been
      printed between the `3' and the `+'.  Similarly,  the  qclass  was  the
      normal  one,  C_IN,  and  omitted.   Any  other  qclass would have been
      printed immediately after the `A'.
      A few anomalies are checked and may result in extra fields enclosed  in
      square  brackets:   If a query contains an answer, authority records or
      additional records section, ancount, nscount, or arcount are printed as
      `[na]', `[nn]' or  `[nau]' where n is the appropriate count.  If any of
      the response bits are set (AA, RA or rcode) or  any  of  the  `must  be
      zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where
      x is the hex value of header bytes two and three.
      UDP Name Server Responses
      Name server responses are formatted as
             src > dst:  id op rcode flags a/n/au type class data (len)
             helios.domain > h2opolo.1538: 3 3/3/7 A (273)
             helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
      In the first example, helios responds to query id 3 from h2opolo with 3
      answer  records,  3  name server records and 7 additional records.  The
      first answer record is type  A  (address)  and  its  data  is  internet
      address   The  total size of the response was 273 bytes,
      excluding UDP and IP headers.  The op (Query) and response code  (NoEr-
      ror) were omitted, as was the class (C_IN) of the A record.
      In  the second example, helios responds to query 2 with a response code
      of non-existent domain (NXDomain) with no answers, one name server  and
      no  authority records.  The `*' indicates that the authoritative answer
      bit was set.  Since there were no answers, no type, class or data  were
      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 te fields  mean  see   or  the  pub/samba/specs/  directory  on  your  favorite mirror site.  The SMB patches were written by Andrew Tridgell

      NFS Requests and Replies
      Sun NFS (Network File System) requests and replies are printed as:
             src.xid > dst.nfs: len op args
             src.nfs > dst.xid: reply stat len op results
             sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
             wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
             sushi.201b > wrl.nfs:
                  144 lookup fh 9,74/4096.6878 "xcolors"
             wrl.nfs > sushi.201b:
                  reply ok 128 lookup fh 9,74/4134.3150
      In  the  first line, host sushi sends a transaction with id 6709 to wrl
      (note that the number following the src host is a transaction  id,  not
      the  source port).  The request was 112 bytes, excluding the UDP and IP
      headers.  The operation was a readlink (read  symbolic  link)  on  file
      handle (fh) 21,24/10.731657119.  (If one is lucky, as in this case, the
      file handle can be interpreted as a  major,minor  device  number  pair,
      followed  by the inode number and generation number.)  Wrl replies `ok'
      with the contents of the link.
      In the third line, sushi asks wrl  to  lookup  the  name  `xcolors'  in
      directory  file  9,74/4096.6878.  Note that the data printed depends on
      the operation type.  The format is intended to be self  explanatory  if
      read in conjunction with an NFS protocol spec.
      If  the  -v (verbose) flag is given, additional information is printed.
      For example:
             sushi.1372a > wrl.nfs:
                  148 read fh 21,11/12.195 8192 bytes @ 24576
             wrl.nfs > sushi.1372a:
                  reply ok 1472 read REG 100664 ids 417/0 sz 29388
      (-v also prints the  IP  header  TTL,  ID,  length,  and  fragmentation
      fields, which have been omitted from this example.)  In the first line,
      sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte  off-
      set  24576.   Wrl  replies `ok'; the packet shown on the second line is
      the first fragment of the reply, and hence is only 1472 bytes long (the
      other bytes will follow in subsequent fragments, but these fragments do
      not have NFS or even UDP headers and so might not be printed, depending
      on  the filter expression used).  Because the -v flag is given, some of
      the file attributes (which are returned in addition to the  file  data)
      are  printed:  the file type (``REG, for regular file), the file mode
      (in octal), the uid and gid, and the file size.
      If the -v flag is given more than once, even more details are  printed.
      Note  that  NFS requests are very large and much of the detail won't be
      printed unless snaplen is increased.  Try using `-s 192' to  watch  NFS
      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 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 packets.
      If the -v flag is given three times, the security index and service  id
      are printed.
      Error  codes  are printed for abort packets, with the exception of Ubik
      beacon packets (because abort packets are used to signify  a  yes  vote
      for the Ubik protocol).
      Note  that  AFS requests are very large and many of the arguments won't
      be printed unless snaplen is increased.  Try using `-s  256'  to  watch
      AFS traffic.
      AFS  reply  packets  do  not  explicitly  identify  the  RPC operation.
      Instead, tcpdump keeps track of ``recent requests, and  matches  them
      to  the  replies using the call number and service ID.  If a reply does
      not closely follow the corresponding request, it might not be parsable.

      KIP AppleTalk (DDP in UDP)
      AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
      and dumped as DDP packets (i.e., all the UDP header information is dis-
      carded).   The file /etc/atalk.names is used to translate AppleTalk net
      and node numbers to names.  Lines in this file have the form
             number    name
             1.254          ether
             16.1      icsd-net
             1.254.110 ace
      The first two lines give the names of AppleTalk  networks.   The  third
      line  gives the name of a particular host (a host is distinguished from
      a net by the 3rd octet in the number -  a  net  number  must  have  two
      octets  and a host number must have three octets.)  The number and name
      should  be   separated   by   whitespace   (blanks   or   tabs).    The
      /etc/atalk.names  file  may contain blank lines or comment lines (lines
      starting with a `#').
      AppleTalk addresses are printed in the form
    > 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.

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

[править] SEE ALSO

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

[править] AUTHORS

      The original authors are:
      Van Jacobson, Craig Leres and  Steven  McCanne,  all  of  the  Lawrence
      Berkeley National Laboratory, University of California, Berkeley, CA.
      It is currently being maintained by
      The current version is available via http:
      The original distribution is available via anonymous ftp:
      IPv6/IPsec  support  is  added by WIDE/KAME project.  This program uses
      Eric Young's SSLeay library, under specific configurations.

[править] BUGS

      Please send problems, bugs, questions, desirable enhancements,  patches
      etc. to:
      NIT doesn't let you watch your own outbound traffic, BPF will.  We rec-
      ommend that you use the latter.
      On Linux systems with 2.0[.x] kernels:
             packets on the loopback device will be seen twice;
             packet filtering cannot be done in the kernel, so that all pack-
             ets  must  be  copied from the kernel in order to be filtered in
             user mode;
             all of a packet, not just the part that's  within  the  snapshot
             length,  will be copied from the kernel (the 2.0[.x] packet cap-
             ture mechanism, if asked to copy only part of a packet to  user-
             land,  will not report the true length of the packet; this would
             cause most IP packets to get an error from tcpdump);
             capturing on some PPP devices won't work correctly.
      We recommend that you upgrade to a 2.2 or later kernel.
      Some attempt should be made to reassemble IP fragments or, at least  to
      compute the right length for the higher level protocol.
      Name server inverse queries are not dumped correctly: the (empty) ques-
      tion section is printed rather than real query in the  answer  section.
      Some  believe  that  inverse queries are themselves a bug and prefer to
      fix the program generating them rather than tcpdump.
      A packet trace that crosses a daylight savings time  change  will  give
      skewed time stamps (the time change is ignored).
      Filter  expressions  on  fields  other than those in Token Ring headers
      will not correctly handle source-routed Token Ring packets.
      Filter expressions on fields other than those in  802.11  headers  will
      not  correctly  handle  802.11 data packets with both To DS and From DS
      ip6 proto should chase header chain, but at this moment  it  does  not.
      ip6 protochain is supplied for this behavior.
      Arithmetic  expression  against  transport  layer headers, like tcp[0],
      does not work against IPv6 packets.  It only looks at IPv4 packets.

                                05 March 2009                      TCPDUMP(1)
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