tcpdump


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

DESCRIPTION
       Tcpdump  prints  out a description of the contents of packets on a net-
       work interface that match the boolean expression.  It can also  be  run
       with the -w flag, which causes it to save the packet data to a file for
       later analysis, and/or with the -r flag, which causes it to read from a
       saved  packet file rather than to read packets from a network interface
       (please note tcpdump is protected via an enforcing apparmor(7)  profile
       in  Ubuntu  which  limits the files tcpdump may access).  In all cases,
       only packets that match expression will be processed by tcpdump.

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

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

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

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

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

       On platforms that  support  the  SIGINFO  signal,  such  as  most  BSDs
       (including  Mac  OS  X)  and  Digital/Tru64  UNIX, it will report those
       counts when it receives a SIGINFO signal (generated,  for  example,  by

       -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
              count).

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

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

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

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

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

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

              Algorithms may  be  des-cbc,  3des-cbc,  blowfish-cbc,  rc3-cbc,
              cast128-cbc,  or  none.  The default is des-cbc.  The ability to
              decrypt packets is only present if  tcpdump  was  compiled  with
              cryptography enabled.
              opened upon receiving the first ESP packet, so any special  per-
              missions  that  tcpdump  may have been given should already have
              been given up.

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

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

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

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

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

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

       -H     Attempt to detect 802.11s draft mesh headers.

       -i     Listen on interface.  If unspecified, tcpdump searches the  sys-
              tem interface list for the lowest numbered, configured up inter-
              face (excluding loopback).  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
              valid for any given interface.

       -J     List the supported time stamp types for the interface and  exit.
              If  the time stamp type cannot be set for the interface, no time
              stamp types are listed.

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

       -l     Make  stdout  line buffered.  Useful if you want to see the data
              while capturing it.  E.g.,
              ``tcpdump  -l  |  tee     dat''     or     ``tcpdump  -l       >
              dat  &  tail  -f  dat''.

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

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

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

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

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

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

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

       -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-
              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
              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 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-
              tion.

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

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

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

              buffer.  In addition, it will name the files with enough leading
              0s to support the maximum number of files, allowing them to sort
              correctly.

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

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

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

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

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

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

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

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

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

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

              This behavior can also be enabled by default at compile time.
              arguments are concatenated with spaces before being parsed.

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

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

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

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

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

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

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

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

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

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

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

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

       Link Level Headers

       If the '-e' option is given, the link level header is printed out.   On
       Ethernets,  the  source and destination addresses, protocol, and packet
       length are printed.
       addresses,  and  the  packet  length.  As on FDDI networks, packets are
       assumed to contain an LLC  packet.   Regardless  of  whether  the  '-e'
       option  is  specified or not, the source routing information is printed
       for source-routed packets.

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

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

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

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

       ARP/RARP Packets

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

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

       If we had done tcpdump -e, the fact that the first packet is  broadcast
       and the second is point-to-point would be visible:
              RTSG Broadcast 0806  64: arp who-has csam tell rtsg
              CSAM RTSG 0806  64: arp reply csam is-at CSAM
       For the first packet this says the Ethernet source address is RTSG, the
       Flags are some combination of S (SYN), F (FIN), P (PUSH),  R  (RST),  U
       (URG),  W  (ECN  CWR), E (ECN-Echo) or `.' (ACK), or `none' if no flags
       are set.  Data-seqno describes the portion of sequence space covered by
       the data in this packet (see example below).  Ack is sequence number of
       the next data expected the other direction on this connection.   Window
       is  the  number  of  bytes  of receive buffer space available the other
       direction on this connection.  Urg indicates there is `urgent' data  in
       the  packet.  Options are tcp options enclosed in angle brackets (e.g.,
       <mss 1024>).

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

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

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

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

       ACK, etc.)

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

                   00010010

       which translates to decimal

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

       Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression,
       because that would select only those packets that have SYN-ACK set, but

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

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

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

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

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

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

       UDP Packets

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

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

       UDP Name Server Requests

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

       Name server requests are formatted as
              src > dst: id op? flags qtype qclass name (len)
              h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
       Host  h2opolo  asked  the domain server on helios for an address record
       (qtype=A) associated with the name ucbvax.berkeley.edu.  The  query  id
       was  `3'.   The  `+' indicates the recursion desired flag was set.  The
       query length was 37 bytes, not including the UDP and IP protocol  head-
       ers.   The  query  operation was the normal one, Query, so the op field
       was omitted.  If the op had been anything  else,  it  would  have  been
       printed  between  the  `3'  and the `+'.  Similarly, the qclass was the
       normal one, C_IN, and  omitted.   Any  other  qclass  would  have  been
       Name server responses are formatted as
              src > dst:  id op rcode flags a/n/au type class data (len)
              helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
              helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
       In the first example, helios responds to query id 3 from h2opolo with 3
       answer records, 3 name server records and 7  additional  records.   The
       first  answer  record  is  type  A  (address)  and its data is internet
       address 128.32.137.3.  The total size of the response  was  273  bytes,
       excluding  UDP and IP headers.  The op (Query) and response code (NoEr-
       ror) were omitted, as was the class (C_IN) of the A record.

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

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

       SMB/CIFS decoding

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

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

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

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
              src.xid > dst.nfs: len op args
              src.nfs > dst.xid: reply stat len op results
              sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
              sushi.201b > wrl.nfs:
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.201b:
                   reply ok 128 lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id 6709  to  wrl
       (note  that  the number following the src host is a transaction id, not
       the source port).  The request was 112 bytes, excluding the UDP and  IP
       headers.   The  operation  was  a readlink (read symbolic link) on file
       handle (fh) 21,24/10.731657119.  (If one is lucky, as in this case, the
       file  handle  can  be  interpreted as a major,minor device number pair,
              wrl.nfs > sushi.1372a:
                   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v  also  prints  the  IP  header  TTL,  ID, length, and fragmentation
       fields, which have been omitted from this example.)  In the first line,
       sushi  asks wrl to read 8192 bytes from file 21,11/12.195, at byte off-
       set 24576.  Wrl replies `ok'; the packet shown on the  second  line  is
       the first fragment of the reply, and hence is only 1472 bytes long (the
       other bytes will follow in subsequent fragments, but these fragments do
       not have NFS or even UDP headers and so might not be printed, depending
       on the filter expression used).  Because the -v flag is given, some  of
       the  file  attributes (which are returned in addition to the file data)
       are printed: the file type (``REG'', for regular file), the  file  mode
       (in octal), the uid and gid, and the file size.

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

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

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

       AFS Requests and Replies

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

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

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

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

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

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

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


       KIP AppleTalk (DDP in UDP)

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

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

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

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

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

       NBP packets are formatted like the following examples:
              icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
              helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
              jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
              helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
              jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
              jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
       Jssmag.209  initiates transaction id 12266 with host helios by request-
       ing up to 8 packets (the `<0-7>').  The hex number at the  end  of  the
       line is the value of the `userdata' field in the request.

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

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


       IP Fragmentation

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

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

       The fragment information is output for each fragment.  The first  frag-
       ment  contains  the  higher  level protocol header and the frag info is
       printed after the protocol info.  Fragments after the first contain  no
       higher  level  protocol  header  and the frag info is printed after the
       source and destination addresses.  For example, here is part of an  ftp
       from  arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't
       appear to handle 576 byte datagrams:
              arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
              arizona > rtsg: (frag 595a:204@328)
              rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
       There are a couple of things to note here:  First, addresses in the 2nd
       line  don't  include  port  numbers.   This is because the TCP protocol
       information is all in the first fragment and we have no idea  what  the
       port  or  sequence numbers are when we print the later fragments.  Sec-
       stamp is the current clock time in the form
              hh:mm:ss.frac
       and is as accurate as the kernel's clock.  The timestamp  reflects  the
       time  the  kernel  first saw the packet.  No attempt is made to account
       for the time lag between when the Ethernet interface removed the packet
       from the wire and when the kernel serviced the `new packet' interrupt.

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

AUTHORS
       The original authors are:

       Van Jacobson, Craig Leres and  Steven  McCanne,  all  of  the  Lawrence
       Berkeley National Laboratory, University of California, Berkeley, CA.

       It is currently being maintained by tcpdump.org.

       The current version is available via http:

              http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

              ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

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

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

              tcpdump-workers@lists.tcpdump.org

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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

       skewed time stamps (the time change is ignored).

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

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

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

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



                                 05 March 2009                      TCPDUMP(8)
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