tcpdump


SYNOPSIS
       tcpdump [ -AdDefIKlLnNOpqRStuUvxX ] [ -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 ]

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.
       In all cases, only packets that match expression will be  processed  by
       tcpdump.

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

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

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

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

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

       On  platforms  that  support  the  SIGINFO  signal,  such  as most BSDs
       (including Mac OS X) and  Digital/Tru64  UNIX,  it  will  report  those
       counts  when  it  receives a SIGINFO signal (generated, for example, by
       typing your ``status'' character, typically control-T, although on some
       -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.

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

       -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.

       -K     Don't  attempt  to  verify  TCP  checksums.   This is useful for
              interfaces that perform the TCP checksum  calculation  in  hard-
              ware;  otherwise,  all outgoing TCP checksums will be flagged as
              bad.

              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-
              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  68  (with SunOS's NIT, the minimum is actually 96).
              68 bytes is adequate for IP, ICMP, TCP and UDP but may  truncate
              protocol  information  from  name  server  and  NFS packets (see
              below).  Packets truncated because of  a  limited  snapshot  are
              indicated  in  the  output with ``[|proto]'', where proto is the
              name of the protocol level at which the truncation has occurred.
              Note  that  taking larger snapshots both increases the amount of
              time it takes to process packets and, effectively, decreases the
              amount  of packet buffering.  This may cause packets to be lost.
              You should limit snaplen to the smallest number that  will  cap-
              ture  the  protocol  information  you're interested in.  Setting
              snaplen to 0 means use the required length to catch whole  pack-
              ets.

       -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).
              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.

       -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 ``-''.

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

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

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

       -xx    When parsing and printing, in addition to printing  the  headers
       -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     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.

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

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

              Expression arguments can be passed to tcpdump as either a single
              argument or as multiple 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.

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)'

              tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

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

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

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

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

       Link Level Headers

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

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

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

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

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

       On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out-
       bound),  packet type, and compression information are printed out.  The
       packet type is printed first.  The three types are ip, utcp, and  ctcp.
       No  further  link information is printed for ip packets.  For TCP pack-
       ets, the connection identifier is printed following the type.   If  the
       packet  is  compressed, its encoded header is printed out.  The special
       cases are printed out as *S+n and *SA+n, where n is the amount by which

       ARP/RARP Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src  and  dst  are  the  source and destination IP addresses and ports.
       Flags are some combination of S (SYN), F (FIN), P (PUSH),  R  (RST),  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
       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

       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
       length]''.

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

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

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

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

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

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

       Recall the structure of a TCP header without options:
       -----------------------------------------------------------------

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

       Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set
       arrives:

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

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

                   00010010

       which translates to decimal

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

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

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

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

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

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

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

       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-
       greek.)

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

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

       UDP Name Server Responses

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

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

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

       Note that name server requests and responses tend to be large  and  the
       default  snaplen  of  68  bytes may not capture enough of the packet to
       print.  Use the -s flag to increase the snaplen if you  need  to  seri-
       ously  investigate  name  server traffic.  `-s 128' has worked well for
       me.


       SMB/CIFS decoding
       samba.org mirror site.  The SMB patches were written by Andrew Tridgell
       (tridge@samba.org).


       NFS Requests and Replies

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

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

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

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

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

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

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

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

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

       Helios  responds  with  8 512-byte packets.  The `:digit' following the
       transaction id gives the packet sequence number in the transaction  and
       the number in parens is the amount of data in the packet, excluding the
       atp header.  The `*' on packet 7 indicates that the EOM bit was set.
       (The  first  form indicates there are more fragments.  The second indi-
       cates this is the last fragment.)

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

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

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

       Timestamps

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

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

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/
              tcpdump-workers@lists.tcpdump.org

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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

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

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

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

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

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

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

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

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



                                07 January 2008                     TCPDUMP(8)
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