Provided by: tcpdump_3.9.8-4_i386 bug

NAME

       tcpdump - dump traffic on a network

SYNOPSIS

       tcpdump [ -AdDeflLnNOpqRStuUvxX ] [ -c count ]
               [ -C file_size ] [ -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 user ]
               [ expression ]

DESCRIPTION

       Tcpdump  prints  out  a  description  of  the  contents of packets on a
       network interface that match the boolean expression.  It  can  also  be
       run with the -w flag, which causes it to save the packet data to a file
       for later analysis, and/or with the -r flag, which causes  it  to  read
       from  a  saved  packet  file rather than to read packets from a network
       interface.  In all cases, only packets that match  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
       signal (typically generated with the kill(1) command); if run with  the
       -c flag, it will capture packets until it is interrupted by a SIGINT or
       SIGTERM signal or the specified number of packets have been  processed.

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

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

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

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

       On platforms that  support  the  SIGINFO  signal,  such  as  most  BSDs
       (including  Mac  OS  X)  and  Digital/Tru64  UNIX, it will report those
       counts when it receives a SIGINFO signal (generated,  for  example,  by
       typing your ‘‘status’’ character, typically control-T, although on some
       platforms, such as Mac OS X, the ‘‘status’’ character  is  not  set  by
       default,  so  you must set it with stty(1) in order to use it) and will
       continue capturing packets.

       Reading packets from a network interface  may  require  that  you  have
       special privileges:

       Under SunOS 3.x or 4.x with NIT or BPF:
              You must have read access to /dev/nit or /dev/bpf*.

       Under Solaris with DLPI:
              You  must  have  read/write access to the network pseudo device,
              e.g.  /dev/le.  On at least some versions of  Solaris,  however,
              this   is   not  sufficient  to  allow  tcpdump  to  capture  in
              promiscuous mode; on those versions  of  Solaris,  you  must  be
              root,  or  tcpdump must be installed setuid to root, in order to
              capture in promiscuous mode.  Note that, on many  (perhaps  all)
              interfaces,  if  you don’t capture in promiscuous mode, you will
              not  see  any  outgoing  packets,  so  a  capture  not  done  in
              promiscuous mode may not be very useful.

       Under HP-UX with DLPI:
              You must be root or tcpdump must be installed setuid to root.

       Under IRIX with snoop:
              You must be root or tcpdump must be installed setuid to root.

       Under Linux:
              You  must  be  root  or tcpdump must be installed setuid to root
              (unless your distribution has a kernel that supports  capability
              bits such as CAP_NET_RAW and code to allow those capability bits
              to be given to particular accounts and to cause those bits to be
              set  on  a  user’s  initial processes when they log in, in which
              case  you   must  have  CAP_NET_RAW  in  order  to  capture  and
              CAP_NET_ADMIN  to  enumerate  network devices with, for example,
              the -D flag).

       Under ULTRIX and Digital UNIX/Tru64 UNIX:
              Any user may capture network traffic with tcpdump.  However,  no
              user  (not  even the super-user) can capture in promiscuous mode
              on an interface unless the super-user has  enabled  promiscuous-
              mode  operation on that interface using pfconfig(8), and no user
              (not even the super-user) can capture unicast  traffic  received
              by  or sent by the machine on an interface unless the super-user
              has enabled copy-all-mode  operation  on  that  interface  using
              pfconfig,  so  useful  packet  capture  on an interface probably
              requires   that   either   promiscuous-mode   or   copy-all-mode
              operation,  or  both  modes  of  operation,  be  enabled on that
              interface.

       Under BSD (this includes Mac OS X):
              You must have read access to /dev/bpf*  on  systems  that  don’t
              have  a  cloning  BPF device, or to /dev/bpf on systems that do.
              On BSDs with a devfs  (this  includes  Mac  OS  X),  this  might
              involve  more  than  just having somebody with super-user access
              setting the ownership or permissions on the  BPF  devices  -  it
              might   involve  configuring  devfs  to  set  the  ownership  or
              permissions every time the system is booted, if the system  even
              supports  that;  if  it  doesn’t support that, you might have to
              find some other way to make that happen at boot time.

       Reading a saved packet file doesn’t require special privileges.

OPTIONS

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

       -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()
              function.

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

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

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

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

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

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

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

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

              The test for ‘foreign’ IPv4 addresses is  done  using  the  IPv4
              address  and  netmask of the 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.

       -i     Listen  on  interface.   If  unspecified,  tcpdump  searches the
              system interface list for the  lowest  numbered,  configured  up
              interface (excluding loopback).  Ties are broken by choosing the
              earliest match.

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

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

       -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 and exit.

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

       -M     Use secret as a shared secret for validating the  digests  found
              in  TCP segments with the TCP-MD5 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     Dont 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
              output lines are shorter.

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

       -r     Read packets from file (which was created with the  -w  option).
              Standard input is used if file is ‘‘-’’.

       -S     Print absolute, rather than relative, TCP sequence numbers.

       -s     Snarf  snaplen  bytes  of  data from each packet rather than the
              default of 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
              capture the protocol information you’re interested in.   Setting
              snaplen  to  0  means  use  the  required  length to catch whole
              packets.

       -T     Force packets selected by "expression"  to  be  interpreted  the
              specified  type.   Currently  known  types  are aodv (Ad-hoc On-
              demand Distance Vector protocol), cnfp (Cisco NetFlow protocol),
              rpc   (Remote   Procedure  Call),  rtp  (Real-Time  Applications
              protocol), rtcp (Real-Time Applications control protocol),  snmp
              (Simple   Network   Management  Protocol),  tftp  (Trivial  File
              Transfer Protocol), vat (Visual Audio Tool), and wb (distributed
              White Board).

       -t     Dont print a timestamp on each dump line.

       -tt    Print an unformatted timestamp on each dump line.

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

       -tttt  Print a timestamp in default format proceeded by  date  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()
              function.

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

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

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

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

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

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

       -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     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.

              The expression consists of one or more  primitives.   Primitives
              usually  consist  of  an  id (name or number) preceded by one or
              more qualifiers.  There are three different kinds of qualifier:

              type   qualifiers say what kind of thing the id name  or  number
                     refers  to.   Possible  types  are  host,  net , port and
                     portrange.  E.g., ‘host foo’,  ‘net  128.3’,  ‘port  20’,
                     ‘portrange  6000-6008’.   If  there is no type qualifier,
                     host is assumed.

              dir    qualifiers specify a  particular  transfer  direction  to
                     and/or from id.  Possible directions are src, dst, src or
                     dst and src and dst.  E.g., ‘src foo’, ‘dst  net  128.3’,
                     ‘src   or  dst  port  ftp-data’.   If  there  is  no  dir
                     qualifier, src or dst is assumed.  For some link  layers,
                     such  as  SLIP and the ‘‘cooked’’ Linux capture mode used
                     for the ‘‘any’’ device and for some other  device  types,
                     the  inbound  and  outbound  qualifiers  can  be  used to
                     specify a desired direction.

              proto  qualifiers restrict the match to a  particular  protocol.
                     Possible protos are: ether, fddi, tr, wlan, ip, ip6, arp,
                     rarp, decnet, tcp and udp.  E.g., ‘ether src  foo’,  ‘arp
                     net 128.3’, ‘tcp port 21’, ‘udp portrange 7000-7009’.  If
                     there is no proto  qualifier,  all  protocols  consistent
                     with the type are assumed.  E.g., ‘src foo’ means ‘(ip or
                     arp or rarp) src foo’ (except the  latter  is  not  legal
                     syntax),  ‘net  bar’  means ‘(ip or arp or rarp) net bar’
                     and ‘port 53’ means ‘(tcp or udp) port 53’.

              [‘fddi’ is actually an alias for ‘ether’; the parser treats them
              identically  as  meaning  ‘‘the  data  link  level  used  on the
              specified network interface.’’  FDDI headers  contain  Ethernet-
              like   source  and  destination  addresses,  and  often  contain
              Ethernet-like packet types, so you  can  filter  on  these  FDDI
              fields just as with the analogous Ethernet fields.  FDDI headers
              also contain other fields, but you cannot name  them  explicitly
              in a filter expression.

              Similarly, ‘tr’ and ‘wlan’ are aliases for ‘ether’; the previous
              paragraph’s statements about FDDI headers also  apply  to  Token
              Ring  and  802.11 wireless LAN headers.  For 802.11 headers, the
              destination address is the DA field and the  source  address  is
              the SA field; the BSSID, RA, and TA fields aren’t tested.]

              In  addition  to  the  above, there are some special ‘primitive’
              keywords that don’t  follow  the  pattern:  gateway,  broadcast,
              less,  greater  and  arithmetic  expressions.   All of these are
              described below.

              More complex filter expressions are built up by using the  words
              and,  or and not to combine primitives.  E.g., ‘host foo and not
              port ftp and not port  ftp-data’.   To  save  typing,  identical
              qualifier lists can be omitted.  E.g., ‘tcp dst port ftp or ftp-
              data or domain’ is exactly the same as ‘tcp dst port ftp or  tcp
              dst port ftp-data or tcp dst port domain’.

              Allowable primitives are:

              dst host host
                     True  if  the  IPv4/v6 destination field of the packet is
                     host, which may be either an address or a name.

              src host host
                     True if the IPv4/v6 source field of the packet is host.

              host host
                     True if either the IPv4/v6 source or destination  of  the
                     packet is host.

                     Any  of  the above host expressions can be prepended with
                     the keywords, ip, arp, rarp, or ip6 as in:
                          ip host host
                     which is equivalent to:
                          ether proto \ip and host host
                     If host is  a  name  with  multiple  IP  addresses,  each
                     address will be checked for a match.

              ether dst ehost
                     True if the Ethernet destination address is ehost.  Ehost
                     may be either a name from /etc/ethers or  a  number  (see
                     ethers(5) for numeric format).

              ether src ehost
                     True if the Ethernet source address is ehost.

              ether host ehost
                     True if either the Ethernet source or destination address
                     is ehost.

              gateway host
                     True if the packet used host as  a  gateway.   I.e.,  the
                     Ethernet  source  or  destination  address  was  host but
                     neither the IP source nor the IP  destination  was  host.
                     Host  must  be  a  name  and  must  be  found both by the
                     machine’s host-name-to-IP-address  resolution  mechanisms
                     (host  name  file,  DNS,  NIS, etc.) and by the machine’s
                     host-name-to-Ethernet-address    resolution     mechanism
                     (/etc/ethers, etc.).  (An equivalent expression is
                          ether host ehost and not host host
                     which can be used with either names or numbers for host /
                     ehost.)   This  syntax  does  not  work  in  IPv6-enabled
                     configuration at this moment.

              dst net net
                     True if the IPv4/v6 destination address of the packet has
                     a network number of net.  Net may be either a  name  from
                     the  networks database (/etc/networks, etc.) or a network
                     number.  An IPv4 network  number  can  be  written  as  a
                     dotted  quad  (e.g.,  192.168.1.0),  dotted triple (e.g.,
                     192.168.1), dotted pair (e.g, 172.16), or  single  number
                     (e.g.,  10);  the netmask is 255.255.255.255 for a dotted
                     quad  (which  means  that  it’s  really  a  host  match),
                     255.255.255.0  for  a  dotted  triple,  255.255.0.0 for a
                     dotted pair, or 255.0.0.0 for a single number.   An  IPv6
                     network  number must be written out fully; the netmask is
                     ff:ff:ff:ff:ff:ff:ff:ff, so IPv6  "network"  matches  are
                     really  always host matches, and a network match requires
                     a netmask length.

              src net net
                     True if the IPv4/v6 source address of the  packet  has  a
                     network number of net.

              net net
                     True  if either the IPv4/v6 source or destination address
                     of the packet has a network number of net.

              net net mask netmask
                     True if the IPv4 address matches net  with  the  specific
                     netmask.   May  be  qualified with src or dst.  Note that
                     this syntax is not valid for IPv6 net.

              net net/len
                     True if the IPv4/v6 address matches net  with  a  netmask
                     len bits wide.  May be qualified with src or dst.

              dst port port
                     True  if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp
                     and has a destination port value of port.  The  port  can
                     be  a  number or a name used in /etc/services (see tcp(7)
                     and udp(7)).  If a name is used, both the port number and
                     protocol  are  checked.  If a number or ambiguous name is
                     used, only the port number is checked (e.g., dst port 513
                     will  print  both  tcp/login traffic and udp/who traffic,
                     and port domain will print both tcp/domain and udp/domain
                     traffic).

              src port port
                     True if the packet has a source port value of port.

              port port
                     True  if  either  the  source  or destination port of the
                     packet is port.

              dst portrange port1-port2
                     True if the packet is ip/tcp, ip/udp, ip6/tcp or  ip6/udp
                     and has a destination port value between port1 and port2.
                     port1 and port2 are interpreted in the  same  fashion  as
                     the port parameter for port.

              src portrange port1-port2
                     True  if the packet has a source port value between port1
                     and port2.

              portrange port1-port2
                     True if either the source  or  destination  port  of  the
                     packet is between port1 and port2.

                     Any  of  the  above port or port range expressions can be
                     prepended with the keywords, tcp or udp, as in:
                          tcp src port port
                     which matches only tcp packets whose source port is port.

              less length
                     True  if  the  packet  has a length less than or equal to
                     length.  This is equivalent to:
                          len <= length.

              greater length
                     True if the packet has a length greater than or equal  to
                     length.  This is equivalent to:
                          len >= length.

              ip proto protocol
                     True  if  the  packet  is  an IPv4 packet (see ip(4P)) of
                     protocol type protocol.  Protocol can be a number or  one
                     of the names icmp, icmp6, igmp, igrp, pim, ah, esp, vrrp,
                     udp, or tcp.  Note that the  identifiers  tcp,  udp,  and
                     icmp  are also keywords and must be escaped via backslash
                     (\),  which  is  \\  in  the  C-shell.   Note  that  this
                     primitive does not chase the protocol header chain.

              ip6 proto protocol
                     True  if  the  packet  is an IPv6 packet of protocol type
                     protocol.  Note that this primitive does  not  chase  the
                     protocol header chain.

              ip6 protochain protocol
                     True  if the packet is IPv6 packet, and contains protocol
                     header with type protocol in its protocol  header  chain.
                     For example,
                          ip6 protochain 6
                     matches  any  IPv6 packet with TCP protocol header in the
                     protocol header  chain.   The  packet  may  contain,  for
                     example,  authentication  header, routing header, or hop-
                     by-hop option header, between IPv6 header and TCP header.
                     The  BPF  code  emitted  by this primitive is complex and
                     cannot be optimized by BPF optimizer code in tcpdump,  so
                     this can be somewhat slow.

              ip protochain protocol
                     Equivalent  to  ip6  protochain protocol, but this is for
                     IPv4.

              ether broadcast
                     True if the packet is an Ethernet broadcast packet.   The
                     ether keyword is optional.

              ip broadcast
                     True  if  the  packet  is  an  IPv4 broadcast packet.  It
                     checks for both the  all-zeroes  and  all-ones  broadcast
                     conventions,   and  looks  up  the  subnet  mask  on  the
                     interface on which the capture is being done.

                     If the subnet mask of the interface on which the  capture
                     is  being  done  is  not  available,  either  because the
                     interface on which capture is being done has  no  netmask
                     or  because  the capture is being done on the Linux "any"
                     interface, which can capture on more than one  interface,
                     this check will not work correctly.

              ether multicast
                     True  if the packet is an Ethernet multicast packet.  The
                     ether  keyword  is  optional.   This  is  shorthand   for
                     ‘ether[0] & 1 != 0’.

              ip multicast
                     True if the packet is an IPv4 multicast packet.

              ip6 multicast
                     True if the packet is an IPv6 multicast packet.

              ether proto protocol
                     True  if  the packet is of ether type protocol.  Protocol
                     can be a number or one of the names ip, ip6,  arp,  rarp,
                     atalk,  aarp,  decnet,  sca, lat, mopdl, moprc, iso, stp,
                     ipx,  or  netbeui.   Note  these  identifiers  are   also
                     keywords and must be escaped via backslash (\).

                     [In  the  case of FDDI (e.g., ‘fddi protocol arp’), Token
                     Ring (e.g., ‘tr protocol arp’), and IEEE 802.11  wireless
                     LANS  (e.g.,  ‘wlan  protocol  arp’),  for  most of those
                     protocols, the protocol  identification  comes  from  the
                     802.2 Logical Link Control (LLC) header, which is usually
                     layered on top of the FDDI, Token Ring, or 802.11 header.

                     When  filtering  for  most  protocol identifiers on FDDI,
                     Token Ring, or 802.11, tcpdump checks only  the  protocol
                     ID  field  of an LLC header in so-called SNAP format with
                     an Organizational Unit Identifier (OUI) of 0x000000,  for
                     encapsulated  Ethernet;  it  doesn’t  check  whether  the
                     packet is in SNAP format with an OUI  of  0x000000.   The
                     exceptions are:

                      iso    tcpdump  checks  the  DSAP  (Destination  Service
                             Access Point) and  SSAP  (Source  Service  Access
                             Point) fields of the LLC header;

                     stp and netbeui
                             tcpdump checks the DSAP of the LLC header;

                      atalk  tcpdump  checks  for a SNAP-format packet with an
                             OUI of 0x080007 and the AppleTalk etype.

                     In the case of Ethernet, tcpdump checks the Ethernet type
                     field for most of those protocols.  The exceptions are:

                     iso, stp, and netbeui
                             tcpdump checks for an 802.3 frame and then checks
                             the LLC header as it does for FDDI,  Token  Ring,
                             and 802.11;

                      atalk  tcpdump checks both for the AppleTalk etype in an
                             Ethernet frame and for a SNAP-format packet as it
                             does for FDDI, Token Ring, and 802.11;

                      aarp   tcpdump  checks  for  the  AppleTalk ARP etype in
                             either an Ethernet frame or an 802.2  SNAP  frame
                             with an OUI of 0x000000;

                      ipx    tcpdump  checks  for the IPX etype in an Ethernet
                             frame, the  IPX  DSAP  in  the  LLC  header,  the
                             802.3-with-no-LLC-header  encapsulation  of  IPX,
                             and the IPX etype in a SNAP frame.

              decnet src host
                     True if the DECNET source address is host, which  may  be
                     an address of the form ‘‘10.123’’, or a DECNET host name.
                     [DECNET host name support is  only  available  on  ULTRIX
                     systems that are configured to run DECNET.]

              decnet dst host
                     True if the DECNET destination address is host.

              decnet host host
                     True  if  either the DECNET source or destination address
                     is host.

              ifname interface
                     True  if  the  packet  was  logged  as  coming  from  the
                     specified  interface  (applies  only to packets logged by
                     OpenBSD’s pf(4)).

              on interface
                     Synonymous with the ifname modifier.

              rnr num
                     True if the packet was logged as matching  the  specified
                     PF  rule  number  (applies  only  to  packets  logged  by
                     OpenBSD’s pf(4)).

              rulenum num
                     Synonymous with the rnr modifier.

              reason code
                     True if the packet  was  logged  with  the  specified  PF
                     reason  code.   The  known  codes are: match, bad-offset,
                     fragment, short, normalize, and memory (applies  only  to
                     packets logged by OpenBSD’s pf(4)).

              rset name
                     True  if  the packet was logged as matching the specified
                     PF ruleset name of an anchored ruleset (applies  only  to
                     packets logged by pf(4)).

              ruleset name
                     Synonymous with the rset modifier.

              srnr num
                     True  if  the packet was logged as matching the specified
                     PF rule number of an anchored ruleset  (applies  only  to
                     packets logged by pf(4)).

              subrulenum num
                     Synonymous with the srnr modifier.

              action act
                     True  if PF took the specified action when the packet was
                     logged.  Known actions are: pass and block (applies  only
                     to packets logged by OpenBSD’s pf(4)).

              ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
                     Abbreviations for:
                          ether proto p
                     where p is one of the above protocols.

              lat, moprc, mopdl
                     Abbreviations for:
                          ether proto p
                     where p is one of the above protocols.  Note that tcpdump
                     does not currently know how to parse these protocols.

              vlan [vlan_id]
                     True if the packet is an IEEE  802.1Q  VLAN  packet.   If
                     [vlan_id]  is  specified, only true if the packet has the
                     specified vlan_id.  Note  that  the  first  vlan  keyword
                     encountered  in  expression  changes the decoding offsets
                     for the remainder of expression on  the  assumption  that
                     the   packet  is  a  VLAN  packet.   The  vlan  [vlan_id]
                     expression may be used more than once, to filter on  VLAN
                     hierarchies.   Each use of that expression increments the
                     filter offsets by 4.

                     For example:
                          vlan 100 && vlan 200
                     filters on VLAN 200 encapsulated within VLAN 100, and
                          vlan && vlan 300 && ip
                     filters  IPv4  protocols   encapsulated   in   VLAN   300
                     encapsulated within any higher order VLAN.

              mpls [label_num]
                     True  if the packet is an MPLS packet.  If [label_num] is
                     specified, only true is  the  packet  has  the  specified
                     label_num.   Note that the first mpls keyword encountered
                     in  expression  changes  the  decoding  offsets  for  the
                     remainder of expression on the assumption that the packet
                     is a MPLS-encapsulated IP packet.  The  mpls  [label_num]
                     expression  may be used more than once, to filter on MPLS
                     hierarchies.  Each use of that expression increments  the
                     filter offsets by 4.

                     For example:
                          mpls 100000 && mpls 1024
                     filters  packets  with  an  outer  label of 100000 and an
                     inner label of 1024, and
                          mpls && mpls 1024 && host 192.9.200.1
                     filters packets to or  from  192.9.200.1  with  an  inner
                     label of 1024 and any outer label.

              pppoed True  if  the  packet  is  a  PPP-over-Ethernet Discovery
                     packet (Ethernet type 0x8863).

              pppoes True if the packet is a PPP-over-Ethernet Session  packet
                     (Ethernet  type  0x8864).   Note  that  the  first pppoes
                     keyword encountered in expression  changes  the  decoding
                     offsets for the remainder of expression on the assumption
                     that the packet is a PPPoE session packet.

                     For example:
                          pppoes && ip
                     filters IPv4 protocols encapsulated in PPPoE.

              tcp, udp, icmp
                     Abbreviations for:
                          ip proto p or ip6 proto p
                     where p is one of the above protocols.

              iso proto protocol
                     True if the packet is an  OSI  packet  of  protocol  type
                     protocol.   Protocol  can be a number or one of the names
                     clnp, esis, or isis.

              clnp, esis, isis
                     Abbreviations for:
                          iso proto p
                     where p is one of the above protocols.

              l1, l2, iih, lsp, snp, csnp, psnp
                     Abbreviations for IS-IS PDU types.

              vpi n  True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris, with a virtual path identifier of n.

              vci n  True  if  the  packet  is  an  ATM  packet, for SunATM on
                     Solaris, with a virtual channel identifier of n.

              lane   True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and is an ATM LANE packet.  Note that the first
                     lane keyword encountered in expression changes the  tests
                     done  in  the  remainder  of expression on the assumption
                     that the packet is either a LANE emulated Ethernet packet
                     or  a  LANE  LE Control packet.  If lane isn’t specified,
                     the tests are done under the assumption that  the  packet
                     is an LLC-encapsulated packet.

              llc    True  if  the  packet  is  an  ATM  packet, for SunATM on
                     Solaris, and is an LLC-encapsulated packet.

              oamf4s True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and  is  a  segment  OAM  F4 flow cell (VPI=0 &
                     VCI=3).

              oamf4e True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and  is an end-to-end OAM F4 flow cell (VPI=0 &
                     VCI=4).

              oamf4  True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and is a segment or end-to-end OAM F4 flow cell
                     (VPI=0 & (VCI=3 | VCI=4)).

              oam    True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and is a segment or end-to-end OAM F4 flow cell
                     (VPI=0 & (VCI=3 | VCI=4)).

              metac  True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and  is  on  a  meta signaling circuit (VPI=0 &
                     VCI=1).

              bcc    True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris, and is on a broadcast signaling circuit (VPI=0 &
                     VCI=2).

              sc     True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris, and is on a signaling circuit (VPI=0 & VCI=5).

              ilmic  True  if  the  packet  is  an  ATM  packet, for SunATM on
                     Solaris, and is on an ILMI circuit (VPI=0 & VCI=16).

              connectmsg
                     True if the packet  is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and  is  on a signaling circuit and is a Q.2931
                     Setup, Call Proceeding, Connect, Connect Ack, Release, or
                     Release Done message.

              metaconnect
                     True  if  the  packet  is  an  ATM  packet, for SunATM on
                     Solaris, and is on a meta  signaling  circuit  and  is  a
                     Q.2931  Setup,  Call  Proceeding,  Connect,  Release,  or
                     Release Done message.

              expr relop expr
                     True if the relation holds, where relop is one of  >,  <,
                     >=,  <=,  =,  !=,  and  expr  is an arithmetic expression
                     composed of integer constants (expressed  in  standard  C
                     syntax),  the  normal binary operators [+, -, *, /, &, |,
                     <<, >>], a  length  operator,  and  special  packet  data
                     accessors.   Note  that  all comparisons are unsigned, so
                     that, for example, 0x80000000 and 0xffffffff are > 0.  To
                     access data inside the packet, use the following syntax:
                          proto [ expr : size ]
                     Proto  is  one of ether, fddi, tr, wlan, ppp, slip, link,
                     ip,  arp,  rarp,  tcp,  udp,  icmp,  ip6  or  radio,  and
                     indicates  the  protocol  layer  for the index operation.
                     (ether, fddi, wlan, tr, ppp, slip and link all  refer  to
                     the  link layer. radio refers to the "radio header" added
                     to some 802.11 captures.)  Note that tcp, udp  and  other
                     upper-layer  protocol  types only apply to IPv4, not IPv6
                     (this will be fixed in the  future).   The  byte  offset,
                     relative  to  the  indicated  protocol layer, is given by
                     expr.  Size is optional and indicates the number of bytes
                     in  the  field of interest; it can be either one, two, or
                     four,  and  defaults  to  one.   The   length   operator,
                     indicated  by  the  keyword  len, gives the length of the
                     packet.

                     For example, ‘ether[0] & 1 != 0’  catches  all  multicast
                     traffic.   The  expression ‘ip[0] & 0xf != 5’ catches all
                     IPv4 packets with options.   The  expression  ‘ip[6:2]  &
                     0x1fff  = 0’ catches only unfragmented IPv4 datagrams and
                     frag zero of fragmented IPv4 datagrams.   This  check  is
                     implicitly  applied  to the tcp and udp index operations.
                     For instance, tcp[0] always means the first byte  of  the
                     TCP  header,  and  never  means  the  first  byte  of  an
                     intervening fragment.

                     Some offsets and field values may be expressed  as  names
                     rather  than  as  numeric values.  The following protocol
                     header field offsets are available: icmptype  (ICMP  type
                     field),  icmpcode  (ICMP  code  field), and tcpflags (TCP
                     flags field).

                     The following ICMP type field values are available: icmp-
                     echoreply,    icmp-unreach,    icmp-sourcequench,   icmp-
                     redirect,     icmp-echo,     icmp-routeradvert,     icmp-
                     routersolicit,   icmp-timxceed,   icmp-paramprob,   icmp-
                     tstamp,  icmp-tstampreply,   icmp-ireq,   icmp-ireqreply,
                     icmp-maskreq, icmp-maskreply.

                     The  following TCP flags field values are available: tcp-
                     fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg.

              Primitives may be combined using:

                     A  parenthesized  group  of  primitives   and   operators
                     (parentheses  are  special  to  the  Shell  and  must  be
                     escaped).

                     Negation (‘!’ or ‘not’).

                     Concatenation (‘&&’ or ‘and’).

                     Alternation (‘||’ or ‘or’).

              Negation has highest precedence.  Alternation and  concatenation
              have  equal  precedence  and associate left to right.  Note that
              explicit and tokens, not juxtaposition,  are  now  required  for
              concatenation.

              If  an  identifier  is  given without a keyword, the most recent
              keyword is assumed.  For example,
                   not host vs and ace
              is short for
                   not host vs and host ace
              which should not be confused with
                   not ( host vs or ace )

              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):
              tcpdumpgateway 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.
              tcpdumptcp[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.)
              tcpdumptcp 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:
              tcpdumpgateway snup and ip[2:2] > 576’

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

       To print all ICMP packets that are not echo requests/replies (i.e., not
       ping packets):
              tcpdumpicmp[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
       datagrams) are ‘async’ packets, with a priority value between 0 and  7;
       for  example,  ‘async4’.   Such packets are assumed to contain an 802.2
       Logical 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
       compression algorithm described in RFC-1144.)

       On SLIP links, a direction indicator  (‘‘I’’  for  inbound,  ‘‘O’’  for
       outbound),  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
       packets, 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 number), and I (packet ID), followed by a delta (+n  or  -n),
       or  a  new  value  (=n).  Finally, the amount of data in the packet and
       compressed header length are printed.

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

       ARP/RARP Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP  addresses  and  ports.
       Flags  are  some  combination of S (SYN), F (FIN), P (PUSH), R (RST), 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
              csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
              csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
              csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
       The  first  line  says that tcp port 1023 on rtsg sent a packet to port
       login on csam.  The S indicates that the SYN flag was set.  The  packet
       sequence  number was 768512 and it contained no data.  (The notation is
       ‘first:last(nbytes)’ which means ‘sequence numbers first up to but  not
       including  last  which  is  nbytes  bytes of user data’.)  There was no
       piggy-backed ack, the available receive window was 4096 bytes and there
       was a max-segment-size option requesting an mss of 1024 bytes.

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

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

       Now, let’s assume that we need to capture SYN  packets,  but  we  don’t
       care  if  ACK  or  any  other  TCP 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
       not those with only SYN set.  Remember that we don’t care if ACK or any
       other control bit is set as long as SYN is set.

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

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

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

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

       This points us to the tcpdump filter expression
                   tcpdump -i xl0tcp[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
       contained 84 bytes of user data.

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

       UDP Name Server Requests

       (N.B.:The following description assumes  familiarity  with  the  Domain
       Service  protocol  described in RFC-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
       headers.   The  query  operation  was  the normal one, Query, so the op
       field was omitted.  If the op had been anything  else,  it  would  have
       been  printed  between  the ‘3’ and the ‘+’.  Similarly, the qclass was
       the normal one, C_IN, and omitted.  Any other qclass  would  have  been
       printed immediately after the ‘A’.

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

       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
       (NoError) were omitted, as was the class (C_IN) of the A record.

       In the second example, helios responds to query 2 with a response  code
       of  non-existent domain (NXDomain) with no answers, one name server and
       no authority records.  The ‘*’ indicates that the authoritative  answer
       bit  was set.  Since there were no answers, no type, class or data were
       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
       seriously  investigate  name  server traffic.  ‘-s 128’ has worked well
       for me.

       SMB/CIFS decoding

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

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

       For  information  on SMB packet formats and what all te 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,
       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
       offset 24576.  Wrl replies ‘ok’; the packet shown on the second line is
       the first fragment of the reply, and hence is only 1472 bytes long (the
       other bytes will follow in subsequent fragments, but these fragments do
       not have NFS or even UDP headers and so might not be printed, depending
       on the filter expression used).  Because the -v flag is given, some  of
       the  file  attributes (which are returned in addition to the file data)
       are printed: the file type (‘‘REG’’, for regular file), the  file  mode
       (in octal), the uid and gid, and the file size.

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

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

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

       AFS Requests and Replies

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

              src.sport > dst.dport: rx packet-type
              src.sport > dst.dport: rx packet-type service call call-name args
              src.sport > dst.dport: rx packet-type service reply call-name args

              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                   new fid 536876964/1/1 ".newsrc"
              pike.afsfs > elvis.7001: rx data fs reply rename
       In the first line, host elvis sends a RX packet to pike.  This was a RX
       data packet to the fs (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
       successful, because it was a data packet and not an abort packet).

       In  general,  all AFS RPCs are decoded at least by RPC call name.  Most
       AFS RPCs have at least some of the arguments  decoded  (generally  only
       the ‘interesting’ arguments, for some definition of 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
       discarded).  The file /etc/atalk.names is used to  translate  AppleTalk
       net and node numbers to names.  Lines in this file have the form
              number    name

              1.254          ether
              16.1      icsd-net
              1.254.110 ace
       The  first  two  lines give the names of AppleTalk networks.  The third
       line gives the name of a 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@*"
              jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
              techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
       The  first  line  is a name lookup request for laserwriters sent by net
       icsd host 112 and broadcast on net jssmag.  The nbp id for  the  lookup
       is  190.   The second line shows a reply for this request (note that it
       has the same id) from host jssmag.209 saying that it has a  laserwriter
       resource  named  "RM1140"  registered  on  port 250.  The third line is
       another reply to the same request saying host techpit  has  laserwriter
       "techpit" registered on port 186.

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

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

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

       IP Fragmentation

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

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

       The fragment information  is  output  for  each  fragment.   The  first
       fragment contains the higher level protocol header and the frag info is
       printed after the protocol info.  Fragments after the first 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.
       Second, the tcp sequence information in the first line is printed as if
       there  were  308  bytes of user data when, in fact, there are 512 bytes
       (308 in the first frag and 204 in the second).  If you are looking  for
       holes  in  the  sequence space or trying to match up acks with packets,
       this can fool you.

       A packet with the IP dont fragment flag  is  marked  with  a  trailing
       (DF).

       Timestamps

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

SEE ALSO

       stty(1), pcap(3), bpf(4), nit(4P), pfconfig(8)

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 configuration.

BUGS

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

              tcpdump-workers@tcpdump.org

       Please send source code contributions, etc. to:

              patches@tcpdump.org

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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won’t work correctly.

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

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

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

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

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

                                 18 April 2005                      TCPDUMP(8)