a2p
accept
access
acct
addftinfo
addr2line
adjtime
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aio_cancel
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aio_return
aio_suspend
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perl
perl56delta
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_exit
__syscall
 
FreeBSD/Linux/UNIX General Commands Manual
Hypertext Man Pages
tcpdump
 
TCPDUMP(1)							    TCPDUMP(1)



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 ]
	       [ -y datalinktype ]
	       [ expression ]

DESCRIPTION
       Tcpdump	prints	out the headers 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 sig-
       nal (typically generated with the kill(1) command); if run with the  -c
       flag,  it  will	capture packets until it is interrupted by a SIGINT or
       SIGTERM signal or the specified number of packets have been  processed.

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

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

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

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

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

       Reading packets from a network interface may require that you have spe-
       cial privileges:

       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 promiscu-
	      ous 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	opera-
	      tion,  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 BSDs	with  a  devfs
	      (this includes Mac OS X), this might involve more than just hav-
	      ing 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 sup-
	      port 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() 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
	      seperation.

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

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

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

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

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

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

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

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

       -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.

       -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  tcp-
	      dump.

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

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

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

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

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

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

       -R     Assume  ESP/AH packets to be based on old specification (RFC1825
	      to RFC1829).  If specified, tcpdump will not print  replay  pre-
	      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).

       -t     Don't 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() 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.

       -x     Print 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 lay-
	      ers  that  pad  (e.g.  Ethernet), the padding bytes will also be
	      printed when  the  higher  layer	packet	is  shorter  than  the
	      required padding.

       -xx    Print each packet, including its link level header, in hex.

       -X     Print  each  packet  (minus  its	link  level header) in hex and
	      ASCII.  This is very handy for analysing new protocols.

       -XX    Print 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  por-
		     trange.   E.g., `host foo', `net 128.3', `port 20', `por-
		     trange 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	quali-
		     fier,  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,  lat,  sca, moprc, mopdl, iso, esis, isis,
		     icmp, icmp6, 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 speci-
	      fied 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 con-
	      tain other fields, but you cannot name them explicitly in a fil-
	      ter 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(3N) 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 nei-
		     ther 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  con-
		     figuration 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
		     /etc/networks  or	a  network number (see networks(4) for
		     details).

	      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(4P)
		     and udp(4P)).  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 traf-
		     fic, 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 pro-
		     tocol 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	primi-
		     tive 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 exam-
		     ple, 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 inter-
		     face 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 inter-
		     face 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  key-
		     words 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 pro-
		     tocols, the protocol identification comes from the  802.2
		     Logical  Link Control (LLC) header, which is usually lay-
		     ered 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 speci-
		     fied  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
		     Synonomous with the rnr modifier.

	      reason code
		     True  if the packet was logged with the specified PF rea-
		     son code.	The known codes are: match, bad-offset,  frag-
		     ment, short, normalize, and memory (applies only to pack-
		     ets 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
		     Synonomous 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
		     Synonomous 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 is 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_id] statement may
		     be  used  more  than once, to filter on vlan hierarchies.
		     each use of the [vlan_id] expression increments the  fil-
		     ter offsets by 4.
		     example(s):
		     "vlan  100  && vlan 200" filters on vlan 200 encapsulated
		     within vlan 100
		     "vlan && vlan 300 && ip" filters IPv4 protocols  encapsu-
		     lated  in	vlan  300 encapsulated within any higher order
		     vlan

	      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 pro-
		     tocol.   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 com-
		     posed of integer constants (expressed in standard C  syn-
		     tax),  the normal binary operators [+, -, *, /, &, |, <<,
		     >>], a length operator, and special  packet  data	acces-
		     sors.   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  indi-
		     cates   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,  indi-
		     cated 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 inter-
		     vening 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-redi-
		     rect, icmp-echo,  icmp-routeradvert,  icmp-routersolicit,
		     icmp-timxceed,  icmp-paramprob,  icmp-tstamp, icmp-tstam-
		     preply, 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 (paren-
		     theses 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):
	      tcpdump 'gateway snup and (port ftp or ftp-data)'

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

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

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

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

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

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

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

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP Packets

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

       This would look less redundant if we had done tcpdump -n:
	      arp who-has 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., ).

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

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

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

       If  the	snapshot was small enough that tcpdump didn't capture the full
       TCP header, it interprets as much of the header	as  it	can  and  then
       reports	``[|tcp]'' to indicate the remainder could not be interpreted.
       If the header contains a bogus option (one with a length that's	either
       too  small  or  beyond  the  end  of the header), tcpdump reports it as
       ``[bad opt]'' and does not interpret any further  options  (since  it's
       impossible  to  tell where they start).	If the header length indicates
       options are present but the IP datagram length is not long  enough  for
       the  options  to  actually  be  there, tcpdump reports it as ``[bad hdr
       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 dec-
       imal value 2", which is exactly what we want.

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

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

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

		   00010010

       which translates to decimal

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

       Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression,
       because that would select only those packets that have SYN-ACK set, but
       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-
       tained 84 bytes of user data.

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

       UDP Name Server Requests

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

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

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

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

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

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

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

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

       AFS Requests and Replies

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

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

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

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

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

       If  the -v flag is given twice, additional information is printed, such
       as the the RX call ID, serial number, and the RX packet flags.  The MTU
       negotiation information is also printed from RX ack packets.

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

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

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

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


       KIP AppleTalk (DDP in UDP)

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

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

       AppleTalk addresses are printed in the form
	      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 request-
       ing up to 8 packets (the `<0-7>').  The hex number at the  end  of  the
       line is the value of the `userdata' field in the request.

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

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


       IP Fragmentation

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

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

       The fragment information is output for each fragment.  The first  frag-
       ment  contains  the  higher  level protocol header and the frag info is
       printed after the protocol info.  Fragments after the first contain  no
       higher  level  protocol	header	and the frag info is printed after the
       source and destination addresses.  For example, here is part of an  ftp
       from  arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't
       appear to handle 576 byte datagrams:
	      arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
	      arizona > rtsg: (frag 595a:204@328)
	      rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
       There are a couple of things to note here:  First, addresses in the 2nd
       line  don't  include  port  numbers.   This is because the TCP protocol
       information is all in the first fragment and we have no idea  what  the
       port  or  sequence numbers are when we print the later fragments.  Sec-
       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
       bpf(4), pcap(3)

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 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.



				 18 April 2005			    TCPDUMP(1)
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