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OpenDarwin 7.2.1 - man page for tcpdump (opendarwin section 1)

TCPDUMP(1)			     General Commands Manual			       TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
	       [ -C file_size ] [ -F file ]
	       [ -i interface ] [ -m module ] [ -r file ]
	       [ -s snaplen ] [ -T type ] [ -w file ]
	       [ -E algo:secret ] [ 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 -b 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 inter-
       rupted 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  ``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 on  other  OSes  it  counts
	      only  packets that were matched by the filter expression and were processed by tcp-
	      dump);

	      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 tcp-
	      dump 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, it will report those
       counts when it receives a SIGINFO signal (generated, for example, by  typing  your  ``sta-
       tus'' character, typically control-T) 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.

       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.

       Under Ultrix and Digital UNIX:
	      Once the super-user has enabled promiscuous-mode operation using	pfconfig(8),  any
	      user may capture network traffic with tcpdump.

       Under BSD:
	      You must have read access to /dev/bpf*.

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

OPTIONS
       -a     Attempt to convert network and broadcast addresses to names.

       -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  2	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).

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

       -E     Use algo:secret for decrypting IPsec ESP packets.  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  cryptogra-
	      phy  enabled.   secret the ascii text for ESP secret key.  We cannot take arbitrary
	      binary value at this moment.  The option assumes RFC2406 ESP, not RFC1827 ESP.  The
	      option  is  only	for  debugging	purposes,  and	the use of this option with truly
	      `secret' key is discouraged.  By presenting IPsec secret key onto command line  you
	      make it visible to others, via ps(1) and other occasions.

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

       -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.

       -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''.

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

       -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 abbrevia-
	      tion 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  pro-
	      tocol  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  out-
	      put  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.   Cur-
	      rently  known types are cnfp (Cisco NetFlow protocol), rpc (Remote Procedure Call),
	      rtp (Real-Time Applications protocol), rtcp (Real-Time Applications control  proto-
	      col),  snmp  (Simple  Network Management 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.

       -v     (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.

       -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 ``-''.

       -x     Print each packet (minus its link level header) in hex.  The smaller of the  entire
	      packet or snaplen bytes will be printed.

       -X     When  printing hex, print ascii too.  Thus if -x is also set, the packet is printed
	      in hex/ascii.  This is very handy for analysing new protocols.  Even if -x  is  not
	      also set, some parts of some packets may be printed in hex/ascii.

	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 differ-
	      ent kinds of qualifier:

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

	      dir    qualifiers  specify a particular transfer direction to and/or from id.  Pos-
		     sible 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 `null' link layers (i.e. point to  point  proto-
		     cols  such as slip) the inbound and outbound qualifiers can be used to spec-
		     ify a desired direction.

	      proto  qualifiers restrict the match to a  particular  protocol.	 Possible  protos
		     are: ether, fddi, tr, ip, ip6, arp, rarp, decnet, tcp and udp.  E.g., `ether
		     src foo', `arp net 128.3', `tcp port 21'.	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' is an alias for `ether'; the previous paragraph's statements about
	      FDDI headers also apply to Token Ring headers.]

	      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 neither the IP source nor the  IP  destina-
		     tion  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 mecha-
		     nism (/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 syn-
		     tax 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 /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 IP address matches net with the specific netmask.  May be quali-
		     fied 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 destina-
		     tion 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 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.  Any of
		     the above port 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 IP 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 IP broadcast packet.  It checks for both	the  all-
		     zeroes  and  all-ones  broadcast  conventions, and looks up the local subnet
		     mask.

	      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 IP 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') and Token Ring  (e.g.,  `tr
		     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 or Token Ring header.

		     When  filtering for most protocol identifiers on FDDI or Token Ring, 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, for which it checks the DSAP (Destination Service
		     Access Point) and SSAP (Source Service  Access  Point)  fields  of  the  LLC
		     header,  stp  and	netbeui,  where it checks the DSAP of the LLC header, and
		     atalk, where it 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, sap,  and  netbeui,  for	which  it
		     checks for an 802.3 frame and then checks the LLC header as it does for FDDI
		     and Token Ring, atalk, where it checks both for the Appletalk  etype  in  an
		     Ethernet  frame  and  for a SNAP-format packet as it does for FDDI and Token
		     Ring, aarp, where it checks for the Appletalk ARP etype in either an  Ether-
		     net  frame or an 802.2 SNAP frame with an OUI of 0x000000, and ipx, where it
		     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.

	      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 speci-
		     fied, 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.

	      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.  Note that tcpdump does an incomplete
		     job of parsing these protocols.

	      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.   To	access	data  inside  the
		     packet, use the following syntax:
			  proto [ expr : size ]
		     Proto  is	one of ether, fddi, tr, ip, arp, rarp, tcp, udp, icmp or ip6, and
		     indicates the protocol layer for the index operation.  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  proto-
		     col  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 expres-
		     sion `ip[0] & 0xf != 5' catches all IP packets with options.  The expression
		     `ip[6:2] & 0x1fff = 0' catches only unfragmented datagrams and frag zero  of
		     fragmented  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-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-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 prece-
	      dence and associate left to right.  Note that explicit and tokens,  not  juxtaposi-
	      tion, 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  argu-
	      ment.  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 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  ethernet  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  gov-
       erns the interpretation of the rest of the packet.  Normal packets (such as those contain-
       ing 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.

       (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 inter-
       net  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 combi-
       nation of S (SYN), F (FIN), P (PUSH) or R (RST) 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 con-
       nection.  Window is the number of bytes of receive buffer space available the other direc-
       tion 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 con-
       tained 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 con-
       versation'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 connec-
       tion 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.  Remem-
       ber 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 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  broad-
       cast, 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  descrip-
       tion 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 proto-
       col 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.

       If  you	are decoding SMB sessions containing unicode strings then you may wish to set the
       environment variable USE_UNICODE to 1.  A patch to auto-detect  unicode	srings	would  be
       welcome.

       For  information on SMB packet formats and what all te fields mean see www.cifs.org or the
       pub/samba/specs/ directory on your favourite 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 num-
       ber 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 num-
       ber 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.  Frag-
       ments 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 ari-
       zona.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 frag-
       ments.  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 don't 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
       traffic(1C), nit(4P), bpf(4), pcap(3)

AUTHORS
       The original authors are:

       Van  Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Labo-
       ratory, 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).

       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 that manipulate FDDI or Token Ring headers assume  that  all	FDDI  and
       Token  Ring packets are SNAP-encapsulated Ethernet packets.  This is true for IP, ARP, and
       DECNET Phase IV, but is not true for protocols such as ISO CLNS.   Therefore,  the  filter
       may inadvertently accept certain packets that do not properly match the filter expression.

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

       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.

					  3 January 2001			       TCPDUMP(1)


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