


TCPDUMP(1)					       TCPDUMP(1)


NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -adeflnNOpqStvx ] [ -c count ] [ -F file ]
	       [ -i interface ] [ -r file ] [ -s snaplen ]
	       [ -T type ] [ -w file ] [ expression ]

DESCRIPTION
       Tcpdump	prints	out  the  headers of packets on a network
       interface that match the boolean expression.

       Under SunOS with nit or bpf: To run tcpdump you must  have
       read  access to /dev/nit or /dev/bpf*.  Under Solaris with
       dlpi: You must have read	 access	 to  the  network  pseudo
       device, e.g.  /dev/le.  Under HP-UX with dlpi: You must be
       root or it must be installed setuid to root.   Under  IRIX
       with  snoop:  You  must	be  root  or it must be installed
       setuid to root.	Under Linux: You must be root or it  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 run tcpdump.
       Under BSD: You must have read access to /dev/bpf*.

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

       -c     Exit after receiving count packets.

       -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 (pre-
	      ceded with a count).

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

       -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  loop-
	      back).   Ties  are  broken by choosing the earliest



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TCPDUMP(1)					       TCPDUMP(1)


	      match.

       -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''.

       -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 infor-
	      mation so output lines are shorter.

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

       -s     Snarf snaplen bytes of data from each packet rather
	      than the default of 68 (with SunOS's NIT, the mini-
	      mum is actually 96).  68 bytes is adequate for  IP,
	      ICMP,  TCP and UDP but may truncate protocol infor-
	      mation  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 proto-
	      col 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 buffer-
	      ing.  This may  cause  packets  to  be  lost.   You
	      should  limit  snaplen  to the smallest number that
	      will capture the protocol information you're inter-
	      ested in.

       -T     Force packets selected by "expression" to be inter-
	      preted the specified type.  Currently  known  types
	      are  rpc	(Remote	 Procedure  Call), rtp (Real-Time
	      Applications protocol),  rtcp  (Real-Time	 Applica-
	      tions  control  protocol), vat (Visual Audio Tool),
	      and wb (distributed White Board).



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TCPDUMP(1)					       TCPDUMP(1)


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

       -t     Don't print a timestamp on each dump line.

       -tt    Print an unformatted timestamp on each dump line.

       -v     (Slightly	 more)	verbose output.	 For example, the
	      time to live and type of service information in  an
	      IP packet is printed.

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

       -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.

	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 expres-
	      sion is `true' will be dumped.

	      The expression consists of one or more  primitives.
	      Primitives  usually  consist of an id (name or num-
	      ber) 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 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.	 Possible
		     directions	 are src, dst, src or dst and src
		     and dst.  E.g., `src foo', `dst net  128.3',
		     `src  or dst port ftp-data'.  If there is no
		     dir qualifier, src or dst is  assumed.   For
		     `null' link layers (i.e. point to point pro-
		     tocols such as slip) the  inbound	and  out-
		     bound  qualifiers	can  be used to specify a
		     desired direction.

	      proto  qualifiers restrict the match to a	 particu-
		     lar  protocol.   Possible protos are: ether,
		     fddi,  ip,	 arp,  rarp,  decnet,  lat,  sca,



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TCPDUMP(1)					       TCPDUMP(1)


		     moprc, mopdl, 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  syn-
		     tax),  `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 Ether-
	      net  fields.   FDDI  headers  also  contain   other
	      fields,  but  you	 cannot name them explicitly in a
	      filter expression.]

	      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  primi-
	      tives.   E.g.,  `host  foo and not port ftp and not
	      port ftp-data'.  To save typing,	identical  quali-
	      fier 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  IP  destination  field  of  the
		     packet  is	 host,	which  may  be	either an
		     address or a name.

	      src host host
		     True if the IP source field of the packet is
		     host.

	      host host
		     True  if either the IP source or destination
		     of the packet is host.   Any  of  the  above
		     host  expressions	can be prepended with the
		     keywords, ip, arp, or rarp as in:
			  ip host host
		     which is equivalent to:
			  ether proto \ip and host host



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TCPDUMP(1)					       TCPDUMP(1)


		     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 desti-
		     nation address is ehost.

	      gateway host
		     True  if  the packet used host as a gateway.
		     I.e., the	ethernet  source  or  destination
		     address  was  host but neither the IP source
		     nor the IP destination was host.  Host  must
		     be	  a  name  and	must  be  found	 in  both
		     /etc/hosts and /etc/ethers.  (An  equivalent
		     expression is
			  ether host ehost and not host host
		     which  can be used with either names or num-
		     bers for host / ehost.)

	      dst net net
		     True if the IP 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 IP source address of the  packet
		     has a network number of net.

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

	      net net mask mask
		     True  if the IP address matches net with the
		     specific netmask.	May be qualified with src
		     or dst.

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



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TCPDUMP(1)					       TCPDUMP(1)


		     dst.

	      dst port port
		     True  if  the packet is ip/tcp or ip/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 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,
		     igrp,  udp, nd, or tcp.  Note that the iden-
		     tifiers tcp, udp, and icmp are also keywords
		     and must be escaped via backslash (\), which
		     is \\ in the C-shell.

	      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



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TCPDUMP(1)					       TCPDUMP(1)


		     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.

	      ether proto protocol
		     True  if  the packet is of ether type proto-
		     col.  Protocol can be a  number  or  a  name
		     like  ip,	arp, or rarp.  Note these identi-
		     fiers are also keywords and must be  escaped
		     via  backslash  (\).   [In	 the case of FDDI
		     (e.g., `fddi protocol  arp'),  the	 protocol
		     identification  comes from the 802.2 Logical
		     Link Control (LLC) header, which is  usually
		     layered  on top of the FDDI header.  Tcpdump
		     assumes,  when  filtering	on  the	 protocol
		     identifier, that all FDDI packets include an
		     LLC header, and that the LLC  header  is  in
		     so-called SNAP format.]

	      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 destina-
		     tion address is host.

	      ip, arp, rarp, decnet
		     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.



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TCPDUMP(1)					       TCPDUMP(1)


	      tcp, udp, icmp
		     Abbreviations for:
			  ip proto p
		     where p is one of the above 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, ip, arp, rarp,
		     tcp, udp, or icmp, and indicates the  proto-
		     col layer for the index operation.	 The byte
		     offset, relative to the  indicated	 protocol
		     layer,  is	 given by expr.	 Size is optional
		     and indicates the number  of  bytes  in  the
		     field  of	interest;  it  can be either one,
		     two, or four,  and	 defaults  to  one.   The
		     length  operator,	indicated  by the keyword
		     len, gives the length of the packet.

		     For example, `ether[0] & 1 != 0' catches all
		     multicast	traffic.  The expression `ip[0] &
		     0xf  !=  5'  catches  all	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.

	      Primitives may be combined using:

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

		     Negation (`!' or `not').

		     Concatenation (`&&' or `and').

		     Alternation (`||' or `or').

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



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TCPDUMP(1)					       TCPDUMP(1)


	      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 sun-
       down:
	      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  pack-
       ets)  of	 each  TCP conversation that involves a non-local
       host.
	      tcpdump 'tcp[13] & 3 != 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'



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TCPDUMP(1)					       TCPDUMP(1)


       To  print   all	 ICMP	packets	  that	 are   not   echo
       requests/replies (i.e., not ping packets):
	      tcpdump 'icmp[0] != 8 and icmp[0] != 0"

OUTPUT FORMAT
       The  output of tcpdump is protocol dependent.  The follow-
       ing 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 Logical Link Control (LLC) packet; the
       LLC  header  is	printed if it is not an ISO datagram or a
       so-called SNAP packet.

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

       On  SLIP	 links, a direction indicator (``I'' for inbound,
       ``O'' for outbound), packet type, and compression informa-
       tion  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 spe-
       cial  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  com-
       pressed	TCP  packet,  with an implicit connection identi-
       fier; 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



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TCPDUMP(1)					       TCPDUMP(1)


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

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

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

       TCP Packets

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

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



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TCPDUMP(1)					       TCPDUMP(1)


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

       Csam replies with a similar packet except  it  includes	a
       piggy-backed  ack  for  rtsg's SYN.  Rtsg then acks csam's
       SYN.  The `.' means no flags were set.	The  packet  con-
       tained  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  pack-
       ets  of	the conversation, the difference between the cur-
       rent packet's sequence number and  this	initial	 sequence
       number is printed.  This means that sequence numbers after
       the first can be interpreted as relative byte positions in
       the  conversation's  data stream (with the first data byte
       each direction being `1').  `-S' will override  this  fea-
       ture,  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 includ-
       ing 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  cap-
       ture  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  pre-
       sent but the IP datagram length is not long enough for the
       options to actually be there, tcpdump reports it as ``[bad



			   30 June 1997			       12





TCPDUMP(1)					       TCPDUMP(1)


       hdr length]''.

       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 data-
       gram to port who on host broadcast, the Internet broadcast
       address.	 The packet contained 84 bytes of user data.

       Some  UDP services are recognized (from the source or des-
       tination port number) and the higher level protocol infor-
       mation	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 ucb-
       vax.berkeley.edu.  The query id was `3'.	  The  `+'  indi-
       cates  the  recursion  desired  flag  was  set.	The query
       length was 37 bytes, not including the UDP and IP protocol
       headers.	  The  query operation was the normal one, Query,
       so the op field was omitted.  If the op had been	 anything
       else,  it  would have been printed between the `3' and the
       `+'.  Similarly, the qclass was the normal one, C_IN,  and
       omitted.	 Any other qclass would have been printed immedi-
       ately after the `A'.

       A few anomalies are checked and may result in extra fields
       enclosed	 in  square  brackets:	 If  a	query contains an
       answer,	name  server  or  authority   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)



			   30 June 1997			       13





TCPDUMP(1)					       TCPDUMP(1)


       In the first example, helios responds to query id  3  from
       h2opolo with 3 answer records, 3 name server records and 7
       authority 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.


       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 han-
       dle (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 gen-
       eration number.)	 Wrl replies `ok' with	the  contents  of
       the link.

       In  the	third  line,  sushi  asks  wrl to lookup the name



			   30 June 1997			       14





TCPDUMP(1)					       TCPDUMP(1)


       `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,  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 oper-
       ation.	 Instead,   tcpdump  keeps  track  of  ``recent''
       requests, and matches them to the replies using the trans-
       action  ID.  If a reply does not closely follow the corre-
       sponding 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.



			   30 June 1997			       15





TCPDUMP(1)					       TCPDUMP(1)


       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  sepa-
       rated	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 indi-
       cated 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



			   30 June 1997			       16





TCPDUMP(1)					       TCPDUMP(1)


	      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 num-
       ber  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 retransmit-
       ted.   Helios  resends  them  then jssmag.209 releases the
       transaction.   Finally,	jssmag.209  initiates  the   next
       request.	  The  `*'  on	the  request  indicates	 that  XO
       (`exactly once') was not set.


       IP Fragmentation

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

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

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



			   30 June 1997			       17





TCPDUMP(1)					       TCPDUMP(1)


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

       The current version is available via anonymous ftp:

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

BUGS
       Please send bug reports to tcpdump@ee.lbl.gov.

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

       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  gener-
       ating them rather than tcpdump.

       Apple  Ethertalk	 DDP packets could be dumped as easily as
       KIP DDP packets but aren't.  Even if we were  inclined  to
       do  anything  to promote the use of Ethertalk (we aren't),
       LBL doesn't allow Ethertalk on any of its networks so we'd
       would have no way of testing this code.




			   30 June 1997			       18





TCPDUMP(1)					       TCPDUMP(1)


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

       Filters	expressions  that  manipulate FDDI headers assume
       that all FDDI packets are 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  fil-
       ter  may	 inadvertently accept certain packets that do not
       properly match the filter expression.
















































			   30 June 1997			       19


