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TCPDUMP(1)							    TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -adeflnNOpqRStvxX ] [ -c count	] [ -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.

       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/write 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 (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 cryptography
	      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 sym-
	      bolically	(this option is	intended to get	around	serious	 brain
	      damage  in Sun's yp server -- usually it hangs forever translat-
	      ing 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 sys-
	      tem interface list for the lowest	numbered, configured up	inter-
	      face (excluding loopback).  Ties are broken by choosing the ear-
	      liest match.

	      On Linux systems with 2.2	or later kernels, an  interface	 argu-
	      ment  of	``any''	can be used to capture packets from all	inter-
	      faces.  Note that	captures on the	``any''	 device	 will  not  be
	      done in promiscuous mode.

       -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''.

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

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

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

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

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

       -T     Force  packets  selected	by  "expression" to be interpreted the
	      specified	type. Currently	known types are	 cnfp  (Cisco  NetFlow
	      protocol),  rpc (Remote Procedure	Call), rtp (Real-Time Applica-
	      tions protocol), rtcp (Real-Time Applications control protocol),
	      snmp  (Simple  Network  Management  Protocol), vat (Visual Audio
	      Tool), and wb (distributed White Board).

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

       -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.

       -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.

       -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.

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

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

       -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	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	quali-
		     fier,  src	 or  dst  is  assumed.	For `null' link	layers
		     (i.e. point to point protocols such as slip) the  inbound
		     and  outbound qualifiers can be used to specify a desired
		     direction.

	      proto  qualifiers	restrict the match to a	 particular  protocol.
		     Possible protos are: ether, fddi, tr, ip, ip6, arp, rarp,
		     decnet, lat, sca, moprc, mopdl, iso,  esis,  isis,	 icmp,
		     icmp6,  tcp  and  udp.   E.g.,  `ether src	foo', `arp net
		     128.3', `tcp port 21'.  If	there is no  proto  qualifier,
		     all  protocols  consistent	 with  the  type  are assumed.
		     E.g., `src	foo' means `(ip	 or  arp  or  rarp)  src  foo'
		     (except  the latter is not	legal syntax), `net bar' means
		     `(ip or arp or rarp) net bar' and `port 53'  means	 `(tcp
		     or	udp) port 53'.

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

	      Similarly, `tr' is an alias  for	`ether';  the  previous	 para-
	      graph'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 nei-
		     ther 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 numbers for	host /
		     ehost.)   This  syntax does not work in IPv6-enabled con-
		     figuration	at this	moment.

	      dst net net
		     True if the IPv4/v6 destination address of	the packet has
		     a	network	 number	 of net. Net may be either a name from
		     /etc/networks or a	network	number	(see  networks(4)  for
		     details).

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

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

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

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

	      dst port port
		     True if the packet	is ip/tcp, ip/udp, ip6/tcp or  ip6/udp
		     and  has  a destination port value	of port.  The port can
		     be	a number or a name used	in /etc/services (see  tcp(4P)
		     and  udp(4P)).   If  a name is used, both the port	number
		     and protocol are checked.	If a number or ambiguous  name
		     is	 used, only the	port number is checked (e.g., dst port
		     513 will print both tcp/login traffic and	udp/who	 traf-
		     fic,  and	port  domain  will  print  both	tcp/domain and
		     udp/domain	traffic).

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

	      port port
		     True if either the	source	or  destination	 port  of  the
		     packet is port.  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	proto-
		     col type protocol.	 Protocol can be a number  or  one  of
		     the  names	icmp, icmp6, igmp, igrp, pim, ah, esp, 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 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	proto-
		     col header	chain.

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

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

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

	      ip broadcast
		     True  if the packet is an IP broadcast packet.  It	checks
		     for both the all-zeroes and  all-ones  broadcast  conven-
		     tions, 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, or iso.
		     Note these	identifiers 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.  The same applies to
		     Token Ring.]

	      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
		     Abbreviations for:
			  ether	proto p
		     where p is	one of the above protocols.

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

	      vlan [vlan_id]
		     True  if  the  packet  is an IEEE 802.1Q VLAN packet.  If
		     [vlan_id] is specified, only true is the packet  has  the
		     specified	vlan_id.   Note	 that  the  first vlan keyword
		     encountered in expression changes	the  decoding  offsets
		     for  the  remainder  of expression	on the assumption that
		     the packet	is a VLAN packet.

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

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

	      clnp, esis, isis
		     Abbreviations for:
			  iso proto p
		     where p is	one of the above protocols.  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  com-
		     posed  of integer constants (expressed in standard	C syn-
		     tax), 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 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 (paren-
		     theses are	special	to the Shell and must be escaped).

		     Negation (`!' or `not').

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

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

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

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

	      Expression arguments can be passed to tcpdump as either a	single
	      argument or as multiple arguments, whichever is more convenient.
	      Generally,  if  the expression contains Shell metacharacters, it
	      is easier	to pass	it as a	 single,  quoted  argument.   Multiple
	      arguments	are concatenated with spaces before being parsed.

EXAMPLES
       To print	all packets arriving at	or departing from sundown:
	      tcpdump host sundown

       To print	traffic	between	helios and either hot or ace:
	      tcpdump host helios and \( hot or	ace \)

       To print	all IP packets between ace and any host	except helios:
	      tcpdump ip host ace and not helios

       To print	all traffic between local hosts	and hosts at Berkeley:
	      tcpdump net ucb-ether

       To  print all ftp traffic through internet gateway snup:	(note that the
       expression is quoted to prevent the shell from  (mis-)interpreting  the
       parentheses):
	      tcpdump 'gateway snup and	(port ftp or ftp-data)'

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

       To  print  the  start and end packets (the SYN and FIN packets) of each
       TCP conversation	that involves a	non-local host.
	      tcpdump 'tcp[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  eth-
       ernet broadcast or multicast:
	      tcpdump 'ether[0]	& 1 = 0	and ip[16] >= 224'

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

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

       Link Level Headers

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

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

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

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

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

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

       ARP/RARP	Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
	      src _ dst: flags data-seqno ack window urgent options
       Src  and	 dst  are  the	source and destination IP addresses and	ports.
       Flags are some combination of S (SYN), F	(FIN), P (PUSH)	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
       connection.  Window is the number of  bytes  of	receive	 buffer	 space
       available  the other direction on this connection.  Urg indicates there
       is `urgent' data	in the packet.	Options	are tcp	 options  enclosed  in
       angle brackets (e.g., <mss 1024>).

       Src,  dst and flags are always present.	The other fields depend	on the
       contents	of the packet's	tcp protocol header and	 are  output  only  if
       appropriate.

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

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

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

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

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

       There are 6 bits	in the control bits section of the TCP header:

	      URG | ACK	| PSH |	RST | SYN | FIN

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

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

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

       Recall the structure of a TCP header without options:

	0			     15				     31
       -----------------------------------------------------------------
       |	  source port	       |       destination port	       |
       -----------------------------------------------------------------
       |			sequence number			       |
       -----------------------------------------------------------------
       |		     acknowledgment number		       |
       -----------------------------------------------------------------
       |  HL   | reserved  |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 fist 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   | reserved  |U|A|P|R|S|F|	window size	       |
       ----------------|---------------|---------------|----------------
       |	       |  13th octet   |	       |	       |

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

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

       We see that this	octet  contains	 2  bytes  from	 the  reserved	field.
       According  to RFC 793 this field	is reserved for	future use and must be
       0. The remaining	6 bits 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:

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

       We already mentioned that bits number 7 and 6 belong  to	 the  reserved
       field,  so  they	must must be 0.	Looking	at the control bits section we
       see that	only bit number	1 (SYN)	is set.

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

	      00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

		   00010010

       which translates	to decimal

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

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

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

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

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

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

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

       Note that you should use	single quotes or a backslash in	the expression
       to hide the AND ('&') special character from the	shell.

       UDP Packets

       UDP format is illustrated by this rwho packet:
	      actinide.who > broadcast.who: udp	84
       This says that port who on host actinide	sent a udp  datagram  to  port
       who on host broadcast, the Internet broadcast address.  The packet con-
       tained 84 bytes of user data.

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

       UDP Name	Server Requests

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

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

       A few anomalies are checked and may result in extra fields enclosed  in
       square brackets:	 If a query contains an	answer,	name server or author-
       ity 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 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	(NoEr-
       ror) were omitted, as was the class (C_IN) of the A record.

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

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

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

       SMB/CIFS	decoding

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

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

       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 number following the src host is a transaction  id,  not
       the  source port).  The request was 112 bytes, excluding	the UDP	and IP
       headers.	 The operation was a readlink (read  symbolic  link)  on  file
       handle (fh) 21,24/10.731657119.	(If one	is lucky, as in	this case, the
       file handle can be interpreted as a  major,minor	 device	 number	 pair,
       followed	 by the	inode number and generation number.)  Wrl replies `ok'
       with the	contents of the	link.

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

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

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

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

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

       AFS Requests and	Replies

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

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

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

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

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

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

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

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

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

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

       KIP Appletalk (DDP in UDP)

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

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

       Appletalk addresses are printed in the form
	      net.host.port

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

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

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

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

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

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

       IP Fragmentation

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

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

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

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

       Timestamps

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

SEE ALSO
       bpf(4), pcap(3)

AUTHORS
       The original authors are:

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

       It is currently being maintained	by tcpdump.org.

       The current version is available	via http:

	      http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

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

       IPv6/IPsec support is added by WIDE/KAME	project.   This	 program  uses
       Eric Young's SSLeay library, under specific configuration.

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

	      tcpdump-workers@tcpdump.org

       Please send source code contributions, etc. to:

	      patches@tcpdump.org

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

       On Linux	systems	with 2.0[.x] kernels:

	      packets on the loopback device will be seen twice;

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

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

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

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

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

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

       Filter expressions 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 inad-
       vertently 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	 pack-
       ets.

       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)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | OUTPUT FORMAT | SEE ALSO | AUTHORS | BUGS

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