Tcpdump man page

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TCPDUMP(1)                                             TCPDUMP(1)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
               [ -C file_size ] [ -F file ]
               [ -i interface ] [ -m module ] [ -r file ]
               [ -s snaplen ] [ -T type ] [ -w file ]
               [ -E algo:secret ] [ expression ]

DESCRIPTION
       Tcpdump  prints  out  the  headers of packets on a network interface that
       match the boolean expression.  It can also be run with the -w flag, which
       causes  it  to  save the packet data to a file for later analysis, and/or
       with the -r flag, which causes it to read from a saved packet file rather
       than  to read packets from a network interface.  In all cases, only pack-
       ets that match expression will be processed by tcpdump.

       Tcpdump will, if not run with the -c  flag,  continue  capturing  packets
       until  it  is  interrupted by a SIGINT signal (generated, for example, by
       typing your interrupt character, typically control-C) or a SIGTERM signal
       (typically  generated with the kill(1) command); if run with the -c flag,
       it will capture packets until it is interrupted by a  SIGINT  or  SIGTERM
       signal or the specified number of packets have been processed.

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

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

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

       On  platforms that support the SIGINFO signal, such as most BSDs, it will
       report those counts when it receives a  SIGINFO  signal  (generated,  for
       example,  by  typing  your ``status'' character, typically control-T) and
       will continue capturing packets.

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

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

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

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

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

       Under Linux:
              You must be root or tcpdump must be installed setuid to root.

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

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

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

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

       -c     Exit after receiving count packets.

       -C     Before  writing a raw packet to a savefile, check whether the file
              is currently larger than file_size and, if so, close  the  current
              savefile  and  open a new one.  Savefiles after the first savefile
              will have the name specified with the -w flag, with a number after
              it,  starting  at 2 and continuing upward.  The units of file_size
              are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

       -d     Dump the compiled packet-matching code in a human readable form to
              standard output and stop.

       -dd    Dump packet-matching code as a C program fragment.

       -ddd   Dump  packet-matching  code  as  decimal  numbers (preceded with a
              count).

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

       -E     Use algo:secret for decrypting IPsec ESP packets.  Algorithms  may
              be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none.
              The default is des-cbc.  The ability to decrypt  packets  is  only
              present if tcpdump was compiled with 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 symbol-
              ically (this option is intended to get around serious brain damage
              in  Sun's  yp  server -- usually it hangs forever translating non-
              local internet numbers).

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

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

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

       -l     Make  stdout  line  buffered.   Useful if you want to see the data
              while capturing it.  E.g.,
              ``tcpdump  -l  |  tee     dat''     or      ``tcpdump  -l        >
              dat  &  tail  -f  dat''.

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

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

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

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

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

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

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

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

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

       -s     Snarf snaplen bytes of data  from  each  packet  rather  than  the
              default  of 68 (with SunOS's NIT, the minimum is actually 96).  68
              bytes is adequate for IP, ICMP, TCP and UDP but may truncate  pro-
              tocol  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 pro-
              tocol level at which the truncation has occurred.  Note that  tak-
              ing larger snapshots both increases the amount of time it takes to
              process packets and, effectively, decreases the amount  of  packet
              buffering.   This  may cause packets to be lost.  You should limit
              snaplen to the smallest number  that  will  capture  the  protocol
              information  you're interested in.  Setting snaplen to 0 means use
              the required length to catch whole packets.

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

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

       -tt    Print an unformatted timestamp on each dump line.

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

       -tttt  Print a timestamp in default format proceeded by date on each dump
              line.

       -u     Print undecoded NFS handles.

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

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

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

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

       -x     Print each packet (minus its  link  level  header)  in  hex.   The
              smaller  of  the  entire  packet or snaplen bytes will be printed.
              Note that this is the entire link-layer packet, so for link layers
              that  pad  (e.g. Ethernet), the padding bytes will also be printed
              when the higher layer packet is shorter than the required padding.

       -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  qualifier,
                     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,  tcp  and  udp.   E.g.,  `ether  src foo', `arp net
                     128.3', `tcp port 21'.  If there is no proto qualifier, all
                     protocols consistent with the type are assumed.  E.g., `src
                     foo' means `(ip or arp or rarp) src foo' (except the latter
                     is  not legal syntax), `net bar' means `(ip or arp or rarp)
                     net bar' and `port 53' means `(tcp or udp) port 53'.

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

              Similarly,  `tr' is an alias for `ether'; the previous paragraph's
              statements about FDDI headers also apply to Token Ring headers.]

              In addition to the above, there are some special `primitive'  key-
              words  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 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 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 eth-
                     ernet source or destination address was  host  but  neither
                     the  IP  source nor the IP destination was host.  Host must
                     be a name and must be found both  by  the  machine's  host-
                     name-to-IP-address  resolution  mechanisms (host name file,
                     DNS, NIS, etc.) and by the machine's host-name-to-Ethernet-
                     address  resolution  mechanism  (/etc/ethers,  etc.).   (An
                     equivalent expression is
                          ether host ehost and not host host
                     which can be used with either names or numbers for  host  /
                     ehost.)  This syntax does not work in IPv6-enabled configu-
                     ration 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 net-
                     work number of net.

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

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

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

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

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

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

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

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

              ip6 proto protocol
                     True if the packet is an IPv6 packet of protocol type  pro-
                     tocol.   Note that this primitive does not chase the 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 conventions,
                     and looks up the local subnet mask.

              ether multicast
                     True if the packet is an ethernet  multicast  packet.   The
                     ether keyword is optional.  This is shorthand for `ether[0]
                     & 1 != 0'.

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

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

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

                     [In the case of FDDI (e.g., `fddi protocol arp') and  Token
                     Ring  (e.g.,  `tr  protocol arp'), for most of those proto-
                     cols, the protocol identification comes from the 802.2 Log-
                     ical Link Control (LLC) header, which is usually layered on
                     top of the FDDI or Token Ring header.

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

                     The  exceptions are iso, for which it checks the DSAP (Des-
                     tination Service Access Point)  and  SSAP  (Source  Service
                     Access  Point)  fields  of the LLC header, stp and netbeui,
                     where it checks the DSAP of  the  LLC  header,  and  atalk,
                     where  it  checks  for  a SNAP-format packet with an OUI of
                     0x080007 and the Appletalk etype.

                     In the case of Ethernet, tcpdump checks the  Ethernet  type
                     field  for most of those protocols; the exceptions are iso,
                     sap, and netbeui, for which it checks for  an  802.3  frame
                     and  then  checks  the  LLC  header as it does for FDDI and
                     Token Ring, atalk, where it checks both for  the  Appletalk
                     etype  in an Ethernet frame and for a SNAP-format packet as
                     it does for FDDI and Token Ring, aarp, where it checks  for
                     the  Appletalk  ARP etype in either an Ethernet frame or an
                     802.2 SNAP frame with an OUI of 0x000000, and ipx, where it
                     checks for the IPX etype in an Ethernet frame, the IPX DSAP
                     in the LLC header, the 802.3 with no LLC header  encapsula-
                     tion of IPX, and the IPX etype in a SNAP frame.]

              decnet src host
                     True  if the DECNET source address is host, which may be an
                     address of the form ``10.123'',  or  a  DECNET  host  name.
                     [DECNET  host name support is only available on Ultrix sys-
                     tems that are configured to run DECNET.]

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

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

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

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

              vlan [vlan_id]
                     True if the packet is  an  IEEE  802.1Q  VLAN  packet.   If
                     [vlan_id]  is  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 proto-
                     col.  Protocol can be a number or one of  the  names  clnp,
                     esis, or isis.

              clnp, esis, isis
                     Abbreviations for:
                          iso proto p
                     where  p  is one of the above protocols.  Note that tcpdump
                     does an incomplete job of parsing these protocols.

              expr relop expr
                     True if the relation holds, where relop is one of >, <, >=,
                     <=, =, !=, and expr is an arithmetic expression composed of
                     integer constants (expressed in  standard  C  syntax),  the
                     normal binary operators [+, -, *, /, &, |], a length opera-
                     tor, 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, ppp, slip, link, ip, arp,
                     rarp, tcp, udp, icmp or ip6,  and  indicates  the  protocol
                     layer for the index operation.  (ether, fddi, tr, ppp, slip
                     and link all refer to the link layer.)  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 off-
                     set,  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.

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

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

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

              Primitives may be combined using:

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

                     Negation (`!' or `not').

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

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

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

              If an identifier is given without a keyword, the most recent  key-
              word 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  argu-
              ments are concatenated with spaces before being parsed.

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

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

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

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

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

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

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

       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  ether-
       net broadcast or multicast:
              tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

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

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

       Link Level Headers

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

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

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

       (N.B.:  The  following description assumes familiarity with the SLIP 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 packets,
       the connection identifier is printed following the type.  If  the  packet
       is  compressed, its encoded header is printed out.  The special cases are
       printed out as *S+n and *SA+n,  where  n  is  the  amount  by  which  the
       sequence number (or sequence number and ack) has changed.  If it is not a
       special case, zero or more changes are printed.  A change is indicated by
       U  (urgent  pointer),  W  (window),  A  (ack), S (sequence number), and I
       (packet ID), followed by a delta  (+n  or  -n),  or  a  new  value  (=n).
       Finally,  the  amount  of data in the packet and compressed header length
       are printed.

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

       ARP/RARP Packets

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

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

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

       TCP Packets

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

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and destination IP addresses and ports.  Flags
       are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a single
       `.'  (no flags).  Data-seqno describes the portion of sequence space cov-
       ered by the data in this packet (see example  below).   Ack  is  sequence
       number  of the next data expected the other direction on this connection.
       Window is the number of bytes of receive buffer space available the other
       direction  on  this  connection.  Urg indicates there is `urgent' data in
       the packet.  Options are tcp options enclosed in  angle  brackets  (e.g.,
       <mss 1024>).

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

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

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

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

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

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

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

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

       Let's assume that we want to watch packets used  in  establishing  a  TCP
       connection.  Recall that TCP uses a 3-way handshake protocol when it ini-
       tializes 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 tcp-
       dump.

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

                   00010010

       which translates to decimal

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

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

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

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

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

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

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

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

       UDP Packets

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

       Some  UDP  services  are  recognized (from the source or destination port
       number) and the higher level protocol information printed.   In  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  Ser-
       vice  protocol  described  in RFC-1035.  If you are not familiar with the
       protocol, the following description will appear to be written in  greek.)

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

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

       UDP Name Server Responses

       Name server responses are formatted as
              src > dst:  id op rcode flags a/n/au type class data (len)
              helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
              helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
       In  the  first example, helios responds to query id 3 from h2opolo with 3
       answer records, 3 name server records  and  7  additional  records.   The
       first  answer record is type A (address) and its data is internet address
       128.32.137.3.  The total size of the response was  273  bytes,  excluding
       UDP  and  IP  headers.   The  op (Query) and response code (NoError) were
       omitted, as was the class (C_IN) of the A record.

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

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

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

       SMB/CIFS decoding

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

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

       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 head-
       ers.  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  direc-
       tory  file  9,74/4096.6878.   Note  that  the data printed depends on the
       operation type.  The format is intended to be self explanatory if read in
       conjunction with an NFS protocol spec.

       If  the  -v  (verbose)  flag is given, additional information is printed.
       For example:

              sushi.1372a > wrl.nfs:
                   148 read fh 21,11/12.195 8192 bytes @ 24576
              wrl.nfs > sushi.1372a:
                   reply ok 1472 read REG 100664 ids 417/0 sz 29388

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

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

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

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

       AFS Requests and Replies

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

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

              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                   new fid 536876964/1/1 ".newsrc"
              pike.afsfs > elvis.7001: rx data fs reply rename

       In  the  first line, host elvis sends a RX packet to pike.  This was a RX
       data packet to the fs (fileserver) service, and is the start  of  an  RPC
       call.   The  RPC  call  was  a  rename, with the old directory file id of
       536876964/1/1 and an old filename of `.newsrc.new', and a  new  directory
       file  id of 536876964/1/1 and a new filename of `.newsrc'.  The host pike
       responds with a RPC reply to  the  rename  call  (which  was  successful,
       because it was a data packet and not an abort packet).

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

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

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

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

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

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

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

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

       KIP Appletalk (DDP in UDP)

       Appletalk DDP packets encapsulated in UDP datagrams  are  de-encapsulated
       and  dumped  as DDP packets (i.e., all the UDP header information is 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 sepa-
       rated by whitespace (blanks or tabs).  The /etc/atalk.names file may con-
       tain blank lines or comment lines (lines starting with a `#').

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

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

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

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

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

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

       Jssmag.209  then  requests  that  packets 3 & 5 be retransmitted.  Helios
       resends them then jssmag.209 releases  the  transaction.   Finally,  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 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 origi-
       nal 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 ari-
       zona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear  to
       handle 576 byte datagrams:
              arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:32
8@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 infor-
       mation  is all in the first fragment and we have no idea what the port or
       sequence numbers are when we print the later fragments.  Second, the  tcp
       sequence  information  in  the first line is printed as if there were 308
       bytes of user data when, in fact, there are 512 bytes (308 in  the  first
       frag  and  204  in  the  second).   If  you  are looking for holes in the
       sequence space or trying to match up acks with  packets,  this  can  fool
       you.

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

       Timestamps

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

SEE ALSO
       traffic(1C), nit(4P), bpf(4), pcap(3)

AUTHORS
       The original authors are:

       Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence  Berke-
       ley 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 recom-
       mend that you use the latter.

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

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

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

       Name server inverse queries are not dumped correctly: the  (empty)  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 proto-
       cols  such  as  ISO CLNS.  Therefore, the filter may inadvertently accept
       certain packets that do not properly match the filter expression.

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

       ip6 proto should chase header chain, but at this moment it does not.  ip6
       protochain is supplied for this behavior.

       Arithmetic expression against transport layer headers, like tcp[0],  does
       not work against IPv6 packets.  It only looks at IPv4 packets.

                          3 January 2001               TCPDUMP(1)
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