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NAME    [Toc]    [Back]

       tcpdump - dump traffic on a network

SYNOPSIS    [Toc]    [Back]

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

DESCRIPTION    [Toc]    [Back]

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

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

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

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

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

       Under Solaris with DLPI:
              You  must  have  read/write access to the network pseudo device,
              e.g.  /dev/le.  On at least some versions of  Solaris,  however,
              this  is not sufficient to allow tcpdump to capture in promiscuous
 mode; on those versions of Solaris, you  must  be  root,  or
              tcpdump must be installed setuid to root, in order to capture in
              promiscuous mode.

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

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

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

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

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

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

OPTIONS    [Toc]    [Back]

       -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

       -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' IPv4 addresses numerically rather than symbolically
 (this option is intended to get around serious brain  damage
  in Sun's NIS 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

              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''.

       -L     List the known data link types for the interface and exit.

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

       -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

       -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 specification,  tcpdump  cannot  deduce  the  version  of
              ESP/AH protocol.

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

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

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

       -T     Force  packets  selected  by  "expression" to be interpreted the
              specified type.  Currently known types are cnfp  (Cisco  NetFlow
              protocol),  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

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

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

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

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

       -y     Set  the  data  link  type  to  use  while  capturing packets to

              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

              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  specified
  network  interface.''   FDDI headers contain Ethernet-like
              source and destination addresses, and  often  contain  Ethernetlike
  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

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

              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 ftpdata
  or domain' is exactly the same as `tcp dst port ftp or tcp
              dst port ftp-data or tcp dst port domain'.

              Allowable primitives are:

              dst host host
                     True if the IPv4/v6 destination field of  the  packet  is
                     host, which may be either an address or a name.

              src host host
                     True if the IPv4/v6 source field of the packet is host.

              host host
                     True  if  either the IPv4/v6 source or destination of the
                     packet is host.  Any of the above host expressions can be
                     prepended with the keywords, ip, arp, rarp, or ip6 as in:
                          ip host host
                     which is equivalent to:
                          ether proto \ip and host host
                     If host is  a  name  with  multiple  IP  addresses,  each
                     address will be checked for a match.

              ether dst ehost
                     True if the ethernet destination address is ehost.  Ehost
                     may be either a name from /etc/ethers or  a  number  (see
                     ethers(3N) for numeric format).

              ether src ehost
                     True if the ethernet source address is ehost.

              ether host ehost
                     True if either the ethernet source or destination address
                     is ehost.

              gateway host
                     True if the packet used host as  a  gateway.   I.e.,  the
                     ethernet  source or destination address was host but neither
 the IP source nor the IP 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-toEthernet-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  configuration
 at this moment.

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

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

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

              net net mask netmask
                     True if the IP address matches net with the specific 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  protocol are checked.  If a number or ambiguous name
                     is used, only the port number is checked (e.g., dst  port
                     513  will  print both tcp/login traffic and udp/who traffic,
 and port  domain  will  print  both  tcp/domain  and
                     udp/domain traffic).

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

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

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

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

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

              ip6 proto protocol
                     True if the packet is an IPv6  packet  of  protocol  type
                     protocol.   Note  that  this primitive does not chase the
                     protocol header chain.

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

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

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

              ip broadcast
                     True if the packet is an IP broadcast packet.  It  checks
                     for  both  the  all-zeroes and all-ones broadcast conventions,
 and looks up the local subnet mask.

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

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

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

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

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

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

                     The  exceptions  are  iso,  for  which it checks the DSAP
                     (Destination Service Access Point) and SSAP (Source  Service
 Access Point) fields of the LLC header, stp and net-
                     beui, 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 encapsulation of IPX, and the
                     IPX etype in a SNAP frame.]

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

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

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

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

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

              vlan [vlan_id]
                     True  if  the  packet  is an IEEE 802.1Q VLAN packet.  If
                     [vlan_id] is 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  composed
  of integer constants (expressed in standard C syntax),
 the normal binary operators [+, -, *, /, &,  |],  a
                     length  operator,  and special packet data accessors.  To
                     access data inside the packet, use the following syntax:
                          proto [ expr : size ]
                     Proto is one of ether, fddi, tr, ip, arp, rarp, tcp, udp,
                     icmp  or  ip6,  and  indicates the protocol layer for the
                     index operation.  Note that tcp,  udp  and  other  upperlayer
  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

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

                     The following ICMP type field values are available: icmp-
                     echoreply,  icmp-unreach,  icmp-sourcequench,  icmp-redi-
                     rect,  icmp-echo,  icmp-routeradvert, icmp-routersolicit,
                     icmp-timxceed, icmp-paramprob,  icmp-tstamp,  icmp-tstam-
                     preply,  icmp-ireq,  icmp-ireqreply,  icmp-maskreq, icmp-

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

              Primitives may be combined using:

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

                     Negation (`!' or `not').

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

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

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

              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    [Toc]    [Back]

       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
              tcpdump 'gateway snup and (port ftp or ftp-data)'

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

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

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

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

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

OUTPUT FORMAT    [Toc]    [Back]

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

       Link Level Headers    [Toc]    [Back]

       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  datagrams)
  are `async' packets, with a priority value between 0 and 7; for
       example, `async4'.  Such packets are assumed to contain an 802.2  Logical
  Link  Control (LLC) packet; the LLC header is printed if it is not
       an ISO datagram or a so-called SNAP packet.

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

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

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

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

       ARP/RARP Packets

       Arp/rarp  output shows the type of request and its arguments.  The format
 is intended to be self explanatory.  Here is a short  sample  taken
       from the start of an `rlogin' from host rtsg to host csam:
              arp who-has csam tell rtsg
              arp reply csam is-at CSAM
       The  first line says that rtsg sent an arp packet asking for the ethernet
 address of 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 tell
              arp reply 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    [Toc]    [Back]

       (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

       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 output.

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

                       |               |
                       |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:

                       |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


       and its decimal representation is

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

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

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

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

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

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

            |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


       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 representation
 of the AND value as well as the result  of  this  operation  is  2
       (binary 00000010), so we know that for packets with SYN set the following
 relation must hold true:

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

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

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

       UDP Packets    [Toc]    [Back]

       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 particular,
  Domain  Name  service  requests (RFC-1034/1035) and Sun RPC calls
       (RFC-1050) to NFS.

       UDP Name Server Requests    [Toc]    [Back]

       (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

       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    [Toc]    [Back]

       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 (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  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

       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

       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

       NFS Requests and Replies    [Toc]    [Back]

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

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

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

       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    [Toc]    [Back]

       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 pac

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