TCPDUMP(8)                                             TCPDUMP(8)

       tcpdump - dump traffic on a network

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

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

       Under  SunOS with nit or bpf: To run tcpdump  have
       read access to /dev/nit or /dev/bpf*.  Under Solaris  with
       dlpi:  You  must  have  read/write  access  to the network
       pseudo device, e.g.  /dev/le.  Under HP-UX with dlpi:  You
       must  be  root  or  it  must  be installed setuid to root.
       Under IRIX with snoop: You must be  root  or  it  must  be
       installed  setuid  to root.  Under Linux: You must be root
       or it must be installed setuid to root.  Under Ultrix  and
       Digital UNIX: Once the super-user has enabled promiscuous-
       mode operation using pfconfig(8), any user  may  run  tcp­
       dump.   Under BSD: You must have read access to /dev/bpf*.

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

       -c     Exit after receiving count packets.

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

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

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

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

       -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 pre­
              sent 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 pur­
              poses,  and  the  use  of  this  option  with truly
              `secret' key is discouraged.  By  presenting  IPsec
              secret key onto command line you make it visible to
              others, via ps(1) and other occasions.

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

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

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

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

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

       -n     Don't convert host addresses to names.  This can be
              used to avoid DNS lookups.

       -nn    Don't  convert  protocol  and  port numbers etc. to
              names either.

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

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

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

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

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

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

       -R     Assume ESP/AH packets to be based on old specifica­
              tion (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.

       -s     Snarf snaplen bytes of data from each packet rather
              than the default of 68 (with SunOS's NIT, the mini­
              mum is actually 96).  68 bytes is adequate for  IP,
              ICMP,  TCP and UDP but may truncate protocol infor­
              mation  from  name  server  and  NFS  packets  (see
              below).   Packets  truncated  because  of a limited
              snapshot  are  indicated   in   the   output   with
              ``[|proto]'',  where  proto  is  the  name  of  the
              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.

       -S     Print absolute, rather than relative, TCP  sequence

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

       -tt    Print an unformatted timestamp on each dump line.

       -U     Drops  root  privileges and changes user ID to user
              and group ID to the primary group of user.

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

       -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  verify­
              ing  the  IP and ICMP header checksum.  SMB packets
              are also printed in full.

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

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

       -w     Write  the  raw packets to file rather than parsing
              and printing them out.  They can later  be  printed
              with  the  -r  option.   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.

              selects  which  packets  will  be  dumped.   If  no
              expression is given, all packets on the net will be
              dumped.  Otherwise, only packets for which  expres­
              sion is `true' will be dumped.

              The  expression consists of one or more primitives.
              Primitives usually consist of an id (name  or  num­
              ber) preceded by one or more qualifiers.  There are
              three different kinds of qualifier:

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

              dir    qualifiers  specify  a  particular  transfer
                     direction   to  and/or  from  id.   Possible
                     directions are src, dst, src or dst and  src
                     and  dst.  E.g., `src foo', `dst net 128.3',
                     `src or dst port ftp-data'.  If there is  no
                     dir  qualifier,  src or dst is assumed.  For
                     `null' link layers (i.e. point to point pro­
                     tocols  such  as  slip) the inbound and out­
                     bound qualifiers can be used  to  specify  a
                     desired direction.

              proto  qualifiers  restrict the match to a particu­
                     lar protocol.  Possible protos  are:  ether,
                     fddi,  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 Ether­
              net  fields.   FDDI  headers  also  contain   other
              fields,  but  you  cannot name them explicitly in a
              filter expression.

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

              In addition to the above, there  are  some  special
              `primitive' keywords that don't follow the pattern:
              gateway, broadcast, less,  greater  and  arithmetic
              expressions.  All of these are described below.

              More  complex  filter  expressions  are built up by
              using the words and, or and not to  combine  primi­
              tives.   E.g.,  `host  foo and not port ftp and not
              port ftp-data'.  To save typing,  identical  quali­
              fier lists can be omitted.  E.g., `tcp dst port ftp
              or ftp-data or domain' is exactly the same as  `tcp
              dst  port  ftp  or tcp dst port ftp-data or tcp dst
              port domain'.

              Allowable primitives are:

              dst host host
                     True if the 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 desti­
                     nation 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

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

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

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

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

              dst port port
                     True  if  the  packet  is  ip/tcp,   ip/udp,
                     ip6/tcp  or  ip6/udp  and  has a destination
                     port value of port.  The port can be a  num­
                     ber  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 traf­
                     fic  and  udp/who  traffic,  and port domain
                     will print both  tcp/domain  and  udp/domain

              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, udp, or
                     tcp.  Note that the  identifiers  tcp,  udp,
                     and  icmp  are  also  keywords  and  must be
                     escaped via backslash (\), which  is  \\  in
                     the  C-shell.  Note that this primitive does
                     not chase protocol header chain.

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

              ip6 protochain protocol
                     True if the packet is IPv6 packet, and  con­
                     tains  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, authentica­
                     tion  header,  routing header, or hop-by-hop
                     option header, between IPv6 header  and  TCP
                     header.  The BPF code emitted by this primi­
                     tive 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

              ip6 multicast
                     True if the  packet  is  an  IPv6  multicast

              ether proto protocol
                     True  if  the packet is of ether type proto­
                     col.  Protocol can be a number or one of the
                     names  ip, ip6, arp, rarp, atalk, aarp, dec­
                     net, sca, lat, mopdl, moprc, or  iso.   Note
                     these identifiers are also keywords and must
                     be escaped via backslash (\).  [In the  case
                     of  FDDI  (e.g.,  `fddi  protocol arp'), the
                     protocol identification comes from the 802.2
                     Logical  Link Control (LLC) header, which is
                     usually layered on top of the  FDDI  header.
                     Tcpdump  assumes, when filtering on the pro­
                     tocol  identifier,  that  all  FDDI  packets
                     include  an  LLC  header,  and  that the LLC
                     header is in  so-called  SNAP  format.   The
                     same applies to Token Ring.]

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

              decnet dst host
                     True if the DECNET  destination  address  is

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

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

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

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

              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 pro­
                     tocol type protocol.  Protocol can be a num­
                     ber 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 pars­
                     ing these protocols.

              expr relop expr
                     True if the relation holds, where  relop  is
                     one  of  >, <, >=, <=, =, !=, and expr is an
                     arithmetic expression  composed  of  integer
                     constants  (expressed in standard C syntax),
                     the normal binary operators [+, -, *, /,  &,
                     |],  a  length  operator, and special packet
                     data accessors.  To access data  inside  the
                     packet, use the following syntax:
                          proto [ expr : size ]
                     Proto  is  one  of ether, fddi, tr, ip, arp,
                     rarp, tcp, udp, icmp or ip6,  and  indicates
                     the  protocol layer for the index operation.
                     Note that tcp,  udp  and  other  upper-layer
                     protocol  types only apply to IPv4, not IPv6
                     (this will be fixed  in  the  future).   The
                     byte  offset, relative to the indicated pro­
                     tocol layer, is  given  by  expr.   Size  is
                     optional  and  indicates the number of bytes
                     in the field of interest; it can  be  either
                     one, two, or four, and defaults to one.  The
                     length operator, indicated  by  the  keyword
                     len, gives the length of the packet.

                     For example, `ether[0] & 1 != 0' catches all
                     multicast traffic.  The expression `ip[0]  &
                     0xf  !=  5'  catches  all  IP  packets  with
                     options. The expression `ip[6:2] & 0x1fff  =
                     0'  catches  only unfragmented datagrams and
                     frag zero  of  fragmented  datagrams.   This
                     check  is  implicitly applied to the tcp and
                     udp index operations.  For instance,  tcp[0]
                     always  means  the  first  byte  of  the TCP
                     header, and never means the first byte of an
                     intervening fragment.

              Primitives may be combined using:

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

                     Negation (`!' or `not').

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

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

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

              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.

       To print all packets arriving at or  departing  from  sun­
              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
              tcpdump ip host ace and not helios

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

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

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

       To  print the start and end packets (the SYN and FIN pack­
       ets) of each TCP conversation that  involves  a  non-local
              tcpdump 'tcp[13] & 3 != 0 and not src and dst net localnet'

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

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

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

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

       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 speci­
       fied or not, the source routing information is printed for
       source-routed packets.

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

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

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

       ARP/RARP Packets

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

       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
              RTSG Broadcast 0806  64: arp who-has csam tell rtsg
              CSAM RTSG 0806  64: arp reply csam is-at CSAM
       For the first packet this says the ethernet source address
       is   RTSG,  the  destination  is  the  ethernet  broadcast
       address,  the  type  field  contained   hex   0806   (type
       ETHER_ARP) and the total length was 64 bytes.

       TCP Packets

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

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

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

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

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

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

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

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

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

              URG | ACK | PSH | RST | SYN | FIN

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

       We see that this octet contains 2 bytes from the  reserved
       field.   According  to  RFC 793 this field is reserved for
       future use and must be 0. The remaining 6 bits are the TCP
       control  bits  we  are interested in. We have numbered the
       bits in this octet from 0 to 7, right to left, so the  PSH
       bit is bit number 3, while the URG bit is number 5.

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

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

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

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


       and its decimal representation is

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

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

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

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

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

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

            |   |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


       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 fil­
       ter expression, because that would select only those pack­
       ets  that  have  SYN-ACK  set, but not those with only SYN
       set. Remember that we don't care if ACK or any other  con­
       trol bit is set as long as SYN is set.

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

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

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

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

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

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

       UDP Packets

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

       Some UDP services are recognized (from the source or  des­
       tination port number) and the higher level protocol infor­
       mation  printed.   In  particular,  Domain  Name   service
       requests  (RFC-1034/1035)  and Sun RPC calls (RFC-1050) to

       UDP Name Server Requests

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

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

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

       UDP Name Server Responses

       Name server responses are formatted as
              src > dst:  id op rcode flags a/n/au type class data (len)
              helios.domain > h2opolo.1538: 3 3/3/7 A (273)
              helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
       In  the  first example, helios responds to query id 3 from
       h2opolo with 3 answer records, 3 name server records and 7
       authority  records.   The  first  answer  record is type A
       (address) and its data is internet  address
       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

       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 me.

       SMB/CIFS decoding

       tcpdump now includes fairly extensive SMB/CIFS/NBT  decod­
       ing  for data on UDP/137, UDP/138 and TCP/139. Some primi­
       tive 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  or  the  pub/samba/specs/
       directory on your favourite mirror site. The SMB
       patches     were     written     by     Andrew    Tridgell

       NFS Requests and Replies

       Sun NFS (Network File System)  requests  and  replies  are
       printed as:
              src.xid > dst.nfs: len op args
              src.nfs > dst.xid: reply stat len op results

              sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
              sushi.201b > wrl.nfs:
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.201b:
                   reply ok 128 lookup fh 9,74/4134.3150

       In  the first line, host sushi sends a transaction with id
       6709 to wrl (note that the number following the  src  host
       is  a  transaction  id, not the source port).  The request
       was 112 bytes, excluding the  UDP  and  IP  headers.   The
       operation was a readlink (read symbolic link) on file han­
       dle (fh) 21,24/10.731657119.  (If one is lucky, as in this
       case,  the file handle can be interpreted as a major,minor
       device number pair, followed by the inode number and  gen­
       eration  number.)   Wrl  replies `ok' with the contents of
       the link.

       In the third line, sushi  asks  wrl  to  lookup  the  name
       `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 frag­
       mentation fields, which have been omitted from this  exam­
       ple.)   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  regu­
       lar  file), the file mode (in octal), the uid and gid, and
       the file size.

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

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

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

       AFS Requests and Replies

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

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

              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ""
                   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 `', 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 excep­
       tion  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 oper­
       ation.    Instead,   tcpdump  keeps  track  of  ``recent''
       requests, and matches them to the replies using  the  call
       number and service ID.  If a reply does not closely follow
       the corresponding request, it might not be parsable.

       KIP Appletalk (DDP in UDP)

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

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

       Appletalk addresses are printed in the form

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

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

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

       ATP packet formatting is  demonstrated  by  the  following
              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 num­
       ber  at the end of the line is the value of the `userdata'
       field in the request.

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

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

       IP Fragmentation

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

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

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


       By  default, all output lines are preceded by a timestamp.
       The timestamp is the current clock time in the form
       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.

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

       The original authors are:

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

       It is currently being maintained by

       The current version is available via http:


       The original distribution is available via anonymous ftp:


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

       Please send problems, bugs, questions, desirable  enhance­
       ments, etc. to:


       Please send source code contributions, etc. to:


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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

       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

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

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

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

       Filter expressions on fields other than those that manipu­
       late  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(8)