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TCPDUMP(8)                                             TCPDUMP(8)

       tcpdump - dump traffic on a network

       tcpdump [ -deflnNOpqStvx ] [ -c count ] [ -F file ]
               [ -i interface ] [ -r file ] [ -s snaplen ]
               [ -T type ] [ -w file ] [ 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 you must have read access
       to /dev/net or /dev/bpf*.  Under Solaris  with  dlpi:  You
       must  have  read access to the network pseudo device, e.g.
       /dev/le.  Under HP-UX with dlpi: You must be  root  or  it
       must  be installed setuid to root.  Under IRIX with snoop:
       You must be root or it must be installed setuid  to  root.
       Under Ultrix: 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*.

       -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  a  decimal  numbers
              (preceded with a count).

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

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

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

       -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

       -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''.

                           22 June 1996                         1

TCPDUMP(8)                                             TCPDUMP(8)

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

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

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

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

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

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

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

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

       -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.

                           22 June 1996                         2

TCPDUMP(8)                                             TCPDUMP(8)

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

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

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

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

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

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

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

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

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

                           22 June 1996                         3

TCPDUMP(8)                                             TCPDUMP(8)

                     `port 53' means `(tcp or udp) port 53'.

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

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

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

              Allowable primitives are:

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

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

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

                           22 June 1996                         4

TCPDUMP(8)                                             TCPDUMP(8)

                     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.)

              dst net net
                     True if the IP destination  address  of  the
                     packet  has a network number of net. Net may
                     be either a name  from  /etc/networks  or  a
                     network    number   (see   networks(5)   for

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

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

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

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

                           22 June 1996                         5

TCPDUMP(8)                                             TCPDUMP(8)

              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,
                     udp,  nd, 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.

              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

              ether proto protocol
                     True  if  the packet is of ether type proto-
                     col.  Protocol can be a  number  or  a  name
                     like  ip,  arp, or rarp.  Note these identi-
                     fiers are also keywords and must be  escaped
                     via  backslash  (\).   [In  the case of FDDI
                     (e.g., `fddi protocol  arp'),  the  protocol

                           22 June 1996                         6

TCPDUMP(8)                                             TCPDUMP(8)

                     identification  comes from the 802.2 Logical
                     Link Control (LLC) header, which is  usually
                     layered  on top of the FDDI header.  Tcpdump
                     assumes,  when  filtering  on  the  protocol
                     identifier, that all FDDI packets include an
                     LLC header, and that the LLC  header  is  in
                     so-called SNAP format.]

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

              decnet 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, arp, rarp, decnet
                     Abbreviations for:
                          ether proto p
                     where p is one of the above protocols.

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

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

              expr relop expr
                     True if the relation holds, where  relop  is
                     one  of  >, <, >=, <=, =, !=, and expr is an
                     arithmetic expression  composed  of  integer
                     constants  (expressed in standard C syntax),
                     the normal binary operators [+, -, *, /,  &,
                     |],  a  length  operator, and special packet
                     data accessors.  To access data  inside  the
                     packet, use the following syntax:
                          proto [ expr : size ]
                     Proto  is one of ether, fddi, ip, arp, rarp,
                     tcp, udp, or icmp, and indicates the  proto-
                     col layer for the index operation.  The byte

                           22 June 1996                         7

TCPDUMP(8)                                             TCPDUMP(8)

                     offset, relative to the  indicated  protocol
                     layer,  is  given by expr.  Size is optional
                     and indicates the number  of  bytes  in  the
                     field  of  interest;  it  can be either one,
                     two, or four,  and  defaults  to  one.   The
                     length  operator,  indicated  by the keyword
                     len, gives the length of the packet.

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

              Primitives may be combined using:

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

                     Negation (`!' or `not').

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

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

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

              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.

                           22 June 1996                         8

TCPDUMP(8)                                             TCPDUMP(8)

       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.

                           22 June 1996                         9

TCPDUMP(8)                                             TCPDUMP(8)

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

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

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

       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

                           22 June 1996                        10

TCPDUMP(8)                                             TCPDUMP(8)

              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

                           22 June 1996                        11

TCPDUMP(8)                                             TCPDUMP(8)

       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.

       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? ucbvax.berkeley.edu. (37)
       Host  h2opolo  asked  the  domain  server on helios for an

                           22 June 1996                        12

TCPDUMP(8)                                             TCPDUMP(8)

       address record (qtype=A) associated  with  the  name  ucb-
       vax.berkeley.edu.   The  query  id was `3'.  The `+' indi-
       cates the recursion  desired  flag  was  set.   The  query
       length was 37 bytes, not including the UDP and IP protocol
       headers.  The query operation was the normal  one,  Query,
       so  the op field was omitted.  If the op had been anything
       else, it would have been printed between the `3'  and  the
       `+'.   Similarly, the qclass was the normal one, C_IN, and
       omitted.  Any other qclass would have been printed immedi-
       ately after the `A'.

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

       UDP Name Server Responses

       Name server responses are formatted as
              src > dst:  id op rcode flags a/n/au type class data (len)
              helios.domain > h2opolo.1538: 3 3/3/7 A (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.

                           22 June 1996                        13

TCPDUMP(8)                                             TCPDUMP(8)

       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, 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.

                           22 June 1996                        14

TCPDUMP(8)                                             TCPDUMP(8)

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

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

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

       KIP Appletalk (DDP in UDP)

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

              1.254          ether
              16.1      icsd-net
              1.254.110 ace
       The  first two lines give the names of appletalk networks.
       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

                           22 June 1996                        15

TCPDUMP(8)                                             TCPDUMP(8)

       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

                           22 June 1996                        16

TCPDUMP(8)                                             TCPDUMP(8)

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

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

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

       Van     Jacobson     (van@ee.lbl.gov),     Craig     Leres
       (leres@ee.lbl.gov)        and        Steven        McCanne
       (mccanne@ee.lbl.gov), all of the Lawrence Berkeley Labora-
       tory, University of California, Berkeley, CA.

                           22 June 1996                        17

TCPDUMP(8)                                             TCPDUMP(8)

       Please send bug reports  to  tcpdump@ee.lbl.gov  or  libp-

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

       tcpdump for Ultrix requires Ultrix version 4.0  or  later;
       the  kernel  has  to have been built with the packetfilter
       pseudo-device driver (see packetfilter(4)).  In  order  to
       watch  either  your  own  outbound or inbound traffic, you
       will need to use Ultrix version 4.2 or later, and you will
       have  to  have  used  the  pfconfig(8)  command  to enable
       ``copyall'' mode.

       Under SunOS 4.1, the packet capture code (or Streams  NIT)
       is  not  what  you'd  call efficient.  Don't plan on doing
       much with your Sun while you're monitoring a busy network.

       On  Sun  systems  prior to release 3.2, NIT is very buggy.
       If run on an old system, tcpdump may crash the machine.

       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.

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

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

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

                           22 June 1996                        18

Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.

(Corrections, notes, and links courtesy of RocketAware.com)

[Detailed Topics]
OpenBSD sources for tcpdump(8)

[Overview Topics]

Up to: Communication Debugging - Status, tracing, and debugging communications and protocols.

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