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


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

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

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

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

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

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

              that, on many  (perhaps  all)  interfaces,  if  you
              don't capture in promiscuous mode, you will not see
              any outgoing packets, so  a  capture  not  done  in
              promiscuous mode may not be very useful.

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

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

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

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

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

       Reading a saved packet file doesn't require special privi­


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

       -c     Exit after receiving count packets.

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

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

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

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

       -O     Do  not  run  the  packet-matching  code optimizer.
              This is useful only if you suspect  a  bug  in  the
              fication, tcpdump  cannot  deduce  the  version  of
              ESP/AH protocol.

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

       -S     Print  absolute, rather than relative, TCP sequence

       -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.   Setting  snaplen  to  0  means use the
              required length to catch whole packets.

       -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

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

       -tt    Print an unformatted timestamp on each dump line.

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

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

       -u     Print undecoded NFS handles.

       -v     (Slightly  more)  verbose output.  For example, the
              time to  live,  identification,  total  length  and
              options  in an IP packet are printed.  Also enables
              file is ``-''.

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

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

              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

              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

                     etc.).  (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 netmask
                     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 with
                     a netmask len bits wide.  May  be  qualified
                     with src or dst.

              dst port port
                     True   if  the  packet  is  ip/tcp,  ip/udp,
                     ip6/tcp or ip6/udp  and  has  a  destination
                     port  value of port.  The port can be a 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.

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

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

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

              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
                     col.  Protocol can be a number or one of the
                     names  ip, ip6, arp, rarp, atalk, aarp, dec­
                     net, sca, lat, mopdl, moprc, iso, stp,  ipx,
                     or netbeui.  Note these identifiers are also
                     keywords and must be escaped  via  backslash

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

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

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

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

              decnet src host
                     True  if  the DECNET source address is host,

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

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

              vlan [vlan_id]
                     True if the packet is an  IEEE  802.1Q  VLAN
                     packet.   If  [vlan_id]  is  specified, only
                     true  is  the  packet  has   the   specified
                     vlan_id.   Note  that the first vlan keyword
                     encountered in expression changes the 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, ppp, slip,
                     link, ip, arp, rarp, tcp, udp, icmp or  ip6,
                     For example, `ether[0] & 1 != 0' catches all
                     multicast traffic.  The expression `ip[0]  &
                     0xf  !=  5'  catches  all  IP  packets  with
                     options.  The expression `ip[6:2] & 0x1fff =
                     0'  catches  only unfragmented datagrams and
                     frag zero  of  fragmented  datagrams.   This
                     check  is  implicitly applied to the tcp and
                     udp index operations.  For instance,  tcp[0]
                     always  means  the  first  byte  of  the TCP
                     header, and never means the first byte of an
                     intervening fragment.

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

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

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

              Primitives may be combined using:

                     A  parenthesized  group  of  primitives  and
                     operators  (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

              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[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

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

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

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


       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.

       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

       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
       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 8 bits in the control bits section  of  the  TCP

              CWR | ECE | 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   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
       |         TCP checksum          |       urgent pointer          |

       A TCP header usually  holds  20  octets  of  data,  unless
       options are present.  The first line of the graph contains
                       |7   5   3     0|

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

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

                       |0 0 0 0 0 0 1 0|
                       |7 6 5 4 3 2 1 0|

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

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


       and its decimal representation is

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

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

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

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

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

       Now, let's assume that we need to capture SYN packets, but
       we  don't  care if ACK or any other TCP control bit is set
       at the same time.  Let's see what happens to octet 13 when
       a TCP datagram with SYN-ACK set arrives:
          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
       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, authority records or additional  records  section,
       ancount, nscount, or arcount are printed as `[na]', `[nn]'
       or  `[nau]' where n is the appropriate count.  If  any  of
       the  response bits are set (AA, RA or rcode) or any of the
       `must be zero' bits  are  set  in  bytes  two  and  three,
       `[b2&3=x]'  is printed, where x is the hex value of header
       bytes two and three.

       UDP Name Server Responses

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

       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

       For  information  on  SMB  packet  formats and what all te
       fields  mean  see  www.cifs.org  or  the  pub/samba/specs/
       directory  on  your  favourite samba.org mirror site.  The
       SMB   patches   were   written    by    Andrew    Tridgell

       NFS Requests and Replies

       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
       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:

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

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

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

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

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

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

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

       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
              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 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)

       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 tcpdump.org.

       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:

              capturing on some PPP devices won't work correctly.

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

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

       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(1)
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