TCPDUMP linux command manual

TCPDUMP(8)                                                         TCPDUMP(8)



NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AdDeflLnNOpqRStuUvxX ] [ -c count ]
               [ -C file_size ] [ -F file ]
               [ -i interface ] [ -m module ] [ -M secret ] [ -r file ]
               [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ] [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -Z user ]
               [ expression ]

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

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

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

              packets  ''captured''  (this is the number of packets that tcp-
              dump has received and processed);

              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  com-
              mand line, on some OSes it counts packets regardless of whether
              they were matched by the filter expression and,  even  if  they
              were  matched  by  the filter expression, regardless of whether
              tcpdump has read and processed  them  yet,  on  other  OSes  it
              counts  only packets that were matched by the filter expression
              regardless of whether tcpdump has read and processed them  yet,
              and  on  other OSes it counts only packets that were matched by
              the filter expression and were processed by tcpdump);

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

       On platforms that support  the  SIGINFO  signal,  such  as  most  BSDs
       (including  Mac  OS  X)  and  Digital/Tru64 UNIX, it will report those
       counts when it receives a SIGINFO signal (generated, for  example,  by
       typing  your  ''status''  character,  typically control-T, although on
       some platforms, such as Mac OS X, the ''status'' character is not  set
       by  default,  so  you must set it with stty(1) in order to use it) and
       will continue capturing packets.

       Reading packets from a network interface may  require  that  you  have
       special privileges:

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

       Under Solaris with DLPI:
              You  must  have read/write access to the network pseudo device,
              e.g.  /dev/le.  On at least some versions of Solaris,  however,
              this is not sufficient to allow tcpdump to capture in promiscu-
              ous mode; on those versions of Solaris, you must  be  root,  or
              tcpdump  must  be installed setuid to root, in order to capture
              in promiscuous mode.  Note that, on many (perhaps  all)  inter-
              faces,  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
              (unless your distribution has a kernel that supports capability
              bits  such  as  CAP_NET_RAW  and code to allow those capability
              bits to be given to particular accounts and to cause those bits
              to  be  set  on a user's initial processes when they log in, in
              which case you  must have CAP_NET_RAW in order to  capture  and
              CAP_NET_ADMIN  to  enumerate network devices with, for example,
              the -D flag).

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

       Under BSD (this includes Mac OS X):
              You must have read access to /dev/bpf*.  On BSDs with  a  devfs
              (this  includes  Mac  OS  X), this might involve more than just
              having somebody with super-user access setting the ownership or
              permissions  on  the BPF devices - it might involve configuring
              devfs to set the ownership or permissions every time the system
              is booted, if the system even supports that; if it doesn't sup-
              port that, you might have to find some other way to  make  that
              happen at boot time.

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

OPTIONS
       -A     Print  each  packet  (minus  its  link  level header) in ASCII.
              Handy for capturing web pages.

       -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  1  and  continuing  upward.   The
              units  of file_size are millions of bytes (1,000,000 bytes, not
              1,048,576 bytes).

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

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

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

       -D     Print the list of the network interfaces available on the  sys-
              tem and on which tcpdump can capture packets.  For each network
              interface, a number and an interface name, possibly followed by
              a text description of the interface, is printed.  The interface
              name or the number can be supplied to the -i flag to specify an
              interface on which to capture.

              This can be useful on systems that don't have a command to list
              them (e.g., Windows systems, or UNIX systems  lacking  ifconfig
              -a);  the  number  can be useful on Windows 2000 and later sys-
              tems, where the interface name is a somewhat complex string.

              The -D flag will not be supported if tcpdump was built with  an
              older  version  of  libpcap  that  lacks the pcap_findalldevs()
              function.

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

       -E     Use spi@ipaddr algo:secret for  decrypting  IPsec  ESP  packets
              that are addressed to addr and contain Security Parameter Index
              value spi. This combination may be repeated with comma or  new-
              line seperation.

              Note  that setting the secret for IPv4 ESP packets is supported
              at this time.

              Algorithms may be  des-cbc,  3des-cbc,  blowfish-cbc,  rc3-cbc,
              cast128-cbc,  or none.  The default is des-cbc.  The ability to
              decrypt packets is only present if tcpdump  was  compiled  with
              cryptography enabled.

              secret  is  the ASCII text for ESP secret key.  If preceeded by
              0x, then a hex value will be read.

              The option assumes RFC2406 ESP, not RFC1827 ESP.  The option is
              only  for debugging purposes, and the use of this option with a
              true 'secret' key is discouraged.  By presenting  IPsec  secret
              key  onto command line you make it visible to others, via ps(1)
              and other occasions.

              In addition to the above syntax, the syntax file  name  may  be
              used  to  have  tcpdump  read the provided file in. The file is
              opened upon receiving the first ESP packet, so any special per-
              missions  that  tcpdump may have been given should already have
              been given up.

       -f     Print 'foreign' IPv4 addresses numerically rather than symboli-
              cally (this option is intended to get around serious brain dam-
              age in Sun's NIS server -- usually it hangs forever  translating
              non-local internet numbers).

              The  test  for  'foreign' IPv4 addresses is done using the IPv4
              address and netmask of the interface on which capture is  being
              done.  If that address or netmask are not available, available,
              either because the interface on which capture is being done has
              no  address  or netmask or because the capture is being done on
              the Linux "any" interface, which can capture on more  than  one
              interface, this option will not work correctly.

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

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

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

              If the -D flag is supported, an interface number as printed  by
              that flag can be used as the interface argument.

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

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

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

       -M     Use  secret as a shared secret for validating the digests found
              in TCP segments with the TCP-MD5 option (RFC 2385), if present.

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

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

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

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

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

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

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

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

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

       -s     Snarf snaplen bytes of data from each packet  rather  than  the
              default  of  68 (with SunOS's NIT, the minimum is actually 96).
              68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate
              protocol  information  from  name  server  and NFS packets (see
              below).  Packets truncated because of a  limited  snapshot  are
              indicated  in  the output with ''[|proto]'', where proto is the
              name  of  the  protocol  level  at  which  the  truncation  has
              occurred.  Note that taking larger snapshots both increases the
              amount of time it takes to process  packets  and,  effectively,
              decreases the amount of packet buffering.  This may cause pack-
              ets to be lost.  You should limit snaplen to the smallest  num-
              ber  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 interpreted the
              specified type.  Currently known types  are  aodv  (Ad-hoc  On-
              demand  Distance  Vector  protocol), cnfp (Cisco NetFlow proto-
              col), rpc (Remote Procedure Call), rtp (Real-Time  Applications
              protocol), rtcp (Real-Time Applications control protocol), snmp
              (Simple Network Management Protocol), tftp (Trivial File Trans-
              fer  Protocol),  vat  (Visual  Audio Tool), and wb (distributed
              White Board).

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

       -tt    Print an unformatted timestamp on each dump line.

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

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

       -u     Print undecoded NFS handles.

       -U     Make output saved via the -w option ''packet-buffered'';  i.e.,
              as each packet is saved, it will be written to the output file,
              rather than being written only when the output buffer fills.

              The -U flag will not be supported if tcpdump was built with  an
              older version of libpcap that lacks the pcap_dump_flush() func-
              tion.

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

              When writing to a file with the -w  option,  report,  every  10
              seconds, the number of packets captured.

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

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

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

       -W     Used  in  conjunction  with  the -C option, this will limit the
              number of files created to  the  specified  number,  and  begin
              overwriting  files  from the beginning, thus creating a 'rotat-
              ing' buffer.  In addition, it will name the files  with  enough
              leading  0s  to  support  the maximum number of files, allowing
              them to sort correctly.

       -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.

       -xx    Print each packet, including its link level header, in hex.

       -X     Print  each  packet  (minus  its  link level header) in hex and
              ASCII.  This is very handy for analysing new protocols.

       -XX    Print each packet, including its link level header, in hex  and
              ASCII.

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

       -Z     Drops privileges (if root) and changes user ID to user and  the
              group ID to the primary group of user.

              This behavior can also be enabled by default at compile time.

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

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

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

              dir    qualifiers  specify  a  particular transfer direction to
                     and/or from id.  Possible directions are src,  dst,  src
                     or  dst  and  src  and  dst.   E.g., 'src foo', 'dst net
                     128.3', 'src or dst port ftp-data'.  If there is no  dir
                     qualifier, src or dst is assumed.  For some link layers,
                     such as SLIP and the ''cooked'' Linux capture mode  used
                     for  the ''any'' device and for some other device types,
                     the inbound and outbound qualifiers can be used to spec-
                     ify a desired direction.

              proto  qualifiers  restrict the match to a particular protocol.
                     Possible protos are: ether, fddi,  tr,  wlan,  ip,  ip6,
                     arp,  rarp, decnet, tcp and udp.  E.g., 'ether src foo',
                     'arp net 128.3', 'tcp port 21'.  If there  is  no  proto
                     qualifier,  all  protocols  consistent with the type are
                     assumed.  E.g., 'src foo' means '(ip or arp or rarp) src
                     foo'  (except the latter is not legal syntax), 'net bar'
                     means '(ip or arp or rarp) net bar' and 'port 53'  means
                     '(tcp or udp) port 53'.

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

              Similarly, 'tr' and 'wlan' are aliases for 'ether'; the  previ-
              ous  paragraph's  statements  about  FDDI headers also apply to
              Token Ring and 802.11 wireless LAN headers.  For  802.11  head-
              ers,  the  destination  address  is the DA field and the source
              address is the SA field; the BSSID, RA, and  TA  fields  aren't
              tested.]

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

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

              Allowable primitives are:

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

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

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

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

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

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

              gateway host
                     True if the packet used host as a  gateway.   I.e.,  the
                     ethernet source or destination address was host but nei-
                     ther the IP source nor  the  IP  destination  was  host.
                     Host  must  be  a  name  and  must  be found both by the
                     machine's host-name-to-IP-address resolution  mechanisms
                     (host  name  file,  DNS, NIS, etc.) and by the machine's
                     host-name-to-Ethernet-address    resolution    mechanism
                     (/etc/ethers, etc.).  (An equivalent expression is
                          ether host ehost and not host host
                     which  can be used with either names or numbers for host
                     / ehost.)  This syntax does  not  work  in  IPv6-enabled
                     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 details).

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

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

              net net mask netmask
                     True if the IP address matches  net  with  the  specific
                     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 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 traf-
                     fic, and port domain  will  print  both  tcp/domain  and
                     udp/domain traffic).

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

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

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

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

              ip proto protocol
                     True if the packet is an IP packet (see ip(4P)) of  pro-
                     tocol 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 primi-
                     tive does not chase the protocol header chain.

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

              ip6 protochain protocol
                     True if the packet is IPv6 packet, and contains protocol
                     header  with type protocol in its protocol header chain.
                     For example,
                          ip6 protochain 6
                     matches any IPv6 packet with TCP protocol header in  the
                     protocol  header  chain.   The  packet  may contain, for
                     example, authentication header, routing header, or  hop-
                     by-hop  option  header,  between  IPv6  header  and  TCP
                     header.  The BPF code emitted by this primitive is  com-
                     plex  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  IPv4  broadcast  packet.   It
                     checks  for  both  the all-zeroes and all-ones broadcast
                     conventions, and looks up the subnet mask on the  inter-
                     face on which the capture is being done.

                     If the subnet mask of the interface on which the capture
                     is being done  is  not  available,  either  because  the
                     interface  on which capture is being done has no netmask
                     or because the capture is being done on the Linux  "any"
                     interface, which can capture on more than one interface,
                     this check will not work correctly.

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

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

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

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

                     [In the case of FDDI (e.g., 'fddi protocol arp'),  Token
                     Ring (e.g., 'tr protocol arp'), and IEEE 802.11 wireless
                     LANS (e.g., 'wlan protocol arp'), for most of those pro-
                     tocols, the protocol identification comes from the 802.2
                     Logical Link Control (LLC) header, which is usually lay-
                     ered on top of the FDDI, Token Ring, or 802.11 header.

                     When  filtering  for  most protocol identifiers on FDDI,
                     Token Ring, or 802.11, 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    tcpdump  checks  the  DSAP  (Destination Service
                             Access Point) and SSAP  (Source  Service  Access
                             Point) fields of the LLC header;

                     stp and netbeui
                             tcpdump checks the DSAP of the LLC header;

                      atalk  tcpdump  checks for a SNAP-format packet with an
                             OUI of 0x080007 and the AppleTalk etype.

                     In the case of Ethernet,  tcpdump  checks  the  Ethernet
                     type  field for most of those protocols.  The exceptions
                     are:

                     iso, sap, and netbeui
                             tcpdump checks  for  an  802.3  frame  and  then
                             checks the LLC header as it does for FDDI, Token
                             Ring, and 802.11;

                      atalk  tcpdump checks both for the AppleTalk  etype  in
                             an  Ethernet  frame and for a SNAP-format packet
                             as it does for FDDI, Token Ring, and 802.11;

                      aarp   tcpdump checks for the AppleTalk  ARP  etype  in
                             either  an Ethernet frame or an 802.2 SNAP frame
                             with an OUI of 0x000000;

                      ipx    tcpdump checks for the IPX etype in an  Ethernet
                             frame,  the  IPX  DSAP  in  the  LLC header, the
                             802.3-with-no-LLC-header encapsulation  of  IPX,
                             and the IPX etype in a SNAP frame.

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

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

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

              ifname interface
                     True  if the packet was logged as coming from the speci-
                     fied  interface  (applies  only  to  packets  logged  by
                     OpenBSD's pf(4)).

              on interface
                     Synonymous with the ifname modifier.

              rnr num
                     True  if the packet was logged as matching the specified
                     PF rule  number  (applies  only  to  packets  logged  by
                     OpenBSD's pf(4)).

              rulenum num
                     Synonomous with the rnr modifier.

              reason code
                     True if the packet was logged with the specified PF rea-
                     son code.  The known codes are: match, bad-offset, frag-
                     ment,  short,  normalize,  and  memory  (applies only to
                     packets logged by OpenBSD's pf(4)).

              rset name
                     True if the packet was logged as matching the  specified
                     PF  ruleset name of an anchored ruleset (applies only to
                     packets logged by pf(4)).

              ruleset name
                     Synonomous with the rset modifier.

              srnr num
                     True if the packet was logged as matching the  specified
                     PF  rule  number of an anchored ruleset (applies only to
                     packets logged by pf(4)).

              subrulenum num
                     Synonomous with the srnr modifier.

              action act
                     True if PF took the specified action when the packet was
                     logged.  Known actions are: pass and block (applies only
                     to packets logged by OpenBSD's pf(4)).

              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  tcp-
                     dump  does  not currently know how to parse these proto-
                     cols.

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

              tcp, udp, icmp
                     Abbreviations for:
                          ip proto p or ip6 proto p
                     where p is one of the above protocols.

              iso proto protocol
                     True  if  the  packet  is an OSI packet of protocol type
                     protocol.  Protocol can be a number or one of the  names
                     clnp, esis, or isis.

              clnp, esis, isis
                     Abbreviations for:
                          iso proto p
                     where p is one of the above protocols.

              l1, l2, iih, lsp, snp, csnp, psnp
                     Abbreviations for IS-IS PDU types.

              vpi n  True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, with a virtual path identifier of n.

              vci n  True if the packet is  an  ATM  packet,  for  SunATM  on
                     Solaris, with a virtual channel identifier of n.

              lane   True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is an ATM LANE packet.  Note that the first
                     lane keyword encountered in expression changes the tests
                     done in the remainder of expression  on  the  assumption
                     that  the  packet  is  either  a  LANE emulated Ethernet
                     packet or a LANE LE Control packet.  If lane isn't spec-
                     ified,  the tests are done under the assumption that the
                     packet is an LLC-encapsulated packet.

              llc    True if the packet is  an  ATM  packet,  for  SunATM  on
                     Solaris, and is an LLC-encapsulated packet.

              oamf4s True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is a segment OAM  F4  flow  cell  (VPI=0  &
                     VCI=3).

              oamf4e True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is an end-to-end OAM F4 flow cell (VPI=0  &
                     VCI=4).

              oamf4  True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is a segment or end-to-end OAM F4 flow cell
                     (VPI=0 & (VCI=3 | VCI=4)).

              oam    True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is a segment or end-to-end OAM F4 flow cell
                     (VPI=0 & (VCI=3 | VCI=4)).

              metac  True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is on a meta  signaling  circuit  (VPI=0  &
                     VCI=1).

              bcc    True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is on a broadcast signaling circuit  (VPI=0
                     & VCI=2).

              sc     True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is on a signaling circuit (VPI=0 &  VCI=5).

              ilmic  True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is on an ILMI circuit (VPI=0 & VCI=16).

              connectmsg
                     True if the packet is  an  ATM  packet,  for  SunATM  on
                     Solaris,  and  is on a signaling circuit and is a Q.2931
                     Setup, Call Proceeding, Connect, Connect  Ack,  Release,
                     or Release Done message.

              metaconnect
                     True  if  the  packet  is  an  ATM packet, for SunATM on
                     Solaris, and is on a meta signaling  circuit  and  is  a
                     Q.2931  Setup,  Call  Proceeding,  Connect,  Release, or
                     Release Done message.

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

                     For  example,  'ether[0] & 1 != 0' catches all multicast
                     traffic.  The expression 'ip[0] & 0xf != 5' catches  all
                     IP  packets  with  options.   The  expression 'ip[6:2] &
                     0x1fff = 0' catches only unfragmented datagrams and frag
                     zero  of fragmented datagrams.  This check is implicitly
                     applied to  the  tcp  and  udp  index  operations.   For
                     instance,  tcp[0] always means the first byte of the TCP
                     header, and never means the first byte of an intervening
                     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-routerso-
                     licit, 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-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
                   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 sin-
              gle argument or as multiple arguments, whichever is more conve-
              nient.  Generally, if the expression contains Shell metacharac-
              ters, it is easier to pass it as  a  single,  quoted  argument.
              Multiple  arguments  are  concatenated with spaces before being
              parsed.

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

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

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

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

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

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

       To print the start and end packets (the SYN and FIN packets)  of  each
       TCP conversation that involves a non-local host.
              tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net 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 eth-
       ernet broadcast or multicast:
              tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

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

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

       Link Level Headers

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

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

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

       On 802.11 networks, the '-e' option causes tcpdump to print the 'frame
       control'  fields,  all  of the addresses in the 802.11 header, and the
       packet length.  As on FDDI networks, packets are assumed to contain an
       LLC 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 information are printed out.
       The packet type is printed first.  The three types are ip,  utcp,  and
       ctcp.  No further link information is printed for ip packets.  For TCP
       packets, the connection identifier is printed following the type.   If
       the packet is compressed, its encoded header is printed out.  The spe-
       cial cases are printed out as *S+n and *SA+n, where n is the amount by
       which  the  sequence  number (or sequence number and ack) has changed.
       If it is not a special case, zero or  more  changes  are  printed.   A
       change  is  indicated  by  U  (urgent pointer), W (window), A (ack), S
       (sequence number), and I (packet ID), followed by a delta (+n or  -n),
       or  a  new  value (=n).  Finally, the amount of data in the packet and
       compressed header length are printed.

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

       ARP/RARP Packets

       Arp/rarp output shows the type of request and its arguments.  The for-
       mat 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 ether-
       net address of internet host csam.  Csam  replies  with  its  ethernet
       address  (in this example, ethernet addresses are in caps and internet
       addresses in lower case).

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

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

       TCP Packets

       (N.B.:The  following description assumes familiarity with the TCP pro-
       tocol described in RFC-793.  If you are not familiar with  the  proto-
       col, 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), R (RST), W
       (ECN CWR) or E (ECN-Echo), or a single  '.'  (no  flags).   Data-seqno
       describes  the  portion  of sequence space covered by the data in this
       packet (see example below).  Ack is sequence number of the  next  data
       expected the other direction on this connection.  Window is the number
       of bytes of receive buffer space available the other direction on this
       connection.   Urg  indicates  there  is  'urgent'  data in the packet.
       Options are tcp options enclosed in angle brackets (e.g., ).

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

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

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

       The expression says "let the 13th octet of a  TCP  datagram  have  the
       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:

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

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

                   00010010

       which translates to decimal

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

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

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


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

       We  see  that  this  AND operation delivers the same result regardless
       whether ACK or another TCP control bit is set.  The decimal  represen-
       tation  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  fol-
       lowing 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 expres-
       sion to hide the AND ('&') special character from the shell.

       UDP Packets

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

       Some UDP services are recognized (from the source or destination  port
       number) and the higher level protocol information printed.  In partic-
       ular, Domain Name service requests (RFC-1034/1035) and Sun  RPC  calls
       (RFC-1050) to NFS.

       UDP Name Server Requests

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

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

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

       UDP Name Server Responses

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

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

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

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


       SMB/CIFS decoding

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

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

       If you are decoding SMB sessions containing unicode strings  then  you
       may wish to set the environment variable USE_UNICODE to 1.  A patch to
       auto-detect unicode strings would be welcome.

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


       NFS Requests and Replies

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

              sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
              sushi.201b > wrl.nfs:
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.201b:
                   reply ok 128 lookup fh 9,74/4134.3150
       In  the first line, host sushi sends a transaction with id 6709 to wrl
       (note that the number following the src host is a transaction id,  not
       the source port).  The request was 112 bytes, excluding the UDP and IP
       headers.  The operation was a readlink (read symbolic  link)  on  file
       handle  (fh)  21,24/10.731657119.   (If one is lucky, as in this case,
       the file handle can be interpreted  as  a  major,minor  device  number
       pair,  followed  by  the  inode  number  and  generation number.)  Wrl
       replies 'ok' with the contents of the link.

       In the third line, sushi asks wrl to  lookup  the  name  'xcolors'  in
       directory  file 9,74/4096.6878.  Note that the data printed depends on
       the operation type.  The format is intended to be self explanatory  if
       read in conjunction with an NFS protocol spec.

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

              sushi.1372a > wrl.nfs:
                   148 read fh 21,11/12.195 8192 bytes @ 24576
              wrl.nfs > sushi.1372a:
                   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v also prints the IP  header  TTL,  ID,  length,  and  fragmentation
       fields,  which  have  been  omitted  from this example.)  In the first
       line, sushi asks wrl to read 8192 bytes  from  file  21,11/12.195,  at
       byte  offset  24576.  Wrl replies 'ok'; the packet shown on the second
       line is the first fragment of the reply, and hence is only 1472  bytes
       long  (the  other bytes will follow in subsequent fragments, but these
       fragments do not have NFS or even UDP headers  and  so  might  not  be
       printed,  depending  on  the  filter expression used).  Because the -v
       flag is given, some of the file  attributes  (which  are  returned  in
       addition  to  the  file data) are printed: the file type (''REG'', for
       regular file), the file mode (in octal), the uid and gid, and the file
       size.

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

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

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

       AFS Requests and Replies

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

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

              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                   new fid 536876964/1/1 ".newsrc"
              pike.afsfs > elvis.7001: rx data fs reply rename
       In the first line, host elvis sends a RX packet to pike.  This  was  a
       RX  data packet to the fs (fileserver) service, and is the start of an
       RPC call.  The RPC call was a rename, with the old directory  file  id
       of  536876964/1/1  and  an  old  filename  of '.newsrc.new', and a new
       directory file id of 536876964/1/1 and a new  filename  of  '.newsrc'.
       The  host pike responds with a RPC reply to the rename call (which was
       successful, because it was a data packet and not an abort packet).

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

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

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

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

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

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

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

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


       KIP AppleTalk (DDP in UDP)

       AppleTalk DDP packets encapsulated in UDP  datagrams  are  de-encapsu-
       lated  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  separated   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
              net.host.port

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

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

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

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

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

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


       IP Fragmentation

       Fragmented Internet datagrams are printed as
              (frag id:size@offset+)
              (frag id:size@offset)
       (The  first form indicates there are more fragments.  The second indi-
       cates 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 frag-
       ment  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 connection 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.  Sec-
       ond, the tcp sequence information in the first line is printed  as  if
       there  were  308 bytes of user data when, in fact, there are 512 bytes
       (308 in the first frag and 204 in the second).  If you are looking for
       holes  in  the sequence space or trying to match up acks with packets,
       this can fool you.

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

       Timestamps

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

SEE ALSO
       stty(1), pcap(3), bpf(4), nit(4P), pfconfig(8)

AUTHORS
       The original authors are:

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

       It is currently being maintained by tcpdump.org.

       The current version is available via http:

              http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

              ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

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

BUGS
       Please  send  problems,  bugs, questions, desirable enhancements, etc.
       to:

              tcpdump-workers@tcpdump.org

       Please send source code contributions, etc. to:

              patches@tcpdump.org

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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

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

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

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

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

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

       Filter  expressions  on fields other than those in 802.11 headers will
       not correctly handle 802.11 data packets with both To DS and  From  DS
       set.

       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.



                                22 March 2004                      TCPDUMP(8)