tcpdump



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 tcpdump.

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

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

              packets ‘‘captured’’ (this is the number of packets that tcpdump
              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 command
              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 spe-
       cial 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)  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
              (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 hav-
              ing 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 system
              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 systems,
              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() func-
              tion.

       -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  newline
              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     Dont  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 packets to be lost.
              You should limit snaplen to the smallest number that  will  cap-
              ture  the  protocol  information  you’re interested in.  Setting
              snaplen to 0 means  use  the  required  length  to  catch  whole
              packets.

       -T     Force  packets  selected  by  "expression" to be interpreted the
              specified type.  Currently known  types  are  aodv  (Ad-hoc  On-
              demand Distance Vector protocol), cnfp (Cisco NetFlow protocol),
              rpc (Remote Procedure Call), rtp (Real-Time Applications  proto-
              col), rtcp (Real-Time Applications control protocol), snmp (Sim-
              ple Network Management Protocol), tftp  (Trivial  File  Transfer
              Protocol),  vat  (Visual  Audio Tool), and wb (distributed White
              Board).

       -t     Dont 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 sec-
              onds, 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 num-
              ber of files created to the specified number,  and  begin  over-
              writing  files  from  the  beginning, thus creating a ’rotating’
              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 lay-
              ers  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 quali-
                     fier, 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 specify 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 quali-
                     fier, 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 speci-
              fied network interface.’’  FDDI  headers  contain  Ethernet-like
              source  and  destination  addresses, and often contain Ethernet-
              like packet types, so you can filter on these FDDI  fields  just
              as  with  the analogous Ethernet fields.  FDDI headers also con-
              tain other fields, but you cannot name them explicitly in a fil-
              ter expression.

              Similarly, ‘tr’ and ‘wlan’ are aliases for ‘ether’; the previous
              paragraph’s statements about FDDI headers also  apply  to  Token
              Ring  and  802.11 wireless LAN headers.  For 802.11 headers, 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 con-
                     figuration 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 net-
                     mask.  May be qualified with src or dst.  Note that  this
                     syntax is not valid for IPv6 net.

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

              dst port port
                     True if the packet is ip/tcp, ip/udp, ip6/tcp or  ip6/udp
                     and  has  a destination port value of port.  The port can
                     be a number or a name used in /etc/services (see  tcp(4P)
                     and  udp(4P)).   If  a name is used, both the port number
                     and protocol are checked.  If a number or ambiguous  name
                     is  used, only the port number is checked (e.g., dst port
                     513 will print both tcp/login traffic and  udp/who  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 proto-
                     col 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 exam-
                     ple, authentication header, routing header, or hop-by-hop
                     option header, between IPv6 header and TCP  header.   The
                     BPF  code emitted by this primitive is complex and cannot
                     be optimized by BPF optimizer code in  tcpdump,  so  this
                     can be somewhat slow.

              ip protochain protocol
                     Equivalent  to  ip6  protochain protocol, but this is for
                     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 inter-
                     face 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 pack-
                     ets 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 tcpdump
                     does not currently know how to parse these protocols.

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

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

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

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

              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 specified,
                     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-redi-
                     rect,  icmp-echo,  icmp-routeradvert, icmp-routersolicit,
                     icmp-timxceed, icmp-paramprob,  icmp-tstamp,  icmp-tstam-
                     preply,  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 (paren-
                     theses are special to the Shell and must be escaped).

                     Negation (‘!’ or ‘not’).

                     Concatenation (‘&&’ or ‘and’).

                     Alternation (‘||’ or ‘or’).

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

              If  an  identifier  is  given without a keyword, the most recent
              keyword is assumed.  For example,
                   not host vs and ace
              is short for
                   not host vs and host ace
              which should not be confused with
                   not ( host vs or ace )

              Expression arguments can be passed to tcpdump as either a single
              argument or as multiple arguments, whichever is more convenient.
              Generally, if the expression contains Shell  metacharacters,  it
              is  easier  to  pass  it as a single, quoted argument.  Multiple
              arguments are concatenated with spaces before being parsed.


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 destination
       addresses,  and  the  packet  length.  As on FDDI networks, packets are
       assumed to contain an LLC  packet.   Regardless  of  whether  the  ’-e’
       option  is  specified or not, the source routing information is printed
       for source-routed packets.

       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 com-
       pression algorithm described in RFC-1144.)

       On SLIP links, a direction indicator (‘‘I’’ for inbound, ‘‘O’’ for out-
       bound),  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 pack-
       ets, the connection identifier is printed following the type.   If  the
       packet  is  compressed, its encoded header is printed out.  The special
       cases are printed out as *S+n and *SA+n, where n is the amount by which
       the sequence number (or sequence number and ack) has changed.  If it is
       not a special case, zero or more changes  are  printed.   A  change  is
       indicated  by U (urgent pointer), W (window), A (ack), S (sequence num-
       ber), 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 contained
       hex 0806 (type ETHER_ARP) and the total length was 64 bytes.

       TCP Packets

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

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src  and  dst  are  the  source and destination IP addresses and ports.
       Flags are some combination of S (SYN), F (FIN), P (PUSH),  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., <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 contained no data so there is  no  data  sequence
       number.  Note that the ack sequence number is a small integer (1).  The
       first time tcpdump sees a tcp ‘conversation’, it  prints  the  sequence
       number from the packet.  On subsequent packets of the conversation, the
       difference between the current packet’s sequence number and  this  ini-
       tial  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  sit-
       ting  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 second
       line shows octets 4 - 7 etc.

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

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

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

                       |               |
                       |---------------|
                       |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 dec-
       imal 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 expression,
       because that would select only those packets that have SYN-ACK set, but
       not those with only SYN set.  Remember that we don’t care if ACK or any
       other control bit is set as long as SYN is set.

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


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

       We  see  that  this  AND  operation delivers the same result regardless
       whether ACK or another TCP control bit is set.  The decimal representa-
       tion  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 follow-
       ing 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  datagram  to  port
       who on host broadcast, the Internet broadcast address.  The packet con-
       tained 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 particu-
       lar, 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  Net-
       BEUI 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  off-
       set  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 success-
       ful, 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-encapsulated
       and dumped as DDP packets (i.e., all the UDP header information is dis-
       carded).   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 jssmag 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 protocol) and
       packet size.

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

       ATP packet formatting is demonstrated by the following 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  dont 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 rec-
       ommend 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 pack-
              ets 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) ques-
       tion  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 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)

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