Provided by: tcpdump_4.5.1-2ubuntu1_amd64 bug


       tcpdump - dump traffic on a network


       tcpdump [ -AbdDefhHIJKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
               [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
               [ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
               [ -P in|out|inout ]
               [ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ]
               [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
               [ expression ]


       Tcpdump  prints  out  a description of the contents 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
       (please  note  tcpdump  is  protected via an enforcing apparmor(7) profile in Ubuntu which
       limits the files tcpdump may access).  It can also be run with the -V flag,  which  causes
       it  to read a list of saved packet files. 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 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 processed.

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

              packets ``captured'' (this is the number of packets that tcpdump has  received  and

              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

       Reading packets from a network interface may require that you have special privileges; see
       the pcap (3PCAP) man page for details.   Reading  a  saved  packet  file  doesn't  require
       special privileges.


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

       -b     Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.

       -B     Set the operating system capture buffer size to buffer_size, in units of KiB  (1024

       -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() function.

       -e     Print  the  link-level header on each dump line.  This can be used, for example, to
              print MAC layer addresses for protocols such as Ethernet and IEEE 802.11.

       -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 separation.

              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 preceded 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 permissions that tcpdump may have been  given  should  already  have
              been given up.

       -f     Print `foreign' IPv4 addresses numerically rather than symbolically (this option is
              intended to get around serious brain damage 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

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

       -G     If   specified,   rotates  the  dump  file  specified  with  the  -w  option  every
              rotate_seconds seconds.  Savefiles will have the name specified by -w which  should
              include  a  time format as defined by strftime(3).  If no time format is specified,
              each new file will overwrite the previous.

              If used in conjunction with  the  -C  option,  filenames  will  take  the  form  of

       -h     Print the tcpdump and libpcap version strings, print a usage message, and exit.

       -H     Attempt to detect 802.11s draft mesh headers.

       -i     Listen  on  interface.   If unspecified, tcpdump searches the system interface list
              for the lowest numbered, configured up interface (excluding  loopback),  which  may
              turn out to be, for example, ``eth0''.

              On Linux systems with 2.2 or later kernels, an interface argument of ``any'' can be
              used to capture packets from all interfaces.  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.

       -I     Put the interface in "monitor mode"; this is supported only on  IEEE  802.11  Wi-Fi
              interfaces, and supported only on some operating systems.

              Note  that  in  monitor  mode  the adapter might disassociate from the network with
              which it's associated, so that you will not be able to use  any  wireless  networks
              with  that  adapter.   This  could  prevent accessing files on a network server, or
              resolving host names or network addresses, if you are capturing in monitor mode and
              are not connected to another network with another adapter.

              This flag will affect the output of the -L flag.  If -I isn't specified, only those
              link-layer types available when not in  monitor  mode  will  be  shown;  if  -I  is
              specified,  only  those  link-layer  types  available  when in monitor mode will be

       -j     Set the time stamp type for the capture to tstamp_type.  The names to use  for  the
              time  stamp  types are given in pcap-tstamp-type(7); not all the types listed there
              will necessarily be valid for any given interface.

       -J     List the supported time stamp types for the interface and exit.  If the time  stamp
              type cannot be set for the interface, no time stamp types are listed.

       -K     Don't  attempt  to verify IP, TCP, or UDP checksums.  This is useful for interfaces
              that perform some or all of those checksum calculation in hardware; otherwise,  all
              outgoing TCP checksums will be flagged as bad.

       -l     Make  stdout line buffered.  Useful if you want to see the data while capturing it.

                     tcpdump -l | tee dat


                     tcpdump -l > dat & tail -f dat

              Note that on Windows,``line buffered'' means ``unbuffered'', so that  WinDump  will
              write each character individually if -l is specified.

              -U  is  similar  to  -l  in  its behavior, but it will cause output to be ``packet-
              buffered'', so that the output is written to stdout  at  the  end  of  each  packet
              rather  than  at the end of each line; this is buffered on all platforms, including

       -L     List the known data link types for the interface, in the specified mode, and  exit.
              The  list  of  known  data  link  types may be dependent on the specified mode; for
              example, on some platforms, a Wi-Fi interface might support one set  of  data  link
              types  when  not  in monitor mode (for example, it might support only fake Ethernet
              headers, or might support 802.11 headers but not support 802.11 headers with  radio
              information)  and another set of data link types when in monitor mode (for example,
              it might support 802.11 headers, or 802.11 headers with radio information, only  in
              monitor mode).

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

       -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 addresses (i.e., host addresses, port numbers, etc.) to names.

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

       -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'.

       -P     Choose  send/receive  direction  direction  for  which  packets should be captured.
              Possible values are `in', `out' and `inout'. Not available on all platforms.

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

       -R     Assume  ESP/AH  packets  to be based on old specification (RFC1825 to RFC1829).  If
              specified, tcpdump will not print replay  prevention  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 65535
              bytes.  Packets truncated because of a limited snapshot are indicated in the output
              with  ``[|proto]'',  where  proto  is  the  name of the protocol level at which the
              truncation has occurred.  Note that taking  larger  snapshots  both  increases  the
              amount  of  time it takes to process packets and, effectively, decreases the amount
              of packet buffering.  This may cause packets to be lost.  You should limit  snaplen
              to the smallest number that will capture the protocol information you're interested
              in.  Setting snaplen  to  0  sets  it  to  the  default  of  65535,  for  backwards
              compatibility with recent older versions of tcpdump.

       -T     Force  packets  selected  by  "expression"  to  be  interpreted the specified type.
              Currently known types are aodv (Ad-hoc On-demand Distance  Vector  protocol),  carp
              (Common  Address  Redundancy  Protocol),  cnfp  (Cisco NetFlow protocol), lmp (Link
              Management Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0 inside
              PGM/EPGM),   radius   (RADIUS),   rpc   (Remote  Procedure  Call),  rtp  (Real-Time
              Applications  protocol),  rtcp  (Real-Time  Applications  control  protocol),  snmp
              (Simple  Network  Management  Protocol), tftp (Trivial File Transfer Protocol), vat
              (Visual Audio Tool), wb (distributed White Board), zmtp1 (ZeroMQ Message  Transport
              Protocol 1.0) and vxlan (Virtual eXtensible Local Area Network).

              Note  that  the  pgm  type above affects UDP interpretation only, the native PGM is
              always recognised as IP protocol 113  regardless.  UDP-encapsulated  PGM  is  often
              called "EPGM" or "PGM/UDP".

              Note  that  the  pgm_zmtp1 type above affects interpretation of both native PGM and
              UDP at once. During the native PGM decoding the application data of an  ODATA/RDATA
              packet  would be decoded as a ZeroMQ datagram with ZMTP/1.0 frames.  During the UDP
              decoding in addition to that any UDP packet would be treated as an encapsulated PGM

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

       -tt    Print an unformatted timestamp on each dump line.

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

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

       -ttttt Print a delta (micro-second resolution) between current and first line on each dump

       -u     Print undecoded NFS handles.

       -U     If  the  -w  option  is  not  specified,  make  the printed packet output ``packet-
              buffered''; i.e., as the description of the contents of each packet is printed,  it
              will  be  written  to  the  standard  output,  rather  than,  when not writing to a
              terminal, being written only when the output buffer fills.

              If the -w  option  is  specified,  make  the  saved  raw  packet  output  ``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() function.

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

       -V     Read a list of filenames from file. Standard input is used if file is ``-''.

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

              This output will be buffered if written to a file or pipe,  so  a  program  reading
              from  the  file  or  pipe may not see packets for an arbitrary amount of time after
              they are received.  Use the -U flag to cause packets to be written as soon as  they
              are received.

              The  MIME  type application/vnd.tcpdump.pcap has been registered with IANA for pcap
              files. The filename extension .pcap appears to be the most commonly used along with
              .cap  and  .dmp.  Tcpdump  itself  doesn't check the extension when reading capture
              files and doesn't add an extension when writing them (it uses magic numbers in  the
              file header instead). However, many operating systems and applications will use the
              extension if it is present and adding one (e.g. .pcap) is recommended.

              See pcap-savefile(5) for a description of the file format.

       -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 '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.

              Used  in conjunction with the -G option, this will limit the number of rotated dump
              files that get created, exiting with status 0 when reaching the limit. If used with
              -C as well, the behavior will result in cyclical files per timeslice.

       -x     When  parsing  and  printing,  in  addition to printing the headers of each packet,
              print the data of 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

       -xx    When parsing and printing, in addition to printing  the  headers  of  each  packet,
              print the data of each packet, including its link level header, in hex.

       -X     When  parsing  and  printing,  in  addition to printing the headers of each packet,
              print the data of each packet (minus its link level header) in hex and ASCII.  This
              is very handy for analysing new protocols.

       -XX    When  parsing  and  printing,  in  addition to printing the headers of each packet,
              print the data of 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     Used in conjunction with the -C or -G options, this will make tcpdump run " command
              file  "  where  file is the savefile being closed after each rotation. For example,
              specifying -z gzip or -z bzip2 will compress each savefile using gzip or bzip2.

              Note that tcpdump will run the command in parallel to the capture, using the lowest
              priority so that this doesn't disturb the capture process.

              And  in  case  you would like to use a command that itself takes flags or different
              arguments, you can always write a shell script that will take the savefile name  as
              the  only argument, make the flags & arguments arrangements and execute the command
              that you want.

       -Z     If tcpdump is running as root, after opening the capture device or input  savefile,
              but  before  opening  any  savefiles for output, change the 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.

              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.

              For the expression syntax, see pcap-filter(7).

              The expression argument can be passed to tcpdump as either a single Shell argument,
              or  as  multiple  Shell arguments, whichever is more convenient.  Generally, if the
              expression contains Shell  metacharacters,  such  as  backslashes  used  to  escape
              protocol names, it is easier to pass it as a single, quoted argument rather than to
              escape the Shell metacharacters.  Multiple arguments are concatenated  with  spaces
              before being parsed.


       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 all IPv4 HTTP packets to and from port 80, i.e. print only packets  that  contain
       data,  not,  for  example,  SYN and FIN packets and ACK-only packets.  (IPv6 is left as an
       exercise for the reader.)
              tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

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

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

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


       The  output of tcpdump is protocol dependent.  The 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 datagrams) are `async' packets, with a priority value between 0 and  7;  for
       example,  `async4'.   Such  packets  are  assumed to contain an 802.2 Logical Link Control
       (LLC) packet; the LLC header is printed if it is not an ISO datagram or a  so-called  SNAP

       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 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 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 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 format is intended to be
       self explanatory.  Here is a short sample taken from the start of an  `rlogin'  from  host
       rtsg to host csam:
              arp who-has csam tell rtsg
              arp reply csam is-at CSAM
       The  first  line  says  that  rtsg  sent  an arp packet asking for the Ethernet address of
       internet host csam.  Csam replies with its Ethernet address  (in  this  example,  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 tell
              arp reply is-at 02:07:01:00:01:c4

       If  we  had done tcpdump -e, the fact that the first packet is broadcast and the second is
       point-to-point would be 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  protocol  described  in
       RFC-793.   If you are not familiar with the protocol, neither this description nor tcpdump
       will be of much use to you.)

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP  addresses  and  ports.   Flags  are  some
       combination  of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
       `.' (ACK), or `none' if no flags are set.  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 the ACK flag was 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 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 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:

                       |               |
                       |7   5   3     0|

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

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

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

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

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


       and its decimal representation is

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

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

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

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

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

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

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

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


       which translates to decimal

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

       Now we can't just use 'tcp[13] == 18' in the tcpdump 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 representation of the AND value as well as the result
       of this operation is 2 (binary 00000010), so we know that for packets  with  SYN  set  the
       following relation must hold true:

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

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

       Some offsets and field values may be expressed as names rather than as numeric values. For
       example tcp[13] may be replaced with tcp[tcpflags]. The following TCP  flag  field  values
       are also available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-act, tcp-urg.

       This can be demonstrated as:
                   tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'

       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 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 particular, 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? (37)
       Host  h2opolo asked the domain server on helios for an address record (qtype=A) associated
       with the name  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 headers.  The query operation was the normal one, Query,  so  the  op  field  was
       omitted.  If the op had been anything else, it would have been printed between the `3' and
       the `+'.  Similarly, the qclass was the normal one, C_IN, and omitted.  Any  other  qclass
       would have been printed 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 (273)
              helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
       In the first example, helios responds to query id 3 from h2opolo with 3 answer records,  3
       name server records and 7 additional records.  The first answer record is type A (address)
       and its data is internet address  The total size of  the  response  was  273
       bytes,  excluding  UDP  and  IP  headers.  The op (Query) and response code (NoError) were
       omitted, as was the class (C_IN) of the A 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.

       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.

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

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
              src.xid > dst.nfs: len op args
              src.nfs > dst.xid: reply stat len op results
              sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
              sushi.201b > wrl.nfs:
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.201b:
                   reply ok 128 lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id  6709  to  wrl  (note  that  the
       number  following the src host is a transaction id, not the source port).  The request was
       112 bytes, excluding the UDP and IP headers.  The operation was a readlink (read  symbolic
       link) on file 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:

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

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

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

       If the -v (verbose) flag is given twice, acknowledgement  packets  and  additional  header
       information  is  printed,  such  as  the  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 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

       KIP AppleTalk (DDP in UDP)

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

     > 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

       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 requesting 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, jssmag.209 initiates the next request.  The
       `*' on the request indicates that XO (`exactly once') was not set.

       IP Fragmentation

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

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

       The  fragment  information  is  output for each fragment.  The first fragment contains the
       higher level protocol header and the  frag  info  is  printed  after  the  protocol  info.
       Fragments  after  the  first  contain no higher level protocol header and the frag info is
       printed after the source and destination addresses.  For example, here is part of  an  ftp
       from  to over a CSNET 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.   Second, the tcp sequence information in the first line is printed as if there
       were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag  and
       204 in the second).  If you are looking for holes in the sequence space or trying to match
       up acks with packets, this can fool you.

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


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


       stty(1), pcap(3PCAP),  bpf(4),  nit(4P),  pcap-savefile(5),  pcap-filter(7),  pcap-tstamp-
       type(7), apparmor(7)



       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

       The current version is available via http:


       The original distribution is available via anonymous ftp:


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


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


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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

                                           12 July 2012                                TCPDUMP(8)