Provided by: tcpdump_4.2.1-1ubuntu2_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 ]
               [ -r 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).  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.

       -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.

       -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).  Ties are
              broken by choosing the earliest match.

              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'' or ``tcpdump  -l   > dat  &  tail  -f  dat''.

       -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'.

       -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),  cnfp
              (Cisco  NetFlow protocol), 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), and wb (distributed White Board).

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

       -tt    Print an unformatted timestamp on each dump line.

       -ttt   Print a delta (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     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() 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.

       -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  ``-''.
              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).

              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.


       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 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

       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.

                                          05 March 2009                                TCPDUMP(8)