Provided by: tcpdump_4.0.0-2ubuntu2_i386 bug


       tcpdump - dump traffic on a network


       tcpdump [ -AdDefIKlLnNOpqRStuUvxX ] [ -B buffer_size ] [ -c count ]
               [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
               [ -i interface ] [ -m module ] [ -M secret ]
               [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ]
               [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -z 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.  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 processed);

              packets ‘‘received by filter’’ (the meaning of this  depends  on
              the  OS on which you’re running tcpdump, and possibly on the way
              the OS was configured - if a filter was specified on the command
              line,  on some OSes it counts packets regardless of whether they
              were matched by the filter expression and,  even  if  they  were
              matched  by the filter expression, regardless of whether tcpdump
              has read and processed them yet, on other OSes  it  counts  only
              packets that were matched by the filter expression regardless of
              whether tcpdump has read and processed them yet,  and  on  other
              OSes  it  counts  only  packets  that were matched by the filter
              expression and were processed by tcpdump);

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

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

       Reading packets from a network interface  may  require  that  you  have
       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 pages.

       -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

       -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()

       -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

              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 correctly.

       -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 ‘file<count>’.

       -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.

       -K     Don’t attempt to verify  TCP  checksums.   This  is  useful  for
              interfaces   that   perform  the  TCP  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.  E.g.,
              ‘‘tcpdump  -l  |  tee     dat’’     or     ‘‘tcpdump  -l       >
              dat  &  tail  -f  dat’’.

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

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

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

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

       -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  68  (with SunOS’s NIT, the minimum is actually 96).
              68 bytes is adequate for IP, ICMP, TCP and UDP but may  truncate
              protocol  information  from  name  server  and  NFS packets (see
              below).  Packets truncated because of  a  limited  snapshot  are
              indicated  in  the  output with ‘‘[|proto]’’, where proto is the
              name of the protocol level at which the truncation has occurred.
              Note  that  taking larger snapshots both increases the amount of
              time it takes to process packets and, effectively, decreases the
              amount  of packet buffering.  This may cause packets to be lost.
              You should limit  snaplen  to  the  smallest  number  that  will
              capture  the protocol information you’re interested in.  Setting
              snaplen to 0 means  use  the  required  length  to  catch  whole

       -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     Dont 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 line.

       -u     Print undecoded NFS handles.

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

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

       -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

       -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 ‘‘-’’.

       -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

              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 padding.

       -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

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

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

              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(4).

              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
              tcpdumpgateway 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.
              tcpdumptcp[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.)
              tcpdumptcp 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:
              tcpdumpgateway snup and ip[2:2] > 576’

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

       To print all ICMP packets that are not echo requests/replies (i.e., not
       ping packets):
              tcpdumpicmp[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 packet.

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

       On 802.11 networks, the ’-e’ option causes tcpdump to print the  ‘frame
       control’  fields,  all  of  the addresses in the 802.11 header, and the
       packet length.  As on FDDI networks, packets are assumed to contain  an
       LLC packet.

       (N.B.:  The  following  description  assumes  familiarity with the SLIP
       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

       The general format of a tcp protocol line is:
              src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and  destination  IP  addresses  and  ports.
       Flags  are  some  combination of S (SYN), F (FIN), P (PUSH), R (RST), W
       (ECN CWR) or E (ECN-Echo), or a  single  ‘.’  (no  flags).   Data-seqno
       describes  the  portion  of  sequence space covered by the data in this
       packet (see example below).  Ack is sequence number of  the  next  data
       expected  the other direction on this connection.  Window is the number
       of bytes of receive buffer space available the other direction on  this
       connection.   Urg  indicates  there  is  ‘urgent’  data  in the packet.
       Options are tcp options enclosed in angle brackets (e.g., <mss  1024>).

       Src,  dst and flags are always present.  The other fields depend on the
       contents of the packet’s tcp protocol header and  are  output  only  if

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

       Csam  replies  with  a similar packet except it includes a piggy-backed
       ack for rtsg’s SYN.  Rtsg then acks csam’s SYN.  The ‘.’ means no flags
       were  set.   The  packet contained no data so there is no data sequence
       number.  Note that the ack sequence number is a small integer (1).  The
       first  time  tcpdump  sees a tcp ‘conversation’, it prints the sequence
       number from the packet.  On subsequent packets of the conversation, the
       difference  between  the  current  packet’s  sequence  number  and this
       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

       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

            |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 xl0tcp[13] & 2 == 2’

       Note that you should use single quotes or a backslash in the expression
       to hide the AND (’&’) special character from the shell.

       UDP Packets

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

       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

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

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

       SMB/CIFS decoding

       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 te 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

       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

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

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

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

              1.254          ether
              16.1      icsd-net
              1.254.110 ace
       The  first  two  lines give the names of AppleTalk networks.  The third
       line gives the name of a particular host (a host is distinguished  from
       a  net  by  the  3rd  octet  in the number - a net number must have two
       octets and a host number must have three octets.)  The number and  name
       should   be   separated   by   whitespace   (blanks   or   tabs).   The
       /etc/atalk.names file may contain blank lines or comment  lines  (lines
       starting with a ‘#’).

       AppleTalk addresses are printed in the form

     > 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

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

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

       ATP packet formatting is demonstrated by the following example:
              jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
              helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
              jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
              helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
              helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
              jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
              jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
       Jssmag.209   initiates   transaction  id  12266  with  host  helios  by
       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 fragment.)

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

       The fragment information  is  output  for  each  fragment.   The  first
       fragment contains the higher level protocol header and the frag info is
       printed after the protocol info.  Fragments after the first contain  no
       higher  level  protocol  header  and the frag info is printed after the
       source and destination addresses.  For example, here is part of an  ftp
       from 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 dont fragment flag  is  marked  with  a  trailing


       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), pcap-filter(4), bpf(4), nit(4P)


       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

       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.

                                07 January 2008                     TCPDUMP(8)