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    Command:

    tcpdump

    
    
    

    SYNOPSIS

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

    DESCRIPTION

           Tcpdump prints out a description of the contents of packets on  a  net-
           work  interface  that match the boolean expression.  It can also be run
           with the -w flag, which causes it to save the packet data to a file for
           later analysis, and/or with the -r flag, which causes it to read from a
           saved packet file rather than to read packets from a network interface.
           In  all  cases, only packets that match expression will be processed by
           tcpdump.
    
           Tcpdump will, if not run with the -c flag, continue  capturing  packets
           until  it is interrupted by a SIGINT signal (generated, for example, by
           typing your interrupt character, typically control-C) or a SIGTERM sig-
           nal  (typically generated with the kill(1) command); if run with the -c
           flag, it will capture packets until it is interrupted by  a  SIGINT  or
           SIGTERM  signal or the specified number of packets have been processed.
    
           When tcpdump finishes capturing packets, it will report counts of:
    
                  packets ''captured'' (this is the number of packets that tcpdump
                  has received and processed);
    
                  packets  ''received  by filter'' (the meaning of this depends on
                  the OS on which you're running tcpdump, and possibly on the  way
                  the OS was configured - if a filter was specified on the command
                  line, on some OSes it counts packets regardless of whether  they
                  were  matched  by  the  filter expression and, even if they were
                  matched by the filter expression, regardless of whether  tcpdump
                  has  read  and  processed them yet, on other OSes it counts only
                  packets that were matched by the filter expression regardless of
                  whether  tcpdump  has  read and processed them yet, and on other
                  OSes it counts only packets that  were  matched  by  the  filter
                  expression and were processed by tcpdump);
    
                  packets  ''dropped  by  kernel''  (this is the number of packets
                  that were dropped, due to a lack of buffer space, by the  packet
                  capture  mechanism in the OS on which tcpdump is running, if the
                  OS reports that information to applications; if not, it will  be
                  reported as 0).
    
           On  platforms  that  support  the  SIGINFO  signal,  such  as most BSDs
           (including Mac OS X) and  Digital/Tru64  UNIX,  it  will  report  those
           -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).
    
                  Note that when used with -Z option (enabled by default),  privi-
                  leges are dropped before opening first savefile.
    
           -d     Dump  the compiled packet-matching code in a human readable form
                  to standard output and stop.
    
           -dd    Dump packet-matching code as a C program fragment.
    
           -ddd   Dump packet-matching code as decimal numbers  (preceded  with  a
                  count).
    
           -D     Print the list of the network interfaces available on the system
                  and on which tcpdump can  capture  packets.   For  each  network
                  interface,  a number and an interface name, possibly followed by
                  a text description of the interface, is printed.  The  interface
                  name  or the number can be supplied to the -i flag to specify an
                  interface on which to capture.
    
                  This can be useful on systems that don't have a command to  list
                  them  (e.g.,  Windows  systems, or UNIX systems lacking ifconfig
                  -a); the number can be useful on Windows 2000 and later systems,
                  where the interface name is a somewhat complex string.
    
                  The  -D  flag will not be supported if tcpdump was built with an
                  older version of libpcap that lacks the pcap_findalldevs() func-
                  tion.
    
           -e     Print the link-level header on each dump line.
    
           -E     Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that
                  are addressed to addr and contain Security Parameter Index value
                  spi.  This  combination  may  be  repeated with comma or newline
                  seperation.
    
                  Note that setting the secret for IPv4 ESP packets  is  supported
                  at this time.
    
                  Algorithms  may  be  des-cbc,  3des-cbc,  blowfish-cbc, rc3-cbc,
                  cast128-cbc, or none.  The default is des-cbc.  The  ability  to
                  decrypt  packets  is  only  present if tcpdump was compiled with
                  missions that tcpdump may have been given  should  already  have
                  been given up.
    
           -f     Print  'foreign' IPv4 addresses numerically rather than symboli-
                  cally (this option is intended to get around serious brain  dam-
                  age  in  Sun's NIS server -- usually it hangs forever translating
                  non-local internet numbers).
    
                  The test for 'foreign' IPv4 addresses is  done  using  the  IPv4
                  address  and  netmask of the interface on which capture is being
                  done.  If that address or netmask are not available,  available,
                  either  because the interface on which capture is being done has
                  no address or netmask or because the capture is  being  done  on
                  the  Linux  "any"  interface, which can capture on more than one
                  interface, this option will not work correctly.
    
           -F     Use file as input for  the  filter  expression.   An  additional
                  expression given on the command line is ignored.
    
           -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  sys-
                  tem interface list for the lowest numbered, configured up inter-
                  face (excluding loopback).  Ties are broken by choosing the ear-
                  liest match.
    
                  On  Linux  systems with 2.2 or later kernels, an interface argu-
                  ment of ''any'' can be used to capture packets from  all  inter-
                  faces.   Note  that  captures  on the ''any'' device will not be
                  done in promiscuous mode.
    
                  If the -D flag is supported, an interface number as  printed  by
                  that flag can be used as the interface argument.
    
           -I     Put  the  interface in "monitor mode"; this is supported only on
                  IEEE 802.11 Wi-Fi interfaces, and supported only on some operat-
                  ing 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.
    
                  stamps  (nanoseconds)  and their actual accuracy is platform and
                  hardware dependent.  Also note that when writing  captures  made
                  with  nanosecond  accuracy  to  a  savefile, the time stamps are
                  written with nanosecond resolution, and the file is written with
                  a  different  magic number, to indicate that the time stamps are
                  in seconds and nanoseconds; not  all  programs  that  read  pcap
                  savefiles will be able to read those captures.
    
                  When  reading  a  savefile, convert time stamps to the precision
                  specified by timestamp_precision, and  display  them  with  that
                  resolution.   If the precision specified is less than the preci-
                  sion of time stamps in the file, the conversion will lose preci-
                  sion.
    
                  The  supported  values  for  timestamp_precision  are  micro for
                  microsecond resolution and nano for nanosecond resolution.   The
                  default is microsecond resolution.
    
           -K     Don't attempt to verify IP, TCP, or UDP checksums.  This is use-
                  ful 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.  E.g.,
                  ''tcpdump  -l  |  tee     dat''     or     ''tcpdump  -l       >
                  dat  &  tail  -f  dat''.
    
           -L     List the known data link types for the interface, in the  speci-
                  fied  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 sup-
                  port 802.11 headers with radio information) and another  set  of
                  data link types when in monitor mode (for example, it might sup-
                  port 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  tcp-
                  dump.
    
           -M     Use  secret  as a shared secret for validating the digests found
                  in TCP segments with the TCP-MD5 option (RFC 2385), if  present.
    
           -n     Don't  convert  host  addresses  to  names.  This can be used to
                  avoid DNS lookups.
    
           -nn    Don't convert protocol and port numbers etc. to names either.
    
           -N     Don't print domain name qualification of host names.   E.g.,  if
    
           -q     Quick (quiet?) output.  Print less protocol information so  out-
                  put lines are shorter.
    
           -R     Assume  ESP/AH packets to be based on old specification (RFC1825
                  to RFC1829).  If specified, tcpdump will not print  replay  pre-
                  vention  field.   Since  there  is  no protocol version field in
                  ESP/AH specification,  tcpdump  cannot  deduce  the  version  of
                  ESP/AH protocol.
    
           -r     Read  packets  from file (which was created with the -w option).
                  Standard input is used if file is ''-''.
    
           -S     Print absolute, rather than relative, TCP sequence numbers.
    
           -s     Snarf snaplen bytes of data from each  packet  rather  than  the
                  default  of 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  pack-
                  ets 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 back-
                  wards 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 proto-
                  col), rtcp (Real-Time Applications control protocol), snmp (Sim-
                  ple  Network  Management  Protocol), tftp (Trivial File Transfer
                  Protocol), vat (Visual Audio Tool), and  wb  (distributed  White
                  Board).
    
           -t     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 pre-
                  vious 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.,
                  onds, the number of packets captured.
    
           -vv    Even  more  verbose  output.  For example, additional fields are
                  printed from NFS  reply  packets,  and  SMB  packets  are  fully
                  decoded.
    
           -vvv   Even more verbose output.  For example, telnet SB ... SE options
                  are printed in full.  With -X Telnet options are printed in  hex
                  as well.
    
           -w     Write  the  raw packets to file rather than parsing and printing
                  them out.  They can later be printed with the -r option.   Stan-
                  dard  output is used if file is ''-''.  See pcap-savefile(5) for
                  a description of the file format.
    
           -W     Used in conjunction with the -C option, this will limit the num-
                  ber  of  files  created to the specified number, and begin over-
                  writing files from the beginning,  thus  creating  a  'rotating'
                  buffer.  In addition, it will name the files with enough leading
                  0s to support the maximum number of files, allowing them to sort
                  correctly.
    
                  Used in conjunction with the -G option, this will limit the num-
                  ber 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
                  datalinktype.
    
           -z     Used in conjunction with the -C or -G options,  this  will  make
           -Z     Drops  privileges  (if root) and changes user ID to user and the
                  group ID to the primary group of user.
    
                  This behavior is enabled by default (-Z  tcpdump),  and  can  be
                  disabled by -Z root.
    
            expression
                  selects  which  packets  will  be  dumped.   If no expression is
                  given, all packets on the net will be dumped.   Otherwise,  only
                  packets for which expression is 'true' will be dumped.
    
                  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.
    
    
    

    EXAMPLES

           To print all packets arriving at or departing from sundown:
                  tcpdump host sundown
    
           To print traffic between helios and either hot or ace:
                  tcpdump host helios and \( hot or ace \)
    
           To print all IP packets between ace and any host except helios:
                  tcpdump ip host ace and not helios
    
           To print all traffic between local hosts and hosts at Berkeley:
                  tcpdump net ucb-ether
    
           To  print all ftp traffic through internet gateway snup: (note that the
           expression is quoted to prevent the shell from  (mis-)interpreting  the
           parentheses):
                  tcpdump 'gateway snup and (port ftp or ftp-data)'
    
           To  print traffic neither sourced from nor destined for local hosts (if
           you gateway to one other net, this stuff should never make it onto your
           local net).
                  tcpdump ip and not net localnet
    
           To  print  the  start and end packets (the SYN and FIN packets) of each
           TCP conversation that involves a non-local host.
                  tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
    
           To print 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)'
    
    
           Link Level Headers
    
           If  the '-e' option is given, the link level header is printed out.  On
           Ethernets, the source and destination addresses, protocol,  and  packet
           length are printed.
    
           On  FDDI  networks, the  '-e' option causes tcpdump to print the 'frame
           control' field,  the source and destination addresses, and  the  packet
           length.   (The  'frame control' field governs the interpretation of the
           rest of the packet.  Normal packets (such as those containing IP  data-
           grams)  are 'async' packets, with a priority value between 0 and 7; for
           example, 'async4'.  Such packets are assumed to contain an 802.2  Logi-
           cal  Link  Control (LLC) packet; the LLC header is printed if it is not
           an ISO datagram or a so-called SNAP packet.
    
           On Token Ring networks, the '-e' option causes  tcpdump  to  print  the
           'access control' and 'frame control' fields, the source and destination
           addresses, and the packet length.  As on  FDDI  networks,  packets  are
           assumed  to  contain  an  LLC  packet.   Regardless of whether the '-e'
           option is specified or not, the source routing information  is  printed
           for source-routed packets.
    
           On  802.11 networks, the '-e' option causes tcpdump to print the 'frame
           control' fields, all of the addresses in the  802.11  header,  and  the
           packet  length.  As on FDDI networks, packets are assumed to contain an
           LLC packet.
    
           (N.B.: The following description assumes familiarity with the SLIP com-
           pression algorithm described in RFC-1144.)
    
           On SLIP links, a direction indicator (''I'' for inbound, ''O'' for out-
           bound), packet type, and compression information are printed out.   The
           packet  type is printed first.  The three types are ip, utcp, and ctcp.
           No further link information is printed for ip packets.  For  TCP  pack-
           ets,  the  connection identifier is printed following the type.  If the
           packet is compressed, its encoded header is printed out.   The  special
           cases are printed out as *S+n and *SA+n, where n is the amount by which
           the sequence number (or sequence number and ack) has changed.  If it is
           not  a  special  case,  zero  or more changes are printed.  A change is
           indicated by U (urgent pointer), W (window), A (ack), S (sequence  num-
           ber), and I (packet ID), followed by a delta (+n or -n), or a new value
           (=n).  Finally, the amount of data in the packet and compressed  header
           length are printed.
    
           For  example,  the  following  line  shows  an  outbound compressed TCP
           packet, with an implicit connection identifier; the ack has changed  by
           6, the sequence number by 49, and the packet ID by 6; there are 3 bytes
           of data and 6 bytes of compressed header:
                  O ctcp * A+6 S+49 I+6 3 (6)
    
           ARP/RARP Packets
    
           If  we had done tcpdump -e, the fact that the first packet is broadcast
           and the second is point-to-point would be visible:
                  RTSG Broadcast 0806  64: arp who-has csam tell rtsg
                  CSAM RTSG 0806  64: arp reply csam is-at CSAM
           For the first packet this says the Ethernet source address is RTSG, the
           destination is the Ethernet broadcast address, the type field contained
           hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
    
           TCP Packets
    
           (N.B.:The following description assumes familiarity with the TCP proto-
           col  described  in RFC-793.  If you are not familiar with the protocol,
           neither this description nor tcpdump will be of much use to you.)
    
           The general format of a tcp protocol line is:
                  src > dst: flags data-seqno ack window urgent options
           Src and dst are the source and  destination  IP  addresses  and  ports.
           Flags  are  some  combination of S (SYN), F (FIN), P (PUSH), R (RST), W
           (ECN CWR) or E (ECN-Echo), or a  single  '.'  (no  flags).   Data-seqno
           describes  the  portion  of  sequence space covered by the data in this
           packet (see example below).  Ack is sequence number of  the  next  data
           expected  the other direction on this connection.  Window is the number
           of bytes of receive buffer space available the other direction on  this
           connection.   Urg  indicates  there  is  'urgent'  data  in the packet.
           Options are tcp options enclosed in angle brackets (e.g., <mss  1024>).
    
           Src,  dst and flags are always present.  The other fields depend on the
           contents of the packet's tcp protocol header and  are  output  only  if
           appropriate.
    
           Here is the opening portion of an rlogin from host rtsg to host csam.
                  rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
                  csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
                  rtsg.1023 > csam.login: . ack 1 win 4096
                  rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
                  csam.login > rtsg.1023: . ack 2 win 4096
                  rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
                  csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
                  csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
                  csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
           The  first  line  says that tcp port 1023 on rtsg sent a packet to port
           login on csam.  The S indicates that the SYN flag was set.  The  packet
           sequence  number was 768512 and it contained no data.  (The notation is
           'first:last(nbytes)' which means 'sequence numbers first up to but  not
           including  last  which  is  nbytes  bytes of user data'.)  There was no
           piggy-backed ack, the available receive window was 4096 bytes and there
           was a max-segment-size option requesting an mss of 1024 bytes.
    
           Csam  replies  with  a similar packet except it includes a piggy-backed
           ack for rtsg's SYN.  Rtsg then acks csam's SYN.  The '.' means no flags
           were  set.   The  packet contained no data so there is no data sequence
           ting in the socket buffer since csam's receive  window  has  gotten  19
           bytes  smaller.   Csam  also  sends  one  byte  of data to rtsg in this
           packet.  On the 8th and 9th lines, csam  sends  two  bytes  of  urgent,
           pushed data to rtsg.
    
           If  the  snapshot was small enough that tcpdump didn't capture the full
           TCP header, it interprets as much of the header  as  it  can  and  then
           reports  ''[|tcp]'' to indicate the remainder could not be interpreted.
           If the header contains a bogus option (one with a length that's  either
           too  small  or  beyond  the  end  of the header), tcpdump reports it as
           ''[bad opt]'' and does not interpret any further  options  (since  it's
           impossible  to  tell where they start).  If the header length indicates
           options are present but the IP datagram length is not long  enough  for
           the  options  to  actually  be  there, tcpdump reports it as ''[bad hdr
           length]''.
    
           Capturing TCP packets with particular flag combinations (SYN-ACK,  URG-
           ACK, etc.)
    
           There are 8 bits in the control bits section of the TCP header:
    
                  CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
    
           Let's  assume  that we want to watch packets used in establishing a TCP
           connection.  Recall that TCP uses a 3-way handshake  protocol  when  it
           initializes  a  new  connection; the connection sequence with regard to
           the TCP control bits is
    
                  1) Caller sends SYN
                  2) Recipient responds with SYN, ACK
                  3) Caller sends ACK
    
           Now we're interested in capturing packets that have only  the  SYN  bit
           set  (Step  1).  Note that we don't want packets from step 2 (SYN-ACK),
           just a plain initial SYN.  What we need is a correct filter  expression
           for tcpdump.
    
           Recall the structure of a TCP header without options:
    
            0                            15                              31
           -----------------------------------------------------------------
           |          source port          |       destination port        |
           -----------------------------------------------------------------
           |                        sequence number                        |
           -----------------------------------------------------------------
           |                     acknowledgment number                     |
           -----------------------------------------------------------------
           |  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
           -----------------------------------------------------------------
           |         TCP checksum          |       urgent pointer          |
           -----------------------------------------------------------------
    
    
                           |               |
                           |---------------|
                           |C|E|U|A|P|R|S|F|
                           |---------------|
                           |7   5   3     0|
    
           These are the TCP control bits we are interested in.  We have  numbered
           the  bits  in  this octet from 0 to 7, right to left, so the PSH bit is
           bit number 3, while the URG bit is number 5.
    
           Recall that we want to capture packets with only SYN  set.   Let's  see
           what happens to octet 13 if a TCP datagram arrives with the SYN bit set
           in its header:
    
                           |C|E|U|A|P|R|S|F|
                           |---------------|
                           |0 0 0 0 0 0 1 0|
                           |---------------|
                           |7 6 5 4 3 2 1 0|
    
           Looking at the control bits section we see that only bit number 1 (SYN)
           is set.
    
           Assuming  that  octet number 13 is an 8-bit unsigned integer in network
           byte order, the binary value of this octet is
    
                  00000010
    
           and its decimal representation is
    
              7     6     5     4     3     2     1     0
           0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2  =  2
    
           We're almost done, because now we know that if only  SYN  is  set,  the
           value  of the 13th octet in the TCP header, when interpreted as a 8-bit
           unsigned integer in network byte order, must be exactly 2.
    
           This relationship can be expressed as
                  tcp[13] == 2
    
           We can use this expression as the filter for tcpdump in order to  watch
           packets which have only SYN set:
                  tcpdump -i xl0 tcp[13] == 2
    
           The expression says "let the 13th octet of a TCP datagram have the dec-
           imal value 2", which is exactly what we want.
    
           Now, let's assume that we need to capture SYN  packets,  but  we  don't
           care  if  ACK  or  any  other  TCP control bit is set at the same time.
           Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set
           arrives:
              7     6     5     4     3     2     1     0
           0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2   = 18
    
           Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression,
           because that would select only those packets that have SYN-ACK set, but
           not those with only SYN set.  Remember that we don't care if ACK or any
           other control bit is set as long as SYN is set.
    
           In order to achieve our goal, we need to logically AND the binary value
           of octet 13 with some other value to preserve the  SYN  bit.   We  know
           that  we  want  SYN  to  be set in any case, so we'll logically AND the
           value in the 13th octet with the binary value of a SYN:
    
                     00010010 SYN-ACK              00000010 SYN
                AND  00000010 (we want SYN)   AND  00000010 (we want SYN)
                     --------                      --------
                =    00000010                 =    00000010
    
           We see that this AND operation  delivers  the  same  result  regardless
           whether ACK or another TCP control bit is set.  The decimal representa-
           tion of the AND value as well as the result  of  this  operation  is  2
           (binary 00000010), so we know that for packets with SYN set the follow-
           ing relation must hold true:
    
                  ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
    
           This points us to the tcpdump filter expression
                       tcpdump -i xl0 'tcp[13] & 2 == 2'
    
           Note that you should use single quotes or a backslash in the expression
           to hide the AND ('&') special character from the shell.
    
           UDP Packets
    
           UDP format is illustrated by this rwho packet:
                  actinide.who > broadcast.who: udp 84
           This  says  that  port who on host actinide sent a udp datagram to port
           who on host broadcast, the Internet broadcast address.  The packet con-
           tained 84 bytes of user data.
    
           Some  UDP  services are recognized (from the source or destination port
           number) and the higher level protocol information printed.  In particu-
           lar,  Domain  Name  service  requests (RFC-1034/1035) and Sun RPC calls
           (RFC-1050) to NFS.
    
           UDP Name Server Requests
    
           (N.B.:The following description assumes  familiarity  with  the  Domain
           Service  protocol  described in RFC-1035.  If you are not familiar with
           the protocol, the following description will appear to  be  written  in
           greek.)
           A  few anomalies are checked and may result in extra fields enclosed in
           square brackets:  If a query contains an answer, authority  records  or
           additional records section, ancount, nscount, or arcount are printed as
           '[na]', '[nn]' or  '[nau]' where n is the appropriate count.  If any of
           the  response  bits  are  set  (AA, RA or rcode) or any of the 'must be
           zero' bits are set in bytes two and three, '[b2&3=x]' is printed, where
           x is the hex value of header bytes two and three.
    
           UDP Name Server Responses
    
           Name server responses are formatted as
                  src > dst:  id op rcode flags a/n/au type class data (len)
                  helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
                  helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
           In the first example, helios responds to query id 3 from h2opolo with 3
           answer records, 3 name server records and 7  additional  records.   The
           first  answer  record  is  type  A  (address)  and its data is internet
           address 128.32.137.3.  The total size of the response  was  273  bytes,
           excluding  UDP and IP headers.  The op (Query) and response code (NoEr-
           ror) were omitted, as was the class (C_IN) of the A record.
    
           In the second example, helios responds to query 2 with a response  code
           of  non-existent domain (NXDomain) with no answers, one name server and
           no authority records.  The '*' indicates that the authoritative  answer
           bit  was set.  Since there were no answers, no type, class or data were
           printed.
    
           Other flag characters that might appear are '-'  (recursion  available,
           RA,  not  set) and '|' (truncated message, TC, set).  If the 'question'
           section doesn't contain exactly one entry, '[nq]' is printed.
    
           SMB/CIFS decoding
    
           tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on
           UDP/137,  UDP/138 and TCP/139.  Some primitive decoding of IPX and Net-
           BEUI SMB data is also done.
    
           By default a fairly minimal decode is done, with a much  more  detailed
           decode  done if -v is used.  Be warned that with -v a single SMB packet
           may take up a page or more, so only use -v if you really want  all  the
           gory details.
    
           For  information  on SMB packet formats and what all te fields mean see
           www.cifs.org  or  the  pub/samba/specs/  directory  on  your   favorite
           samba.org mirror site.  The SMB patches were written by Andrew Tridgell
           (tridge@samba.org).
    
           NFS Requests and Replies
    
           Sun NFS (Network File System) requests and replies are printed as:
           file  handle  can  be  interpreted as a major,minor device number pair,
           followed by the inode number and generation number.)  Wrl replies  'ok'
           with the contents of the link.
    
           In  the  third  line,  sushi  asks  wrl to lookup the name 'xcolors' in
           directory file 9,74/4096.6878.  Note that the data printed  depends  on
           the  operation  type.  The format is intended to be self explanatory if
           read in conjunction with an NFS protocol spec.
    
           If the -v (verbose) flag is given, additional information  is  printed.
           For example:
    
                  sushi.1372a > wrl.nfs:
                       148 read fh 21,11/12.195 8192 bytes @ 24576
                  wrl.nfs > sushi.1372a:
                       reply ok 1472 read REG 100664 ids 417/0 sz 29388
           (-v  also  prints  the  IP  header  TTL,  ID, length, and fragmentation
           fields, which have been omitted from this example.)  In the first line,
           sushi  asks wrl to read 8192 bytes from file 21,11/12.195, at byte off-
           set 24576.  Wrl replies 'ok'; the packet shown on the  second  line  is
           the first fragment of the reply, and hence is only 1472 bytes long (the
           other bytes will follow in subsequent fragments, but these fragments do
           not have NFS or even UDP headers and so might not be printed, depending
           on the filter expression used).  Because the -v flag is given, some  of
           the  file  attributes (which are returned in addition to the file data)
           are printed: the file type (''REG'', for regular file), the  file  mode
           (in octal), the uid and gid, and the file size.
    
           If  the -v flag is given more than once, even more details are printed.
    
           Note that NFS requests are very large and much of the detail  won't  be
           printed  unless  snaplen is increased.  Try using '-s 192' to watch NFS
           traffic.
    
           NFS reply  packets  do  not  explicitly  identify  the  RPC  operation.
           Instead,  tcpdump  keeps track of ''recent'' requests, and matches them
           to the replies using the transaction ID.  If a reply does  not  closely
           follow the corresponding request, it might not be parsable.
    
           AFS Requests and Replies
    
           Transarc AFS (Andrew File System) requests and replies are printed as:
    
                  src.sport > dst.dport: rx packet-type
                  src.sport > dst.dport: rx packet-type service call call-name args
                  src.sport > dst.dport: rx packet-type service reply call-name args
    
                  elvis.7001 > pike.afsfs:
                       rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                       new fid 536876964/1/1 ".newsrc"
                  pike.afsfs > elvis.7001: rx data fs reply rename
           In the first line, host elvis sends a RX packet to pike.  This was a RX
    
           If  the  -v  (verbose) flag is given twice, acknowledgement packets and
           additional header information is printed, such as the the RX  call  ID,
           call number, sequence number, serial number, and the RX packet flags.
    
           If  the -v flag is given twice, additional information is printed, such
           as the the RX call ID, serial number, and the RX packet flags.  The MTU
           negotiation information is also printed from RX ack packets.
    
           If  the -v flag is given three times, the security index and service id
           are printed.
    
           Error codes are printed for abort packets, with the exception  of  Ubik
           beacon  packets  (because  abort packets are used to signify a yes vote
           for the Ubik protocol).
    
           Note that AFS requests are very large and many of the  arguments  won't
           be  printed  unless  snaplen is increased.  Try using '-s 256' to watch
           AFS traffic.
    
           AFS reply  packets  do  not  explicitly  identify  the  RPC  operation.
           Instead,  tcpdump  keeps track of ''recent'' requests, and matches them
           to the replies using the call number and service ID.  If a  reply  does
           not closely follow the corresponding request, it might not be parsable.
    
           KIP AppleTalk (DDP in UDP)
    
           AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
           and dumped as DDP packets (i.e., all the UDP header information is dis-
           carded).  The file /etc/atalk.names is used to translate AppleTalk  net
           and node numbers to names.  Lines in this file have the form
                  number    name
    
                  1.254          ether
                  16.1      icsd-net
                  1.254.110 ace
           The  first  two  lines give the names of AppleTalk networks.  The third
           line gives the name of a particular host (a host is distinguished  from
           a  net  by  the  3rd  octet  in the number - a net number must have two
           octets and a host number must have three octets.)  The number and  name
           should   be   separated   by   whitespace   (blanks   or   tabs).   The
           /etc/atalk.names file may contain blank lines or comment  lines  (lines
           starting with a '#').
    
           AppleTalk addresses are printed in the form
                  net.host.port
    
                  144.1.209.2 > icsd-net.112.220
                  office.2 > icsd-net.112.220
                  jssmag.149.235 > icsd-net.2
           (If  the /etc/atalk.names doesn't exist or doesn't contain an entry for
    
           NBP packets are formatted like the following examples:
                  icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
                  jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
                  techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
           The  first  line  is a name lookup request for laserwriters sent by net
           icsd host 112 and broadcast on net jssmag.  The nbp id for  the  lookup
           is  190.   The second line shows a reply for this request (note that it
           has the same id) from host jssmag.209 saying that it has a  laserwriter
           resource  named  "RM1140"  registered  on  port 250.  The third line is
           another reply to the same request saying host techpit  has  laserwriter
           "techpit" registered on port 186.
    
           ATP packet formatting is demonstrated by the following example:
                  jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
                  helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
                  jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
                  helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
                  helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
                  jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
                  jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
           Jssmag.209  initiates transaction id 12266 with host helios by request-
           ing up to 8 packets (the '<0-7>').  The hex number at the  end  of  the
           line is the value of the 'userdata' field in the request.
    
           Helios  responds  with  8 512-byte packets.  The ':digit' following the
           transaction id gives the packet sequence number in the transaction  and
           the number in parens is the amount of data in the packet, excluding the
           atp header.  The '*' on packet 7 indicates that the EOM bit was set.
    
           Jssmag.209 then requests that packets 3 & 5 be  retransmitted.   Helios
           resends  them  then jssmag.209 releases the transaction.  Finally, jss-
           mag.209 initiates the next request.  The '*' on the  request  indicates
           that XO ('exactly once') was not set.
    
           IP Fragmentation
    
           Fragmented Internet datagrams are printed as
                  (frag id:size@offset+)
                  (frag id:size@offset)
           (The  first  form indicates there are more fragments.  The second indi-
           cates this is the last fragment.)
    
           Id is the fragment id.  Size is the fragment size (in bytes)  excluding
           line  don't  include  port  numbers.   This is because the TCP protocol
           information is all in the first fragment and we have no idea  what  the
           port  or  sequence numbers are when we print the later fragments.  Sec-
           ond, the tcp sequence information in the first line is  printed  as  if
           there  were  308  bytes of user data when, in fact, there are 512 bytes
           (308 in the first frag and 204 in the second).  If you are looking  for
           holes  in  the  sequence space or trying to match up acks with packets,
           this can fool you.
    
           A packet with the IP don't fragment flag  is  marked  with  a  trailing
           (DF).
    
           Timestamps
    
           By  default,  all output lines are preceded by a timestamp.  The times-
           tamp is the current clock time in the form
                  hh:mm:ss.frac
           and is as accurate as the kernel's clock.  The timestamp  reflects  the
           time  the  kernel  first saw the packet.  No attempt is made to account
           for the time lag between when the Ethernet interface removed the packet
           from  the wire and when the kernel serviced the 'new packet' interrupt.
    
    
    

    SEE ALSO

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

    AUTHORS

           The original authors are:
    
           Van  Jacobson,  Craig  Leres  and  Steven  McCanne, all of the Lawrence
           Berkeley National Laboratory, University of California, Berkeley, CA.
    
           It is currently being maintained by tcpdump.org.
    
           The current version is available via http:
    
                  http://www.tcpdump.org/
    
           The original distribution is available via anonymous ftp:
    
                  ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
    
           IPv6/IPsec support is added by WIDE/KAME project.   This  program  uses
           Eric Young's SSLeay library, under specific configurations.
    
    
    

    BUGS

           Please  send problems, bugs, questions, desirable enhancements, patches
           etc. to:
    
                  tcpdump-workers@lists.tcpdump.org
    
           NIT doesn't let you watch your own outbound traffic, BPF will.  We rec-
           ommend that you use the latter.
    
                  capturing on some PPP devices won't work correctly.
    
           We recommend that you upgrade to a 2.2 or later kernel.
    
           Some  attempt should be made to reassemble IP fragments or, at least to
           compute the right length for the higher level protocol.
    
           Name server inverse queries are not dumped correctly: the (empty) ques-
           tion  section  is printed rather than real query in the answer section.
           Some believe that inverse queries are themselves a bug  and  prefer  to
           fix the program generating them rather than tcpdump.
    
           A  packet  trace  that crosses a daylight savings time change will give
           skewed time stamps (the time change is ignored).
    
           Filter expressions on fields other than those  in  Token  Ring  headers
           will not correctly handle source-routed Token Ring packets.
    
           Filter  expressions  on  fields other than those in 802.11 headers will
           not correctly handle 802.11 data packets with both To DS  and  From  DS
           set.
    
           ip6  proto  should  chase header chain, but at this moment it does not.
           ip6 protochain is supplied for this behavior.
    
           Arithmetic expression against transport  layer  headers,  like  tcp[0],
           does not work against IPv6 packets.  It only looks at IPv4 packets.
    
                                     05 March 2009                      TCPDUMP(8)
    
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