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NetBSD 6.1.5 - man page for tcpdump (netbsd section 8)

TCPDUMP(8)									       TCPDUMP(8)

       tcpdump - dump traffic on a network

       tcpdump [ -AbdDefIKlLnNOpqRStuUvxX ] [ -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 inter-
       rupted by a SIGINT signal (generated, for example, by  typing  your  interrupt  character,
       typically  control-C)  or a SIGTERM signal (typically generated with the kill(1) command);
       if run with the -c flag, it will capture packets until it is interrupted by  a  SIGINT  or
       SIGTERM signal or the specified number of packets have been processed.

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

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

	      packets ``received by filter'' (the meaning of this depends  on  the  OS	on  which
	      you're running tcpdump, and possibly on the way the OS was configured - if a filter
	      was specified on the command line, on some OSes it  counts  packets  regardless  of
	      whether  they  were matched by the filter expression and, even if they were matched
	      by the filter expression, regardless of whether tcpdump has read and processed them
	      yet,  on	other OSes it counts only packets that were matched by the filter expres-
	      sion 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 tcp-
	      dump 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  (gener-
       ated, for example, by typing your ``status'' character, typically control-T) and will con-
       tinue capturing packets.

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

       -A     Print each packet (minus its link level header) in ASCII.  Handy for capturing  web
	      pages.  -a Attempt to convert network and broadcast addresses to names.

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

       -B     Set the operating system capture buffer size to buffer_size.

       -c     Exit after receiving count packets.

       -C     Before  writing  a  raw  packet  to a savefile, check whether the file is currently
	      larger than file_size and, if so, close the current savefile and open  a	new  one.
	      Savefiles  after	the first savefile will have the name specified with the -w flag,
	      with a number after it,  starting  at  1	and  continuing  upward.   The	units  of
	      file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

       -d     Dump  the compiled packet-matching code in a human readable form to standard output
	      and stop.

       -dd    Dump packet-matching code as a C program fragment.

       -ddd   Dump packet-matching code as decimal numbers (preceded with a count).

       -D     Print the list of the network interfaces available on the system and on which  tcp-
	      dump  can  capture  packets.  For each network interface, a number and an interface
	      name, possibly followed by a text description of the interface,  is  printed.   The
	      interface name or the number can be supplied to the -i flag to specify an interface
	      on which to capture.

	      This can be useful on systems that don't have a command to list them (e.g., Windows
	      systems,	or UNIX systems lacking ifconfig -a); the number can be useful on Windows
	      2000 and later systems, where the interface name is a somewhat complex string.

	      The -D flag will not be supported if tcpdump was built with  an  older  version  of
	      libpcap that lacks the pcap_findalldevs() function.

       -e     Print the link-level header on each dump line.

       -E     Use  spi@ipaddr  algo:secret for decrypting IPsec ESP packets that are addressed to
	      addr and contain Security Parameter  Index  value  spi.  This  combination  may  be
	      repeated with comma or newline seperation.

	      Note that setting the secret for IPv4 ESP packets is supported at this time.

	      Algorithms  may  be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none.
	      The default is des-cbc.  The ability to decrypt packets is only present if  tcpdump
	      was compiled with cryptography enabled.

	      secret  is  the ASCII text for ESP secret key.  If 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, 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" inter-
	      face, which can capture on more than one interface, this option will not work  cor-

       -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_sec-
	      onds  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

       -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.

	      If the -D flag is supported, an interface number as printed by  that  flag  can  be
	      used as the interface argument.

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

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

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

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

       -l     Make stdout line buffered.  Useful if you want to see the data while capturing  it.
	      ``tcpdump  -l  |	tee dat'' or ``tcpdump	-l   > dat  &  tail  -f  dat''.

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

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

       -M     Use secret as a shared secret for validating the digests found in TCP segments with
	      the TCP-MD5 option (RFC 2385), if present.

       -n     Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.

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

       -O     Do  not run the packet-matching code optimizer.  This is useful only if you suspect
	      a bug in the optimizer.

       -p     Don't put the interface into promiscuous mode.  Note that the interface might be in
	      promiscuous  mode for some other reason; hence, `-p' cannot be used as an abbrevia-
	      tion for `ether host {local-hw-addr} or ether broadcast'.

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

       -R     Assume  ESP/AH  packets  to be based on old specification (RFC1825 to RFC1829).  If
	      specified, tcpdump will not print replay prevention field.  Since there is no  pro-
	      tocol  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 trun-
	      cation has occurred.  Note that taking larger snapshots both increases  the  amount
	      of  time	it  takes  to  process	packets and, effectively, decreases the amount of
	      packet buffering.  This may cause packets to be lost.  You should limit snaplen  to
	      the  smallest  number  that will capture the protocol information you're interested
	      in.  Setting snaplen to 0 sets it to the default of 65535, for  backwards  compati-
	      bility with recent older versions of tcpdump.

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

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

       -tt    Print an unformatted timestamp on each dump line.

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

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

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

       -u     Print undecoded NFS handles.

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

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

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

	      When writing to a file with the -w option, report, every 10 seconds, the number  of
	      packets captured.

       -vv    Even  more  verbose  output.   For  example, additional fields are printed from NFS
	      reply packets, and SMB packets are fully decoded.

       -vvv   Even more verbose output.  For example, telnet SB ... SE	options  are  printed  in
	      full.  With -X Telnet options are printed in hex as well.

       -w     Write  the raw packets to file rather than parsing and printing them out.  They can
	      later be printed with the -r option.  Standard output is used  if  file  is  ``-''.
	      See pcap-savefile(5) for a description of the file format.

       -W     Used in conjunction with the -C option, this will limit the number of files created
	      to the specified number, and begin overwriting files from the beginning, thus  cre-
	      ating a 'rotating' buffer.  In addition, it will name the files with enough leading
	      0s to support the maximum number of files, allowing them to sort correctly.

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

       -x     When parsing and printing, in addition to printing  the  headers	of  each  packet,
	      print the data of each packet (minus its link level header) in hex.  The smaller of
	      the entire packet or snaplen bytes will be printed.  Note that this is  the  entire
	      link-layer  packet,  so for link layers that pad (e.g. Ethernet), the padding bytes
	      will also be printed when the higher layer packet is shorter than the required pad-

       -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.  The avail-
	      able data link types may be found using the -L option.

       -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     By default, tcpdump operates in NetBSD under the privileges  of  the  user  ``_tcp-
	      dump''.	Before	the  user  ID and the corresponding primary group ID are changed,
	      tcpdump will change the root directory to /var/chroot/tcpdump.  By using the option
	      -Z the real and effective user and group IDs can be changed to ``user'' instead.

	      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 argu-
	      ment.  Multiple arguments are concatenated with spaces before being parsed.

       To print all packets arriving at or departing from sundown:
	      tcpdump host sundown

       To print traffic between helios and either hot or ace:
	      tcpdump host helios and \( hot or ace \)

       To print all IP packets between ace and any host except helios:
	      tcpdump ip host ace and not helios

       To print all traffic between local hosts and hosts at Berkeley:
	      tcpdump net ucb-ether

       To print all ftp traffic through internet gateway  snup:  (note	that  the  expression  is
       quoted to prevent the shell from (mis-)interpreting the parentheses):
	      tcpdump 'gateway snup and (port ftp or ftp-data)'

       To  print traffic neither sourced from nor destined for local hosts (if you gateway to one
       other net, this stuff should never make it onto your local net).
	      tcpdump ip and not net localnet

       To print the start and end packets (the SYN and FIN packets) of each TCP conversation that
       involves a non-local host.
	      tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

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

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

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

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

       The output of tcpdump is protocol dependent.  The following gives a brief description  and
       examples of most of the formats.

       Link Level Headers

       If  the	'-e'  option  is  given, the link level header is printed out.	On Ethernets, the
       source and destination addresses, protocol, and packet length are printed.

       On FDDI networks, the  '-e' option causes tcpdump to print the `frame control' field,  the
       source  and destination addresses, and the packet length.  (The `frame control' field gov-
       erns the interpretation of the rest of the packet.  Normal packets (such as those contain-
       ing 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 inter-
       net host csam.  Csam  replies  with  its  Ethernet  address  (in  this  example,  Ethernet
       addresses are in caps and internet addresses in lower case).

       This would look less redundant if we had done tcpdump -n:
	      arp who-has tell
	      arp reply is-at 02:07:01:00:01:c4

       If  we  had done tcpdump -e, the fact that the first packet is broadcast and the second is
       point-to-point would be visible:
	      RTSG Broadcast 0806  64: arp who-has csam tell rtsg
	      CSAM RTSG 0806  64: arp reply csam is-at CSAM
       For the first packet this says the Ethernet source address is RTSG, the destination is the
       Ethernet  broadcast  address,  the  type field contained hex 0806 (type ETHER_ARP) and the
       total length was 64 bytes.

       TCP Packets

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

       The general format of a tcp protocol line is:
	      src > dst: flags data-seqno ack window urgent options
       Src and dst are the source and destination IP addresses and ports.  Flags are some  combi-
       nation  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  con-
       tained 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 con-
       versation's  data  stream  (with the first data byte each direction being `1').	`-S' will
       override this feature, causing the original sequence numbers to be output.

       On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the rtsg ->  csam
       side  of  the  conversation).   The PUSH flag is set in the packet.  On the 7th line, csam
       says it's received data sent by rtsg up to but not including byte 21.  Most of  this  data
       is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes
       smaller.  Csam also sends one byte of data to rtsg in this packet.  On  the  8th  and  9th
       lines, csam sends two bytes of urgent, pushed data to rtsg.

       If  the	snapshot  was  small  enough  that tcpdump didn't capture the full TCP header, it
       interprets as much of the header as it can and then reports  ``[|tcp]''	to  indicate  the
       remainder  could  not  be  interpreted.	If the header contains a bogus option (one with a
       length that's either too small or beyond the end of the header),  tcpdump  reports  it  as
       ``[bad  opt]''  and  does not interpret any further options (since it's impossible to tell
       where they start).  If the header length indicates options are present but the IP datagram
       length  is  not	long  enough  for the options to actually be there, tcpdump reports it as
       ``[bad hdr length]''.

       Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)

       There are 8 bits in the control bits section of the TCP header:

	      CWR | ECE | URG | ACK | PSH | RST | SYN | FIN

       Let's assume that we want to watch packets used in establishing a TCP connection.   Recall
       that TCP uses a 3-way handshake protocol when it initializes a new connection; the connec-
       tion sequence with regard to the TCP control bits is

	      1) Caller sends SYN
	      2) Recipient responds with SYN, ACK
	      3) Caller sends ACK

       Now we're interested in capturing packets that have only the SYN bit set (Step  1).   Note
       that  we don't want packets from step 2 (SYN-ACK), just a plain initial SYN.  What we need
       is a correct filter expression for tcpdump.

       Recall the structure of a TCP header without options:

	0			     15 			     31
       |	  source port	       |       destination port        |
       |			sequence number 		       |
       |		     acknowledgment number		       |
       |  HL   | rsvd  |C|E|U|A|P|R|S|F|	window size	       |
       |	 TCP checksum	       |       urgent pointer	       |

       A TCP header usually holds 20 octets of data, unless options are present.  The first  line
       of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc.

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

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

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

		       |	       |
		       |7   5	3     0|

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

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

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

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

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


       and its decimal representation is

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

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

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

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

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

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

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

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


       which translates to decimal

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

       Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression, because that would
       select only those packets that have SYN-ACK set, but not those with only SYN set.   Remem-
       ber that we don't care if ACK or any other control bit is set as long as SYN is set.

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

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

       We  see that this AND operation delivers the same result regardless whether ACK or another
       TCP control bit is set.	The decimal representation of the AND value as well as the result
       of  this  operation  is	2 (binary 00000010), so we know that for packets with SYN set the
       following relation must hold true:

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

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

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

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

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

       UDP Packets

       UDP format is illustrated by this rwho packet:
	      actinide.who > broadcast.who: udp 84
       This  says  that  port who on host actinide sent a udp datagram to port who on host broad-
       cast, 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 descrip-
       tion will appear to be written in greek.)

       Name server requests are formatted as
	      src > dst: id op? flags qtype qclass name (len)
	      h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu.(37)
       Host h2opolo asked the domain server on helios for an address record (qtype=A)  associated
       with the name ucbvax.berkeley.edu.  The query id was `3'.  The `+' indicates the recursion
       desired flag was set.  The query length was 37 bytes, not including the UDP and IP  proto-
       col  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
	      helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0(97)
       In  the first example, helios responds to query id 3 from h2opolo with 3 answer records, 3
       name server records and 7 additional records.  The first answer record is type A (address)
       and  its  data  is  internet address  The total size of the response was 273
       bytes, excluding UDP and IP headers.  The op (Query)  and  response  code  (NoError)  were
       omitted, as was the class (C_IN) of the A record.

       In  the	second	example,  helios responds to query 2 with a response code of non-existent
       domain (NXDomain) with no answers, one name server and  no  authority  records.	 The  `*'
       indicates  that	the  authoritative  answer  bit was set.  Since there were no answers, no
       type, class or data were printed.

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

       SMB/CIFS decoding

       tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on	UDP/137,  UDP/138
       and TCP/139.  Some primitive decoding of IPX and NetBEUI SMB data is also done.

       By default a fairly minimal decode is done, with a much more detailed decode done if -v is
       used.  Be warned that with -v a single SMB packet may take up a page or more, so only  use
       -v if you really want all the gory details.

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

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
	      src.xid > dst.nfs: len op args
	      src.nfs > dst.xid: reply stat len op results
	      sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
	      wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
	      sushi.201b > wrl.nfs:
		   144 lookup fh 9,74/4096.6878 "xcolors"
	      wrl.nfs > sushi.201b:
		   reply ok 128 lookup fh 9,74/4134.3150
       In  the first line, host sushi sends a transaction with id 6709 to wrl (note that the num-
       ber 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 num-
       ber and generation number.)  Wrl replies `ok' with the contents of the link.

       In the third line, sushi  asks  wrl  to	lookup	the  name  `xcolors'  in  directory  file
       9,74/4096.6878.	 Note that the data printed depends on the operation type.  The format is
       intended to be self explanatory if read in conjunction with an NFS protocol spec.

       If the -v (verbose) flag is given, additional information is printed.  For example:
	      sushi.1372a > wrl.nfs:
		   148 read fh 21,11/12.195 8192 bytes @ 24576
	      wrl.nfs > sushi.1372a:
		   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v also prints the IP header TTL, ID, length, and fragmentation fields, which  have  been
       omitted	from  this  example.)	In the first line, sushi asks wrl to read 8192 bytes from
       file 21,11/12.195, at byte offset 24576.  Wrl replies `ok'; the packet shown on the second
       line  is  the  first  fragment  of the reply, and hence is only 1472 bytes long (the other
       bytes will follow in subsequent fragments, but these fragments do not have NFS or even UDP
       headers	and  so  might not be printed, depending on the filter expression used).  Because
       the -v flag is given, some of the file attributes (which are returned in addition  to  the
       file  data)  are  printed:  the	file  type (``REG'', for regular file), the file mode (in
       octal), the uid and gid, and the file size.

       If the -v flag is given more than once, even more details are printed.

       Note that NFS requests are very large and much of  the  detail  won't  be  printed  unless
       snaplen is increased.  Try using `-s 192' to watch NFS traffic.

       NFS  reply  packets  do not explicitly identify the RPC operation.  Instead, tcpdump keeps
       track of ``recent'' requests, and matches them to the replies using  the  transaction  ID.
       If a reply does not closely follow the corresponding request, it might not be parsable.

       AFS Requests and Replies

       Transarc AFS (Andrew File System) requests and replies are printed as:

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

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

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

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

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

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

       AppleTalk addresses are printed in the form
	      net.host.port > icsd-net.112.220
	      office.2 > icsd-net.112.220
	      jssmag.149.235 > icsd-net.2
       (If  the  /etc/atalk.names  doesn't  exist  or doesn't contain an entry for some AppleTalk
       host/net number, addresses are printed in numeric form.)  In the first example,	NBP  (DDP
       port  2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd
       node 112.  The second line is the same except the full name of the source  node	is  known
       (`office').  The third line is a send from port 235 on net jssmag node 149 to broadcast on
       the icsd-net NBP port (note that the broadcast address(255) is indicated by  a	net  name
       with  no  host  number - for this reason it's a good idea to keep node names and net names
       distinct in /etc/atalk.names).

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

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

       ATP packet formatting is demonstrated by the following example:
	      jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
	      helios.132 > jssmag.209.165: atp-resp 12266:0(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:1(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:2(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:3(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:4(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:5(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:6(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp*12266:7(512) 0xae040000
	      jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
	      helios.132 > jssmag.209.165: atp-resp 12266:3(512) 0xae040000
	      helios.132 > jssmag.209.165: atp-resp 12266:5(512) 0xae040000
	      jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
	      jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
       Jssmag.209  initiates  transaction id 12266 with host helios by requesting up to 8 packets
       (the `<0-7>').  The hex number at the end of the line is the value of the `userdata' field
       in the request.

       Helios  responds with 8 512-byte packets.  The `:digit' following the transaction id gives
       the packet sequence number in the transaction and the number in parens is  the  amount  of
       data  in the packet, excluding the atp header.  The `*' on packet 7 indicates that the EOM
       bit was set.

       Jssmag.209 then requests that packets 3 & 5 be retransmitted.  Helios  resends  them  then
       jssmag.209 releases the transaction.  Finally, jssmag.209 initiates the next request.  The
       `*' on the request indicates that XO (`exactly once') was not set.

       IP Fragmentation

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

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

       The fragment information is output for each fragment.  The  first  fragment  contains  the
       higher  level protocol header and the frag info is printed after the protocol info.  Frag-
       ments 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 ari-
       zona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear to handle	576  byte
	      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 frag-
       ments.  Second, the tcp sequence information in the first line is printed as if there were
       308  bytes  of user data when, in fact, there are 512 bytes (308 in the first frag and 204
       in the second).	If you are looking for holes in the sequence space or trying to match  up
       acks with packets, this can fool you.

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


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

       pcap(3), bpf(4), pcap-savefile(5), pcap-filter(7)

       The original authors are:

       Van  Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Labo-
       ratory, University of California, Berkeley, CA.

       It is currently being maintained by tcpdump.org.

       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:


       Some  attempt  should be made to reassemble IP fragments or, at least to compute the right
       length for the higher level protocol.

       Name server inverse queries are not dumped correctly:  the  (empty)  question  section  is
       printed	rather	than real query in the answer section.	Some believe that inverse queries
       are themselves a bug and prefer to fix the program generating them rather than tcpdump.

       A packet trace that crosses a daylight savings time change will give  skewed  time  stamps
       (the time change is ignored).

       Filter  expressions  on	fields	other than those in Token Ring headers will not correctly
       handle source-routed Token Ring packets.

       Filter expressions on fields other than those in 802.11 headers will not correctly  handle
       802.11 data packets with both To DS and From DS set.

       ip6  proto  should  chase header chain, but at this moment it does not.	ip6 protochain is
       supplied for this behavior.

       Arithmetic expression against transport layer headers, like tcp[0], does not work  against
       IPv6 packets.  It only looks at IPv4 packets.

					 17 December 2010			       TCPDUMP(8)

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