👤
Home Man
Search
Today's Posts
Register

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

Linux 2.6 - man page for tcpdump (linux section 8)

TCPDUMP(8)									       TCPDUMP(8)

NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       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 ]

DESCRIPTION
       Tcpdump	prints	out  a description of the contents of packets on a network interface that
       match the boolean expression.  It can also be run with the -w flag,  which  causes  it  to
       save  the  packet data to a file for later analysis, and/or with the -r flag, which causes
       it to read from a saved packet file rather than to read packets from a  network	interface
       (please	note  tcpdump  is  protected via an enforcing apparmor(7) profile in Ubuntu which
       limits the files tcpdump may access).  In all cases, only packets  that	match  expression
       will be processed by tcpdump.

       Tcpdump	will,  if not run with the -c flag, continue capturing packets until it is 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
	      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 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, although on
       some platforms, such as Mac OS X, the ``status'' character is not set by default,  so  you
       must set it with stty(1) in order to use it) and will continue capturing packets.

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

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

       -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 separation.

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

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

	      secret is the ASCII text for ESP secret key.  If preceded by 0x, then a  hex  value
	      will be read.

	      The  option assumes RFC2406 ESP, not RFC1827 ESP.  The option is only for debugging
	      purposes, and the use of this option with a true `secret' key is	discouraged.   By
	      presenting  IPsec  secret  key onto command line you make it visible to others, via
	      ps(1) and other occasions.

	      In addition to the above syntax, the syntax file name may be used to  have  tcpdump
	      read  the provided file in. The file is opened upon receiving the first ESP packet,
	      so any special permissions that tcpdump may have been  given  should  already  have
	      been given up.

       -f     Print `foreign' IPv4 addresses numerically rather than symbolically (this option is
	      intended to get around serious brain damage in Sun's NIS server -- usually it hangs
	      forever translating non-local internet numbers).

	      The test for `foreign' IPv4 addresses is done using the IPv4 address and netmask of
	      the interface on which capture is being done.  If that address or netmask  are  not
	      available,  available,  either because the interface on which capture is being done
	      has no address or netmask or because the capture is being done on the  Linux  "any"
	      interface,  which can capture on more than one interface, this option will not work
	      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_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
	      `file<count>'.

       -i     Listen  on  interface.   If unspecified, tcpdump searches the system interface list
	      for the lowest numbered, configured up interface (excluding  loopback).	Ties  are
	      broken by choosing the earliest match.

	      On Linux systems with 2.2 or later kernels, an interface argument of ``any'' can be
	      used to capture packets from all interfaces.  Note that  captures  on  the  ``any''
	      device will not be done in promiscuous mode.

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

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

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

	      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.
	      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 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
	      shorter.

       -R     Assume ESP/AH packets to be based on old specification (RFC1825  to  RFC1829).   If
	      specified,  tcpdump will not print replay prevention field.  Since there is no 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
	      line.

       -u     Print undecoded NFS handles.

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

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

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

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

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

       -y     Set the data link type to use while capturing packets to datalinktype.

       -z     Used in conjunction with the -C or -G options, this will make tcpdump run " command
	      file  "  where  file is the savefile being closed after each rotation. For example,
	      specifying -z gzip or -z bzip2 will compress each savefile using gzip or bzip2.

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

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

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

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

	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 argu-
	      ment.  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)'

       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
       multicast:
	      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'

OUTPUT FORMAT
       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 128.3.254.6 tell 128.3.254.68
	      arp reply 128.3.254.6 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), U (URG), W (ECN CWR), E (ECN-Echo) or `.'
       (ACK), or `none' if no flags are set.  Data-seqno describes the portion of sequence  space
       covered	by  the  data  in this packet (see example below).  Ack is sequence number of the
       next data expected the other direction on this connection.  Window is the number of  bytes
       of  receive  buffer space available the other direction on this connection.  Urg indicates
       there is `urgent' data in the packet.  Options are tcp options enclosed in angle  brackets
       (e.g., <mss 1024>).

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

       Here is the opening portion of an rlogin from host rtsg to host csam.
	      rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
	      csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
	      rtsg.1023 > csam.login: . ack 1 win 4096
	      rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
	      csam.login > rtsg.1023: . ack 2 win 4096
	      rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
	      csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
	      csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
	      csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
       The first line says that tcp port 1023 on rtsg sent a packet to port login on csam.  The S
       indicates  that	the  SYN flag was set.	The packet sequence number was 768512 and it 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 the ACK flag was set.	The packet  contained  no
       data  so  there	is no data sequence number.  Note that the ack sequence number is a small
       integer(1).  The first time tcpdump sees a tcp `conversation',	it  prints  the  sequence
       number from the packet.	On subsequent packets of the conversation, the difference between
       the current packet's sequence number and this initial sequence number  is  printed.   This
       means  that sequence numbers after the first can be interpreted as relative byte positions
       in the conversation's data stream (with the first data byte  each  direction  being  `1').
       `-S' will override this feature, causing the original sequence numbers to be output.

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

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

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

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

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

       Let's  assume that we want to watch packets used in establishing a TCP connection.  Recall
       that TCP uses a 3-way handshake protocol when it initializes a new connection; the 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:

		       |	       |
		       |---------------|
		       |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 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:

	    |C|E|U|A|P|R|S|F|
	    |---------------|
	    |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

		   00010010

       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 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 (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
       parsable.

       KIP AppleTalk (DDP in UDP)

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

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

       AppleTalk addresses are printed in the form
	      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  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
       186.

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

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

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

       IP Fragmentation

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

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

       The  fragment  information  is  output for each fragment.  The first fragment contains the
       higher level protocol header and the frag info is printed after the protocol info.   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
       datagrams:
	      arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
	      arizona > rtsg: (frag 595a:204@328)
	      rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
       There are a couple of things to note here:  First, addresses in the 2nd line don't include
       port  numbers.	This is because the TCP protocol information is all in the first fragment
       and we have no idea what the port or sequence numbers are when we print	the  later  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).

       Timestamps

       By default, all output lines are preceded by a timestamp.  The timestamp  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), apparmor(7)

AUTHORS
       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:

	      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 recommend that you  use
       the latter.

       On Linux systems with 2.0[.x] kernels:

	      packets on the loopback device will be seen twice;

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

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

	      capturing on some PPP devices won't work correctly.

       We recommend that you upgrade to a 2.2 or later kernel.

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

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

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

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

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

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

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

					  05 March 2009 			       TCPDUMP(8)


All times are GMT -4. The time now is 07:43 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
×
UNIX.COM Login
Username:
Password:  
Show Password