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TCPDUMP(8)                  System Manager's Manual                 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.  In all cases, only packets that match expression will be
       processed by tcpdump.

       Tcpdump will, if not run with the -c flag, continue capturing packets
       until it is interrupted by a SIGINT signal (generated, for example, by
       typing your interrupt character, typically control-C) or a SIGTERM
       signal (typically generated with the kill(1) command); if run with the
       -c flag, it will capture packets until it is interrupted by a SIGINT or
       SIGTERM signal or the specified number of packets have been processed.

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

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

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

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

       On platforms that support the SIGINFO signal, such as most BSDs
       (including Mac OS X) and Digital/Tru64 UNIX, it will report those
       counts when it receives a SIGINFO signal (generated, for example, by
       typing your ``status'' character, typically control-T) and will
       continue 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 privileges.

OPTIONS
       -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 tcpdump can capture packets.  For each network
              interface, a number and an interface name, possibly followed by
              a text description of the interface, is printed.  The interface
              name or the number can be supplied to the -i flag to specify an
              interface on which to capture.

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

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

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

       -E     Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that
              are addressed to addr and contain Security Parameter Index value
              spi. This combination may be repeated with comma or newline
              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" interface, which can capture on more than one
              interface, this option will not work correctly.

       -F     Use file as input for the filter expression.  An additional
              expression given on the command line is ignored.

       -G     If specified, rotates the dump file specified with the -w option
              every rotate_seconds seconds.  Savefiles will have the name
              specified by -w which should include a time format as defined by
              strftime(3). If no time format is specified, each new file will
              overwrite the previous.

              If used in conjunction with the -C option, filenames will take
              the form of `file<count>'.

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

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

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

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

              This flag will affect the output of the -L flag.  If -I isn't
              specified, only those link-layer types available when not in
              monitor mode will be shown; if -I is specified, only those link-
              layer types available when in monitor mode will be 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 example, on some
              platforms, a Wi-Fi interface might support one set of data link
              types when not in monitor mode (for example, it might support
              only fake Ethernet headers, or might support 802.11 headers but
              not support 802.11 headers with radio information) and another
              set of data link types when in monitor mode (for example, it
              might support 802.11 headers, or 802.11 headers with radio
              information, only in monitor mode).

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

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

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

       -N     Don't print domain name qualification of host names.  E.g., if
              you give this flag then tcpdump will print ``nic'' instead of
              ``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 abbreviation for `ether host
              {local-hw-addr} or ether broadcast'.

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

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

       -r     Read packets from file (which was created with the -w option).
              Standard input is used if file is ``-''.

       -S     Print absolute, rather than relative, TCP sequence numbers.

       -s     Snarf snaplen bytes of data from each packet rather than the
              default of 65535 bytes.  Packets truncated because of a limited
              snapshot are indicated in the output with ``[|proto]'', where
              proto is the name of the protocol level at which the truncation
              has occurred.  Note that taking larger snapshots both increases
              the amount of time it takes to process packets and, effectively,
              decreases the amount of packet buffering.  This may cause
              packets to be lost.  You should limit snaplen to the smallest
              number that will capture the protocol information you're
              interested in.  Setting snaplen to 0 sets it to the default of
              65535, for backwards compatibility with recent older versions of
              tcpdump.

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

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

       -tt    Print an unformatted timestamp on each dump line.

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

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

       -ttttt Print a delta (micro-second resolution) between current and
              first line on each dump 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 creating a 'rotating'
              buffer.  In addition, it will name the files with enough leading
              0s to support the maximum number of files, allowing them to sort
              correctly.

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

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

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

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

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

       -y     Set the data link type to use while capturing packets to
              datalinktype.  The available 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 ``_tcpdump''.  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.

        expression
              selects which packets will be dumped.  If no expression is
              given, all packets on the net will be dumped.  Otherwise, only
              packets for which expression is `true' will be dumped.

              For the expression syntax, see pcap-filter(7).

              Expression arguments can be passed to tcpdump as either a single
              argument or as multiple arguments, whichever is more convenient.
              Generally, if the expression contains Shell metacharacters, it
              is easier to pass it as a single, quoted argument.  Multiple
              arguments are concatenated with spaces before being parsed.

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

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

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

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

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

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

       To print the start and end packets (the SYN and FIN packets) of each
       TCP conversation that involves a non-local host.
              tcpdump 'tcp[tcpflags] &&amp; (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]&&amp;0xf)<&lt;<&lt;2)) - ((tcp[12]&&amp;0xf0)>&gt;>&gt;2)) != 0)'

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

       To print IP broadcast or multicast packets that were not sent via
       Ethernet broadcast or multicast:
              tcpdump 'ether[0] &&amp; 1 = 0 and ip[16] >&gt;= 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 governs the interpretation of the
       rest of the packet.  Normal packets (such as those containing IP
       datagrams) are `async' packets, with a priority value between 0 and 7;
       for example, `async4'.  Such packets are assumed to contain an 802.2
       Logical Link Control (LLC) packet; the LLC header is printed if it is
       not an ISO datagram or a so-called SNAP packet.

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

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

       (N.B.: The following description assumes familiarity with the SLIP
       compression algorithm described in RFC-1144.)

       On SLIP links, a direction indicator (``I'' for inbound, ``O'' for
       outbound), packet type, and compression information are printed out.
       The packet type is printed first.  The three types are ip, utcp, and
       ctcp.  No further link information is printed for ip packets.  For TCP
       packets, the connection identifier is printed following the type.  If
       the packet is compressed, its encoded header is printed out.  The
       special cases are printed out as *S+n and *SA+n, where n is the amount
       by which the sequence number (or sequence number and ack) has changed.
       If it is not a special case, zero or more changes are printed.  A
       change is indicated by U (urgent pointer), W (window), A (ack), S
       (sequence number), and I (packet ID), followed by a delta (+n or -n),
       or a new value (=n).  Finally, the amount of data in the packet and
       compressed header length are printed.

       For example, the following line shows an outbound compressed TCP
       packet, with an implicit connection identifier; the ack has changed by
       6, the sequence number by 49, and the packet ID by 6; there are 3 bytes
       of data and 6 bytes of compressed header:
              O ctcp * A+6 S+49 I+6 3 (6)
       ARP/RARP Packets

       Arp/rarp output shows the type of request and its arguments.  The
       format is intended to be self explanatory.  Here is a short sample
       taken from the start of an `rlogin' from host rtsg to host csam:
              arp who-has csam tell rtsg
              arp reply csam is-at CSAM
       The first line says that rtsg sent an arp packet asking for the
       Ethernet address of internet host csam.  Csam replies with its Ethernet
       address (in this example, Ethernet addresses are in caps and internet
       addresses in lower case).

       This would look less redundant if we had done tcpdump -n:
              arp who-has 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 &gt; dst: flags data-seqno ack window urgent options
       Src and dst are the source and destination IP addresses and ports.
       Flags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), W
       (ECN CWR) or E (ECN-Echo), or a single `.' (no flags).  Data-seqno
       describes the portion of sequence space covered by the data in this
       packet (see example below).  Ack is sequence number of the next data
       expected the other direction on this connection.  Window is the number
       of bytes of receive buffer space available the other direction on this
       connection.  Urg indicates there is `urgent' data in the packet.
       Options are tcp options enclosed in angle brackets (e.g., <mss 1024>).

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

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

       Csam replies with a similar packet except it includes a piggy-backed
       ack for rtsg's SYN.  Rtsg then acks csam's SYN.  The `.' means no flags
       were set.  The packet contained no data so there is no data sequence
       number.  Note that the ack sequence number is a small integer (1).  The
       first time tcpdump sees a tcp `conversation', it prints the sequence
       number from the packet.  On subsequent packets of the conversation, the
       difference between the current packet's sequence number and this
       initial sequence number is printed.  This means that sequence numbers
       after the first can be interpreted as relative byte positions in the
       conversation's data stream (with the first data byte each direction
       being `1').  `-S' will override this feature, causing the original
       sequence numbers to be output.

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

                       |               |
                       |---------------|
                       |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.  Remember that we don't care if ACK or any
       other control bit is set as long as SYN is set.

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


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

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

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

       This points us to the tcpdump filter expression
                   tcpdump -i xl0 'tcp[13] &&amp; 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] &&amp; tcp-push != 0'

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

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

       Some UDP services are recognized (from the source or destination port
       number) and the higher level protocol information printed.  In
       particular, Domain Name service requests (RFC-1034/1035) and Sun RPC
       calls (RFC-1050) to NFS.
       UDP Name Server Requests

       (N.B.:The following description assumes familiarity with the Domain
       Service protocol described in RFC-1035.  If you are not familiar with
       the protocol, the following description will appear to be written in
       greek.)

       Name server requests are formatted as
              src &gt; 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 protocol
       headers.  The query operation was the normal one, Query, so the op
       field was omitted.  If the op had been anything else, it would have
       been printed between the `3' and the `+'.  Similarly, the qclass was
       the normal one, C_IN, and omitted.  Any other qclass would have been
       printed immediately after the `A'.

       A few anomalies are checked and may result in extra fields enclosed in
       square brackets:  If a query contains an answer, authority records or
       additional records section, ancount, nscount, or arcount are printed as
       `[na]', `[nn]' or  `[nau]' where n is the appropriate count.  If any of
       the response bits are set (AA, RA or rcode) or any of the `must be
       zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where
       x is the hex value of header bytes two and three.
       UDP Name Server Responses

       Name server responses are formatted as
              src &gt; 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
       (tridgeATsamba.org).

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
              src.xid &gt; dst.nfs: len op args
              src.nfs &gt; dst.xid: reply stat len op results

              sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
              sushi.201b > wrl.nfs:
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.201b:
                   reply ok 128 lookup fh 9,74/4134.3150

       In the first line, host sushi sends a transaction with id 6709 to wrl
       (note that the number following the src host is a transaction id, not
       the source port).  The request was 112 bytes, excluding the UDP and IP
       headers.  The operation was a readlink (read symbolic link) on file
       handle (fh) 21,24/10.731657119.  (If one is lucky, as in this case, the
       file handle can be interpreted as a major,minor device number pair,
       followed by the inode number and generation number.)  Wrl replies `ok'
       with the contents of the link.

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

       If the -v (verbose) flag is given, additional information is printed.
       For example:

              sushi.1372a > wrl.nfs:
                   148 read fh 21,11/12.195 8192 bytes @ 24576
              wrl.nfs > sushi.1372a:
                   reply ok 1472 read REG 100664 ids 417/0 sz 29388

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

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

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

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

       Transarc AFS (Andrew File System) requests and replies are printed as:
              src.sport &gt; dst.dport: rx packet-type
              src.sport &gt; dst.dport: rx packet-type service call call-name args
              src.sport &gt; 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.  Fragments after the first contain no
       higher level protocol header and the frag info is printed after the
       source and destination addresses.  For example, here is part of an ftp
       from arizona.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 fragments.
       Second, the tcp sequence information in the first line is printed as if
       there were 308 bytes of user data when, in fact, there are 512 bytes
       (308 in the first frag and 204 in the second).  If you are looking for
       holes in the sequence space or trying to match up acks with packets,
       this can fool you.

       A packet with the IP don't fragment flag is marked with a trailing
       (DF).
       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
       pcap(3), bpf(4), pcap-savefile(5), pcap-filter(7)

AUTHORS
       The original authors are:

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

       It is currently being maintained by tcpdump.org.

       The current version is available via http:

              http://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

              ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

       IPv6/IPsec support is added by WIDE/KAME project.  This program uses
       Eric Young's SSLeay library, under specific configurations.

BUGS
       Please send problems, bugs, questions, desirable enhancements, patches
       etc. to:

              tcpdump-workersATlists.org

       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)