tcpdump

TCPDUMP(1)                   General Commands Manual                  TCPDUMP(1)



NAME
       tcpdump - dump traffic on a network

SYNOPSIS
       tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]
               [ -c count ]
               [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
               [ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
               [ --number ] [ -Q in|out|inout ]
               [ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]
               [ -W filecount ]
               [ -E spi@ipaddr algo:secret,...  ]
               [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
               [ --time-stamp-precision=tstamp_precision ]
               [ --immediate-mode ] [ --version ]
               [ expression ]

DESCRIPTION
       Tcpdump prints out a description of the contents of packets on a network
       interface that match the boolean expression; the description is preceded
       by a time stamp, printed, by default, as hours, minutes, seconds, and
       fractions of a second since midnight.  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.  It can
       also be run with the -V flag, which causes it to read a list of saved
       packet files. In all cases, only packets that match expression will be
       processed by tcpdump.

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

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

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

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

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

       On platforms that support the SIGINFO signal, such as most BSDs
       (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
       when it receives a SIGINFO signal (generated, for example, by typing your
       ``status'' character, typically control-T, although on some platforms,
       such as Mac OS X, the ``status'' character is not set by default, so you
       must set it with stty(1) in order to use it) and will continue capturing
       packets. On platforms that do not support the SIGINFO signal, the same
       can be achieved by using the SIGUSR1 signal.

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

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 buffer_size
       --buffer-size=buffer_size
              Set the operating system capture buffer size to buffer_size, in
              units of KiB (1024 bytes).

       -c count
              Exit after receiving count packets.

       -C file_size
              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
       --list-interfaces
              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.  This can be used,
              for example, to print MAC layer addresses for protocols such as
              Ethernet and IEEE 802.11.

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

       -G rotate_seconds
              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>'.

       -h
       --help Print the tcpdump and libpcap version strings, print a usage
              message, and exit.

       --version
              Print the tcpdump and libpcap version strings and exit.

       -H     Attempt to detect 802.11s draft mesh headers.

       -i interface
       --interface=interface
              Listen on interface.  If unspecified, tcpdump searches the system
              interface list for the lowest numbered, configured up interface
              (excluding loopback), which may turn out to be, for example,
              ``eth0''.

              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, if no interface
              on the system has that number as a name.

       -I
       --monitor-mode
              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.

       --immediate-mode
              Capture in "immediate mode".  In this mode, packets are delivered
              to tcpdump as soon as they arrive, rather than being buffered for
              efficiency.  This is the default when printing packets rather than
              saving packets to a ``savefile'' if the packets are being printed
              to a terminal rather than to a file or pipe.

       -j tstamp_type
       --time-stamp-type=tstamp_type
              Set the time stamp type for the capture to tstamp_type.  The names
              to use for the time stamp types are given in pcap-tstamp(7); not
              all the types listed there will necessarily be valid for any given
              interface.

       -J
       --list-time-stamp-types
              List the supported time stamp types for the interface and exit.
              If the time stamp type cannot be set for the interface, no time
              stamp types are listed.

       --time-stamp-precision=tstamp_precision
              When capturing, set the time stamp precision for the capture to
              tstamp_precision.  Note that availability of high precision time
              stamps (nanoseconds) and their actual accuracy is platform and
              hardware dependent.  Also note that when writing captures made
              with nanosecond accuracy to a savefile, the time stamps are
              written with nanosecond resolution, and the file is written with a
              different magic number, to indicate that the time stamps are in
              seconds and nanoseconds; not all programs that read pcap savefiles
              will be able to read those captures.

       When reading a savefile, convert time stamps to the precision specified
       by timestamp_precision, and display them with that resolution.  If the
       precision specified is less than the precision of time stamps in the
       file, the conversion will lose precision.

       The supported values for timestamp_precision are micro for microsecond
       resolution and nano for nanosecond resolution.  The default is
       microsecond resolution.

       -K
       --dont-verify-checksums
              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

              Note that on Windows,``line buffered'' means ``unbuffered'', so
              that WinDump will write each character individually if -l is
              specified.

              -U is similar to -l in its behavior, but it will cause output to
              be ``packet-buffered'', so that the output is written to stdout at
              the end of each packet rather than at the end of each line; this
              is buffered on all platforms, including Windows.

       -L
       --list-data-link-types
              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 module
              Load SMI MIB module definitions from file module.  This option can
              be used several times to load several MIB modules into tcpdump.

       -M secret
              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''.

       -#
       --number
              Print an optional packet number at the beginning of the line.

       -O
       --no-optimize
              Do not run the packet-matching code optimizer.  This is useful
              only if you suspect a bug in the optimizer.

       -p
       --no-promiscuous-mode
              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 direction
       --direction=direction
              Choose send/receive direction direction for which packets should
              be captured. Possible values are `in', `out' and `inout'. Not
              available on all platforms.

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

       -r file
              Read packets from file (which was created with the -w option or by
              other tools that write pcap or pcap-ng files).  Standard input is
              used if file is ``-''.

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

       -s snaplen
       --snapshot-length=snaplen
              Snarf snaplen bytes of data from each packet rather than the
              default of 262144 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 262144, for
              backwards compatibility with recent older versions of tcpdump.

       -T type
              Force packets selected by "expression" to be interpreted the
              specified type.  Currently known types are aodv (Ad-hoc On-demand
              Distance Vector protocol), carp (Common Address Redundancy
              Protocol), cnfp (Cisco NetFlow protocol), lmp (Link Management
              Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0
              inside PGM/EPGM), resp (REdis Serialization Protocol), radius
              (RADIUS), 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), wb (distributed White Board),
              zmtp1 (ZeroMQ Message Transport Protocol 1.0) and vxlan (Virtual
              eXtensible Local Area Network).

              Note that the pgm type above affects UDP interpretation only, the
              native PGM is always recognised as IP protocol 113 regardless.
              UDP-encapsulated PGM is often called "EPGM" or "PGM/UDP".

              Note that the pgm_zmtp1 type above affects interpretation of both
              native PGM and UDP at once. During the native PGM decoding the
              application data of an ODATA/RDATA packet would be decoded as a
              ZeroMQ datagram with ZMTP/1.0 frames.  During the UDP decoding in
              addition to that any UDP packet would be treated as an
              encapsulated PGM packet.

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

       -tt    Print the timestamp, as seconds since January 1, 1970, 00:00:00,
              UTC, and fractions of a second since that time, on each dump line.

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

       -tttt  Print a timestamp, as hours, minutes, seconds, and fractions of a
              second since midnight, preceded by the 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
       --packet-buffered
              If the -w option is not specified, make the printed packet output
              ``packet-buffered''; i.e., as the description of the contents of
              each packet is printed, it will be written to the standard output,
              rather than, when not writing to a terminal, being written only
              when the output buffer fills.

              If the -w option is specified, make the saved raw packet output
              ``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.

       -V file
              Read a list of filenames from file. Standard input is used if file
              is ``-''.

       -w file
              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 ``-''.

              This output will be buffered if written to a file or pipe, so a
              program reading from the file or pipe may not see packets for an
              arbitrary amount of time after they are received.  Use the -U flag
              to cause packets to be written as soon as they are received.

              The MIME type application/vnd.tcpdump.pcap has been registered
              with IANA for pcap files. The filename extension .pcap appears to
              be the most commonly used along with .cap and .dmp. Tcpdump itself
              doesn't check the extension when reading capture files and doesn't
              add an extension when writing them (it uses magic numbers in the
              file header instead). However, many operating systems and
              applications will use the extension if it is present and adding
              one (e.g. .pcap) is recommended.

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

       -z postrotate-command
              Used in conjunction with the -C or -G options, this will make
              tcpdump run " postrotate-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 user
       --relinquish-privileges=user
              If tcpdump is running as root, after opening the capture device or
              input savefile, but before opening any savefiles for output,
              change the user ID to user and the group ID to the primary group
              of user.

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

        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).

              The expression argument can be passed to tcpdump as either a
              single Shell argument, or as multiple Shell arguments, whichever
              is more convenient.  Generally, if the expression contains Shell
              metacharacters, such as backslashes used to escape protocol names,
              it is easier to pass it as a single, quoted argument rather than
              to escape the Shell metacharacters.  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.

       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 applied a time stamp to the packet.  No attempt is made
       to account for the time lag between when the network interface finished
       receiving the packet from the network and when the kernel applied a time
       stamp to the packet; that time lag could include a delay between the time
       when the network interface finished receiving a packet from the network
       and the time when an interrupt was delivered to the kernel to get it to
       read the packet and a delay between the time when the kernel serviced the
       `new packet' interrupt and the time when it applied a time stamp to the
       packet.

       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.

       IPv4 Packets

       If the link-layer header is not being printed, for IPv4 packets, IP is
       printed after the time stamp.

       If the -v flag is specified, information from the IPv4 header is shown in
       parentheses after the IP or the link-layer header.  The general format of
       this information is:
              tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
       tos is the type of service field; if the ECN bits are non-zero, those are
       reported as ECT(1), ECT(0), or CE.  ttl is the time-to-live; it is not
       reported if it is zero.  id is the IP identification field.  offset is
       the fragment offset field; it is printed whether this is part of a
       fragmented datagram or not.  flags are the MF and DF flags; + is reported
       if MF is set, and DFP is reported if F is set.  If neither are set, . is
       reported.  proto is the protocol ID field.  length is the total length
       field.  options are the IP options, if any.

       Next, for TCP and UDP packets, the source and destination IP addresses
       and TCP or UDP ports, with a dot between each IP address and its
       corresponding port, will be printed, with a > separating the source and
       destination.  For other protocols, the addresses will be printed, with a
       > separating the source and destination.  Higher level protocol
       information, if any, will be printed after that.

       For fragmented IP datagrams, the first fragment contains the higher level
       protocol header; fragments after the first contain no higher level
       protocol header.  Fragmentation information will be printed only with the
       -v flag, in the IP header information, as described above.

       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, this
       description will not be of much use to you.)

       The general format of a TCP protocol line is:
              src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
       Src and dst are the source and destination IP addresses and ports.
       Tcpflags are some combination 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).  Ackno 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.
       Opts are TCP options (e.g., mss 1024).  Len is the length of payload
       data.

       Iptype, 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.
              IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
              IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
              IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
              IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
              IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
              IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
              IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
              IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
              IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 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' which means `sequence numbers first up to but not including
       last.)  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 or length.  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] & 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 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 > 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 > 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 the 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.sport > dst.nfs: NFS request xid xid len op args
              src.nfs > dst.dport: NFS reply xid xid reply stat len op results
              sushi.1023 > wrl.nfs: NFS request xid 26377
                   112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.1023: NFS reply xid 26377
                   reply ok 40 readlink "../var"
              sushi.1022 > wrl.nfs: NFS request xid 8219
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.1022: NFS reply xid 8219
                   reply ok 128 lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id 26377 to wrl.
       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.) In the second line, wrl replies `ok'
       with the same transaction id and the contents of the link.

       In the third line, sushi asks (using a new transaction id) wrl to lookup
       the name `xcolors' in directory file 9,74/4096.6878. In the fourth line,
       wrl sends a reply with the respective transaction id.

       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.  Also note that older versions of tcpdump printed NFS
       packets in a slightly different format: the transaction id (xid) would be
       printed instead of the non-NFS port number of the packet.

       If the -v (verbose) flag is given, additional information is printed.
       For example:
              sushi.1023 > wrl.nfs: NFS request xid 79658
                   148 read fh 21,11/12.195 8192 bytes @ 24576
              wrl.nfs > sushi.1023: NFS reply xid 79658
                   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 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 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.


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

              http://www.iana.org/assignments/media-
              types/application/vnd.tcpdump.pcap

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:

              https://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

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

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

BUGS
       To report a security issue please send an e-mail to security@tcpdump.org.

       To report bugs and other problems, contribute patches, request a feature,
       provide generic feedback etc please see the file CONTRIBUTING in the
       tcpdump source tree root.

       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.



                                 2 February 2017                      TCPDUMP(1)