tcpdump(1) — Linux manual page

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TCPDUMP(1)               General Commands Manual              TCPDUMP(1)

NAME         top

       tcpdump - dump traffic on a network

SYNOPSIS         top

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

DESCRIPTION         top

       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 macOS) 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 macOS, 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.

       Using the SIGUSR2 signal along with the -w flag will forcibly
       flush the packet buffer into the output file.

       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         top

       -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.

       --count
              Print only on stderr the packet count when reading capture
              file(s) instead of parsing/printing the packets. If a
              filter is specified on the command line, tcpdump counts
              only packets that were matched by the filter expression.

       -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.

              Please mind that although code compilation is always DLT-
              specific, typically it is impossible (and unnecessary) to
              specify which DLT to use for the dump because tcpdump uses
              either the DLT of the input pcap file specified with -r,
              or the default DLT of the network interface specified with
              -i, or the particular DLT of the network interface
              specified with -y and -i respectively. In these cases the
              dump shows the same exact code that would filter the input
              file or the network interface without -d.

              However, when neither -r nor -i is specified, specifying
              -d prevents tcpdump from guessing a suitable network
              interface (see -i).  In this case the DLT defaults to
              EN10MB and can be set to another valid value manually with
              -y.

       -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,
              are 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(3PCAP) 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.
              Whenever a generated filename is not unique, tcpdump will
              overwrite the pre-existing data; providing a time
              specification that is coarser than the capture period is
              therefore not advised.

              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, report the list of link-layer types, report the
              list of time stamp types, or report the results of
              compiling a filter expression on interface.  If
              unspecified and if the -d flag is not given, 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(@MAN_MISC_INFO@); 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.

       --micro
       --nano Shorthands for --time-stamp-precision=micro or
              --time-stamp-precision=nano, adjusting the time stamp
              precision accordingly.  When reading packets from a
              savefile, using --micro truncates time stamps if the
              savefile was created with nanosecond precision.  In
              contrast, a savefile created with microsecond precision
              will have trailing zeroes added to the time stamp when
              --nano is used.

       -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'.

       --print
              Print parsed packet output, even if the raw packets are
              being saved to a file with the -w flag.

       -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 pcapng 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.  Note also that taking
              smaller snapshots will discard data from protocols above
              the transport layer, which loses information that may be
              important.  NFS and AFS requests and replies, for example,
              are very large, and much of the detail won't be available
              if a too-short snapshot length is selected.

              If you need to reduce the snapshot size below the default,
              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), domain (Domain Name System), lmp (Link
              Management Protocol), pgm (Pragmatic General Multicast),
              pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM), ptp (Precision Time
              Protocol), radius (RADIUS), resp (REdis Serialization
              Protocol), rpc (Remote Procedure Call), rtcp (Real-Time
              Applications control protocol), rtp (Real-Time
              Applications protocol), snmp (Simple Network Management
              Protocol), someip (SOME/IP), tftp (Trivial File Transfer
              Protocol), vat (Visual Audio Tool), vxlan (Virtual
              eXtensible Local Area Network), wb (distributed White
              Board) and zmtp1 (ZeroMQ Message Transport Protocol 1.0).

              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 (microsecond or nanosecond resolution
              depending on the --time-stamp-precision option) between
              current and previous line on each dump line.  The default
              is microsecond resolution.

       -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 (microsecond or nanosecond resolution
              depending on the --time-stamp-precision option) between
              current and first line on each dump line.  The default is
              microsecond resolution.

       -u     Print undecoded NFS handles.

       -U
       --packet-buffered
              If the -w option is not specified, or if it is specified
              but the --print flag is also 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(3PCAP) 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 and at the same
              time not reading from a file with the -r option, report to
              stderr, once per second, the number of packets captured.
              In Solaris, FreeBSD and possibly other operating systems
              this periodic update currently can cause loss of captured
              packets on their way from the kernel to tcpdump.

       -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(@MAN_FILE_FORMATS@) for a description of
              the file format.

       -W filecount
              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 in conjunction with both -C and -G, the -W option
              will currently be ignored, and will only affect the file
              name.

       -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 (see
              -L) or just compiling and dumping packet-matching code
              (see -d) 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(@MAN_MISC_INFO@).

              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         top

       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 the TCP packets with flags RST and ACK both set.  (i.e.
       select only the RST and ACK flags in the flags field, and if the
       result is "RST and ACK both set", match)
              tcpdump 'tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)'

       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         top

       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 DF
       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-ack, 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.

       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).

       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         top

       stty(1), pcap(3PCAP), bpf(4), nit(4P),
       pcap-savefile(@MAN_FILE_FORMATS@), pcap-filter(@MAN_MISC_INFO@),
       pcap-tstamp(@MAN_MISC_INFO@)

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

AUTHORS         top

       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 HTTPS:

              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 OpenSSL/LibreSSL, under specific configurations.

BUGS         top

       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 userspace, 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.

COLOPHON         top

       This page is part of the tcpdump (a command-line network packet
       analyzer) project.  Information about the project can be found at
       ⟨http://www.tcpdump.org/⟩.  If you have a bug report for this
       manual page, see ⟨http://www.tcpdump.org/#patches⟩.  This page
       was obtained from the project's upstream Git repository
       ⟨git://bpf.tcpdump.org/tcpdump⟩ on 2020-12-18.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2020-12-15.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

                            15 September 2020                 TCPDUMP(1)

Pages that refer to this page: pcap(3pcap)pcap_dump_open(3pcap)pcap_open_offline(3pcap)netsniff-ng(8)