tcpdump(8) — Linux manual page


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

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

       -c count
              Exit after receiving count packets.

              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

              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

              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>'.

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

              Print the tcpdump and libpcap version strings and exit.

       -H     Attempt to detect 802.11s draft mesh headers.

       -i 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.

              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.

              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
              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.

              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.

              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

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

       --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.

              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


                     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

              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

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

       -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

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

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

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

       -Q 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 ``-''.

              Print absolute, rather than relative, TCP sequence numbers.

       -s 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

              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

       -tttt  Print a timestamp, as hours, minutes, seconds, and fractions
              of a second since midnight, preceded by the date, on each dump

       -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

       -u     Print undecoded NFS handles.

              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)

       -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

       -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

              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
              Set the data link type to use while capturing packets (see -L)
              or just compiling and dumping packet-matching code (see -d) to

       -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
              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.

              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.


       By default, all output lines are preceded by a timestamp.  The
       timestamp is the current clock time in the form
       and is as accurate as the kernel's clock.  The timestamp reflects the
       time the kernel 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 tell
              arp reply is-at 02:07:01:00:01:c4

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

       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

       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

       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:

                       |               |
                       |7   5   3     0|

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

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

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

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

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


       and its decimal representation is

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

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

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

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

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

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

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

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


       which translates to decimal

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

       Now we can't just use 'tcp[13] == 18' in the tcpdump filter
       expression, because that would select only those packets that have
       SYN-ACK set, but not those with only SYN set.  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

       Name server requests are formatted as
              src > dst: id op? flags qtype qclass name (len)
              h2opolo.1538 > helios.domain: 3+ A? (37)
       Host h2opolo asked the domain server on helios for an address record
       (qtype=A) associated with the name  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 (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  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 or the pub/samba/specs/ directory on your favorite mirror site.  The SMB patches were written by Andrew
       Tridgell (

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
     > 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

       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

     > dst.dport: rx packet-type
     > dst.dport: rx packet-type service call call-name args
     > dst.dport: rx packet-type service reply call-name args
              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ""
                   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 `', 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

       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

     > 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@),


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

       The current version is available via HTTPS:


       The original distribution is available via anonymous ftp:


       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

       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

       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 
       ⟨⟩.  If you have a bug report for this manual
       page, see ⟨⟩.  This page was obtained
       from the project's upstream Git repository
       ⟨git://⟩ on 2020-11-01.  (At that time, the
       date of the most recent commit that was found in the repository was
       2020-11-01.)  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

                              15 September 2020                   TCPDUMP(1)

Pages that refer to this page: ovs-pcap(1)ovs-tcpundump(1)socket(7)BPF(8)csysdig(8)netsniff-ng(8)ovs-tcpdump(8)sysdig(8)tc-bpf(8)traceroute(8)