Provided by: tcpdump_4.9.3-2_amd64 bug

NAME

       tcpdump - dump traffic on a network

SYNOPSIS

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

DESCRIPTION

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

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

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

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

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

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

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

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

OPTIONS

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

       -b     Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.

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

       -c count
              Exit after receiving count packets.

       -C file_size
              Before writing a raw packet to a savefile, check  whether  the  file  is  currently
              larger  than  file_size  and, if so, close the current savefile and open a new one.
              Savefiles after the first savefile will have the name specified with the  -w  flag,
              with  a  number  after  it,  starting  at  1  and  continuing upward.  The units of
              file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

              Note that when used with -Z option (enabled by  default),  privileges  are  dropped
              before opening first savefile.

       -d     Dump  the compiled packet-matching code in a human readable form to standard output
              and stop.

       -dd    Dump packet-matching code as a C program fragment.

       -ddd   Dump packet-matching code as decimal numbers (preceded with a count).

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

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

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

       -e     Print  the  link-level header on each dump line.  This can be used, for example, to
              print MAC layer addresses for protocols such as Ethernet and IEEE 802.11.

       -E     Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are  addressed  to
              addr  and  contain  Security  Parameter  Index  value  spi. This combination may be
              repeated with comma or newline separation.

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

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

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

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

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

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

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

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

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

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

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

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

       -H     Attempt to detect 802.11s draft mesh headers.

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

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

              If  the  -D  flag  is supported, an interface number as printed by that flag can be
              used as the interface argument, if no interface on the system has that number as  a
              name.

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

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

              This flag will affect the output of the -L flag.  If -I isn't specified, only those
              link-layer  types  available  when  not  in  monitor  mode  will be shown; if -I is
              specified, only those link-layer types available  when  in  monitor  mode  will  be
              shown.

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

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

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

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

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

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

       -K
       --dont-verify-checksums
              Don't attempt to verify IP, TCP, or UDP checksums.  This is useful  for  interfaces
              that  perform some or all of those checksum calculation in hardware; otherwise, all
              outgoing TCP checksums will be flagged as bad.

       -l     Make stdout line buffered.  Useful if you want to see the data while capturing  it.
              E.g.,

                     tcpdump -l | tee dat

              or

                     tcpdump -l > dat & tail -f dat

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

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

       -L
       --list-data-link-types
              List  the known data link types for the interface, in the specified mode, and exit.
              The list of known data link types may be  dependent  on  the  specified  mode;  for
              example,  on  some  platforms, a Wi-Fi interface might support one set of data link
              types when not in monitor mode (for example, it might support  only  fake  Ethernet
              headers,  or might support 802.11 headers but not support 802.11 headers with radio
              information) and another set of data link types when in monitor mode (for  example,
              it  might support 802.11 headers, or 802.11 headers with radio information, only in
              monitor mode).

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

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

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

       -N     Don't print domain name qualification of host names.  E.g., if you give  this  flag
              then tcpdump will print ``nic'' instead of ``nic.ddn.mil''.

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

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

       -p
       --no-promiscuous-mode
              Don't put the interface into promiscuous mode.  Note that the interface might be in
              promiscuous  mode  for  some  other  reason;  hence,  `-p'  cannot  be  used  as an
              abbreviation for `ether host {local-hw-addr} or ether broadcast'.

       -Q direction
       --direction=direction
              Choose send/receive direction direction  for  which  packets  should  be  captured.
              Possible values are `in', `out' and `inout'. Not available on all platforms.

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

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

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

       -s snaplen
       --snapshot-length=snaplen
              Snarf  snaplen  bytes  of  data  from each packet rather than the default of 262144
              bytes.  Packets truncated because of a limited snapshot are indicated in the output
              with  ``[|proto]'',  where  proto  is  the  name of the protocol level at which the
              truncation has occurred.  Note that taking  larger  snapshots  both  increases  the
              amount  of  time it takes to process packets and, effectively, decreases the amount
              of packet buffering.  This may cause packets to be lost.  You should limit  snaplen
              to the smallest number that will capture the protocol information you're interested
              in.  Setting snaplen to  0  sets  it  to  the  default  of  262144,  for  backwards
              compatibility with recent older versions of tcpdump.

       -T type
              Force  packets  selected  by  "expression"  to  be  interpreted the specified type.
              Currently known types are aodv (Ad-hoc On-demand Distance  Vector  protocol),  carp
              (Common  Address  Redundancy  Protocol),  cnfp  (Cisco NetFlow protocol), lmp (Link
              Management Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1 (ZMTP/1.0 inside
              PGM/EPGM),  resp  (REdis  Serialization  Protocol),  radius  (RADIUS),  rpc (Remote
              Procedure  Call),  rtp   (Real-Time   Applications   protocol),   rtcp   (Real-Time
              Applications  control  protocol),  snmp  (Simple Network Management Protocol), tftp
              (Trivial File Transfer Protocol), vat (Visual Audio Tool),  wb  (distributed  White
              Board), zmtp1 (ZeroMQ Message Transport Protocol 1.0) and vxlan (Virtual eXtensible
              Local Area Network).

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

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

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

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

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

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

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

       -u     Print undecoded NFS handles.

       -U
       --packet-buffered
              If the -w option is  not  specified,  make  the  printed  packet  output  ``packet-
              buffered'';  i.e., as the description of the contents of each packet is printed, it
              will be written to the  standard  output,  rather  than,  when  not  writing  to  a
              terminal, being written only when the output buffer fills.

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

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

       -v     When parsing and printing, produce (slightly more) verbose  output.   For  example,
              the  time  to  live,  identification,  total length and options in an IP packet are
              printed.  Also enables additional packet integrity checks such as verifying the  IP
              and ICMP header checksum.

              When  writing to a file with the -w option, report, every 10 seconds, the number of
              packets captured.

       -vv    Even more verbose output.  For example, additional  fields  are  printed  from  NFS
              reply packets, and SMB packets are fully decoded.

       -vvv   Even  more  verbose  output.   For example, telnet SB ... SE options are printed in
              full.  With -X Telnet options are printed in hex as well.

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

       -w file
              Write the raw packets to file rather than parsing and printing them out.  They  can
              later be printed with the -r option.  Standard output is used if file is ``-''.

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

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

              See pcap-savefile(5) for a description of the file format.

       -W     Used in conjunction with the -C option, this will limit the number of files created
              to the specified number, and begin  overwriting  files  from  the  beginning,  thus
              creating  a  'rotating'  buffer.   In  addition, it will name the files with enough
              leading 0s to support the maximum number of files, allowing them to sort correctly.

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

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

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

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

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

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

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

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

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

       -Z user
       --relinquish-privileges=user
              If tcpdump is running as root, after opening the capture device or input  savefile,
              change the user ID to user and the group ID to the primary group of user.

              This behavior is enabled by default (-Z tcpdump), and can be disabled by -Z root.

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

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

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

EXAMPLES

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

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

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

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

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

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

       To print the start and end packets (the SYN and FIN packets) of each TCP conversation that
       involves a non-local host.
              tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

       To  print  all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain
       data, not, for example, SYN and FIN packets and ACK-only packets.  (IPv6  is  left  as  an
       exercise for the reader.)
              tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

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

       To  print  IP  broadcast or multicast packets that were not sent via Ethernet broadcast or
       multicast:
              tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

       To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
              tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

OUTPUT FORMAT

       The output of tcpdump is protocol dependent.  The following gives a brief description  and
       examples of most of the formats.

       Timestamps

       By  default,  all  output lines are preceded by a timestamp.  The timestamp is the current
       clock time in the form
              hh:mm:ss.frac
       and is as accurate as the kernel's clock.  The timestamp  reflects  the  time  the  kernel
       applied  a  time  stamp  to  the  packet.   No attempt is made to account for the time lag
       between when the network interface finished receiving the packet from the network and when
       the kernel applied a time stamp to the packet; that time lag could include a delay between
       the time when the network interface finished receiving a packet from the network  and  the
       time  when  an  interrupt  was  delivered to the kernel to get it to read the packet and a
       delay between the time when the kernel serviced the `new packet' interrupt  and  the  time
       when it applied a time stamp to the packet.

       Link Level Headers

       If  the  '-e'  option  is  given, the link level header is printed out.  On Ethernets, the
       source and destination addresses, protocol, and packet length are printed.

       On FDDI networks, the  '-e' option causes tcpdump to print the `frame control' field,  the
       source  and  destination  addresses,  and  the  packet length.  (The `frame control' field
       governs the interpretation of the rest of the  packet.   Normal  packets  (such  as  those
       containing  IP  datagrams) are `async' packets, with a priority value between 0 and 7; for
       example, `async4'.  Such packets are assumed to contain  an  802.2  Logical  Link  Control
       (LLC)  packet;  the LLC header is printed if it is not an ISO datagram or a so-called SNAP
       packet.

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

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

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

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

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

       ARP/RARP Packets

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

       This would look less redundant if we had done tcpdump -n:
              arp who-has 128.3.254.6 tell 128.3.254.68
              arp reply 128.3.254.6 is-at 02:07:01:00:01:c4

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

       IPv4 Packets

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

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

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

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

       TCP Packets

       (N.B.:The following description assumes familiarity with the  TCP  protocol  described  in
       RFC-793.   If you are not familiar with the protocol, this description will not be of much
       use to you.)

       The general format of a TCP protocol line is:
              src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
       Src and dst are the source and destination IP addresses  and  ports.   Tcpflags  are  some
       combination  of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
       `.' (ACK), or `none' if no flags are set.  Data-seqno describes the  portion  of  sequence
       space covered by the data in this packet (see example below).  Ackno is sequence number of
       the next data expected the other direction on this connection.  Window is  the  number  of
       bytes  of  receive  buffer  space  available  the other direction on this connection.  Urg
       indicates there is `urgent' data in the packet.  Opts are TCP options  (e.g.,  mss  1024).
       Len is the length of payload data.

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

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

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

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

                       |               |
                       |---------------|
                       |C|E|U|A|P|R|S|F|
                       |---------------|
                       |7   5   3     0|

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

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

                       |C|E|U|A|P|R|S|F|
                       |---------------|
                       |0 0 0 0 0 0 1 0|
                       |---------------|
                       |7 6 5 4 3 2 1 0|

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

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

            |C|E|U|A|P|R|S|F|
            |---------------|
            |0 0 0 1 0 0 1 0|
            |---------------|
            |7 6 5 4 3 2 1 0|

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

                   00010010

       which translates to decimal

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

       Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression, because that would
       select  only  those  packets  that  have  SYN-ACK  set,  but  not those with only SYN set.
       Remember that we don't care if ACK or any other control bit is set as long as SYN is set.

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

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

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

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

       This points us to the tcpdump filter expression
                   tcpdump -i xl0 'tcp[13] & 2 == 2'

       Some offsets and field values may be expressed as names rather than as numeric values. For
       example tcp[13] may be replaced with tcp[tcpflags]. The following TCP  flag  field  values
       are also available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-act, tcp-urg.

       This can be demonstrated as:
                   tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'

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

       UDP Packets

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

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

       UDP Name Server Requests

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

       Name server requests are formatted as
              src > dst: id op? flags qtype qclass name (len)
              h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
       Host  h2opolo asked the domain server on helios for an address record (qtype=A) associated
       with the name ucbvax.berkeley.edu.  The query id was `3'.  The `+' indicates the recursion
       desired  flag  was  set.   The  query  length  was  37 bytes, not including the UDP and IP
       protocol headers.  The query operation was the normal one, Query,  so  the  op  field  was
       omitted.  If the op had been anything else, it would have been printed between the `3' and
       the `+'.  Similarly, the qclass was the normal one, C_IN, and omitted.  Any  other  qclass
       would have been printed immediately after the `A'.

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

       UDP Name Server Responses

       Name server responses are formatted as
              src > dst:  id op rcode flags a/n/au type class data (len)
              helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
              helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
       In the first example, helios responds to query id 3 from h2opolo with 3 answer records,  3
       name server records and 7 additional records.  The first answer record is type A (address)
       and its data is internet address 128.32.137.3.  The total size of  the  response  was  273
       bytes,  excluding  UDP  and  IP  headers.  The op (Query) and response code (NoError) were
       omitted, as was the class (C_IN) of the A record.

       In the second example, helios responds to query 2 with a  response  code  of  non-existent
       domain  (NXDomain)  with  no  answers,  one name server and no authority records.  The `*'
       indicates that the authoritative answer bit was set.  Since  there  were  no  answers,  no
       type, class or data were printed.

       Other flag characters that might appear are `-' (recursion available, RA, not set) and `|'
       (truncated message, TC, set).  If the  `question'  section  doesn't  contain  exactly  one
       entry, `[nq]' is printed.

       SMB/CIFS decoding

       tcpdump  now  includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138
       and TCP/139.  Some primitive decoding of IPX and NetBEUI SMB data is also done.

       By default a fairly minimal decode is done, with a much more detailed decode done if -v is
       used.   Be warned that with -v a single SMB packet may take up a page or more, so only use
       -v if you really want all the gory details.

       For information on SMB packet formats and what all the fields mean see www.cifs.org or the
       pub/samba/specs/  directory  on your favorite samba.org mirror site.  The SMB patches were
       written by Andrew Tridgell (tridge@samba.org).

       NFS Requests and Replies

       Sun NFS (Network File System) requests and replies are printed as:
              src.sport > dst.nfs: NFS request xid xid len op args
              src.nfs > dst.dport: NFS reply xid xid reply stat len op results
              sushi.1023 > wrl.nfs: NFS request xid 26377
                   112 readlink fh 21,24/10.73165
              wrl.nfs > sushi.1023: NFS reply xid 26377
                   reply ok 40 readlink "../var"
              sushi.1022 > wrl.nfs: NFS request xid 8219
                   144 lookup fh 9,74/4096.6878 "xcolors"
              wrl.nfs > sushi.1022: NFS reply xid 8219
                   reply ok 128 lookup fh 9,74/4134.3150
       In the first line, host sushi sends a transaction with id 26377 to wrl.  The  request  was
       112  bytes, excluding the UDP and IP headers.  The operation was a readlink (read symbolic
       link) on file handle (fh) 21,24/10.731657119.  (If one is lucky, as in this case, the file
       handle  can  be  interpreted  as  a  major,minor device number pair, followed by the inode
       number and generation number.) In  the  second  line,  wrl  replies  `ok'  with  the  same
       transaction id and the contents of the link.

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

       Note  that  the  data printed depends on the operation type.  The format is intended to be
       self explanatory if read in conjunction with an NFS protocol spec.  Also note  that  older
       versions of tcpdump printed NFS packets in a slightly different format: the transaction id
       (xid) would be printed instead of the non-NFS port number of the packet.

       If the -v (verbose) flag is given, additional information is printed.  For example:
              sushi.1023 > wrl.nfs: NFS request xid 79658
                   148 read fh 21,11/12.195 8192 bytes @ 24576
              wrl.nfs > sushi.1023: NFS reply xid 79658
                   reply ok 1472 read REG 100664 ids 417/0 sz 29388
       (-v also prints the IP header TTL, ID, length, and fragmentation fields, which  have  been
       omitted  from  this  example.)   In the first line, sushi asks wrl to read 8192 bytes from
       file 21,11/12.195, at byte offset 24576.  Wrl replies `ok'; the packet shown on the second
       line  is  the  first  fragment  of the reply, and hence is only 1472 bytes long (the other
       bytes will follow in subsequent fragments, but these fragments do not have NFS or even UDP
       headers  and  so  might not be printed, depending on the filter expression used).  Because
       the -v flag is given, some of the file attributes (which are returned in addition  to  the
       file  data)  are  printed:  the  file  type (``REG'', for regular file), the file mode (in
       octal), the uid and gid, and the file size.

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

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

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

       AFS Requests and Replies

       Transarc AFS (Andrew File System) requests and replies are printed as:

              src.sport > dst.dport: rx packet-type
              src.sport > dst.dport: rx packet-type service call call-name args
              src.sport > dst.dport: rx packet-type service reply call-name args
              elvis.7001 > pike.afsfs:
                   rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                   new fid 536876964/1/1 ".newsrc"
              pike.afsfs > elvis.7001: rx data fs reply rename
       In the first line, host elvis sends a RX packet to pike.  This was a RX data packet to the
       fs (fileserver) service, and is the start of an RPC call.  The RPC call was a rename, with
       the old directory file id of 536876964/1/1 and an old filename of `.newsrc.new', and a new
       directory file id of 536876964/1/1 and  a  new  filename  of  `.newsrc'.   The  host  pike
       responds  with a RPC reply to the rename call (which was successful, because it was a data
       packet and not an abort packet).

       In general, all AFS RPCs are decoded at least by RPC call name.  Most  AFS  RPCs  have  at
       least  some of the arguments decoded (generally only the `interesting' arguments, for some
       definition of interesting).

       The format is intended to be self-describing, but it will probably not be useful to people
       who are not familiar with the workings of AFS and RX.

       If  the  -v  (verbose)  flag is given twice, acknowledgement packets and additional header
       information is printed, such as the RX call  ID,  call  number,  sequence  number,  serial
       number, and the RX packet flags.

       If  the -v flag is given twice, additional information is printed, such as the RX call ID,
       serial number, and the RX packet flags.  The MTU negotiation information is  also  printed
       from RX ack packets.

       If the -v flag is given three times, the security index and service id are printed.

       Error  codes  are  printed  for  abort  packets, with the exception of Ubik beacon packets
       (because abort packets are used to signify a yes vote for the Ubik protocol).

       Note that AFS requests are very large and many of the arguments won't  be  printed  unless
       snaplen is increased.  Try using `-s 256' to watch AFS traffic.

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

       KIP AppleTalk (DDP in UDP)

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

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

       AppleTalk addresses are printed in the form
              net.host.port

              144.1.209.2 > icsd-net.112.220
              office.2 > icsd-net.112.220
              jssmag.149.235 > icsd-net.2
       (If  the  /etc/atalk.names  doesn't  exist  or doesn't contain an entry for some AppleTalk
       host/net number, addresses are printed in numeric form.)  In the first example,  NBP  (DDP
       port  2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd
       node 112.  The second line is the same except the full name of the source  node  is  known
       (`office').  The third line is a send from port 235 on net jssmag node 149 to broadcast on
       the icsd-net NBP port (note that the broadcast address (255) is indicated by  a  net  name
       with  no  host  number - for this reason it's a good idea to keep node names and net names
       distinct in /etc/atalk.names).

       NBP (name binding protocol) and ATP (AppleTalk transaction protocol)  packets  have  their
       contents  interpreted.   Other protocols just dump the protocol name (or number if no name
       is registered for the protocol) and packet size.

       NBP packets are formatted like the following examples:
              icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
              jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
              techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
       The first line is a name lookup request for laserwriters sent by net  icsd  host  112  and
       broadcast on net jssmag.  The nbp id for the lookup is 190.  The second line shows a reply
       for this request (note that it has the same id) from host jssmag.209 saying that it has  a
       laserwriter  resource  named  "RM1140"  registered on port 250.  The third line is another
       reply to the same request saying host techpit has laserwriter "techpit" registered on port
       186.

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

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

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

SEE ALSO

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

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

AUTHORS

       The original authors are:

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

       It is currently being maintained by tcpdump.org.

       The current version is available via http:

              https://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

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

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

BUGS

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

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

       NIT  doesn't let you watch your own outbound traffic, BPF will.  We recommend that you use
       the latter.

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

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

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

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

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

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

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

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

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

                                         2 February 2017                               TCPDUMP(8)