Provided by: tcpdump_4.99.0-2_amd64 bug

NAME

       tcpdump - dump traffic on a network

SYNOPSIS

       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

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

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

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

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

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

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

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

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

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

OPTIONS

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

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

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

       -c count
              Exit after receiving count packets.

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

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

              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.

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

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

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

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

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

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

              The  -D  flag  will  not be supported if tcpdump was built with an older version of
              libpcap that lacks the pcap_findalldevs(3PCAP) function.

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

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

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

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

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

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

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

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

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

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

       -G rotate_seconds
              If  specified,  rotates  the  dump  file  specified  with  the  -w   option   every
              rotate_seconds  seconds.  Savefiles will have the name specified by -w which should
              include a time format as defined by strftime(3).  If no time format  is  specified,
              each  new  file  will overwrite the previous.  Whenever a generated filename is not
              unique,  tcpdump  will  overwrite  the  pre-existing   data;   providing   a   time
              specification that is coarser than the capture period is therefore not advised.

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

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

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

       -H     Attempt to detect 802.11s draft mesh headers.

       -i interface
       --interface=interface
              Listen, report the list of link-layer types, report the list of time  stamp  types,
              or  report  the  results  of  compiling  a  filter  expression  on  interface.   If
              unspecified and if the -d flag is not given, tcpdump searches the system  interface
              list  for  the lowest numbered, configured up interface (excluding loopback), which
              may turn out to be, for example, ``eth0''.

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

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

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

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

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

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

       -j tstamp_type
       --time-stamp-type=tstamp_type
              Set  the  time stamp type for the capture to tstamp_type.  The names to use for the
              time stamp types are given in pcap-tstamp(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.

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

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

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

                     tcpdump -l | tee dat

              or

                     tcpdump -l > dat & tail -f dat

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -s snaplen
       --snapshot-length=snaplen
              Snarf snaplen bytes of data from each packet rather  than  the  default  of  262144
              bytes.  Packets truncated because of a limited snapshot are indicated in the output
              with ``[|proto]'', where proto is the name of  the  protocol  level  at  which  the
              truncation has occurred.

              Note  that  taking  larger  snapshots both increases the amount of time it takes to
              process packets and, effectively, decreases the amount of packet  buffering.   This
              may cause packets to be lost.  Note also that taking smaller snapshots will discard
              data from protocols above the transport layer, which loses information that may  be
              important.  NFS and AFS requests and replies, for example, are very large, and much
              of the detail won't be available if a too-short snapshot length is selected.

              If you need to reduce the snapshot size below the default, you should limit snaplen
              to the smallest number that will capture the protocol information you're interested
              in.  Setting snaplen to  0  sets  it  to  the  default  of  262144,  for  backwards
              compatibility with recent older versions of tcpdump.

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

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

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

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

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

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

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

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

       -u     Print undecoded NFS handles.

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

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

              The  -U  flag  will  not be supported if tcpdump was built with an older version of
              libpcap that lacks the pcap_dump_flush(3PCAP) function.

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

              When  writing  to a file with the -w option and at the same time not reading from a
              file with the -r option, report to stderr, once per second, the number  of  packets
              captured.  In  Solaris,  FreeBSD and possibly other operating systems this periodic
              update currently can cause loss of captured packets on their way from the kernel to
              tcpdump.

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

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

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

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

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

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

              See pcap-savefile(5) 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.  In the current implementation this flag may have the same effect  as  -xx
              if the packet is truncated.

       -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.  In the current implementation this flag
              may have the same effect as -XX if the packet is truncated.

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

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

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

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

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

       -Z user
       --relinquish-privileges=user
              If tcpdump is running as root, after opening the capture device or input  savefile,
              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  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

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

       Timestamps

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

       Link Level Headers

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

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

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

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

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

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

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

       ARP/RARP Packets

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

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

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

       IPv4 Packets

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

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

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

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

       TCP Packets

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

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

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

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

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

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

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

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

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

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

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

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

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

       Recall the structure of a TCP header without options:

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

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

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

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

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

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

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

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

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

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

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

              00000010

       and its decimal representation is

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

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

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

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

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

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

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

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

                   00010010

       which translates to decimal

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

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

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

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

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

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

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

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

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

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

       UDP Packets

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

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

       UDP Name Server Requests

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

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

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

       UDP Name Server Responses

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

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

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

       SMB/CIFS decoding

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

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

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

       NFS Requests and Replies

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

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

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

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

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

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

       AFS Requests and Replies

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

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

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

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

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

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

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

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

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

       KIP AppleTalk (DDP in UDP)

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

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

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

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

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

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

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

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

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

SEE ALSO

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

              https://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 HTTPS:

              https://www.tcpdump.org/

       The original distribution is available via anonymous ftp:

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

       IPv6/IPsec support is added by WIDE/KAME project.   This  program  uses  OpenSSL/LibreSSL,
       under specific configurations.

BUGS

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

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

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

       On Linux systems with 2.0[.x] kernels:

              packets on the loopback device will be seen twice;

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

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

              capturing on some PPP devices won't work correctly.

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

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

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

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

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

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

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

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

                                         21 December 2020                              TCPDUMP(8)