linux-func-tcpdump

安装

yum

yum install tcpdump

源码安装

# flex
yum -y install flex

# bison
yum -y install bison

wget http://www.tcpdump.org/release/libpcap-1.5.3.tar.gz
wget http://www.tcpdump.org/release/tcpdump-4.5.1.tar.gz
tar -zxvf libpcap-1.5.3.tar.gz
cd libpcap-1.5.3
./configure
sudo make install

cd ..
tar -zxvf tcpdump-4.5.1.tar.gz
cd tcpdump-4.5.1
./configure
sudo make install

yum -y install bison

文档

TCPDUMP(8)                           System Manager's Manual                          TCPDUMP(8)



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|-P 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  inter‐
       rupted  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  fil‐
              ter  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 pro‐
              cessed 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  expres‐
              sion 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 (gener‐
       ated, for example, by typing your ``status'' character, typically control-T, although  on
       some  platforms, such as Mac OS X, the ``status'' character is not set by default, so you
       must set it with stty(1) in order to use it) and  will  continue  capturing  packets.  On
       platforms  that  do not support the SIGINFO signal, the same can be achieved by using the
       SIGUSR1 signal.

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

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

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

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

       -c count
              Exit after receiving count packets.

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

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

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

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

       -D
       --list-interfaces
              Print the list of the network interfaces available on the system and on which tcp‐
              dump 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  inter‐
              face on which to capture.

              This  can  be useful on systems that don't have a command to list them (e.g., Win‐
              dows 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_sec‐
              onds 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_pre‐
       cision, 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 host addresses to names.  This can be used to avoid DNS lookups.

       -nn    Don't convert protocol and port numbers etc. to names either.

       -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 abbre‐
              viation for `ether host {local-hw-addr} or ether broadcast'.

       -Q|-P direction
       --direction=direction
              Choose send/receive direction direction for which packets should be captured. Pos‐
              sible 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  out‐
              put  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 inter‐
              ested 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.  Cur‐
              rently 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 eXtensi‐
              ble 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  frac‐
              tions 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  termi‐
              nal, 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  cap‐
              ture  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 cre‐
              ated 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  cor‐
              rectly.

              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 rota‐
              tion. 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 low‐
              est priority so that this doesn't disturb the capture process.

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

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

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

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

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

              The  expression  argument  can be passed to tcpdump as either a single Shell argu‐
              ment, 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 spa‐
              ces 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 con‐
       taining  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 infor‐
       mation, 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 con‐
       tained  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 differ‐
       ence 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  data‐
       gram  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 con‐
       nection 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 val‐
       ues 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 broad‐
       cast, 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 descrip‐
       tion 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 recur‐
       sion 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 trans‐
       action id and the contents of the link.

       In the third line, sushi asks (using a new transaction id) wrl to lookup the name  `xcol‐
       ors'  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 sec‐
       ond 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 peo‐
       ple 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 num‐
       ber, 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" regis‐
       tered 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 Labo‐
       ratory, University of California, Berkeley, CA.

       It is currently being maintained by tcpdump.org.

       The current version is available via http:

              http://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)

错误

• tcpdump: packet printing is not supported for link type NFLOG: use -w

默认监听eth0网卡的网络信息，如果没有eth0网卡，且没用-i指定网卡，那么tcpdump是运行不起来的。

例子

# 监视指定网络接口的数据包
tcpdump -i eth0

# 截获所有 47.92.36.227 的主机收到的和发出的所有的数据包
tcpdump host 47.92.36.227

# 截获主机 47.92.36.227 和主机47.92.36.228 或47.92.36.229的通信
tcpdump host 47.92.36.227 and \ (47.92.36.228 or 47.92.36.229 \)

# 截获主机47.92.36.227除了和主机47.92.36.228之外所有主机通信的ip包
tcpdump ip host 47.92.36.227 and ! 47.92.36.228

# 截获主机hostname发送的所有数据
tcpdump -i eth0 src host hostname

# 截获所有送到主机hostname的数据包
tcpdump -i eth0 dst host hostname

# 对本机的tcp 7003端口的数据包
tcpdump tcp port 7003