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tcpdump - dump traffic on a network

 

NAME

        tcpdump - dump traffic on a network
 

SYNOPSIS

        tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
                [ -C file_size ] [ -F file ]
                [ -i interface ] [ -m module ] [ -r file ]
                [ -s snaplen ] [ -T type ] [ -w file ]
                [ -E algo:secret ] [ expression ]
 

DESCRIPTION

        Tcpdump  prints  out the headers of packets on a network interface that
        match the boolean expression.  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 -b flag, which causes it to read from  a  saved  packet
        file  rather  than  to  read  packets from a network interface.  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 sig‐
        nal (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  ‘‘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 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, it
        will report those counts when it receives a SIGINFO signal  (generated,
        for  example, by typing your ‘‘status’’ character, typically control-T)
        and will continue capturing packets.
 
        Reading packets from a network interface may require that you have spe‐
        cial privileges:
 
        Under SunOS 3.x or 4.x with NIT or BPF:
               You must have read access to /dev/nit or /dev/bpf*.
 
        Under Solaris with DLPI:
               You  must  have  read/write access to the network pseudo device,
               e.g.  /dev/le.  On at least some versions of  Solaris,  however,
               this  is not sufficient to allow tcpdump to capture in promiscu‐
               ous mode; on those versions of Solaris, you  must  be  root,  or
               tcpdump must be installed setuid to root, in order to capture in
               promiscuous mode.
 
        Under HP-UX with DLPI:
               You must be root or tcpdump must be installed setuid to root.
 
        Under IRIX with snoop:
               You must be root or tcpdump must be installed setuid to root.
 
        Under Linux:
               You must be root or tcpdump must be installed setuid to root.
 
        Under Ultrix and Digital UNIX:
               Once the super-user has enabled promiscuous-mode operation using
               pfconfig(8),  any user may capture network traffic with tcpdump.
 
        Under BSD:
               You must have read access to /dev/bpf*.
 
        Reading a saved packet file doesn’t require special privileges.
 

OPTIONS

        -a     Attempt to convert network and broadcast addresses to names.
 
        -c     Exit after receiving count packets.
 
        -C     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 2 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).
 
        -e     Print the link-level header on each dump line.
 
        -E     Use algo:secret for decrypting IPsec  ESP  packets.   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 the ascii text for ESP secret key.   We  cannot
               take  arbitrary binary value at this moment.  The option assumes
               RFC2406 ESP, not RFC1827 ESP.  The option is only for  debugging
               purposes,  and the use of this option with truly ‘secret’ key is
               discouraged.  By presenting IPsec secret key onto  command  line
               you make it visible to others, via ps(1) and other occasions.
 
        -f     Print  ‘foreign’ internet addresses numerically rather than sym‐
               bolically (this option is intended to get around  serious  brain
               damage in Sun’s yp server — usually it hangs forever translating
               non-local internet numbers).
 
        -F     Use file as input for  the  filter  expression.   An  additional
               expression given on the command line is ignored.
 
        -i     Listen  on interface.  If unspecified, tcpdump searches the sys‐
               tem interface list for the lowest numbered, configured up inter‐
               face (excluding loopback).  Ties are broken by choosing the ear‐
               liest match.
 
               On Linux systems with 2.2 or later kernels, an  interface  argu‐
               ment  of  ‘‘any’’ can be used to capture packets from all inter‐
               faces.  Note that captures on the ‘‘any’’  device  will  not  be
               done in promiscuous mode.
 
        -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’’.
 
        -m     Load  SMI  MIB module definitions from file module.  This option
               can be used several times to load several MIB modules into  tcp‐
               dump.
 
        -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’’.
 
        -O     Do not run the packet-matching code optimizer.  This  is  useful
               only if you suspect a bug in the optimizer.
 
        -p     Don’t  put  the  interface into promiscuous mode.  Note that the
               interface might be in promiscuous mode for  some  other  reason;
               hence,  ‘-p’  cannot  be used as an abbreviation for ‘ether host
               {local-hw-addr} or ether broadcast’.
 
        -q     Quick (quiet?) output.  Print less protocol information so  out‐
               put lines are shorter.
 
        -R     Assume  ESP/AH packets to be based on old specification (RFC1825
               to RFC1829).  If specified, tcpdump will not print  replay  pre‐
               vention  field.   Since  there  is  no protocol version field in
               ESP/AH specification,  tcpdump  cannot  deduce  the  version  of
               ESP/AH protocol.
 
        -r     Read  packets  from file (which was created with the -w option).
               Standard input is used if file is ‘‘-’’.
 
        -S     Print absolute, rather than relative, TCP sequence numbers.
 
        -s     Snarf snaplen bytes of data from each  packet  rather  than  the
               default  of  68  (with SunOS’s NIT, the minimum is actually 96).
               68 bytes is adequate for IP, ICMP, TCP and UDP but may  truncate
               protocol  information  from  name  server  and  NFS packets (see
               below).  Packets truncated because of  a  limited  snapshot  are
               indicated  in  the  output with ‘‘[|proto]’’, where proto is the
               name of the protocol level at which the truncation has occurred.
               Note  that  taking larger snapshots both increases the amount of
               time it takes to process packets and, effectively, decreases the
               amount  of packet buffering.  This may cause packets to be lost.
               You should limit snaplen to the smallest number that  will  cap‐
               ture  the  protocol  information  you’re interested in.  Setting
               snaplen to 0 means use the required length to catch whole  pack‐
               ets.
 
        -T     Force  packets  selected  by  "expression" to be interpreted the
               specified type.  Currently known types are cnfp  (Cisco  NetFlow
               protocol),  rpc (Remote Procedure Call), rtp (Real-Time Applica‐
               tions protocol), rtcp (Real-Time Applications control protocol),
               snmp  (Simple  Network  Management  Protocol), vat (Visual Audio
               Tool), and wb (distributed White Board).
 
        -t     Don’t print a timestamp on each dump line.
 
        -tt    Print an unformatted timestamp on each dump line.
 
        -ttt   Print a delta (in micro-seconds) between  current  and  previous
               line on each dump line.
 
        -tttt  Print  a  timestamp  in default format proceeded by date on each
               dump line.  -u Print undecoded NFS handles.
 
        -v     (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.
 
        -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.
 
        -w     Write  the  raw packets to file rather than parsing and printing
               them out.  They can later be printed with the -r option.   Stan‐
               dard output is used if file is ‘‘-’’.
 
        -x     Print  each  packet  (minus  its link level header) in hex.  The
               smaller of the entire packet or snaplen bytes will be printed.
 
        -X     When printing hex, print ascii too.  Thus if -x is also set, the
               packet  is  printed  in  hex/ascii.   This  is  very  handy  for
               analysing new protocols.  Even if -x is not also set, some parts
               of some packets may be printed in hex/ascii.
 
         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.
 
               The  expression  consists of one or more primitives.  Primitives
               usually consist of an id (name or number)  preceded  by  one  or
               more qualifiers.  There are three different kinds of qualifier:
 
               type   qualifiers  say  what kind of thing the id name or number
                      refers to.  Possible types are host, net and port.  E.g.,
                      ‘host  foo’, ‘net 128.3’, ‘port 20’.  If there is no type
                      qualifier, host is assumed.
 
               dir    qualifiers specify a  particular  transfer  direction  to
                      and/or from id.  Possible directions are src, dst, src or
                      dst and src and dst.  E.g., ‘src foo’, ‘dst  net  128.3’,
                      ‘src  or  dst  port ftp-data’.  If there is no dir quali‐
                      fier, src or dst is  assumed.   For  ‘null’  link  layers
                      (i.e.  point to point protocols such as slip) the inbound
                      and outbound qualifiers can be used to specify a  desired
                      direction.
 
               proto  qualifiers  restrict  the match to a particular protocol.
                      Possible protos are: ether, fddi, tr, ip, ip6, arp, rarp,
                      decnet,  lat,  sca,  moprc, mopdl, iso, esis, isis, icmp,
                      icmp6, tcp and udp.  E.g.,  ‘ether  src  foo’,  ‘arp  net
                      128.3’,  ‘tcp  port 21’.  If there is no proto qualifier,
                      all protocols  consistent  with  the  type  are  assumed.
                      E.g.,  ‘src  foo’  means  ‘(ip  or  arp or rarp) src foo’
                      (except the latter is not legal syntax), ‘net bar’  means
                      ‘(ip  or  arp or rarp) net bar’ and ‘port 53’ means ‘(tcp
                      or udp) port 53’.
 
               [‘fddi’ is actually an alias for ‘ether’; the parser treats them
               identically  as meaning ‘‘the data link level used on the speci‐
               fied network interface.’’  FDDI  headers  contain  Ethernet-like
               source  and  destination  addresses, and often contain Ethernet-
               like packet types, so you can filter on these FDDI  fields  just
               as  with  the analogous Ethernet fields.  FDDI headers also con‐
               tain other fields, but you cannot name them explicitly in a fil‐
               ter expression.
 
               Similarly,  ‘tr’  is  an  alias  for ‘ether’; the previous para‐
               graph’s statements about FDDI headers also apply to  Token  Ring
               headers.]
 
               In  addition  to  the  above, there are some special ‘primitive’
               keywords that don’t  follow  the  pattern:  gateway,  broadcast,
               less,  greater  and  arithmetic  expressions.   All of these are
               described below.
 
               More complex filter expressions are built up by using the  words
               and,  or and not to combine primitives.  E.g., ‘host foo and not
               port ftp and not port  ftp-data’.   To  save  typing,  identical
               qualifier lists can be omitted.  E.g., ‘tcp dst port ftp or ftp-
               data or domain’ is exactly the same as ‘tcp dst port ftp or  tcp
               dst port ftp-data or tcp dst port domain’.
 
               Allowable primitives are:
 
               dst host host
                      True  if  the  IPv4/v6 destination field of the packet is
                      host, which may be either an address or a name.
 
               src host host
                      True if the IPv4/v6 source field of the packet is host.
 
               host host
                      True if either the IPv4/v6 source or destination  of  the
                      packet is host.  Any of the above host expressions can be
                      prepended with the keywords, ip, arp, rarp, or ip6 as in:
                           ip host host
                      which is equivalent to:
                           ether proto \ip and host host
                      If  host  is  a  name  with  multiple  IP addresses, each
                      address will be checked for a match.
 
               ether dst ehost
                      True if the ethernet destination address is ehost.  Ehost
                      may  be  either  a name from /etc/ethers or a number (see
                      ethers(3N) for numeric format).
 
               ether src ehost
                      True if the ethernet source address is ehost.
 
               ether host ehost
                      True if either the ethernet source or destination address
                      is ehost.
 
               gateway host
                      True  if  the  packet  used host as a gateway.  I.e., the
                      ethernet source or destination address was host but  nei‐
                      ther the IP source nor the IP destination was host.  Host
                      must be a name and must be found both  by  the  machine’s
                      host-name-to-IP-address  resolution mechanisms (host name
                      file, DNS, NIS, etc.) and by the machine’s  host-name-to-
                      Ethernet-address   resolution   mechanism   (/etc/ethers,
                      etc.).  (An equivalent expression is
                           ether host ehost and not host host
                      which can be used with either names or numbers for host /
                      ehost.)   This  syntax does not work in IPv6-enabled con‐
                      figuration at this moment.
 
               dst net net
                      True if the IPv4/v6 destination address of the packet has
                      a  network  number of net.  Net may be either a name from
                      /etc/networks or a network number  (see  networks(4)  for
                      details).
 
               src net net
                      True  if  the  IPv4/v6 source address of the packet has a
                      network number of net.
 
               net net
                      True if either the IPv4/v6 source or destination  address
                      of the packet has a network number of net.
 
               net net mask netmask
                      True if the IP address matches net with the specific net‐
                      mask.  May be qualified with src or dst.  Note that  this
                      syntax is not valid for IPv6 net.
 
               net net/len
                      True  if  the  IPv4/v6 address matches net with a netmask
                      len bits wide.  May be qualified with src or dst.
 
               dst port port
                      True if the packet is ip/tcp, ip/udp, ip6/tcp or  ip6/udp
                      and  has  a destination port value of port.  The port can
                      be a number or a name used in /etc/services (see  tcp(4P)
                      and  udp(4P)).   If  a name is used, both the port number
                      and protocol are checked.  If a number or ambiguous  name
                      is  used, only the port number is checked (e.g., dst port
                      513 will print both tcp/login traffic and  udp/who  traf‐
                      fic,  and  port  domain  will  print  both tcp/domain and
                      udp/domain traffic).
 
               src port port
                      True if the packet has a source port value of port.
 
               port port
                      True if either the source  or  destination  port  of  the
                      packet is port.  Any of the above port expressions can be
                      prepended with the keywords, tcp or udp, as in:
                           tcp src port port
                      which matches only tcp packets whose source port is port.
 
               less length
                      True  if  the  packet  has a length less than or equal to
                      length.  This is equivalent to:
                           len <= length.
 
               greater length
                      True if the packet has a length greater than or equal  to
                      length.  This is equivalent to:
                           len >= length.
 
               ip proto protocol
                      True if the packet is an IP packet (see ip(4P)) of proto‐
                      col type protocol.  Protocol can be a number  or  one  of
                      the  names  icmp,  icmp6, igmp, igrp, pim, ah, esp, vrrp,
                      udp, or tcp.  Note that the  identifiers  tcp,  udp,  and
                      icmp  are also keywords and must be escaped via backslash
                      (\), which is \\ in the C-shell.  Note that  this  primi‐
                      tive does not chase the protocol header chain.
 
               ip6 proto protocol
                      True  if  the  packet  is an IPv6 packet of protocol type
                      protocol.  Note that this primitive does  not  chase  the
                      protocol header chain.
 
               ip6 protochain protocol
                      True  if the packet is IPv6 packet, and contains protocol
                      header with type protocol in its protocol  header  chain.
                      For example,
                           ip6 protochain 6
                      matches  any  IPv6 packet with TCP protocol header in the
                      protocol header chain.  The packet may contain, for exam‐
                      ple, authentication header, routing header, or hop-by-hop
                      option header, between IPv6 header and TCP  header.   The
                      BPF  code emitted by this primitive is complex and cannot
                      be optimized by BPF optimizer code in  tcpdump,  so  this
                      can be somewhat slow.
 
               ip protochain protocol
                      Equivalent  to  ip6  protochain protocol, but this is for
                      IPv4.
 
               ether broadcast
                      True if the packet is an ethernet broadcast packet.   The
                      ether keyword is optional.
 
               ip broadcast
                      True  if the packet is an IP broadcast packet.  It checks
                      for both the all-zeroes and  all-ones  broadcast  conven‐
                      tions, and looks up the local subnet mask.
 
               ether multicast
                      True  if the packet is an ethernet multicast packet.  The
                      ether  keyword  is  optional.   This  is  shorthand   for
                      ‘ether[0] & 1 != 0’.
 
               ip multicast
                      True if the packet is an IP multicast packet.
 
               ip6 multicast
                      True if the packet is an IPv6 multicast packet.
 
               ether proto protocol
                      True  if  the packet is of ether type protocol.  Protocol
                      can be a number or one of the names ip, ip6,  arp,  rarp,
                      atalk,  aarp,  decnet,  sca, lat, mopdl, moprc, iso, stp,
                      ipx, or netbeui.  Note these identifiers  are  also  key‐
                      words and must be escaped via backslash (\).
 
                      [In  the  case  of  FDDI  (e.g., ‘fddi protocol arp’) and
                      Token Ring (e.g., ‘tr protocol arp’), for most  of  those
                      protocols,  the  protocol  identification  comes from the
                      802.2 Logical Link Control (LLC) header, which is usually
                      layered on top of the FDDI or Token Ring header.
 
                      When  filtering  for most protocol identifiers on FDDI or
                      Token Ring, tcpdump checks only the protocol ID field  of
                      an  LLC header in so-called SNAP format with an Organiza‐
                      tional Unit Identifier (OUI) of  0x000000,  for  encapsu‐
                      lated Ethernet; it doesn’t check whether the packet is in
                      SNAP format with an OUI of 0x000000.
 
                      The exceptions are iso, for  which  it  checks  the  DSAP
                      (Destination  Service Access Point) and SSAP (Source Ser‐
                      vice Access Point) fields of the LLC header, stp and net‐
                      beui,  where  it  checks  the DSAP of the LLC header, and
                      atalk, where it checks for a SNAP-format packet  with  an
                      OUI of 0x080007 and the Appletalk etype.
 
                      In the case of Ethernet, tcpdump checks the Ethernet type
                      field for most of those  protocols;  the  exceptions  are
                      iso,  sap,  and netbeui, for which it checks for an 802.3
                      frame and then checks the LLC header as it does for  FDDI
                      and  Token  Ring,  atalk,  where  it  checks both for the
                      Appletalk etype in an Ethernet frame and for a  SNAP-for‐
                      mat  packet  as  it  does  for FDDI and Token Ring, aarp,
                      where it checks for the Appletalk ARP etype in either  an
                      Ethernet  frame  or  an  802.2  SNAP frame with an OUI of
                      0x000000, and ipx, where it checks for the IPX  etype  in
                      an  Ethernet  frame,  the IPX DSAP in the LLC header, the
                      802.3 with no LLC header encapsulation of  IPX,  and  the
                      IPX etype in a SNAP frame.]
 
               decnet src host
                      True  if  the DECNET source address is host, which may be
                      an address of the form ‘‘10.123’’, or a DECNET host name.
                      [DECNET  host  name  support  is only available on Ultrix
                      systems that are configured to run DECNET.]
 
               decnet dst host
                      True if the DECNET destination address is host.
 
               decnet host host
                      True if either the DECNET source or  destination  address
                      is host.
 
               ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
                      Abbreviations for:
                           ether proto p
                      where p is one of the above protocols.
 
               lat, moprc, mopdl
                      Abbreviations for:
                           ether proto p
                      where p is one of the above protocols.  Note that tcpdump
                      does not currently know how to parse these protocols.
 
               vlan [vlan_id]
                      True if the packet is an IEEE  802.1Q  VLAN  packet.   If
                      [vlan_id]  is  specified, only true is the packet has the
                      specified vlan_id.  Note  that  the  first  vlan  keyword
                      encountered  in  expression  changes the decoding offsets
                      for the remainder of expression on  the  assumption  that
                      the packet is a VLAN packet.
 
               tcp, udp, icmp
                      Abbreviations for:
                           ip proto p or ip6 proto p
                      where p is one of the above protocols.
 
               iso proto protocol
                      True if the packet is an OSI packet of protocol type pro‐
                      tocol.  Protocol can be a number  or  one  of  the  names
                      clnp, esis, or isis.
 
               clnp, esis, isis
                      Abbreviations for:
                           iso proto p
                      where p is one of the above protocols.  Note that tcpdump
                      does an incomplete job of parsing these protocols.
 
               expr relop expr
                      True if the relation holds, where relop is one of  >,  <,
                      >=,  <=, =, !=, and expr is an arithmetic expression com‐
                      posed of integer constants (expressed in standard C  syn‐
                      tax),  the  normal binary operators [+, -, *, /, &, |], a
                      length operator, and special packet data  accessors.   To
                      access data inside the packet, use the following syntax:
                           proto [ expr : size ]
                      Proto is one of ether, fddi, tr, ip, arp, rarp, tcp, udp,
                      icmp or ip6, and indicates the  protocol  layer  for  the
                      index  operation.   Note  that  tcp, udp and other upper-
                      layer protocol types only apply to IPv4, not  IPv6  (this
                      will  be fixed in the future).  The byte offset, relative
                      to the indicated protocol layer, is given by expr.   Size
                      is  optional  and  indicates  the  number of bytes in the
                      field of interest; it can be either one,  two,  or  four,
                      and  defaults  to one.  The length operator, indicated by
                      the keyword len, gives the length of the packet.
 
                      For example, ‘ether[0] & 1 != 0’  catches  all  multicast
                      traffic.   The  expression ‘ip[0] & 0xf != 5’ catches all
                      IP packets  with  options.   The  expression  ‘ip[6:2]  &
                      0x1fff  = 0’ catches only unfragmented datagrams and frag
                      zero of fragmented datagrams.  This check  is  implicitly
                      applied  to  the  tcp  and  udp  index  operations.   For
                      instance, tcp[0] always means the first byte of  the  TCP
                      header,  and never means the first byte of an intervening
                      fragment.
 
                      Some offsets and field values may be expressed  as  names
                      rather  than  as  numeric values.  The following protocol
                      header field offsets are available: icmptype  (ICMP  type
                      field),  icmpcode  (ICMP  code  field), and tcpflags (TCP
                      flags field).
 
                      The following ICMP type field values are available: icmp-
                      echoreply,  icmp-unreach,  icmp-sourcequench,  icmp-redi     
                      rect, icmp-echo,  icmp-routeradvert,  icmp-routersolicit,
                      icmp-timxceed,  icmp-paramprob,  icmp-tstamp, icmp-tstam     
                      preply, icmp-ireq,  icmp-ireqreply,  icmp-maskreq,  icmp-
                      maskreply.
 
                      The  following TCP flags field values are available: tcp-
                      fin, tcp-syn, tcp-rst, tcp-push, tcp-push, tcp-ack,  tcp-
                      urg.
 
               Primitives may be combined using:
 
                      A parenthesized group of primitives and operators (paren‐
                      theses are special to the Shell and must be escaped).
 
                      Negation (‘!’ or ‘not’).
 
                      Concatenation (‘&&’ or ‘and’).
 
                      Alternation (‘||’ or ‘or’).
 
               Negation has highest precedence.  Alternation and  concatenation
               have  equal  precedence  and associate left to right.  Note that
               explicit and tokens, not juxtaposition,  are  now  required  for
               concatenation.
 
               If  an  identifier  is  given without a keyword, the most recent
               keyword is assumed.  For example,
                    not host vs and ace
               is short for
                    not host vs and host ace
               which should not be confused with
                    not ( host vs or ace )
 
               Expression arguments can be passed to tcpdump as either a single
               argument or as multiple arguments, whichever is more convenient.
               Generally, if the expression contains Shell  metacharacters,  it
               is  easier  to  pass  it as a single, quoted argument.  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 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 eth‐
        ernet 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     
        The  output  of  tcpdump  is protocol dependent.  The following gives a
        brief description and examples of most of the formats.
 
        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 data‐
        grams) are ‘async’ packets, with a priority value between 0 and 7;  for
        example,  ‘async4’.  Such packets are assumed to contain an 802.2 Logi‐
        cal 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.
 
        (N.B.: The following description assumes familiarity with the SLIP com‐
        pression algorithm described in RFC-1144.)
 
        On SLIP links, a direction indicator (‘‘I’’ for inbound, ‘‘O’’ for out‐
        bound), 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  pack‐
        ets,  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  num‐
        ber), 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  for‐
        mat  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  ether‐
        net  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.
 
        TCP Packets
 
        (N.B.:The following description assumes familiarity with the TCP proto‐
        col  described  in RFC-793.  If you are not familiar with the protocol,
        neither this description nor tcpdump will be of much use to you.)
 
        The general format of a tcp protocol line is:
               src > dst: flags data-seqno ack window urgent options
        Src and dst are the source and  destination  IP  addresses  and  ports.
        Flags  are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or
        a single ‘.’ (no flags).  Data-seqno describes the portion of  sequence
        space  covered  by the data in this packet (see example below).  Ack 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.  Options are tcp options enclosed in
        angle brackets (e.g., <mss 1024>).
 
        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.
               rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
               csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
               rtsg.1023 > csam.login: . ack 1 win 4096
               rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
               csam.login > rtsg.1023: . ack 2 win 4096
               rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
               csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
               csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
               csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 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(nbytes)’  which means ‘sequence numbers first up to but not
        including last which is nbytes bytes of  user  data’.)   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 no flags
        were set.  The packet contained no data so there is  no  data  sequence
        number.  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  ini‐
        tial  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  sit‐
        ting  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 dec‐
        imal 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 representa‐
        tion  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 follow‐
        ing 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     
 
        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 con‐
        tained 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 particu‐
        lar, 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 head‐
        ers.  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  (NoEr‐
        ror) 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.
 
        Note  that  name server requests and responses tend to be large and the
        default snaplen of 68 bytes may not capture enough  of  the  packet  to
        print.   Use  the  -s flag to increase the snaplen if you need to seri‐
        ously investigate name server traffic.  ‘-s 128’ has  worked  well  for
        me.
 
        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  Net‐
        BEUI 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.
 
        If you are decoding SMB sessions containing unicode  strings  then  you
        may  wish to set the environment variable USE_UNICODE to 1.  A patch to
        auto-detect unicode srings would be welcome.
 
        For information on SMB packet formats and what all te fields  mean  see
        www.cifs.org  or  the  pub/samba/specs/  directory  on  your  favourite
        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.xid > dst.nfs: len op args
               src.nfs > dst.xid: reply stat len op results
 
               sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
               wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
               sushi.201b > wrl.nfs:
                    144 lookup fh 9,74/4096.6878 "xcolors"
               wrl.nfs > sushi.201b:
                    reply ok 128 lookup fh 9,74/4134.3150
        In  the  first line, host sushi sends a transaction with id 6709 to wrl
        (note that the number following the src host is a transaction  id,  not
        the  source port).  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.)  Wrl replies ‘ok’
        with the contents of the link.
 
        In the third line, sushi asks wrl  to  lookup  the  name  ‘xcolors’  in
        directory  file  9,74/4096.6878.  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.
 
        If  the  -v (verbose) flag is given, additional information is printed.
        For example:
 
               sushi.1372a > wrl.nfs:
                    148 read fh 21,11/12.195 8192 bytes @ 24576
               wrl.nfs > sushi.1372a:
                    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  off‐
        set  24576.   Wrl  replies ‘ok’; the packet shown on the second line is
        the first fragment of the reply, and hence is only 1472 bytes long (the
        other bytes will follow in subsequent fragments, but these fragments do
        not have NFS or even UDP headers and so might not be printed, depending
        on  the filter expression used).  Because the -v flag is given, some of
        the file attributes (which are returned in addition to the  file  data)
        are  printed:  the file type (‘‘REG’’, for regular file), the file mode
        (in octal), the uid and gid, and the file size.
 
        If the -v flag is given more than once, even more details are  printed.
 
        Note  that  NFS requests are very large and much of the detail won’t be
        printed unless snaplen is increased.  Try using ‘-s 192’ to  watch  NFS
        traffic.
 
        NFS  reply  packets  do  not  explicitly  identify  the  RPC operation.
        Instead, tcpdump keeps track of ‘‘recent’’ requests, and  matches  them
        to  the  replies using the transaction ID.  If a reply does not closely
        follow the corresponding request, it might not be parsable.
 
        AFS Requests and Replies
 
        Transarc AFS (Andrew File System) requests and replies are printed as:
 
               src.sport > dst.dport: rx packet-type
               src.sport > dst.dport: rx packet-type service call call-name args
               src.sport > dst.dport: rx packet-type service reply call-name args
 
               elvis.7001 > pike.afsfs:
                    rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
                    new fid 536876964/1/1 ".newsrc"
               pike.afsfs > elvis.7001: rx data fs reply rename
        In the first line, host elvis sends a RX packet to pike.  This was a RX
        data  packet to the fs (fileserver) service, and is the start of an RPC
        call.  The RPC call was a rename, with the old  directory  file  id  of
        536876964/1/1 and an old filename of ‘.newsrc.new’, and a new directory
        file id of 536876964/1/1 and a new filename  of  ‘.newsrc’.   The  host
        pike  responds  with a RPC reply to the rename call (which was success‐
        ful, 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 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 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 dis‐
        carded).   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  request‐
        ing  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,  jss‐
        mag.209  initiates  the next request.  The ‘*’ on the request indicates
        that XO (‘exactly once’) was not set.
 
        IP Fragmentation
 
        Fragmented Internet datagrams are printed as
               (frag id:size@offset+)
               (frag id:size@offset)
        (The first form indicates there are more fragments.  The  second  indi‐
        cates this is the last fragment.)
 
        Id  is the fragment id.  Size is the fragment size (in bytes) excluding
        the IP header.  Offset is this fragment’s  offset  (in  bytes)  in  the
        original datagram.
 
        The  fragment information is output for each fragment.  The first frag‐
        ment contains the higher level protocol header and  the  frag  info  is
        printed  after the protocol info.  Fragments after the first contain no
        higher level protocol header and the frag info  is  printed  after  the
        source  and destination addresses.  For example, here is part of an ftp
        from arizona.edu to lbl-rtsg.arpa over a CSNET connection that  doesn’t
        appear to handle 576 byte datagrams:
               arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
               arizona > rtsg: (frag 595a:204@328)
               rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
        There are a couple of things to note here:  First, addresses in the 2nd
        line don’t include port numbers.  This  is  because  the  TCP  protocol
        information  is  all in the first fragment and we have no idea what the
        port or sequence numbers are when we print the later  fragments.   Sec‐
        ond,  the  tcp  sequence information in the first line is printed as if
        there were 308 bytes of user data when, in fact, there  are  512  bytes
        (308  in the first frag and 204 in the second).  If you are looking for
        holes in the sequence space or trying to match up  acks  with  packets,
        this can fool you.
 
        A  packet  with  the  IP  don’t fragment flag is marked with a trailing
        (DF).
 
        Timestamps
 
        By default, all output lines are preceded by a timestamp.   The  times‐
        tamp 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 first saw the packet.  No attempt is  made  to  account
        for the time lag between when the ethernet interface removed the packet
        from the wire and when the kernel serviced the ‘new packet’  interrupt.
        bpf(4), pcap(3)
 

AUTHORS

        The original authors are:
 
        Van  Jacobson,  Craig  Leres  and  Steven  McCanne, all of the Lawrence
        Berkeley National Laboratory, University of California, Berkeley, CA.
 
        It is currently being maintained by tcpdump.org.
 
        The current version is available via http:
 
               http://www.tcpdump.org/
 
        The original distribution is available via anonymous ftp:
 
               ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
 
        IPv6/IPsec support is added by WIDE/KAME project.   This  program  uses
        Eric Young’s SSLeay library, under specific configuration.
 

BUGS

        Please send problems, bugs, questions, desirable enhancements, etc. to:
 
               tcpdump-workers@tcpdump.org
 
        Please send source code contributions, etc. to:
 
               patches@tcpdump.org
 
        NIT doesn’t let you watch your own outbound traffic, BPF will.  We rec‐
        ommend 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 pack‐
               ets 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  cap‐
               ture  mechanism, if asked to copy only part of a packet to user‐
               land, will not report the true length of the packet; this  would
               cause most IP packets to get an error from tcpdump).
 
        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) ques‐
        tion  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 that manipulate FDDI or Token  Ring  headers  assume
        that  all  FDDI  and  Token Ring packets are SNAP-encapsulated Ethernet
        packets.  This is true for IP, ARP, and DECNET Phase  IV,  but  is  not
        true  for  protocols such as ISO CLNS.  Therefore, the filter may inad‐
        vertently accept certain packets that do not properly match the  filter
        expression.
 
        Filter  expressions  on  fields  other than those that manipulate Token
        Ring headers will not correctly handle source-routed Token  Ring  pack‐
        ets.
 
        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.
 
                                 3 January 2001                      TCPDUMP(1)
 

Sections

Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.