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NAME    [Toc]    [Back]

     tcpdump - dump traffic on a network

SYNOPSIS    [Toc]    [Back]

     tcpdump [-adeflnNoOpqStvxX] [-c count] [-E  [espalg:]espkey]
[-F file]
             [-i  interface] [-r file] [-s snaplen] [-T type] [-w

DESCRIPTION    [Toc]    [Back]

     tcpdump prints out the headers of packets on a  network  interface that
     match  the boolean expression.  You must have read access to

     The options are as follows:

     -a        Attempt to convert network and broadcast addresses
to names.

     -c count  Exit after receiving count packets.

     -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

     -e        Print the link-level header on each dump line.

     -E [espalg:]espkey
               Try to decrypt RFC 2406 ESP (Encapsulating Security Payload)
               traffic  using the specified hex key espkey.  Supported algorithms
  for  espalg  are:  aes128,  aes128-hmac96,
               blowfish-hmac96,    cast,    cast-hmac96,    des3,
des3-hmac96, des and
               des-hmac96.     The    algorithm    defaults    to
aes128-hmac96.  This option
  should be used for debugging only, since the
key will show
               up in ps(1) output.

     -f        Print ``foreign'' internet  addresses  numerically
rather than
               symbolically.   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  file   Use file as input for the filter expression.  Any
               expressions given on the command line are ignored.

     -i interface
               Listen  on  interface.   If  unspecified,  tcpdump
searches the system
 interface list for the lowest  numbered,  configured ``up''
               interface  (excluding  loopback).  Ties are broken
by choosing
               the earliest match.

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

                     # tcpdump -l | tee dat
                     # tcpdump -l > dat & tail -f dat

     -n         Do  not  convert  addresses (host addresses, port
numbers, etc.)
               to names.

     -N        Do not print domain  name  qualification  of  host
names.  For example,
  if you specify this flag then tcpdump will
print ``nic''
               instead of ``nic.ddn.mil''.

     -o        Print a guess of the possible operating  system(s)
of hosts that
               sent TCP SYN packets.  See pf.os(5) for a description of the
               passive operating system fingerprints.

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

     -p         Do  not  put the interface into promiscuous mode.
The interface
               might be in promiscuous mode for some  other  reason; hence, -p
               cannot be used as an abbreviation for ``ether host
 or ``ether broadcast''.

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

     -r  file    Read  packets from a file which was created with
the -w option.
               Standard input is used if file is `-'.

     -s snaplen
               Analyze at most the first snaplen  bytes  of  data
from each packet
 rather than the default of 96.  96 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 snaplen are  indicated  in  the
output with
               ``[|proto]'',  where proto is the name of the protocol level at
               which the truncation has occurred.  Taking  larger
               both increases the amount of time it takes to process packets
               and, effectively, decreases the amount  of  packet
               This  may  cause  packets  to be lost.  You should
limit snaplen to
               the smallest number that will capture the protocol
               you're interested in.

     -S        Print absolute, rather than relative, TCP sequence

     -t        Do not print a timestamp on each dump line.

     -tt       Print an unformatted timestamp on each dump  line.

     -ttt      Print day and month in timestamp.

     -tttt     Print timestamp difference between packets.

     -ttttt    Print timestamp difference since the first packet.

     -T type   Force packets selected by expression to be  interpreted as the
               specified  type.   Currently  known types are vrrp
(Virtual Router
               Redundancy protocol), cnfp (Cisco  NetFlow  protocol), rpc
               (Remote  Procedure  Call), rtp (Real-Time Applications protocol),
               rtcp (Real-Time  Applications  control  protocol),
sack (RFC 2018
               TCP  Selective Acknowledgements Options), vat (Visual Audio
               Tool), and wb (distributed White Board).

     -v        (Slightly more) verbose output.  For example,  the
time to live
               (TTL)  and type of service (ToS) information in an
IP packet are

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

     -w  file   Write the raw packets to file rather than parsing
and printing
               them out.  They can be analyzed later with the  -r
               Standard 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         Like  -x  but dumps the packet in emacs-hexl like

     expression selects which packets  will  be  dumped.   If  no
expression is
     given,  all  packets  on the net will be dumped.  Otherwise,
only packets
     satisfying expression 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
     There are three different kinds of qualifiers:

     type   Specify which kind of address component 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     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 qualifier, src  or  dst  is  assumed.
For null link
            layers  (i.e.,  point-to-point protocols such as SLIP
(Serial Line
            Internet  Protocol)  or  the  pflog(4)  header),  the
inbound and
            outbound  qualifiers can be used to specify a desired

     proto  Restrict the match to a particular protocol.   Possible protocols
            are: arp, decnet, ether, fddi, ip, lat, mopdl, moprc,
rarp, tcp,
            and udp.  E.g., ``ether src foo'', ``arp net 128.3'',
``tcp port
            21''.   If there is no protocol 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
            network interface".  FDDI (Fiber Distributed Data Interface) 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 contain other fields, but you cannot
name them explicitly
 in a filter expression.

     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 IP destination field of the
packet is
                        host, which may be either an address or a

     src host host      True if the IP source field of the packet
is host.

     host host          True if either the IP source or  destination of the
                        packet is host.

                        Any  of the above host expressions can be
                        with the keywords, ip, arp,  or  rarp  as

                              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(3) for a numeric format).

     ether  src  ehost     True if the Ethernet source address is

     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
                        neither the IP source nor the IP destination was host.
                        host must be a name and must be found  in
                        /etc/hosts  and  /etc/ethers.  An equivalent expression

                              ether host ehost and not host host

                        which can be used with  either  names  or
numbers for

     dst net net        True if the IP 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(5) for

     src net net        True if the  IP  source  address  of  the
packet has a network
 number of net.

     net  net            True if either the IP source or destination address of
                        the packet has a network number of net.

     dst port port      True if the packet is  IP/TCP  or  IP/UDP
and has a destination
  port  value  of port.  The port
can be a number
                        or a  name  used  in  /etc/services  (see
tcp(4) and
                        udp(4)).   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  traffic, and ``dst 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
                        with the keywords tcp or udp, as in:

                              tcp src port port

                        which  matches  only  TCP  packets  whose
source port is

     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 proto     True if the packet is an IP  packet  (see
ip(4)) of protocol
  type proto.  proto can be a number
or one of the
                        names icmp, udp, nd, or tcp.  The identifiers tcp,
                        udp, and icmp are also shell keywords and
must be escaped.

     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  allones broadcast
                        conventions and looks up the local subnet

     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

     ether  proto  proto   True  if  the  packet is of ether type
proto.  proto can
                        be a number or a name like  ip,  arp,  or
rarp.  These
                        identifiers  are  also shell keywords and
must be escaped.
  In the case of FDDI (e.g., ``fddi
                        arp''), the protocol identification comes
from the
                        802.2 Logical Link Control (LLC)  header,
which is usually
  layered  on top of the FDDI header.
tcpdump assumes,
 when  filtering  on  the  protocol
identifier, that
                        all  FDDI  packets include an LLC header,
and that the
                        LLC header is in so-called SNAP format.

     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
                        systems  that  are configured to run DECNET.

     decnet dst host    True if the DECNET destination address is

     decnet  host host   True if either the DECNET source or destination address
 is host.

     ifname interface   True if the packet was logged  as  coming
from the specified
  interface (applies only to packets
logged by

     on interface       Synonymous with the ifname modifier.

     rnr num            True if the packet was logged as matching
the specified
  PF  rule number in the main ruleset
(applies only
                        to packets logged by pf(4)).

     rulenum num        Synonymous with the rnr modifier.

     reason code        True if the packet was  logged  with  the
specified PF
                        reason code.  The known codes are: match,
                        fragment,      bad-timestamp,      short,
normalize, and memory
                        (applies   only   to  packets  logged  by

     rset name          True if the packet was logged as matching
the specified
 PF ruleset name of an anchored ruleset (applies
                        only to packets logged by pf(4)).

     ruleset name       Synonymous with the rset modifier.

     srnr num           True if the packet was logged as matching
the specified
  PF rule number of an anchored ruleset (applies
                        only to packets logged by pf(4)).

     subrulenum num     Synonymous with the srnr modifier.

     action act         True if PF took the specified action when
the packet
                        was logged.  Known actions are: pass, and
block (applies
 only to packets logged by pf(4)).

     ip, arp, rarp, decnet, lat, moprc, mopdl
                        Abbreviations for:

                              ether proto p

                        where p is one of  the  above  protocols.
tcpdump does
                        not  currently  know  how  to  parse lat,
moprc, or mopdl.

     tcp, udp, icmp     Abbreviations for: ip proto p where p  is
one of the
                        above protocols.

     expr  relop  expr    True if the relation holds, where relop
is one of `>',
                        `<', `>=', `<=', `=', `!=', and  expr  is
an arithmetic
                        expression  composed of integer constants
(expressed in
                        standard C syntax), the normal binary operators (`+',
                        `-', `*', `/', `&', `|'), a length operator, and special
 packet data  accessors.   To  access
data inside the
                        packet, use the following syntax:


                        proto  is  one  of  ether, fddi, ip, arp,
rarp, tcp, udp,
                        or icmp, and indicates the protocol layer
for the index
 operation.  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

                        For example, ``ether[0] & 1 != 0'' catches all multicast
 traffic.  The expression  ``ip[0]  &
0xf != 5''
                        catches all IP packets with options.  The
                        ``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.

     Primitives  may  be  combined using a parenthesized group of
primitives and
     operators.  Parentheses are special to the shell and must be
     Allowable primitives and operators are:

           Negation (``!'' or ``not'')

           Concatenation (``&&'' or ``and'')

           Alternation (``||'' or ``or'')

     Negation has highest precedence.  Alternation and concatenation have
     equal precedence and associate left to right.  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    [Toc]    [Back]

     To print all packets arriving at or departing from sundown:

           # tcpdump host sundown

     To print traffic between helios and either hot or  ace  (the
expression is
     quoted to prevent the shell from mis-interpreting the parentheses):

           # 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:

           # 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
     connection that involves a non-local host:

           #  tcpdump  'tcp[13]  & 3 != 0 and not src and dst net

     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[0] != 8 and icmp[0] != 0'

     To print and decrypt all ESP packets with SPI 0x00001234:

           #   tcpdump   -E   des3-hmac96:ab...def   'ip[20:4]  =

OUTPUT FORMAT    [Toc]    [Back]

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

   Link Level Headers    [Toc]    [Back]
     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  the  packet  filter  logging interface pflog(4), logging
reason (rule
     match, bad-offset, fragment, short, normalize, memory),  action taken
     (pass/block),  direction  (in/out) and interface information
are printed
     out for each packet.

     On FDDI networks, the -e option causes tcpdump to print  the
frame control
     field,  the source and destination addresses, and the packet
length.  The
     frame control field governs the interpretation of  the  rest
of the packet.
     Normal  packets  (such as those containing IP datagrams) are
     packets, with a priority value between 0 and 7; for example,
     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 socalled
 SNAP packet.

     The  following description assumes familiarity with the SLIP
     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

     In this example, Ethernet addresses are in caps and internet
addresses in
     lower case.  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 CSAM.

     This would look less redundant if we had done tcpdump -n:

           arp who-has tell
           arp reply is-at 02:07:01:00:01:c4

     If we had done tcpdump -e, the fact that the first packet is
     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    [Toc]    [Back]
     The following description assumes familiarity with  the  TCP
protocol 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 src-os data-seqno ack  window  urgent

     src  and dst are the source and destination IP addresses and
ports.  flags
     is some combination of `S' (SYN), `F' (FIN), `P' (PUSH),  or
`R' (RST),
     `W'  (congestion Window reduced), `E' (ecn ECHO) or a single
`.' (no
     flags).  src-os will list a guess of the source host's operating system
     if   the  -o  command  line  flag  was  passed  to  tcpdump.
data-seqno describes
     the portion of sequence space covered by the  data  in  this
packet (see
     example below).  ack is the sequence number of the next data
expected by
     the other end of this connection.  window is the  number  of
bytes of receive
  buffer  space available at the other end of 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
       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  host 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 piggybacked
  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.
     The ack sequence number is a 32-bit integer.  The first time
tcpdump sees
     a TCP connection, it prints the  sequence  number  from  the
packet.  On subsequent
  packets  of  the connection, the difference between
the current
     packet's sequence number and this initial sequence number is
     This  means that sequence numbers after the first can be interpreted as
     relative byte positions  in  the  connection'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 connection).  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.

   UDP Packets    [Toc]    [Back]
     UDP format is illustrated by this rwho packet:

           actinide.who > broadcast.who: udp 84

     This says that port who on host actinide sent a UDP datagram
to port who
     on host broadcast,  the  Internet  broadcast  address.   The
packet contained
     84 bytes of user data.

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

   UDP Name Server Requests    [Toc]    [Back]
     The following description assumes familiarity with  the  Domain Service
     protocol  described  in  RFC  1035.  If you are not familiar
with the protocol,
 the following description will appear to be written  in

     Name server requests are formatted as

           src > dst: id op? flags qtype qclass name (len)


           h2opolo.1538  >  helios.domain:  3+  A?  ucbvax.berkeley.edu. (37)

     Host h2opolo asked the domain server on helios  for  an  address record
     (qtype=A) associated with the name ucbvax.berkeley.edu.  The
query id was
     3.  The `+' indicates the recursion desired  flag  was  set.
The query
     length  was  37 bytes, not including the UDP and IP protocol
headers.  The
     query operation was the normal one (Query) so the  op  field
was omitted.
     If 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 was
     omitted.   Any  other qclass would have been printed immediately after the

     A few anomalies are checked and may result in  extra  fields
enclosed in
     square  brackets: if a query contains an answer, name server
or authority
     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    [Toc]    [Back]
     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
           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 authority
records.  The first
     answer record is type A (address and its data  is  internet)
address  The total size of the response was 273 bytes,
     UDP and IP headers.  The op (Query) and rcode (NoError) were
omitted, as
     was the class (C_IN) of the A record.

     In the second example, helios responds to query op 2 with an
rcode 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.

     Name  server requests and responses tend to be large and the
     snaplen of 96 bytes may not capture enough of the packet  to
print.  Use
     the -s flag to increase the snaplen if you need to seriously
     name server traffic.  ``-s 128'' has worked well for me.

   NFS Requests and Replies    [Toc]    [Back]
     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.
     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
     op was a readlink (read symbolic link) on  fh  (``file  handle'')
     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 with a stat
of ok and
     the contents of the link.

     In the third line, sushi  asks  wrl  to  look  up  the  name
``xcolors'' in directory
  file  9,74/4096.6878.   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
     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,  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  with  a  stat of ok; the packet shown on the second
line is the
     first fragment of the reply, and hence is  only  1472  bytes
long.  The other
  bytes  will  follow  in  subsequent fragments, but these
fragments do not
     have NFS or even UDP headers and so might  not  be  printed,
depending on
     the  filter  expression used.  Because the -v flag is given,
some of the
     file attributes (which are returned in addition to the  file
data) are
     printed:  the  file type (`REG', for regular file), the file
mode (in
     octal), the UID and GID, and the file size.

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

     NFS  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 xid (transaction ID).  If a reply does not
closely follow
 the corresponding request, it might not be parsable.

   KIP AppleTalk (DDP in UDP)    [Toc]    [Back]
     AppleTalk DDP packets encapsulated in UDP datagrams are  deencapsulated
     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


  > icsd-net.112.220
           office.2 > icsd-net.112.220
           jssmag.149.235 > icsd-net.2

     If /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-net 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.   The  broadcast
address (255)
     is  indicated  by  a  net name with no host number; for this
reason it is a
     good idea to keep node  names  and  net  names  distinct  in

     NBP  (name  binding protocol) and ATP (AppleTalk transaction
     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 icsdi-net
 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>
           helios.132  >  jssmag.209.165:  atp-resp 12266:0 (512)
           helios.132 > jssmag.209.165:  atp-resp  12266:1  (512)
           helios.132  >  jssmag.209.165:  atp-resp 12266:2 (512)
           helios.132 > jssmag.209.165:  atp-resp  12266:3  (512)
           helios.132  >  jssmag.209.165:  atp-resp 12266:4 (512)
           helios.132 > jssmag.209.165:  atp-resp  12266:5  (512)
           helios.132  >  jssmag.209.165:  atp-resp 12266:6 (512)
           helios.132 >  jssmag.209.165:  atp-resp*12266:7  (512)
           jssmag.209.165   >   helios.132:  atp-req   12266<3,5>
           helios.132 > jssmag.209.165:  atp-resp  12266:3  (512)
           helios.132  >  jssmag.209.165:  atp-resp 12266:5 (512)
           jssmag.209.165  >  helios.132:   atp-rel    12266<0-7>
           jssmag.209.133   >   helios.132:  atp-req*  12267<0-7>

     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 ``:n'' following the transaction
  ID  gives the packet sequence number in the transaction and the
     number in parentheses 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.

   IP Fragmentation    [Toc]    [Back]
     Fragmented Internet datagrams are printed as

           (frag id : size @ offset [+])

     A `+' indicates there are more fragments.  The last fragment
will have no

     id is the fragment ID.  size is the fragment size (in bytes)
     the  IP header.  offset is this fragment's offset (in bytes)
in the original

     The fragment information is output for each  fragment.   The
first fragment
     contains  the  higher level protocol header and the fragment
info is printed
 after the protocol info.  Fragments after the first  contain no higher
     level protocol header and the fragment 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.
Second, 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

   Timestamps    [Toc]    [Back]
     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 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.

SEE ALSO    [Toc]    [Back]

     ethers(3),  pcap(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4),
     networks(5), pf.os(5)

     Transmission Control Protocol, RFC 793, September 1981.

     Domain Names - Concepts and Facilities, RFC  1034,  November

     Domain  Names  - Implementation and Specification, RFC 1035,
November 1987.

     RPC: Remote Procedure Call, RFC 1050, April 1988.

     Compressing TCP/IP Headers for Low-Speed Serial  Links,  RFC
1144, February

     TCP  Selective  Acknowledgement  Options,  RFC 2018, October

     IP Encapsulating Security Payload (ESP), RFC 2406,  November

AUTHORS    [Toc]    [Back]

     Van Jacobson <van@ee.lbl.gov>,
     Craig Leres <leres@ee.lbl.gov>, and
     Steven  McCanne  <mccanne@ee.lbl.gov>,  all  of the Lawrence
Berkeley Laboratory,
 University of California, Berkeley, CA.

BUGS    [Toc]    [Back]

     Please send bug reports to  <tcpdump@ee.lbl.gov>  or  <libpcap@ee.lbl.gov>.

     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 the 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.

     Apple Ethertalk DDP packets could be dumped as easily as KIP
DDP packets
     but aren't.  Even if we were inclined to do anything to promote the use
     of  Ethertalk (we aren't, LBL doesn't allow Ethertalk on any
of its networks
 so we'd have no way of testing this code).

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

     Filter  expressions that manipulate FDDI headers assume that
all FDDI
     packets are 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 inadvertently  accept  certain  packets
that do not
     properly match the filter expression.

OpenBSD      3.6                           May      25,      1999
[ Back ]
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