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

NAME
     tcpdump - dump traffic on a network

SYNOPSIS
     tcpdump [-adefILlNnOopqStvXx] [-c count] [-D direction]
             [-E [espalg:]espkey] [-F file] [-i interface] [-r file]
             [-s snaplen] [-T type] [-w file] [-y datalinktype] [expression]

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

     The options are as follows:

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

     -c count  Exit after receiving count packets.

     -D direction
               Select packets flowing in the specified direction.  Valid di-
               rections are: in and out.  The default is to accept packets
               flowing in any direction.

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

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

     -F file   Use file as input for the filter expression.  Any additional
               expressions given on the command line are ignored.

     -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.

     -I        Print the interface on each dump line.

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

     -L        List the supported data link types for the interface and exit.

     -l        Make stdout line buffered.  Useful if you want to see the data
               while capturing it.  For example:

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

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

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

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

     -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.

     -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 "{local-hw-
               addr}"'' or ``ether broadcast''.

     -q        Quick (quiet?) output.  Print less protocol information so out-
               put 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        Print absolute, rather than relative, TCP sequence numbers.

     -s snaplen
               Analyze at most the first snaplen bytes of data from each pack-
               et rather than the default of 116.  116 bytes is adequate for
               IPv6, ICMP, TCP, and UDP, but may truncate protocol information
               from name server and NFS packets (see below).  Packets truncat-
               ed because of a limited snaplen are indicated in the output
               with ``[|proto]'', where proto is the name of the protocol lev-
               el at which the truncation has occurred.  Taking larger snap-
               shots 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 lim-
               it snaplen to the smallest number that will capture the proto-
               col information you're interested in.

     -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), tcp (Transmission
               Control Protocol), vat (Visual Audio Tool), and wb (distributed
               White Board).

     -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.

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

     -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 option.
               Standard output is used if file is `-'.

     -X        Print each packet in hex and ASCII.  If the -e option is also
               specified, the link-level header will be included.  The smaller
               of the entire packet or snaplen bytes will be printed.

     -x        Print each packet in hex.  If the -e option is also specified,
               the link-level header will be included.  The smaller of the en-
               tire packet or snaplen bytes will be printed.

     -y datalinktype
               Set the data link type to use while capturing to datalinktype.
               Commonly used types include EN10MB, IEEE802_11, and
               IEEE802_11_RADIO.  The choices applicable to a particular de-
               vice can be listed using -L.

     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 qualifiers.
     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.  Possi-
            ble directions are src, dst, src or dst, src and dst, addr1,
            addr2, addr3, and addr4.  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.  The addr1, addr2, addr3, and addr4 qualifiers
            are only valid for IEEE 802.11 Wireless LAN link layers.  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 direction.

     proto  Restrict the match to a particular protocol.  Possible protocols
            are: ah, arp, atalk, decnet, esp, ether, fddi, icmp, icmp6, igmp,
            igrp, ip, ip6, lat, mopdl, moprc, pim, rarp, sca, stp, tcp, udp,
            and wlan.  E.g., ``ether src foo'', ``arp net 128.3'', ``tcp port
            21'', ``wlan addr1 0:2:3:4:5:6''.  If there is no protocol quali-
            fier, 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 iden-
            tically as meaning "the data link level used on the specified
            network interface".  FDDI (Fiber Distributed Data Interface) head-
            ers 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.  FD-
            DI headers also contain other fields, but you cannot name them ex-
            plicitly 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 arith-
     metic 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 omit-
     ted 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 name.

     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 prepended
                        with the keywords, ip, arp, or rarp 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 ad-
                        dress 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 num-
                        ber (see ethers(3) for a numeric format).

     ether src ehost    True if the Ethernet source address is ehost.

     ether host ehost   True if either the Ethernet source or destination ad-
                        dress 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 both
                        /etc/hosts and /etc/ethers.  An equivalent expression
                        is

                              ether host ehost and not host host

                        which can be used with either names or numbers for
                        host/ehost.

     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
                        details).

     src net net        True if the IP source address of the packet has a net-
                        work 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 des-
                        tination port value of port.  The port can be a number
                        or name from services(5) (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 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 proto     True if the packet is an IP packet (see ip(4)) of pro-
                        tocol type proto.  proto can be a number or name from
                        protocols(5), such as icmp, udp, or tcp.  These iden-
                        tifiers are also keywords and must be escaped using a
                        backslash character (`\').

     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
                        conventions 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.

     ether proto proto  True if the packet is of ether type proto.  proto can
                        be a number or one of the names ip, ip6, arp, rarp,
                        atalk, atalkarp, decnet, decdts, decdns, lanbridge,
                        lat, mopdl, moprc, pup, sca, sprite, stp, vexp, vprod,
                        or xns.  These identifiers are also keywords and must
                        be escaped using a backslash character (`\').  In the
                        case of FDDI (e.g., ``fddi protocol arp''), the proto-
                        col 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 in-
                        clude 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 host.

     decnet host host   True if either the DECNET source or destination ad-
                        dress is host.

     ifname interface   True if the packet was logged as coming from the spec-
                        ified interface (applies only to packets logged by
                        pf(4)).

     on interface       Synonymous with the ifname modifier.

     rnr num            True if the packet was logged as matching the speci-
                        fied 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, bad-offset,
                        fragment, short, normalize, memory, bad-timestamp,
                        congestion, ip-option, proto-cksum, state-mismatch,
                        state-insert, state-limit, src-limit, and synproxy
                        (applies only to packets logged by pf(4)).

     rset name          True if the packet was logged as matching the speci-
                        fied 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 speci-
                        fied 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.  Valid actions are: pass, block, and match
                        (applies only to packets logged by pf(4)).

     wlan addr1 ehost   True if the first IEEE 802.11 address is ehost.

     wlan addr2 ehost   True if the second IEEE 802.11 address is ehost.

     wlan addr3 ehost   True if the third IEEE 802.11 address is ehost.

     wlan addr4 ehost   True if the fourth IEEE 802.11 address is ehost.  The
                        fourth address field is only used for WDS (Wireless
                        Distribution System) frames.

     wlan host ehost    True if either the first, second, third, or fourth
                        IEEE 802.11 address is ehost.

     type type          True if the IEEE 802.11 frame type matches the speci-
                        fied type.  Valid types are: data, mgt, ctl, or a nu-
                        meric value.

     subtype subtype    True if the IEEE 802.11 frame subtype matches the
                        specified subtype.  Valid subtypes are: assocreq,
                        assocresp, reassocreq, reassocresp, probereq,
                        proberesp, beacon, atim, disassoc, auth, deauth, data,
                        or a numeric value.

     dir dir            True if the IEEE 802.11 frame direction matches the
                        specified dir.  Valid directions are: nods, tods,
                        fromds, dstods, or a numeric value.

     atalk, ip, ip6, arp, decnet, lat, moprc, mopdl, rarp, sca
                        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.

     ah, esp, icmp, icmp6, igmp, igrp, pim, tcp, udp
                        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 spe-
                        cial packet data accessors.  To access data inside the
                        packet, use the following syntax:

                              proto[expr:size]

                        proto is one of ether, fddi, ip, arp, rarp, tcp, udp,
                        or icmp, and indicates the protocol layer for the in-
                        dex operation.  The byte offset, relative to the indi-
                        cated protocol layer, is given by expr.  size is op-
                        tional 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 multi-
                        cast 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.

     Primitives may be combined using a parenthesized group of primitives and
     operators.  Parentheses are special to the shell and must be escaped.
     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
     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 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 Ether-
     net 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] = 0x00001234'

OUTPUT FORMAT
     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 Eth-
     ernets, 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, bad-timestamp, 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 ``async''
     packets, with a priority value between 0 and 7; for example, async4.
     Such packets are assumed to contain an 802.2 Logical Link Control (LLC)
     packet; the LLC header is printed if it is not an ISO datagram or a so-
     called SNAP packet.

     The following description assumes familiarity with the SLIP compression
     algorithm described in RFC 1144.

     On SLIP links, a direction indicator (`I' for inbound, `O' for outbound),
     packet type, and compression information are printed out.  The packet
     type is printed first.  The three types are ip, utcp, and ctcp.  No fur-
     ther 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 se-
     quence 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 se-
     quence 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 Ether-
     net address CSAM.

     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
     The following description assumes familiarity with the TCP protocol de-
     scribed 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 options

     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 re-
     ceive 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 ap-
     propriate.

     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 lo-
     gin 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 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.
     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 sub-
     sequent packets of the connection, the difference between the current
     packet's sequence number and this initial sequence number is printed.
     This means that sequence numbers after the first can be interpreted as
     relative byte positions in the connection's data stream (with the first
     data byte each direction being 1).  -S will override this feature, caus-
     ing 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 small-
     er.  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
     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 particu-
     lar, Domain Name service requests (RFC 1034/1035) and Sun RPC calls (RFC
     1050) to NFS.

   UDP Name Server Requests
     The following description assumes familiarity with the Domain Service
     protocol described in RFC 1035.  If you are not familiar with the proto-
     col, 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)

     For example:

           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.

     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
     Name server responses are formatted as

           src > dst: id op rcode flags a / n / au type class data (len)

     For example:

           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 authority 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 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 print-
     ed.

     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 default
     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 investigate
     name server traffic.  ``-s 128'' has worked well for me.

   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.
     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 di-
     rectory file 9,74/4096.6878.  The data printed depends on the operation
     type.  The format is intended to be self-explanatory if read in conjunc-
     tion 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, 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 oth-
     er 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 un-
     less 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 fol-
     low the corresponding request, it might not be parsable.

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

           number            name
           1.254             ether
           16.1              icsd-net
           1.254.110         ace

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

     AppleTalk addresses are printed in the form

           net.host.  port

     For example:

           144.1.209.2 > 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 what-
     ever 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 /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 ics-
     di-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 re-
     source named RM1140 registered on port 250.  The third line is another
     reply to the same request saying host techpit has laserwriter techpit
     registered on port 186.

     ATP packet formatting is demonstrated by the following example:

           jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
           helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
           jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
           helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
           jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
           jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002

     Jssmag.209 initiates transaction ID 12266 with host helios by requesting
     up to 8 packets (the``<0-7>'').  The hex number at the end of the line is
     the value of the userdata field in the request.

     Helios responds with 8 512-byte packets.  The ``:n'' following the trans-
     action 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 re-
     sends 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
     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) excluding
     the IP header.  offset is this fragment's offset (in bytes) in the origi-
     nal datagram.

     The fragment information is output for each fragment.  The first fragment
     contains the higher level protocol header and the fragment info is print-
     ed 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 ari-
     zona.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 infor-
     mation 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 se-
     quence 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 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 ker-
     nel serviced the ``new packet'' interrupt.

   IP Checksum Offload
     Some network cards support IP checksum offload.  Packet headers for such
     interfaces erroneously indicate a bad checksum, since the checksum is not
     calculated until after tcpdump sees the packet.

SEE ALSO
     ethers(3), pcap(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4), udp(4),
     networks(5), pf.os(5), protocols(5), services(5)

     Transmission Control Protocol, RFC 793, September 1981.

     Domain Names - Concepts and Facilities, RFC 1034, November 1987.

     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
     1990.

     TCP Selective Acknowledgement Options, RFC 2018, October 1996.

     IP Encapsulating Security Payload (ESP), RFC 2406, November 1998.

AUTHORS
     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
     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) ques-
     tion 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 net-
     works 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.  There-
     fore, the filter may inadvertently accept certain packets that do not
     properly match the filter expression.

OpenBSD 4.7                    January 14, 2010                             15
