tcpdump



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


NAME

     tcpdump - dump traffic on a network


SYNOPSIS

     tcpdump [-AadefILlNnOopqStvXx] [-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        Print each packet in 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.

     -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
               directions 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 4835 ESP (Encapsulating Security Payload)
               traffic using the specified hex key espkey.  Supported
               algorithms 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
               option 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
               system 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
               example, 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
               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        Print absolute, rather than relative, TCP sequence numbers.

     -s snaplen
               Analyze at most the first snaplen bytes of data from each
               packet 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 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 snapshots both increases the amount of time it takes to
               process packets and, effectively, decreases the amount of
               packet buffering.  This may cause packets to be lost.  You
               should limit snaplen to the smallest number that will capture
               the protocol information you're interested in.

     -T type   Force packets selected by expression to be interpreted as the
               specified type.  Currently known types are:

                     cnfp   Cisco NetFlow protocol
                     gre    Generic Routing Encapsulation over UDP
                     mpls   Multiprocol Label Switching over UDP
                     rpc    Remote Procedure Call
                     rtcp   Real-Time Applications control protocol
                     rtp    Real-Time Applications protocol
                     sack   RFC 2018 TCP Selective Acknowledgements Options
                     tcp    Transmission Control Protocol
                     tftp   Trivial File Transfer Protocol
                     vat    Visual Audio Tool
                     vrrp   Virtual Router Redundancy protocol
                     vxlan  Virtual eXtensible Local Area Network
                     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
               entire 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
               device 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.
            Possible 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 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 specified
            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 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
                        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 ehost.

     ether host ehost   True if either the Ethernet source or destination
                        address is ehost.

     gateway host       True if the packet used host as a gateway; i.e., the
                        Ethernet source or destination address was host but
                        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/hosts or a network number (see hosts(5) for
                        details).

     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 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
                        protocol type proto.  proto can be a number or name
                        from protocols(5), such as icmp, udp, or tcp.  These
                        identifiers 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 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 host.

     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
                        pf(4)).

     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, 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
                        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.  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
                        specified type.  Valid types are: data, mgt, ctl, or a
                        numeric 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
                        special 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
                        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 packet.

                        For example, "ether[0] & 1 != 0" catches all multicast
                        traffic.  The expression "ip[0] & 0xf != 5" catches
                        all IP packets with options.  The expression "ip[6:2]
                        & 0x1fff = 0" catches only unfragmented datagrams and
                        frag zero of fragmented datagrams.  This check is
                        implicitly applied to the tcp and udp index
                        operations.  For instance, "tcp[0]" always means the
                        first byte of the TCP header, and never means the
                        first byte of an intervening fragment.

     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 misinterpreting 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 network
     192.168.7.0/24 (if you gateway to one other net, this stuff should never
     make it onto your local network):

           # tcpdump ip and not net 192.168.7.0/24

     To print the start and end packets (the SYN and FIN packets) of each TCP
     connection that involves a host that is not in local network
     192.168.7.0/24:

           # tcpdump 'tcp[13] & 3 != 0 and not src and dst net 192.168.7.0/24'

     To print only the SYN packets of HTTP connections:

           # tcpdump 'tcp[tcpflags] = tcp-syn and port http'

     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 only echo request ICMP packets:

           # tcpdump 'icmp[icmptype] = icmp-echo'

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

           # tcpdump -E des3-hmac96:ab...def 'ip[20:4] = 0x00001234'

     To print raw wireless frames passing the iwn0 interface:
           # tcpdump -i iwn0 -y IEEE802_11_RADIO -v


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
     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, 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
     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 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
     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 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
     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 1024>
       csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
       rtsg.1023 > csam.login: . ack 1 win 4096
       rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
       csam.login > rtsg.1023: . ack 2 win 4096
       rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
       csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
       csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
       csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1

     The first line says that TCP port 1023 on rtsg sent a packet to port
     login on 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
     subsequent 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,
     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
     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
     The following description assumes familiarity with the Domain Service
     protocol described in RFC 1035.  If you are not familiar with the
     protocol, the following description will appear to be written in Greek.

     Name server requests are formatted as

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

     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
     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 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
     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 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
     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.

   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
     original datagram.

     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 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
     kernel serviced the "new packet" interrupt.

   IP and Protocol Checksum Offload
     Some network cards support IP and/or protocol 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), pcap-filter(3), bpf(4), ip(4), pf(4), pflog(4),
     tcp(4), udp(4), hosts(5), pf.os(5), protocols(5), services(5)


STANDARDS

     Transmission Control Protocol, RFC 793, September 1981.

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

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

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

     V. Jacobson, Compressing TCP/IP Headers for Low-Speed Serial Links, RFC
     1144, February 1990.

     M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow, TCP Selective
     Acknowledgement Options, RFC 2018, October 1996.

     V. Manral, Cryptographic Algorithm Implementation Requirements for
     Encapsulating Security Payload (ESP) and Authentication Header (AH), RFC
     4835, April 2007.


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

     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.

     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 6.4                      July 6, 2018                      OpenBSD 6.4

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