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    #REDIRECT [[Wikipedia:Image:Autofellatio_2.jpg]]
    TCPDUMP(1) TCPDUMP(1)

    NAME
    tcpdump - dump traffic on a network

    SYNOPSIS
    tcpdump [ -adeflnNOpqStvx ] [ -c count ] [ -F file ]
    [ -i interface ] [ -r file ] [ -s snaplen ]
    [ -T type ] [ -w file ] [ expression ]

    DESCRIPTION
    Tcpdump prints out the headers of packets on a network
    interface that match the boolean expression.

    Under SunOS with nit or bpf: To run tcpdump you must have
    read access to /dev/nit or /dev/bpf*. Under Solaris with
    dlpi: You must have read access to the network pseudo
    device, e.g. /dev/le. Under HP-UX with dlpi: You must be
    root or it must be installed setuid to root. Under IRIX
    with snoop: You must be root or it must be installed
    setuid to root. Under Linux: You must be root or it must
    be installed setuid to root. Under Ultrix and Digital
    UNIX: Once the super-user has enabled promiscuous-mode
    operation using pfconfig(8), any user may run tcpdump.
    Under BSD: You must have read access to /dev/bpf*.

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

    -c Exit after receiving count packets.

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

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

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

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

    -f Print `foreign' internet addresses numerically
    rather than symbolically (this option is intended
    to get around serious brain damage in Sun's yp
    server -- usually it hangs forever translating non-
    local internet numbers).

    -F Use file as input for the filter expression. An
    additional expression given on the command line is
    ignored.

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

    -l Make stdout line buffered. Useful if you want to
    see the data while capturing it. E.g.,
    ``tcpdump -l | tee dat'' or ``tcpdump -l >
    dat & tail -f dat''.

    -n Don't convert addresses (i.e., host addresses, port
    numbers, etc.) to names.

    -N Don't print domain name qualification of host
    names. E.g., if you give this flag then tcpdump
    will print ``nic'' instead of ``nic.ddn.mil''.

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

    -p Don't put the interface into promiscuous mode.
    Note that the interface might be in promiscuous
    mode for some other reason; hence, `-p' cannot be
    used as an abbreviation for `ether host {local-hw-
    addr} or ether broadcast'.

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

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

    -s Snarf snaplen bytes of data from each packet rather
    than the default of 68 (with SunOS's NIT, the mini-
    mum is actually 96). 68 bytes is adequate for IP,
    ICMP, TCP and UDP but may truncate protocol infor-
    mation from name server and NFS packets (see
    below). Packets truncated because of a limited
    snapshot are indicated in the output with
    ``[|proto]'', where proto is the name of the proto-
    col level at which the truncation has occurred.
    Note that taking larger snapshots both increases
    the amount of time it takes to process packets and,
    effectively, decreases the amount of packet buffer-
    ing. This may cause packets to be lost. You
    should limit snaplen to the smallest number that
    will capture the protocol information you're inter-
    ested in.

    -T Force packets selected by "expression" to be inter-
    preted the specified type. Currently known types
    are rpc (Remote Procedure Call), rtp (Real-Time
    Applications protocol), rtcp (Real-Time Applica-
    tions control protocol), vat (Visual Audio Tool),
    and wb (distributed White Board).

    -S Print absolute, rather than relative, TCP sequence
    numbers.

    -t Don't print a timestamp on each dump line.

    -tt Print an unformatted timestamp on each dump line.

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

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

    -w Write the raw packets to file rather than parsing
    and printing them out. They can later be printed
    with the -r option. Standard output is used if
    file is ``-''.

    -x Print each packet (minus its link level header) in
    hex. The smaller of the entire packet or snaplen
    bytes will be printed.

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

    The expression consists of one or more primitives.
    Primitives usually consist of an id (name or num-
    ber) preceded by one or more qualifiers. There are
    three different kinds of qualifier:

    type qualifiers say what kind of thing the id
    name or number refers to. Possible types
    are host, net and port. E.g., `host foo',
    `net 128.3', `port 20'. If there is no type
    qualifier, host is assumed.

    dir qualifiers specify a particular transfer
    direction to and/or from id. Possible
    directions are src, dst, src or dst and src
    and dst. E.g., `src foo', `dst net 128.3',
    `src or dst port ftp-data'. If there is no
    dir qualifier, src or dst is assumed. For
    `null' link layers (i.e. point to point pro-
    tocols such as slip) the inbound and out-
    bound qualifiers can be used to specify a
    desired direction.

    proto qualifiers restrict the match to a particu-
    lar protocol. Possible protos are: ether,
    fddi, ip, arp, rarp, decnet, lat, sca,
    moprc, mopdl, iso, esis, isis, tcp and udp.
    E.g., `ether src foo', `arp net 128.3', `tcp
    port 21'. If there is no proto qualifier,
    all protocols consistent with the type are
    assumed. E.g., `src foo' means `(ip or arp
    or rarp) src foo' (except the latter is not
    legal syntax), `net bar' means `(ip or arp
    or rarp) net bar' and `port 53' means `(tcp
    or udp) port 53'.

    [`fddi' is actually an alias for `ether'; the
    parser treats them identically as meaning ``the
    data link level used on the specified network
    interface.'' FDDI headers contain Ethernet-like
    source and destination addresses, and often contain
    Ethernet-like packet types, so you can filter on
    these FDDI fields just as with the analogous Ether-
    net 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 primi-
    tives. E.g., `host foo and not port ftp and not
    port ftp-data'. To save typing, identical quali-
    fier 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(3N) for
    numeric format).

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

    ether host ehost
    True if either the ethernet source or desti-
    nation 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 num-
    bers 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(4) 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.

    net net mask mask
    True if the IP address matches net with the
    specific netmask. May be qualified with src
    or dst.

    net net/len
    True if the IP address matches net a netmask
    len bits wide. May be qualified with src or
    dst.

    dst port port
    True if the packet is ip/tcp or ip/udp and
    has a destination port value of port. The
    port can be a number or a name used in
    /etc/services (see tcp(4P) and udp(4P)). If
    a name is used, both the port number and
    protocol are checked. If a number or
    ambiguous name is used, only the port number
    is checked (e.g., dst port 513 will print
    both tcp/login traffic and udp/who traffic,
    and port domain will print both tcp/domain
    and udp/domain traffic).

    src port port
    True if the packet has a source port value
    of port.

    port port
    True if either the source or destination
    port of the packet is port. Any of the
    above port expressions can be prepended with
    the keywords, tcp or udp, as in:
    tcp src port port
    which matches only tcp packets whose source
    port is port.

    less length
    True if the packet has a length less than or
    equal to length. This is equivalent to:
    len <= length.

    greater length
    True if the packet has a length greater than
    or equal to length. This is equivalent to:
    len >= length.

    ip proto protocol
    True if the packet is an ip packet (see
    ip(4P)) of protocol type protocol. Protocol
    can be a number or one of the names icmp,
    igrp, udp, nd, or tcp. Note that the iden-
    tifiers tcp, udp, and icmp are also keywords
    and must be escaped via backslash (), which
    is \ in the C-shell.

    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 protocol
    True if the packet is of ether type proto-
    col. Protocol can be a number or a name
    like ip, arp, or rarp. Note these identi-
    fiers are also keywords and must be escaped
    via backslash (). [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
    Ultrix systems that are configured to run
    DECNET.]

    decnet dst host
    True if the DECNET destination address is
    host.

    decnet host host
    True if either the DECNET source or destina-
    tion address is host.

    ip, arp, rarp, decnet, iso
    Abbreviations for:
    ether proto p
    where p is one of the above protocols.

    lat, moprc, mopdl
    Abbreviations for:
    ether proto p
    where p is one of the above protocols. Note
    that tcpdump does not currently know how to
    parse these protocols.

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

    esis, isis
    Abbreviations for:
    iso proto p
    where p is one of the above protocols. Note
    that tcpdump does an incomplete job of pars-
    ing these 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 proto-
    col 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).

    Negation (`!' or `not').

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

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

    Negation has highest precedence. Alternation and
    concatenation have equal precedence and associate
    left to right. Note that explicit and tokens, not
    juxtaposition, are now required for concatenation.

    If an identifier is given without a keyword, the
    most recent keyword is assumed. For example,
    not host vs and ace
    is short for
    not host vs and host ace
    which should not be confused with
    not ( host vs or ace )

    Expression arguments can be passed to tcpdump as
    either a single argument or as multiple arguments,
    whichever is more convenient. Generally, if the
    expression contains Shell metacharacters, it is
    easier to pass it as a single, quoted argument.
    Multiple arguments are concatenated with spaces
    before being parsed.

    EXAMPLES
    To print all packets arriving at or departing from sun-
    down:
    tcpdump host sundown

    To print traffic between helios and either hot or ace:
    tcpdump host helios and ( hot or ace )

    To print all IP packets between ace and any host except
    helios:
    tcpdump ip host ace and not helios

    To print all traffic between local hosts and hosts at
    Berkeley:
    tcpdump net ucb-ether

    To print all ftp traffic through internet gateway snup:
    (note that the expression is quoted to prevent the shell
    from (mis-)interpreting the parentheses):
    tcpdump 'gateway snup and (port ftp or ftp-data)'

    To print traffic neither sourced from nor destined for
    local hosts (if you gateway to one other net, this stuff
    should never make it onto your local net).
    tcpdump ip and not net localnet

    To print the start and end packets (the SYN and FIN pack-
    ets) of each TCP conversation 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 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'

    OUTPUT FORMAT
    The output of tcpdump is protocol dependent. The follow-
    ing gives a brief description and examples of most of the
    formats.

    Link Level Headers

    If the '-e' option is given, the link level header is
    printed out. On ethernets, the source and destination
    addresses, protocol, and packet length are printed.

    On FDDI networks, the '-e' option causes tcpdump to print
    the `frame control' field, the source and destination
    addresses, and the packet length. (The `frame control'
    field governs the interpretation of the rest of the
    packet. Normal packets (such as those containing IP data-
    grams) are `async' packets, with a priority value between
    0 and 7; for example, `async4'. Such packets are assumed
    to contain an 802.2 Logical Link Control (LLC) packet; the
    LLC header is printed if it is not an ISO datagram or a
    so-called SNAP packet.

    (N.B.: The following description assumes familiarity with
    the SLIP compression algorithm described in RFC-1144.)

    On SLIP links, a direction indicator (``I'' for inbound,
    ``O'' for outbound), packet type, and compression informa-
    tion 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 spe-
    cial 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 com-
    pressed TCP packet, with an implicit connection identi-
    fier; 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 argu-
    ments. The format is intended to be self explanatory.
    Here is a short sample taken from the start of an `rlogin'
    from host rtsg to host csam:
    arp who-has csam tell rtsg
    arp reply csam is-at CSAM
    The first line says that rtsg sent an arp packet asking
    for the ethernet address of internet host csam. Csam
    replies with its ethernet address (in this example, ether-
    net addresses are in caps and internet addresses in lower
    case).

    This would look less redundant if we had done tcpdump -n:
    arp who-has 128.3.254.6 tell 128.3.254.68
    arp reply 128.3.254.6 is-at 02:07:01:00:01:c4

    If we had done tcpdump -e, the fact that the first packet
    is broadcast and the second is point-to-point would be
    visible:
    RTSG Broadcast 0806 64: arp who-has csam tell rtsg
    CSAM RTSG 0806 64: arp reply csam is-at CSAM
    For the first packet this says the ethernet source address
    is RTSG, the destination is the ethernet broadcast
    address, the type field contained hex 0806 (type
    ETHER_ARP) and the total length was 64 bytes.

    TCP Packets

    (N.B.:The following description assumes familiarity with
    the TCP 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 data-seqno ack window urgent options
    Src and dst are the source and destination IP addresses
    and ports. Flags are some combination of S (SYN), F
    (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
    Data-seqno describes the portion of sequence space covered
    by the data in this packet (see example below). Ack is
    sequence number of the next data expected the other direc-
    tion on this connection. Window is the number of bytes of
    receive buffer space available the other direction on this
    connection. Urg indicates there is `urgent' data in the
    packet. Options are tcp options enclosed in angle brack-
    ets (e.g., <mss 1024>).

    Src, dst and flags are always present. The other fields
    depend on the contents of the packet's tcp protocol header
    and are output only if appropriate.

    Here is the opening portion of an rlogin from host rtsg to
    host csam.
    rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
    csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
    rtsg.1023 > csam.login: . ack 1 win 4096
    rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
    csam.login > rtsg.1023: . ack 2 win 4096
    rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
    csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
    csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
    csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
    The first line says that tcp port 1023 on rtsg sent a
    packet to port login on csam. The S indicates that the
    SYN flag was set. The packet sequence number was 768512
    and it contained no data. (The notation is
    `first:last(nbytes)' which means `sequence numbers first
    up to but not including last which is nbytes bytes of user
    data'.) There was no piggy-backed ack, the available
    receive window was 4096 bytes and there was a max-segment-
    size option requesting an mss of 1024 bytes.

    Csam replies with a similar packet except it includes a
    piggy-backed ack for rtsg's SYN. Rtsg then acks csam's
    SYN. The `.' means no flags were set. The packet con-
    tained no data so there is no data sequence number. Note
    that the ack sequence number is a small integer (1). The
    first time tcpdump sees a tcp `conversation', it prints
    the sequence number from the packet. On subsequent pack-
    ets of the conversation, the difference between the cur-
    rent packet's sequence number and this initial sequence
    number is printed. This means that sequence numbers after
    the first can be interpreted as relative byte positions in
    the conversation's data stream (with the first data byte
    each direction being `1'). `-S' will override this fea-
    ture, causing the original sequence numbers to be output.

    On the 6th line, rtsg sends csam 19 bytes of data (bytes 2
    through 20 in the rtsg -> csam side of the conversation).
    The PUSH flag is set in the packet. On the 7th line, csam
    says it's received data sent by rtsg up to but not includ-
    ing byte 21. Most of this data is apparently sitting in
    the socket buffer since csam's receive window has gotten
    19 bytes smaller. Csam also sends one byte of data to
    rtsg in this packet. On the 8th and 9th lines, csam sends
    two bytes of urgent, pushed data to rtsg.

    If the snapshot was small enough that tcpdump didn't cap-
    ture the full TCP header, it interprets as much of the
    header as it can and then reports ``[|tcp]'' to indicate
    the remainder could not be interpreted. If the header
    contains a bogus option (one with a length that's either
    too small or beyond the end of the header), tcpdump
    reports it as ``[bad opt]'' and does not interpret any
    further options (since it's impossible to tell where they
    start). If the header length indicates options are pre-
    sent but the IP datagram length is not long enough for the
    options to actually be there, tcpdump reports it as ``[bad
    hdr length]''.

    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 data-
    gram 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 des-
    tination port number) and the higher level protocol infor-
    mation printed. In particular, Domain Name service
    requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to
    NFS.

    UDP Name Server Requests

    (N.B.:The following description assumes familiarity with
    the Domain Service protocol described in RFC-1035. If you
    are not familiar with the protocol, the following descrip-
    tion will appear to be written in greek.)

    Name server requests are formatted as
    src > dst: id op? flags qtype qclass name (len)
    h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
    Host h2opolo asked the domain server on helios for an
    address record (qtype=A) associated with the name ucb-
    vax.berkeley.edu. The query id was `3'. The `+' indi-
    cates 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 the op had been anything
    else, it would have been printed between the `3' and the
    `+'. Similarly, the qclass was the normal one, C_IN, and
    omitted. Any other qclass would have been printed immedi-
    ately 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)
    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 response code
    (NoError) were omitted, as was the class (C_IN) of the A
    record.

    In the second example, helios responds to query 2 with a
    response code of non-existent domain (NXDomain) with no
    answers, one name server and no authority records. The
    `*' indicates that the authoritative answer bit was set.
    Since there were no answers, no type, class or data were
    printed.

    Other flag characters that might appear are `-' (recursion
    available, RA, not set) and `|' (truncated message, TC,
    set). If the `question' section doesn't contain exactly
    one entry, `[nq]' is printed.

    Note that name server requests and responses tend to be
    large and the default snaplen of 68 bytes may not capture
    enough of the packet to print. Use the -s flag to
    increase the snaplen if you need to 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 (note that the number following the src host
    is a transaction id, not the source port). The request
    was 112 bytes, excluding the UDP and IP headers. The
    operation was a readlink (read symbolic link) on file han-
    dle (fh) 21,24/10.731657119. (If one is lucky, as in this
    case, the file handle can be interpreted as a major,minor
    device number pair, followed by the inode number and gen-
    eration number.) Wrl replies `ok' with the contents of
    the link.

    In the third line, sushi asks wrl to lookup the name
    `xcolors' in directory file 9,74/4096.6878. Note that the
    data printed depends on the operation type. The format is
    intended to be self explanatory if read in conjunction
    with an NFS protocol spec.

    If the -v (verbose) flag is given, additional information
    is printed. For example:

    sushi.1372a > wrl.nfs:
    148 read fh 21,11/12.195 8192 bytes @ 24576
    wrl.nfs > sushi.1372a:
    reply ok 1472 read REG 100664 ids 417/0 sz 29388

    (-v also prints the IP header TTL, ID, and fragmentation
    fields, which have been omitted from this example.) In
    the first line, sushi asks wrl to read 8192 bytes from
    file 21,11/12.195, at byte offset 24576. Wrl replies
    `ok'; the packet shown on the second line is the first
    fragment of the reply, and hence is only 1472 bytes long
    (the other bytes will follow in subsequent fragments, but
    these fragments do not have NFS or even UDP headers and so
    might not be printed, depending on the filter expression
    used). Because the -v flag is given, some of the file
    attributes (which are returned in addition to the file
    data) are printed: the file type (``REG'', for regular
    file), the file mode (in octal), the uid and gid, and the
    file size.

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

    Note that NFS requests are very large and much of the
    detail won't be printed unless snaplen is increased. Try
    using `-s 192' to watch NFS traffic.

    NFS reply packets do not explicitly identify the RPC oper-
    ation. Instead, tcpdump keeps track of ``recent''
    requests, and matches them to the replies using the trans-
    action ID. If a reply does not closely follow the corre-
    sponding 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 contain blank lines or comment
    lines (lines starting with a `#').

    Appletalk addresses are printed in the form
    net.host.port

    144.1.209.2 > icsd-net.112.220
    office.2 > icsd-net.112.220
    jssmag.149.235 > icsd-net.2
    (If the /etc/atalk.names doesn't exist or doesn't contain
    an entry for some appletalk host/net number, addresses are
    printed in numeric form.) In the first example, NBP (DDP
    port 2) on net 144.1 node 209 is sending to whatever is
    listening on port 220 of net icsd node 112. The second
    line is the same except the full name of the source node
    is known (`office'). The third line is a send from port
    235 on net jssmag node 149 to broadcast on the icsd-net
    NBP port (note that the broadcast address (255) is indi-
    cated by a net name with no host number - for this reason
    it's a good idea to keep node names and net names distinct
    in /etc/atalk.names).

    NBP (name binding protocol) and ATP (Appletalk transaction
    protocol) packets have their contents interpreted. Other
    protocols just dump the protocol name (or number if no
    name is registered for the protocol) and packet size.

    NBP packets are formatted like the following examples:
    icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
    jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
    techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
    The first line is a name lookup request for laserwriters
    sent by net icsd host 112 and broadcast on net jssmag.
    The nbp id for the lookup is 190. The second line shows a
    reply for this request (note that it has the same id) from
    host jssmag.209 saying that it has a laserwriter resource
    named "RM1140" registered on port 250. The third line is
    another reply to the same request saying host techpit has
    laserwriter "techpit" registered on port 186.

    ATP packet formatting is demonstrated by the following
    example:
    jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
    helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
    jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
    helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
    helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
    jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
    jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
    Jssmag.209 initiates transaction id 12266 with host helios
    by requesting up to 8 packets (the `<0-7>'). The hex num-
    ber at the end of the line is the value of the `userdata'
    field in the request.

    Helios responds with 8 512-byte packets. The `:digit'
    following the transaction id gives the packet sequence
    number in the transaction and the number in parens is the
    amount of data in the packet, excluding the atp header.
    The `*' on packet 7 indicates that the EOM bit was set.

    Jssmag.209 then requests that packets 3 & 5 be retransmit-
    ted. Helios resends them then jssmag.209 releases the
    transaction. Finally, jssmag.209 initiates the next
    request. The `*' on the request indicates that XO
    (`exactly once') was not set.

    IP Fragmentation

    Fragmented Internet datagrams are printed as
    (frag id:size@offset+)
    (frag id:size@offset)
    (The first form indicates there are more fragments. The
    second indicates this is the last fragment.)

    Id is the fragment id. Size is the fragment size (in
    bytes) excluding the IP header. Offset is this fragment's
    offset (in bytes) in the original datagram.

    The fragment information is output for each fragment. The
    first fragment contains the higher level protocol header
    and the frag info is printed after the protocol info.
    Fragments after the first contain no higher level protocol
    header and the frag info is printed after the source and
    destination addresses. For example, here is part of an
    ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connec-
    tion 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.

    SEE ALSO
    bpf(4), pcap(3)

    AUTHORS
    Van Jacobson, Craig Leres and Steven McCanne, all of the
    Lawrence Berkeley National Laboratory, University of Cali-
    fornia, Berkeley, CA.

    The current version is available via anonymous ftp:

    ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

    BUGS
    Please send bug reports to tcpdump@ee.lbl.gov.

    NIT doesn't let you watch your own outbound traffic, BPF
    will. We recommend that you use the latter.

    Some attempt should be made to reassemble IP fragments or,
    at least to compute the right length for the higher level
    protocol.

    Name server inverse queries are not dumped correctly: The
    (empty) question section is printed rather than real query
    in the answer section. Some believe that inverse queries
    are themselves a bug and prefer to fix the program gener-
    ating them rather than tcpdump.

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

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

    Filters 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 fil-
    ter may inadvertently accept certain packets that do not
    properly match the filter expression.

    30 June 1997 1


    [[Category:Computer]]
    [[Category:Networking]]
    [[Category:Programs]]
    [[Category:Hacking]]

    Revision as of 03:02, 30 March 2005

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