HELPLIB.HLB  —  traceroute
    SYNOPSIS

    traceroute parameters

    The traceroute command displays the route that packets take to a
    network host.

1  –  FLAGS

    -A         Looks up the AS-number (Autonomous System) for each
               hop's network address at the whois server specified by
               the -h option.
    -a         If the destination host has multiple addresses,
               traceroute probes all addresses if this option is
               set. Normally only the first address as returned by
               the resolver is attempted.
    -c         Specifies a delay (in seconds) to pause between
    stoptime   probe packets. This may be necessary if the final
               destination is a router that does not accept
               undeliverable packets in bursts.
    -f         Disables IP fragmentation. If the given packetsize
               is too big to be handled unfragmented by a machine
               along the route, a "fragmentation needed" status
               is returned and the indicator !F is printed. If a
               gate-way returns the value of the proper MTU size
               to be used, traceroute decreases the packet size
               automatically to this new value. If the proper MTU
               size is not returned, traceroute chooses a shorter
               packet size.
    -g         Enables the IP LSRR (Loose Source Record Route)
    gateway    option. This is useful for asking how somebody else,
               at the specified gateway, reaches a particular target.
    -h server  Specifies the name or IP address of the whois server
               that is contacted for the AS-number lookup, if the -A
               option is given.
    -i         Sets the starting time-to-live value to initial_ttl,
    initial_   to override the default value of 1. Effectively this
    ttl        skips processing for those intermediate hosts that are
               less than initial_ttl hops away.
    -k         Keeps the connection to the whois server permanently
               open. This makes lookups considerably quicker, because
               connection setup for each individual lookup is not
               necessary. However, all whois servers do not support
               this feature.
    -l         Prints the value of the ttl field in each received
               packet (this can be used to help detect asymmetric
               routing).
    -m max_    Sets the max time-to-live (max number of hops) used in
    ttl        outgoing probe packets. The default is 30 hops, which
               is the same default used for TCP connections.
    -N         Displays the network name for each hop. If a BIND
               resolver cannot be reached, network names are
               retrieved just from the /etc/networks file.
    -n         Prints hop IP addresses numerically rather than both
               symbolically and numerically. This saves a nameserver
               address-to-name lookup for each gateway found on the
               path. It also prevents a reverse lookup for numeric
               dotted quad addresses given on the command line
               (destination host, or -g gateway addresses).
    -p port    Sets the base UDP port number used in probes (default
               is 33434). The traceroute command presumes that
               nothing is listening on UDP ports base to base+nhops-1
               at the destination host (so an ICMP "port unreachable"
               message is returned to terminate the route tracing).
               If another process is listening on a port in the
               default range, use this option to pick an unused port
               range.
    -Q         Stops probing this hop after maxquit consecutive
    maxquit    timeouts are detected. The default value is 5. Useful
               in combination with -S if you have specified a big
               nqueries probe count.
    -q         Sets the number of probes launched at each ttl setting
    nqueries   (default is 3).
    -r         Bypasses the normal routing tables and sends directly
               to a host on an attached network. If the host is
               not on a directly-attached network, an error is
               returned. This option can be used to ping a local
               host through an interface that has no route through
               it (for example, after the interface was dropped by
               routed(8) or gated(8)).
    -S         Prints a per-hop minimum/average/maximum rtt (round-
               trip time) statistics summary. This suppresses the
               per-probe rtt and ttl reporting. For better statistics
               you need to increase the default nqueries probe count.
               See also the -Q option.
    -s         Uses the following IP address (which must be given as
    source_    an IP number, not a hostname) as the source address
    addr       in outgoing probe packets. On hosts with more than
               one IP address, use this option to force the source
               address to be something other than the IP address of
               the interface on which the probe packet is sent. If
               the IP address is not one of this machine's interface
               addresses, an error is returned and nothing is sent.
    -t tos     Sets the type-of-service in probe packets to the
               following value (default zero). The value must be a
               decimal integer in the range 0 to 255. Use this option
               to determine if different types-of-service result in
               different paths. Not all values of TOS are legal or
               meaningful. See the IP specification for definitions.
               Useful values are probably -t 16 (low delay) and -t 8
               (high throughput).
    -v         Produces verbose output. Lists any received ICMP
               packets other than "time exceeded" and "unreachable".
    -w         Sets the time (in seconds) to wait for a response to a
    waittime   probe. The default is 3 seconds.

2  –  DESCRIPTION

    The Internet is a large and complex aggregation of network
    hardware, connected together by gateways. The traceroute command
    tracks the route packets follow from gateway to gateway. The
    command uses the IP protocol `time to live' field and attempts to
    elicit an ICMP "time exceeded" response from each gateway along
    the path to a particular host.

    The only mandatory parameter is the destination host name or IP
    number. The default probe datagram length is 38 bytes, but you
    can increase this by specifying a packet size (in bytes) after
    the destination host name. This is useful when the -f option is
    given for MTU discovery along the route. You should start with
    the maximum packet size for your own network interface (if the
    given value is even bigger, traceroute attempts to select a more
    appropriate value). If no packet size is given when using the -f
    option, traceroute determines the initial MTU automatically.

    To track the route of an IP packet, traceroute launches UDP probe
    packets with a small ttl (time to live) and then listens for an
    ICMP "time exceeded" reply from a gateway. Probes start with
    a ttl of one and increase by one until either an ICMP "port
    unreachable" is returned (indicating that the packet reached
    the host) or the maximum number of hops is exceeded (the default
    is 30 hops and can be changed with the -m option). At each ttl
    setting, traceroute launches three probes (you can change the
    number with the -q option) and prints a line showing the ttl,
    address of the gateway, and round trip time of each probe. If the
    probe answers come from different gateways, traceroute prints the
    address of each responding system. If there is no response within
    a 3 second timeout interval (which you can change with the -w
    option), an asterisk (*) is printed for that probe.

    To prevent the destination host from processing the UDP probe
    packets, the destination port is set to an unlikely value. You
    can change the destination port value with the -p option, if
    necessary.

3  –  Special Annotations

    Other possible annotations after the time are:

    !H    Host is unreachable.
    !N    Network is unreachable.
    !P    Protocol is unreachable.
    !F    Fragmentation needed.

          This indicator may show up if the -f command line option
          is being used, and the associated gateway requires further
          fragmentation. In case the desired new MTU size is known,
          it is indicated.
    !S    Source route failed.

          This should not occur under normal circumstances and the
          associated gateway might be broken if you see one.
    !T    Host or network is unreachable for the given tos.
    !U    Destination is unreachable.

          This indicator is printed for some of the new unreachable
          subcodes as defined in RFC 1812.
    !A    Some routers fail to generate an ICMP "port unreachable"
          message, but send an ICMP "time exceeded" message instead
          if they are the target host. The indicator is printed if
          this is detected.
    !G    Some routers erroneously generate ICMP "port unreachable"
          instead of "time exceeded" if they are specified as loose
          source route gateway hosts. The indicator is printed if
          this is detected.

          If all the probes result in an unreachable status,
          traceroute stops sending probes and exits.

4  –  TTL Indication

    (ttl=n!)

    This indicates that the ttl value in the ICMP "time exceeded"
    packet that we received was unexpected. We expected some initial
    value, for example, the number of routers between our system and
    another system. In other words, if the path from hop 5 to us is
    the same as the path from us to hop 5, we expect to receive a ttl
    value of 4.

    There are several common initial values for ICMP ttls: 255, 60,
    59, 30 and 29. 4.3 tahoe BSD and Cisco routers use 255, Proteon
    routers use either 59 or 29 depending on software release,
    several other implementations use 60 and 30. Tru64 UNIX uses an
    initial ttl of 64. The traceroute command checks against all of
    these, making it hard to detect some small routing asymmetries.
    If you want to see the ttl values in all the packets, use the -l
    option.

                                   NOTE

       This program is intended for use in network testing,
       measurement and management. It should be used primarily
       for manual fault isolation. Because of the load it could
       impose on the network, do not use traceroute during normal
       operations or from automated scripts.

5  –  Examples

    1. The following command traces the route a packet takes from
       localhost to the host nis.nsf.net:

       localhost> traceroute nis.nsf.net

       traceroute to nis.nsf.net (35.1.1.48), 30 hops max, 56 byte packet

        1  helios.ee.lbl.gov (128.3.112.1)  19 ms  19 ms  0 ms
        2  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  39 ms 19 ms
        3  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  39 ms 19 ms
        4  ccngw-ner-cc.Berkeley.EDU (128.32.136.23)  39 ms  40 ms  39 ms
        5  ccn-nerif22.Berkeley.EDU (128.32.168.22)  39 ms  39 ms  39 ms
        6  128.32.197.4 (128.32.197.4)  40 ms  59 ms  59 ms
        7  131.119.2.5 (131.119.2.5)  59 ms  59 ms  59 ms
        8  129.140.70.13 (129.140.70.13)  99 ms  99 ms  80 ms
        9  129.140.71.6 (129.140.71.6)  139 ms  239 ms  319 ms
       10  129.140.81.7 (129.140.81.7)  220 ms  199 ms  199 ms
       11  nic.merit.edu (35.1.1.48)  239 ms  239 ms  239 ms
 Note that lines 2 and 3 are identical. This is due to a bug in the
 kernel on the 2nd hop system, lbl-csam.arpa, that forwards packets
 with a zero ttl (a bug in the distributed version of 4.3BSD). The
 NSFNet (129.140) does not supply address-to-name translations for
 its NSSes. Therefore, you cannot be certain of the path the packets
 take cross-country.

    2. The following is another example of output from the
       traceroute com mand. Packets from localhost to the host
       allspice.lcs.mit.edu are being traced:

       localhost> traceroute allspice.lcs.mit.edu

       traceroute to allspice.lcs.mit.edu (18.26.0.115), 30 hops max

        1  helios.ee.lbl.gov (128.3.112.1)  0 ms  0 ms  0 ms
        2  lilac-dmc.Berkeley.EDU (128.32.216.1)  19 ms  19 ms 19 ms
        3  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  19 ms 19 ms
        4  ccngw-ner-cc.Berkeley.EDU (128.32.136.23)  19 ms  39 ms  39 ms
        5  ccn-nerif22.Berkeley.EDU (128.32.168.22)  20 ms  39 ms  39 ms
        6  128.32.197.4 (128.32.197.4)  59 ms  119 ms  39 ms
        7  131.119.2.5 (131.119.2.5)  59 ms  59 ms  39 ms
        8  129.140.70.13 (129.140.70.13)  80 ms  79 ms  99 ms
        9  129.140.71.6 (129.140.71.6)  139 ms  139 ms  159 ms
       10  129.140.81.7 (129.140.81.7)  199 ms  180 ms  300 ms
       11  129.140.72.17 (129.140.72.17)  300 ms  239 ms  239 ms
       12  * * *
       13  128.121.54.72 (128.121.54.72)  259 ms  499 ms  279 ms
       14  * * *
       15  * * *
       16  * * *
       17  * * *
       18  ALLSPICE.LCS.MIT.EDU (18.26.0.115)  339 ms  279 ms 279 ms

       Note that the gateways 12, 14, 15, 16 and 17 hops away either
       do not send ICMP "time exceeded" messages or send them with
       a ttl too small to reach localhost. Further investigation is
       required to determine the cause. For example, by contacting
       the system administrators for gateways 14 through 17, you
       could discover that these gateways are running the MIT C
       Gateway code that does not send "time exceeded" messages.

       The silent gateway 12 in the example may be the result of
       a bug in the 4.[23]BSD network code (and its derivatives):
       4.x (x <= 3) sends an unreachable message using whatever ttl
       remains in the original datagram. Since, for gateways, the
       remaining ttl is zero, the ICMP "time exceeded" is guaranteed
       to not make it back to us.

       When this bug appears on the destination system it behaves as
       follows:

        1  helios.ee.lbl.gov (128.3.112.1)  0 ms  0 ms  0 ms
        2  lilac-dmc.Berkeley.EDU (128.32.216.1)  39 ms  19 ms 39 ms
        3  lilac-dmc.Berkeley.EDU (128.32.216.1)  19 ms  39 ms 19 ms
        4  ccngw-ner-cc.Berkeley.EDU (128.32.136.23)  39 ms  40 ms  19 ms
        5  ccn-nerif35.Berkeley.EDU (128.32.168.35)  39 ms  39 ms  39 ms
        6  csgw.Berkeley.EDU (128.32.133.254)  39 ms  59 ms  39 ms
        7  * * *
        8  * * *
        9  * * *
       10  * * *
       11  * * *
       12  * * *
       13  rip.Berkeley.EDU (128.32.131.22)  59 ms !  39 ms ! 39 ms !

       Note that there are 12 gateways (13 is the final destination
       and the last half of them are missing. What is happening is
       that the host rip (a Sun-3 running Sun OS3.5) is using the ttl
       from our arriving datagram as the ttl in its ICMP reply. The
       reply will time out on the return path (with no notice sent to
       anyone since ICMPs are not sent for ICMPs) until we probe with
       a ttl that is at least twice the path length. This means that
       the host rip is really only 7 hops away. A reply that returns
       with a ttl of 1 is a clue this problem exists. The traceroute
       command prints a ! after the time if the ttl is less than or
       equal to 1. Since many systems continue to run obsolete or
       non-standard software, expect to see this problem frequently.
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