traceroute

SYNOPSIS

     traceroute [-m max_ttl] [-n] [-p port] [-q nqueries] [-r] [-s src_addr]
                [-t tos] [-w waittime] host [packetsize]

DESCRIPTION

     The Internet is a large and complex aggregation of network hardware, con-
     nected together by gateways.  Tracking the route one's packets follow (or
     finding the miscreant gateway that's discarding your packets) can be dif-
     ficult.  Traceroute utilizes the IP protocol `time to live' field and at-
     tempts to elicit an ICMP TIME_EXCEEDED response from each gateway along
     the path to some host.

     The only mandatory parameter is the destination host name or IP number.
     The default probe datagram length is 38 bytes, but this may be increased
     by specifying a packet size (in bytes) after the destination host name.

     Other options are:

     -m max_ttl
             Set the max time-to-live (max number of hops) used in outgoing
             probe packets.  The default is 30 hops (the same default used for
             TCP connections).

     -n      Print hop addresses numerically rather than symbolically and nu-
             merically (saves a nameserver address-to-name lookup for each
             gateway found on the path).

     -p port
             Set the base UDP port number used in probes (default is 33434).
             Traceroute hopes that nothing is listening on UDP ports base to
             base+nhops-1 at the destination host (so an ICMP PORT_UNREACHABLE
             message will be returned to terminate the route tracing).  If
             something is listening on a port in the default range, this op-
             tion can be used to pick an unused port range.

     -q nqueries
             Set the number of probes per ``ttl'' to nqueries (default is
             three probes).

     -r      Bypass the normal routing tables and send 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
             (e.g., after the interface was dropped by routed(8)).

     -s src_addr
             Use the following IP address (which must be given as an IP num-
             ber, not a hostname) as the source address in outgoing probe
             packets.  On hosts with more than one IP address, this option can
             be used to force the source address to be something other than
             the IP address of the interface the probe packet is sent on.  If
             the IP address is not one of this machine's interface addresses,
             an error is returned and nothing is sent.


     -w      Set the time (in seconds) to wait for a response to a probe (de-
             fault 3 sec.).

     This program attempts to trace the route an IP packet would follow to
     some internet host by launching UDP probe packets with a small ttl (time
     to live) then listening for an ICMP "time exceeded" reply from a gateway.
     We start our probes with a ttl of one and increase by one until we get an
     ICMP "port unreachable" (which means we got to "host") or hit a max
     (which defaults to 30 hops & can be changed with the -m flag).  Three
     probes (changed with -q flag) are sent at each ttl setting and a line is
     printed showing the ttl, address of the gateway and round trip time of
     each probe.  If the probe answers come from different gateways, the ad-
     dress of each responding system will be printed.  If there is no response
     within a 3 sec. timeout interval (changed with the -w flag), a "*" is
     printed for that probe.

     We don't want the destination host to process the UDP probe packets so
     the destination port is set to an unlikely value (if some clod on the
     destination is using that value, it can be changed with the -p flag).

     A sample use and output might be:

     [yak 71]% 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 & 3 are the same.  This is due to a buggy kernel on the
     2nd hop system - lbl-csam.arpa - that forwards packets with a zero ttl (a
     bug in the distributed version of 4.3 BSD). Note that you have to guess
     what path the packets are taking cross-country since the NSFNet (129.140)
     doesn't supply address-to-name translations for its NSSes.

     A more interesting example is:

     [yak 72]% 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
     Note that the gateways 12, 14, 15, 16 & 17 hops away either don't send
     ICMP "time exceeded" messages or send them with a ttl too small to reach
     us.  14 - 17 are running the MIT C Gateway code that doesn't send "time
     exceeded"s.  God only knows what's going on with 12.

     The silent gateway 12 in the above may be the result of a bug in the
     4.[23] BSD network code (and its derivatives):  4.x (x <= 3) sends an un-
     reachable 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.  The behavior of this bug is
     slightly more interesting when it appears on the destination system:

     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 !

     Notice that there are 12 "gateways" (13 is the final destination) and ex-
     actly the last half of them are "missing".  What's really happening is
     that rip (a Sun-3 running Sun OS3.5) is using the ttl from our arriving
     datagram as the ttl in its ICMP reply.  So, the reply will time out on
     the return path (with no notice sent to anyone since ICMP's aren't sent
     for ICMP's) until we probe with a ttl that's at least twice the path
     length.  I.e., rip is really only 7 hops away.  A reply that returns with
     a ttl of 1 is a clue this problem exists.  Traceroute prints a "!" after
     the time if the ttl is <= 1.  Since vendors ship a lot of obsolete (DEC's
     Ultrix, Sun 3.x) or non-standard (HPUX) software, expect to see this
     problem frequently and/or take care picking the target host of your
     probes.  Other possible annotations after the time are !H, !N, !P (got a
     host, network or protocol unreachable, respectively), !S or !F (source
     route failed or fragmentation needed - neither of these should ever occur
     and the associated gateway is busted if you see one).  If almost all the
     probes result in some kind of unreachable, traceroute will give up and
     exit.

     This program is intended for use in network testing, measurement and man-
     agement.  It should be used primarily for manual fault isolation.  Be-
     cause of the load it could impose on the network, it is unwise to use
     traceroute during normal operations or from automated scripts.

AUTHOR

     Implemented by Van Jacobson from a suggestion by Steve Deering.  Debugged
     by a cast of thousands with particularly cogent suggestions or fixes from
     C. Philip Wood, Tim Seaver and Ken Adelman.