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

       traceroute - print the route packets take to network host

       traceroute [ -aDFPIdlMnrvx ] [ -f first_ttl ]
               [ -g gateway ] [ -i iface ] [ -m max_ttl ]
               [ -p port ] [ -q nqueries ] [ -s src_addr ]
               [ -t tos ] [ -w waittime ] [ -z pausemsecs
        ] ] [ -A as_server ]
               host [ packetlen ]

       The  Internet  is  a large and complex aggregation of network hardware,
       connected together by  gateways.   Tracking  the  route  one's  packets
       follow  (or  finding  the  miscreant  gateway  that's  discarding  your
       packets) can be difficult.  Traceroute uses the IP  protocol  `time  to
       live'  field and attempts 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  40  bytes,  but  this may be
       increased  by  specifying  a  packet  length  (in  bytes)   after   the
       destination host name.

       Other options are:

       -a     Turn on AS# lookups for each hop encountered.

       -A     Turn  on  AS#  lookups  and  use the given server instead of the

       -d     Turn on socket-level debugging.

       -D     Dump the packet data to standard error before transmitting it.

       -f     Set the initial time-to-live used in the  first  outgoing  probe

       -F     Set the "don't fragment" bit.

       -g     Specify a loose source route gateway (8 maximum).

       -i     Specify  a network interface to obtain the source IP address for
              outgoing probe packets. This is normally only useful on a multi-
              homed host. (See the -s flag for another way to do this.)

       -I     Use ICMP ECHO instead of UDP datagrams.

       -l     Display  the  ttl  value of the returned packet.  This is useful
              for checking for asymmetric routing.

       -m     Set the max time-to-live (max number of hops) used  in  outgoing
              probe   packets.    The   default   value   is  taken  from  the
              net.inet.ip.ttl sysctl(3) variable.

       -M     If found, show the MPLS Label and the Experimental (EXP) bit for
              the hop.

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

       -p     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 option can be used to pick an unused port range.

       -P     Set the "don't fragment" bit, and use the next hop mtu each time
              we  get  the  "need  fragmentation" error, thus probing the path

       -q     Set the number of probe packets sent for each hop.  By  default,
              traceroute sends three probe packets.

       -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     Use  the  following  IP address (which usually is given as an IP
              number, not a hostname) as the source address in outgoing  probe
              packets.   On  multi-homed  hosts  (those  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. (See the -i flag for another way to do this.)

       -t     Set 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.  This option can be used  to  see  if  different
              types-of-service  result  in  different  paths.  (If you are not
              running 4.4BSD, this may be academic since  the  normal  network
              services  like  telnet  and  ftp don't let you control the TOS).
              Not all values of TOS are legal or meaningful - see the IP  spec
              for definitions.  Useful values are probably `-t 16' (low delay)
              and `-t 8' (high throughput).

       -v     Verbose output.  Received ICMP packets other than  TIME_EXCEEDED
              and UNREACHABLEs are listed.

       -w     Set  the  time  (in  seconds)  to wait for a response to a probe
              (default 5 sec.).

       -x     Toggle ip checksums. Normally,  this  prevents  traceroute  from
              calculating  ip  checksums.  In some cases, the operating system
              can overwrite parts of the outgoing packet but  not  recalculate
              the  checksum  (so in some cases the default is to not calculate
              checksums and using -x causes them to be calculated). Note  that
              checksums  are usually required for the last hop when using ICMP
              ECHO probes (-I).  So they  are  always  calculated  when  using

       -z     Set  the time (in milliseconds) to pause between probes (default
              0).  Some systems such as Solaris and  routers  such  as  Ciscos
              rate  limit icmp messages. A good value to use with this this is
              500 (e.g. 1/2 second).

       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 (change 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  address  of each responding system will be printed.  If
       there is no response within a 5 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
              traceroute to (, 30 hops max, 38 byte packet
               1 (  19 ms  19 ms  0 ms
               2  lilac-dmc.Berkeley.EDU (  39 ms  39 ms  19 ms
               3  lilac-dmc.Berkeley.EDU (  39 ms  39 ms  19 ms
               4  ccngw-ner-cc.Berkeley.EDU (  39 ms  40 ms  39 ms
               5  ccn-nerif22.Berkeley.EDU (  39 ms  39 ms  39 ms
               6 (  40 ms  59 ms  59 ms
               7 (  59 ms  59 ms  59 ms
               8 (  99 ms  99 ms  80 ms
               9 (  139 ms  239 ms  319 ms
              10 (  220 ms  199 ms  199 ms
              11 (  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 - lilac-dmc.Berkeley.EDU  -  that  forwards  packets
       with  a  zero  ttl  (a bug in the distributed version of 4.3BSD).  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
              traceroute to (, 30 hops max
               1 (  0 ms  0 ms  0 ms
               2  lilac-dmc.Berkeley.EDU (  19 ms  19 ms  19 ms
               3  lilac-dmc.Berkeley.EDU (  39 ms  19 ms  19 ms
               4  ccngw-ner-cc.Berkeley.EDU (  19 ms  39 ms  39 ms
               5  ccn-nerif22.Berkeley.EDU (  20 ms  39 ms  39 ms
               6 (  59 ms  119 ms  39 ms
               7 (  59 ms  59 ms  39 ms
               8 (  80 ms  79 ms  99 ms
               9 (  139 ms  139 ms  159 ms
              10 (  199 ms  180 ms  300 ms
              11 (  300 ms  239 ms  239 ms
              12  * * *
              13 (  259 ms  499 ms  279 ms
              14  * * *
              15  * * *
              16  * * *
              17  * * *
              18  ALLSPICE.LCS.MIT.EDU (  339 ms  279 ms  279 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
       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.  The behavior
       of this bug is  slightly  more  interesting  when  it  appears  on  the
       destination system:

               1 (  0 ms  0 ms  0 ms
               2  lilac-dmc.Berkeley.EDU (  39 ms  19 ms  39 ms
               3  lilac-dmc.Berkeley.EDU (  19 ms  39 ms  19 ms
               4  ccngw-ner-cc.Berkeley.EDU (  39 ms  40 ms  19 ms
               5  ccn-nerif35.Berkeley.EDU (  39 ms  39 ms  39 ms
               6  csgw.Berkeley.EDU (  39 ms  59 ms  39 ms
               7  * * *
               8  * * *
               9  * * *
              10  * * *
              11  * * *
              12  * * *
              13  rip.Berkeley.EDU (  59 ms !  39 ms !  39 ms !

       Notice  that  there are 12 "gateways" (13 is the final destination) and
       exactly 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  (HP-UX)
       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,  or  !P  (host,
       network  or  protocol unreachable), !S (source route failed), !F-<pmtu>
       (fragmentation needed  -  the  RFC1191  Path  MTU  Discovery  value  is
       displayed),  !X  (communication  administratively prohibited), !V (host
       precedence violation), !C (precedence cutoff in effect),  or  !N  (ICMP
       unreachable  code num).  These are defined by RFC1812 (which supersedes
       RFC1716).  If almost all the probes result in some kind of unreachable,
       traceroute will give up and exit.

              traceroute -g

       will show the path from the Cambridge Mailbridge to PSC, while

              traceroute -g -g

       will show the path from the Cambridge Mailbridge to Merit, using PSC to
       reach the Mailbridge.

       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, it is unwise to use
       traceroute during normal operations or from automated scripts.

       netstat(1), ping(8)

       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.

       The current version is available via anonymous ftp:


       Please send bug reports to

       The  AS  number  capability  reports  information that may sometimes be
       inaccurate due to discrepancies between the  contents  of  the  routing
       database server and the current state of the Internet.

4.3 Berkeley Distribution      21 September 2000                 TRACEROUTE(8)