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

       traceroute - print the route packets take to network host

       traceroute [ -aDFPIdlnrvx ] [ -f first_ttl ]
               [ -g gateway ] [ -i iface ] [ -m max_ttl ]
               [ -p port ] [ -q nqueries ] [ -s src_addr ]
               [ -t tos ] [ -w waittime ] [ -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  fol-
       low  (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  destina-
       tion 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 is 30 hops (the same default used
              for TCP connections).

       -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 checksums. Normally, this prevents traceroute from calcu-
              lating checksums. In some cases, the operating system can  over-
              write  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).

       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 nis.nsf.net.
              traceroute to nis.nsf.net (, 30 hops max, 38 byte packet
               1  helios.ee.lbl.gov (  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  nic.merit.edu (  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 4.3BSD).  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

       A more interesting example is:

              [yak 72]% traceroute allspice.lcs.mit.edu.
              traceroute to allspice.lcs.mit.edu (, 30 hops max
               1  helios.ee.lbl.gov (  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  data-
       gram.   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 destina-
       tion system:

               1  helios.ee.lbl.gov (  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 tar-
       get host of your probes.

       Other  possible  annotations  after  the  time are !H, !N, or !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), !X  (commu-
       nication administratively prohibited), or !N (ICMP unreachable code N).
       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 tracerouteATee.gov.

       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        22 April 1997                   TRACEROUTE(8)