Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
TRACEROUTE(8)		    System Manager's Manual		 TRACEROUTE(8)

       traceroute - print the route packets take to network host

       traceroute [ -dFISdnrvx ] [ -f first_ttl	] [ -g gateway ]
	       [ -i iface ] [ -M first_ttl ]
	       [ -m max_ttl ] [	-P proto ] [ -p	port ]
	       [ -q nqueries ] [ -s src_addr ] [ -t tos	]
	       [ -w waittime ] [ -z pausemsecs ]
	       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 utilizes 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 in-
       creased by specifying a packet length (in bytes)	after the  destination
       host name.

       Other options are:

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

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

       -d     Enable socket level debugging.

       -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.	 (A  synonym  for  "-P

       -M     Set  the initial time-to-live value used in outgoing probe pack-
	      ets.  The	default	is 1, i.e., start with the first hop.

       -m     Set the max time-to-live (max number of hops) used  in  outgoing
	      probe  packets.	The  default is	net.inet.ip.ttl	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     Send packets of specified	IP protocol. The  currently  supported
	      protocols	 are: UDP, TCP,	GRE and	ICMP. Other protocols may also
	      be specified (either by name or by  number),  though  traceroute
	      does  not	 implement  any	special	knowledge of their packet for-
	      mats. This option	is useful for determining which	router along a
	      path  may	 be  blocking packets based on IP protocol number. But
	      see BUGS below.

       -p     Protocol specific. For UDP and TCP, sets the  base  port	number
	      used  in probes (default is 33434).  Traceroute hopes that noth-
	      ing 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  lis-
	      tening  on  a port in the	default	range, this option can be used
	      to pick an unused	port range.

       -q     Set the number of	probes per hop (default	is 3).

       -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(8C)).

       -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  ad-
	      dress),  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.)

       -S     Print  a	summary	 of how	many probes were not answered for each

       -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 (de-
	      fault 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 un-
       til we get an ICMP "port	unreachable" (which means we got to "host") or
       hit a max (which	defaults to net.inet.ip.ttl 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 dif-
       ferent 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	(, 64	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 - - 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
	      traceroute to (,	64 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 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 (  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 ar-
       riving 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)	 soft-
       ware,  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, net-
       work  or	 protocol  unreachable),  !S  (source route failed), !F-<pmtu>
       (fragmentation needed - the RFC1191 Path	MTU Discovery  value  is  dis-
       played),	 !X  (communication  administratively  prohibited),  !V	 (host
       precedence violation), !C (precedence  cutoff  in  effect),  or	!<num>
       (ICMP unreachable code <num>).  These are defined by RFC1812 (which su-
       persedes	RFC1716).  If almost all the probes result in some kind	of un-
       reachable, traceroute will give up and exit.

       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.

       pathchar(8), netstat(1),	ping(8)

       Implemented by Van Jacobson from	a suggestion by	 Steve	Deering.   De-
       bugged  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:

       When using protocols other than UDP, functionality is reduced.  In par-
       ticular,	 the  last  packet  will often appear to be lost, because even
       though it reaches the destination host, there's no way to know that be-
       cause no	ICMP message is	sent back.  In the TCP case, traceroute	should
       listen for a RST	from the destination host (or an  intermediate	router
       that's filtering	packets), but this is not implemented yet.

       Please send bug reports to

4.3 Berkeley Distribution      21 September 2000		 TRACEROUTE(8)


Want to link to this manual page? Use this URL:

home | help