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
increased by specifying a packet length (in bytes) after the destina-
tion 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
numerically (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
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.)
-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
(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 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 gate-
way 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 (18.104.22.168), 64 hops max, 38 byte packet
1 helios.ee.lbl.gov (22.214.171.124) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (188.8.131.52) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (184.108.40.206) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (220.127.116.11) 39 ms 39 ms 39 ms
6 18.104.22.168 (22.214.171.124) 40 ms 59 ms 59 ms
7 126.96.36.199 (188.8.131.52) 59 ms 59 ms 59 ms
8 184.108.40.206 (220.127.116.11) 99 ms 99 ms 80 ms
9 18.104.22.168 (22.214.171.124) 139 ms 239 ms 319 ms
10 126.96.36.199 (188.8.131.52) 220 ms 199 ms 199 ms
11 nic.merit.edu (184.108.40.206) 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 (220.127.116.11), 64 hops max
1 helios.ee.lbl.gov (18.104.22.168) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (22.214.171.124) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (126.96.36.199) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (188.8.131.52) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (184.108.40.206) 20 ms 39 ms 39 ms
6 220.127.116.11 (18.104.22.168) 59 ms 119 ms 39 ms
7 22.214.171.124 (126.96.36.199) 59 ms 59 ms 39 ms
8 188.8.131.52 (184.108.40.206) 80 ms 79 ms 99 ms
9 220.127.116.11 (18.104.22.168) 139 ms 139 ms 159 ms
10 22.214.171.124 (126.96.36.199) 199 ms 180 ms 300 ms
11 188.8.131.52 (184.108.40.206) 300 ms 239 ms 239 ms
12 * * *
13 220.127.116.11 (18.104.22.168) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (22.214.171.124) 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.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-
1 helios.ee.lbl.gov (126.96.36.199) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (188.8.131.52) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (184.108.40.206) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (220.127.116.11) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (18.104.22.168) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (22.214.171.124) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (126.96.36.199) 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 (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
supersedes RFC1716). 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
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.
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:
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
because 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 tracerouteATee.gov.
4.3 Berkeley Distribution 21 September 2000 TRACEROUTE(8)