traceroute – print the route packets take to network host




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 gate-
way 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 destination host name.

Other options are:

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


in outgoing probe packets. The default is 30 hops
(the same default used for TCP connections).

-n Print hop addresses numerically rather than symbol-
ically and numerically (saves a nameserver address-

-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_UNREACH-
ABLE 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.

-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

-s Use the following IP address (which must be given
as an IP number, 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
this machine’s interface addresses, an error is
returned and nothing is sent.

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 nor-
mal 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 defini-
tions. Useful values are probably `-t 16′ (low
delay) and `-t 8′ (high throughput).

-v Verbose output. Received ICMP packets other than

a probe (default 5 sec.).

This program attempts to trace the route an IP packet
would follow to some internet host by launching UDP probe
an ICMP “time exceeded” reply from a gateway. We start
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
showing the ttl, address of the gateway and round trip
ferent gateways, the address of each responding system
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

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
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

buggy kernel on the 2nd hop system – – that
forwards packets with a zero ttl (a bug in the distributed
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
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

either don’t send ICMP “time exceeded” messages or send
them with a ttl too small to reach us. 14 – 17 are run-
ning the MIT C Gateway code that doesn’t send “time
exceeded”s. God only knows what’s going on with 12.

4.x (x < = 3) sends an unreachable message using whatever ttl remains in the original datagram. Since, for gate- ways, 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 des- tination) 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) 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 (got a host, network or protocol unreachable, respec- tively), !S or !F (source route failed or fragmentation needed - neither of these should ever occur and the asso- ciated gateway is busted if you see one), !X (communica- tion administratively prohibited), or ! (ICMP unreach- able code N). 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, mea- surement 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 dur- ing normal operations or from automated scripts.SEE ALSOnetstat(1), ping(8)


Implemented by Van Jacobson from a suggestion by Steve
Deering. Debugged by a cast of thousands with particu-
larly 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

27 September 1996


Leave a Comment