NAME
traceroute - print the route packets take to network host

SYNOPSIS
traceroute [ -m max_ttl ] [ -n ] [ -p port ] [ -q nqueries ] [ -r ]
[ -s src_addr ] [ -t tos ] [ -w ] [ -w waittime ] host
[ packetsize ]

DESCRIPTION
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 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 38 bytes, but this
may be increased by specifying a packet size (in bytes) after the
destination host name.

Other options are:

-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.

-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 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 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.

-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 3 sec.).

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 3 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 (35.1.1.48), 30 hops max, 56 byte packet
1 helios.ee.lbl.gov (128.3.112.1) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 39 ms 39 ms 39 ms
6 128.32.197.4 (128.32.197.4) 40 ms 59 ms 59 ms
7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 59 ms
8 129.140.70.13 (129.140.70.13) 99 ms 99 ms 80 ms
9 129.140.71.6 (129.140.71.6) 139 ms 239 ms 319 ms
10 129.140.81.7 (129.140.81.7) 220 ms 199 ms 199 ms
11 nic.merit.edu (35.1.1.48) 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 NSSes.

A more interesting example is:

[yak 72]% traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (18.26.0.115), 30 hops max
1 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (128.32.168.22) 20 ms 39 ms 39 ms
6 128.32.197.4 (128.32.197.4) 59 ms 119 ms 39 ms
7 131.119.2.5 (131.119.2.5) 59 ms 59 ms 39 ms
8 129.140.70.13 (129.140.70.13) 80 ms 79 ms 99 ms
9 129.140.71.6 (129.140.71.6) 139 ms 139 ms 159 ms
10 129.140.81.7 (129.140.81.7) 199 ms 180 ms 300 ms
11 129.140.72.17 (129.140.72.17) 300 ms 239 ms 239 ms
12 * * *
13 128.121.54.72 (128.121.54.72) 259 ms 499 ms 279 ms
14 * * *
15 * * *
16 * * *
17 * * *
18 ALLSPICE.LCS.MIT.EDU (18.26.0.115) 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 helios.ee.lbl.gov (128.3.112.1) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (128.32.216.1) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (128.32.216.1) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (128.32.136.23) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (128.32.168.35) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (128.32.133.254) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * *
10 * * *
11 * * *
12 * * *
13 rip.Berkeley.EDU (128.32.131.22) 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) 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, !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). 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.

AUTHOR
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.