/usr/sbin/traceroute [-m max_ttl] [-n] [-p port] [-q nqueries] [-r] [-s src_addr] [ -w] [-w waittime] host [packetsize]
The Internet is a large and complex aggregation of network hardware connected together by gateways. The traceroute command tracks the route packets follow from gateway to gateway. The command uses the IP protocol `time to live' field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to a particular 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.
To track the route of an IP packet, traceroute launches UDP probe packets with a small ttl (time to live) and then listens for an ICMP "time exceeded" reply from a gateway. Probes start with a ttl of one and increase by one until either an ICMP "port unreachable" is returned (indicating that the packet reached the host) or the maximum number of hops is exceeded (the default is 30 hops and can be changed with the -m flag). At each ttl setting, three probes are launched (the number can be changed with the -q flag) and traceroute prints a line showing the ttl, address of the gateway, and round trip time of each probe. If the probe answers come from different gateways, traceroute prints the address of each responding system. If there is no response within a 3 second timeout interval (which can be changed with the -w flag), an asterisk (*) is printed for that probe.
To prevent the destination host from processing the UDP probe packets, the destination port is set to an unlikely value. The destination port value can be changed with the -p flag, if necessary.
Additional traceroute options are: Sets the max time-to-live (max number of hops) used in outgoing probe packets. The default is 30 hops which is the same default used for TCP connections. Prints hop IP addresses using dotted decimal notation. This saves a nameserver address-to-name lookup for each gateway found on the path. Sets the base UDP port number used in probes (default is 33434). The traceroute command presumes 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 another process is listening on a port in the default range, this option can be used to pick an unused port range. Sets the number of probes launched at each ttl setting (default is 3). Bypasses the normal routing tables and sends 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 (for example, after the interface was dropped by routed(8) or gated(8)). Uses 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 on which the probe packet is sent. If the IP address is not one of this machine's interface addresses, an error is returned and nothing is sent. Lists any received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs. Sets the time (in seconds) to wait for a response to a probe. The default is 3 seconds.
The following command traces the route a packet takes from localhost to the host nis.nsf.net: localhost> traceroute nis.nsf.net
traceroute to nis.nsf.net (126.96.36.199), 30 hops max, 56 byte packet
1 helios.ee.lbl.gov (188.8.131.52) 19 ms 19 ms 0 ms
2 lilac-dmc.Berkeley.EDU (184.108.40.206) 39 ms 39 ms 19 ms
3 lilac-dmc.Berkeley.EDU (220.127.116.11) 39 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (18.104.22.168) 39 ms 40 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (22.214.171.124) 39 ms 39 ms 39 ms
6 126.96.36.199 (188.8.131.52) 40 ms 59 ms 59 ms
7 184.108.40.206 (220.127.116.11) 59 ms 59 ms 59 ms
8 18.104.22.168 (22.214.171.124) 99 ms 99 ms 80 ms
9 126.96.36.199 (188.8.131.52) 139 ms 239 ms 319 ms 10 184.108.40.206 (220.127.116.11) 220 ms 199 ms 199 ms 11 nic.merit.edu (18.104.22.168) 239 ms 239 ms 239 ms Note that lines 2 and 3 are identical. This is due to a bug in the 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). The NSFNet (129.140) does not supply address-to-name translations for its NSSes. Therefore, you cannot be certain of the path the packets take cross-country.
The following is another example of output from the traceroute command. Packets from localhost to the host allspice.lcs.mit.edu are being traced: localhost> traceroute allspice.lcs.mit.edu.
traceroute to allspice.lcs.mit.edu (22.214.171.124), 30 hops max
1 helios.ee.lbl.gov (126.96.36.199) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (188.8.131.52) 19 ms 19 ms 19 ms
3 lilac-dmc.Berkeley.EDU (184.108.40.206) 39 ms 19 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (220.127.116.11) 19 ms 39 ms 39 ms
5 ccn-nerif22.Berkeley.EDU (18.104.22.168) 20 ms 39 ms 39 ms
6 22.214.171.124 (126.96.36.199) 59 ms 119 ms 39 ms
7 188.8.131.52 (184.108.40.206) 59 ms 59 ms 39 ms
8 220.127.116.11 (18.104.22.168) 80 ms 79 ms 99 ms
9 22.214.171.124 (126.96.36.199) 139 ms 139 ms 159 ms 10 188.8.131.52 (184.108.40.206) 199 ms 180 ms 300 ms 11 220.127.116.11 (18.104.22.168) 300 ms 239 ms 239 ms 12 * * * 13 22.214.171.124 (126.96.36.199) 259 ms 499 ms 279 ms 14 * * * 15 * * * 16 * * * 17 * * * 18 ALLSPICE.LCS.MIT.EDU (188.8.131.52) 339 ms 279 ms 279 ms Note that the gateways 12, 14, 15, 16 and 17 hops away either do not send ICMP "time exceeded" messages or send them with a ttl too small to reach localhost. Further investigation is required to determine the cause. For example, by contacting the system administrator(s) for gateways 14 through 17, you could discover that these gateways are running the MIT C Gateway code that does not send "time exceeded" messages.
The silent gateway 12 in the example 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 datagram. Since, for gateways, the remaining ttl is zero, the ICMP "time exceeded" is guaranteed to not make it back to us.
When this bug appears on the destination system
it behaves as follows:
1 helios.ee.lbl.gov (184.108.40.206) 0 ms 0 ms 0 ms
2 lilac-dmc.Berkeley.EDU (220.127.116.11) 39 ms 19 ms 39 ms
3 lilac-dmc.Berkeley.EDU (18.104.22.168) 19 ms 39 ms 19 ms
4 ccngw-ner-cc.Berkeley.EDU (22.214.171.124) 39 ms 40 ms 19 ms
5 ccn-nerif35.Berkeley.EDU (126.96.36.199) 39 ms 39 ms 39 ms
6 csgw.Berkeley.EDU (188.8.131.52) 39 ms 59 ms 39 ms
7 * * *
8 * * *
9 * * * 10 * * * 11 * * * 12 * * * 13 rip.Berkeley.EDU (184.108.40.206) 59 ms ! 39 ms ! 39 ms ! Note that there are 12 "gateways" (13 is the final destination) and the last half of them are "missing". What is happening is that the host rip (a Sun-3 running Sun OS3.5) is using the ttl from our arriving datagram as the ttl in its ICMP reply. The reply will time out on the return path (with no notice sent to anyone since ICMP's are not sent for ICMP's) until we probe with a ttl that is at least twice the path length. This means that the host rip is really only 7 hops away.
A reply that returns with a ttl of 1 is a clue this problem exists. The traceroute prints a "!" after the time if the ttl is less than or equal to 1. Since many systems continue to run obsolete or non-standard software, expect to see this problem frequently.
Other possible annotations after the time are !H, !N, !P (host, network, or protocol unreachable, respectively), !S or !F (source route failed or fragmentation needed - if either of these occurs, the associated gateway is broken). If almost all the probes result in some kind of unreachable, the traceroute command 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, you should not use traceroute during normal operations or from automated scripts.
Commands: netstat(1), ping(8)
Daemons: gated(8), routed(8) delim off