traceroute

by Chris Kranz on Jul.07, 2009, under Manual Pages

NAME

traceroute – print the route packets take to network host

SYNOPSIS

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

The Internet is a large and complex aggregation of network hardware, connected together by gateways. An intranet or local net may also be complex. Tracking the route one’s packets follow (or finding the 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 addressto-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_port to base_port+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 ).

-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-ofservice result in different paths. (This may be academic, since the normal network services on the appliance 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.).

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 and 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 5 second 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 service on the destination is using that value, it can be changed with the -p flag).

A sample use and output might be:

toaster> traceroute nis.nsf.net. traceroute to nis.nsf.net (35.1.1.48), 30 hops max, 38 byte packet 1 internal-router.mycorp.com (10.17.12.34) 1.177 ms 1.448 ms 0.663 ms 2 10.16.105.1 (10.16.105.1) 1.141 ms 0.771 ms 0.722 ms 3 10.12.12.19 (10.12.12.19) 0.659 ms 0.614 ms 0.591 ms 4 10.12.12.20 (10.12.12.20) 1.22 ms 3.479 ms 1.788 ms 5 firewall.mycorp.com (10.25.91.101) 2.253 ms * 7.092 ms 6 isp-router.mycorp.com (198.92.178.1) 5.97 ms 5.522 ms 4.846 ms 7 isp-pop1.isp.net (4.12.88.205) 50.091 ms 75.644 ms 54.489 ms 8 isp-mycity1.isp.net (4.12.16.7) 137.352 ms 128.624 ms 107.653 ms 9 router1.mycity1-nbr1.isp.net (4.12.55.17) 69.458 ms 94.687 ms 58.282 ms 10 router2.city2.isp.net (4.12.68.141) 108.603 ms 73.293 ms 73.454 ms 11 router3.city2.isp.net (4.12.8.45) 89.773 ms 77.354 ms 86.19 ms 12 core6-hssi5-0-0.SanFrancisco.cw.net (204.70.10.213) 64.212 ms 72.039 ms 33.971 ms 13 corerouter2.SanFrancisco.cw.net (204.70.9.132) 15.747 ms 18.744 ms 21.543 ms 14 bordercore2.NorthRoyalton.cw.net (166.48.224.1) 69.559 ms 73.967 ms 68.042 ms 15 merit.NorthRoyalton.cw.net (166.48.225.250) 83.99 ms 130.937 ms 129.694 ms 16 198.108.23.145 (198.108.23.145) 147.379 ms 75.614 ms 82.193 ms 17 nic.merit.edu (198.108.1.48) 116.747 ms 163.204 ms *

After the time, there may appear annotations; the annotations printed by traceroute are:

!: reply returned with a TTL <= 1

!H: host unreachable

!N: network unreachable

!P: protocol unreachable

!S: source route failed

!F: fragmentation needed

Neither of the latter two should ever occur; if you see one, it means the associated gateway is malfunctioning. 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.