Tracing IP routes with TRACEROUTE command
August 20, 1999
The traceroute command traces the route to a given ip address or host. It shows the ip address of each hop along the way. The traceroute command functions by sending udp packets to the specified host and listens for ICMP messages returning.
Traceroute is a useful tool when debugging routing problems. Often packets get to where they are going but do not return due to problems in routing. If you can determine that packets are arriving at their destination (perhaps by using the ptrace command) then from that destination you can traceroute back to the origin and see where the routes go off course.
In the case when routing has broken down and a network can no longer see out to the Internet you can issue a traceroute from the isolated LAN to the internet and get the last ip address seen. Then traceroute from the internet to the isolated LAN and get the last ip seen and the problem will generally lie between those two ip addresses.
traceroute [ip address, hostname] traceroute # terminates traceroute