My last post about Traceroute got some interesting conversation going on in the comments. Basically there is quite a big difference in the way in which Windows and Linux handle traceroute. I tested on both Windows 7 and Ubuntu 10.04, but my guess is that all Windows follow the same format as do all *nix’s (please let me know if otherwise though!) I would recommend reading the above post again quickly to get all the basics out the way before we delve into the differences. Step-wise, this is what happens on Windows: The OS send a DNS PTR request to 1.

Traceroute is a powerful tool. Extremely useful when checking the path of a packet through the network. But how does it ACTUALLY work? What is REALLY going on? Layer3 packets all have a TTL. A Time To Live. If a router receives a packet with a TTL of 1 (and the packet is addresses to a host not directly connected to this router) it will drop the packet. It will also then create an ICMP error packet and send it back to the original source of the packet to let it know that the address was unreachable this time. If you ping another machine, the OS will generally create a TTL of 255 for sent packets, though it doesn’t HAVE to be 255.