Traceroute is a computer network diagnostic tool for displaying the route (path) and measuring transit delays of packets across an Internet Protocol (IP) network. It utilizes the IP protocol’s time to live (TTL) field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to the host.
This tool verifies the path by which our packet should reach the destination, without actually sending the data. This post is not anything about how this tool works it’s rather about the concept this tools uses for fulfilling our purpose.
We can always refer the Linux man and info pages for gaining the knowledge about how to use this tool.
You should kow the basics first
Each IP packet that we send on the internet has a field called as TTL. TTL stands for Time To Live. we can measure TTL by the no of hops. Its the maximum number of hops that a packet can travel through across the internet, before its discarded. Hops are the computers, routers, or any devices that comes in between the source and the destination. If there is no TTL in an IP packet, the packet will flow endlessly from one router to another and on and on forever searching for the destination. Operating System automatically handles TTL value, although we can change it with the help of few tools.
Now If the destination is not found after traveling through too many hops, the receiving router will drop the packet and informs the original sender. Let’s say I need to reach 126.96.36.199 Ip address, and my default TTL value is 30 hops. Which means i can travel a maximum of 30 hops to reach my destination, before which the packet is dropping. Each router that comes in between the source and destination will go on reducing the TTL value before sending to the next router.
Which means if i have a default TTL value of 30, then my first router will reduce it to 29 and then send that to the next router across the path. Again the receiving router will make it 28 and send to the next and so on. If a router receives a packet with TTl of 1, the packet will be discarded. But the router which discards the packet will inform the original sender that the TTL value has exceeded. Once the receiver gets the message he will come to know about the sender. Read more