There may be scenarios where one would like to know path that packet is following. We can trace path of packet by traceroute and tracert command.
Traceroute command is used in linux and tracert command is used under windows. These commands come handy when you are troubleshooting network related issues.
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Syntax:
traceroute <destination url>
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Output:
It will print complete list routers that packet visited to reach destination. In output, it lists out IP of routers.
Sometimes you will see '*' symbol in output of traceroute. '*' field in output depicts that required field is unable to fetch by traceroute. There are different reasons for getting '*' field in output but it will successfully reach to a destination.
If in the last line of output, destination is not visible then you can be sure that system is not able to reach to destination. Check following screenshot of traceroute command.
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How it works:
Traceroute uses TTL(Time to live) field from IP packet. For those who are not aware of TTL field, this field describes lifetime of a packet. Usually value of TTL is set to 32 or 64. Each time a router receives any packet it decreases TTL value by one field. When TTL value becomes 0 then packet gets discarded and corresponding router sends error reply to a destination.
By using above situation, traceroute sends a packet using TTL value 1. Next router receives packet and sends error message to destination by decrementing TTL value to 0. This way traceroute go on by increasing TTL value and list out all the IP addresses between source and destination.
Modifying default options.
We can modify default option in traceroute command like TTL value, queries per hop etc. You can refer following link to modify default options in traceroute.
http://www.thegeekstuff.com/2012/05/traceroute-examples/
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example: traceroute gmail.com
