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Charly's Column – traceroute

Article from Issue 235/2020
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Like every admin, Charly regularly uses the classic traceroute tool. If unfriendly digital natives interfere with an ICMP filter, he simply switches to a clever alternative like LFT.

Practically every admin uses the classic traceroute tool at more or less regular intervals. This gets me all the more irritated when I find myself in a hotel with a WiFi network where the admin has completely disabled ICMP. Apart from the fact that this causes more trouble than benefits in what is by definition a public network, it can be easily circumvented.

The first version of traceroute was written in 1988 by a certain Van Jacobsen – Van is his first name, not an honorific. To be able to trace the path of packets through the web, Jacobsen came up with a clever method. He sent test packets through the Internet to a defined destination and increased the time to live (TTL) value for each packet.

The first packet is assigned a TTL of one. Each router that transports the packet further reduces the TTL by one. Once the TTL reaches a value of zero, the router sends it back with an ICMP TTL exceeded message. By successively increasing the TTL, Jacobsen got the packets back from routers that were further and further away and was able to follow the path of the packet until it finally reached its destination.

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