Traceroute and random UDP ports

Glen Kent glen.kent at
Tue Aug 12 18:54:17 CDT 2008


The outgoing packets from traceroute are sent towards the destination
using UDP and very high port numbers, typically in the range of 32,768
and higher. This is because no one is gernally expected to run UDP
services up there, so when the packet finally reaches the destination,
traceroute can tell that it got to the end (because the ICMP changes
from "TTL exceeded" to "port unreachable").

My question is: What if the receiver is actually listening on one of
the "random" UDP ports? What would happen in such cases?

Also, why do we increase the UDP port number with each subsequent
traceroute packet that is sent?


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