Traceroute and random UDP ports

Patrick W. Gilmore patrick at
Tue Aug 12 18:57:21 CDT 2008

On Aug 12, 2008, at 7:54 PM, Glen Kent wrote:

> 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?

Depends on what is running there.

Given people randomizing things like DNS ephemeral ports, if they're  
not careful, it will probably happen more often.

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

So you know which hop sent the packet back.


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