Subject: TTL/hopcount becomes 0, what to send back, original packet with TTL = 1 or the one with TTL = 0?

From: Jeroen Massar <[email-masked]>
Date: Wed, 22 Jun 2011 20:27:43 +0200

Hi,

I was wondering about the question in the subject.

One gets a packet, the TTL/Hopcount is one, as I am the router, I
subtract 1, then realize it is 0 and have to send out an ICMP
unreachable as I want to actually forward it on to the next router/host.

Now, the question is, do I need to send back the _original_ header
containing TTL=1 in the payload of the ICMP or the new header with a TTL
of 0?

The question becomes more interesting when one receives a packet with
TTL=0 from some malicious sender, what to do with that one?

Greets,
 Jeroen