ICMP Unreachables

Hello all,

I've been doing some work with ICMP unreachable (type 3) packets, and I've run into a few problems I was hoping you could help with.

First of all, when dealing with type 3, code 1 (host unreachable) messages, the payload must be formatted in a specific fashion to correctly respond to a host unreachable error condition.

This what I've managed to figure out based on the sample/icmp_unreach.c code:

unsigned char payload[8] = { a, b, c, d, e, f, g, h };

'a' and 'b' represent the source port of the original packet, while 'c' and 'd' represent the destination port of the original packet.

That's easy enough to manufacture, however, what is the purpose of the last 4 bytes, and how are they calculated?

Any advice/help/general information would be very helpful.

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Thanks,

Darren