A cursory look at RFC 1009 indicates there is no *requirement* that
gateways return an ICMP x Unreachable in the case at hand. In any
case, both the network and the host are reachable --- just not for
all applications. There is no ICMP message defined specifically for
this case, although there is always the catch-all Parameter Problem
message.
1009 is fairly out of date; I should check the drafts directories for
a new router requirements document.
Anyway, the right code is Destination Unreachable, code 10. That value
is defined by RFC1122 for ``administratively prohibited''. Unfortunately,
4.3bsd, and likely many of its descendants, will ignore any ICMP message
with a code value they consider invalid. Thus, Cisco routers send back
``host unreachable''.
--Steve Bellovin
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