[c-nsp] Help settling an argument :-)

[lists at hojmark.org]

> If you ping the BC subnet interface of B from A, what address 
> should B use to source the ICMP response?

I'm pretty sure RFC1122 (Host Requirements) says that the host
SHOULD send the response sourced from the original destination.

> This is OS-dependant, some OS will source with B:s BC interface
> IP, some will do it with the B:s AB interface IP adress.

"SHOULD" is pretty strong in RFC terms... A host can, and often
will, chose a different interface, but normally won't use another
address.

> This also applies to ICMP unreachable or TTL expired 
> messages.

No that's a bit different because, strictly speaking, it's not
replies. Typically, a host will source it based on its (static)
routing table, but it may be configurable.

For IOS, of cause, we all know that the source addresses are
(mostly?) configurable. (It's just a shame that you can't do
it all in one place).

But then, IOS is typically used on routers, not hosts.

-A