RFC 1918 addresses

While this is technically correct, actually changing the addresses in ICMP
payloads violates my first rule of debugging complex systems:

  Diagnostics should be a simple as possible and never be altered by any
non-diagnostic system.

(Which begs the question of should people be using these addresses locally
at all. Which is kind of metaphysical except that it *does* preserve
address space, which is a universal good. Some people do this for security
reasons too, although it's at best only moderately effective security
measure and could produce a false sense of security inside such an
addressing realm.)

I agree that ever having a source or destination IP that's RFC1918 outside
the domain is a very bad thing.