> Oh, and the other thing where someone commented that on the
> majority of NATd
> setups the NAT router would actually spot the destination IP
> address as its
> own and route the request locally instead of via the internet ...
> well, that
> wouldn't matter at all, surely?

That assertion was incorrect.  See below.

> If aaa.bbb.ccc.ddd is the IP of the NAT
> router then whether the request goes via the internet and back, or whether
> the NAT box realises it points to itself, doesn't matter at all - the IP
> address is still the correct IP address, the scheme still works
> as described
> and intended.

No, it won't work at all either way, because of how most NAT routers work:

Typical NAT routers don't handle self-referencing IPs well.  In other words,
if my NAT router's WAN address is 123.45.67.89 and a LAN machine (say,
10.10.10.1) tries to talk to 123.45.67.89:4321, it will fail.  I'm using a
SonicWall SOHO firewall/router, and this is the case with it.  I believe
Linksys and the other really common ones behave similarly.  It's annoying,
but true.

The bottom line is:  A local machine with NAT cannot reliably figure out
what routable IP it has without outside help.  Luckily, we have a network of
outside help, and seed nodes are required for anything to work, so we just
need our protocol to handle some form of, "What IP do I look like?"  "You
look like IP such-and-such."

-glenn


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