stream close reasons should distinguish DNS failure from nonexistent host
When we next revamp the spec, we should arrange that the RELAY_END reason bytes (spec 6.3) distinguish the two fundamentally different kinds of DNS failure.
Currently, REASON_RESOLVEFAILED seems to do double duty for all of:
-
The DNS worked properly and told us that the host definitely doesn't exist (
NXDOMAIN) or that this isn't a hostname we can use (NODATApseudo-rcode) -
The DNS isn't working properly and we weren't able to find out about this host.
-
The question we asked the DNS server was invalid (eg, wrong hostname syntax, name too long, etc.)
I don't think we absolutely need to distinguish 3; it can be lumped in with 1, and it shoudl be rare. But we should distinguish 1 from 2. They correspond, from (say) a Tor Browser user's pov, "this website cannot be reached due to some network or computer not working right" and "you have a wrong (or out of date) URL".