Handle empty DNS reply without error as NOERROR

This re-introduces an error code to handle returning empty DNS replies instead of sending NXDOMAIN error, which fixes resolving IPv6 only destinations when first sending an A type DNS request, as most applications do. As discussed in #40984 (closed) the previous commit was reverted. This is the exact same code as the previously merged one, but as suggested in the issue uses an error code outside the 0-255 range, which is safe for internal use in the C Tor implementation. Fixes #40248 (closed)

changed the description

requested review from @ahf

changed milestone to %Tor: 0.4.9.x-freeze

This to me looks good and plausible. Essentially, if the requests is successful but simply returns no A and AAAA, we send back NOERROR leading to a 0x00 error code returned which matches the RFC1035 no error condition.

@ahf, you fine with this?

@dgoulet if we think it solves the Arch Linux issue, then sure. There was also a bigger Spec discussion around this in torspec#278 (closed)

Are we finding all of this to be in an OK state at this point?

The code follows that discussion to not make a new public code, keep it internal and return the RFC1035 code in that case.

This seems fine to me. Not returning a DNS error when there are no address is logical. We have an 049 to test this :)

approved this merge request

merged

mentioned in commit 13cb056a