On 5 Nov 2018, at 9:44, RW wrote:
I created an A-record at Namecheap for a_b.mydomain.tld and neither firefox nor chromium had a problem with it.
That's interesting and unfortunate because 'a_b' is unequivocally a violation of the syntax for hostnames. It may be acceptable as a DNS label, but it isn't a valid hostname.
FWIW, BIND 9.x (since 9.4-ish) will parse and load a zone with such an A in it, but complains and does not serve the record: NXDOMAIN for a normal query, no hint of it in a zone transfer. Deep in the mists of time, the resolver for 'classic' MacOS (not derived from any other resolver) got an update that made it no longer resolve hostnames with underscores and while there was a brief bit of grumbling, they never reversed that stringency. I would guess that with some authoritative servers refusing to serve invalid names and some resolvers refusing to resolve them, it would be a low-yield tactic to use them to evade filtering.
