Always returning the same records is too restrictive

[DavidSchinazi]

The document currently states:

Servers operating in Split Mode SHOULD have DNS configured to return the same A (or AAAA) record for all ESNI-enabled servers they service. This yields an anonymity set of cardinality equal to the number of ESNI-enabled server domains supported by a given client-facing server. Thus, even with SNI encryption, an attacker which can enumerate the set of ESNI-enabled domains supported by a client-facing server can guess the correct SNI with probability at least 1/K, where K is the size of this ESNI-enabled server anonymity set. This probability may be increased via traffic analysis or other mechanisms.

I think this is too restrictive. If I have 100000 names and 100 IPs and for all queries I randomly return four out of my 100 IPs chosen randomly, I still provide the same anonymity set but I violate the SHOULD. How about: Content providers operating in Split Mode SHOULD ensure that the A and AAAA records for ESNI-enabled server names do not allow identifying the server name from the IP address. This can for example be achieved by always returning the same records for all ESNI-enabled names, or by having the function that picks addresses from a pool not depend on the server name. This yields an anonymity set...

[enygren]

Agreed. At a minimum this should reference a set of A and AAAA records rather than "the record" (as many records across the A and AAAA rrsets is very common).

I think @DavidSchinazi's proposed text is fine.

[ghedo]

@DavidSchinazi sgtm, do you plan to open a PR? I made https://github.com/ghedo/draft-ietf-tls-esni/commit/c9570cf4a00925bead63d46ecd6744288209146e but I think your wording is better.

[DavidSchinazi]

Created #157