Code change:
Clients using Opportunistic Discovery (opportunistic) MUST be limited to cases where the Unencrypted DNS Resolver and Designated Resolver <del> have the same IP address.  Clients which do not follow Opportunistic Discovery (opportunistic) and instead try to connect without first checking for a designation run the possible risk of being intercepted by an attacker hosting an Encrypted DNS Resolver on an IP address of an Unencrypted DNS Resolver where the attacker has failed to gain </del> <ins> have the same IP address, which SHOULD be a private or local IP address.  Clients which do not follow Opportunistic Discovery (opportunistic) and instead try to connect without first checking for a designation run the possible risk of being intercepted by an attacker hosting an Encrypted DNS Resolver on an IP address of an Unencrypted DNS Resolver where the attacker has failed to gain </ins> control of the Unencrypted DNS Resolver. The constraints on the use of Designated Resolvers specified here

Related discussion:
text: And section 6.5 states DNR trumps DDR. address, clients MAY choose to opportunistically use the Designated Resolver even without this certificate check (Section 4.3). Why only when the IP is the same? Why not for when the IP is different? What's to lose, since the only fallback option is stick to using the unencrypted resolver? yields an IP address that was not presented in the Additional Answers section or ipv4hint or ipv6hint fields of the original SVCB query, the connection made to that IP address MUST pass the same TLS certificate checks before being allowed to replace a previously known and validated IP address for the same Designated Resolver name. How does the appearance of an (unsigned) Additional Answer entry convey any kind of real world trust/authentication? In other words, why should it not ALWAYS pass the same TLS certificate checks ? is to use unencrypted DNS, why not just use it anyway? use DDR to discover details about all supported encrypted DNS protocols. It's a little odd to mention this as the DNR draft really tries hard to say to only use the network's offered encrypted DNS servers, so this option is in conflict with the DNR draft. Unfortunately, all currently deployed software does not know this, so it will be very common that this happens. URL SHOULD treat URL as a locally served zone per [RFC6303] Why is this not a MUST ? queries being dropped either maliciously or unintentionally, clients can re-send their SVCB queries periodically. I don't see how this would improve security for the client. It also mixes up behaviour of proper DNS clients that have their own retransmit logic for if they get no answer. attack where an attacker can block connections to the encrypted DNS server, and recommends that clients prevent it by switching to SVCB- reliant behavior once SVCB resolution does succeed. For DDR, this means that once a client discovers a compatible Designated Resolver, it SHOULD NOT use unencrypted DNS until the SVCB record expires I wonder which attacker can block encrypted DNS connections but not spoof (or block!) an unsigned SVCB record to the local client? And even if that is the case, this would be a denial of service attack if the DNS client cannot fallback to unencrypted DNS.