Discovery of Designated Resolvers

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Table of Contents

1. Introduction

When DNS clients wish to use encrypted DNS protocols such as DNS-over-TLS (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [RFC8484], they require additional information beyond the IP address of the DNS server, such as the resolver's hostname, non-standard ports, or URL paths. However, common configuration mechanisms only provide the resolver's IP address during configuration. Such mechanisms include network provisioning protocols like DHCP [RFC2132] and IPv6 Router Advertisement (RA) options [RFC8106], as well as manual configuration.¶

This document defines two mechanisms for clients to discover designated resolvers using DNS server Service Binding (SVCB, [I-D.ietf-dnsop-svcb-https]) records:¶

1. When only an IP address of an Unencrypted Resolver is known, the client queries a special use domain name to discover DNS SVCB records associated with the Unencrypted Resolver (Section 4).¶
2. When the hostname of an encrypted DNS server is known, the client requests details by sending a query for a DNS SVCB record. This can be used to discover alternate encrypted DNS protocols supported by a known server, or to provide details if a resolver name is provisioned by a network (Section 5).¶

Both of these approaches allow clients to confirm that a discovered Encrypted Resolver is designated by the originally provisioned resolver. "Designated" in this context means that the resolvers are operated by the same entity or cooperating entities; for example, the resolvers are accessible on the same IP address, or there is a certificate that claims ownership over both resolvers.¶

2. Terminology

This document defines the following terms:¶

DDR:

Discovery of Designated Resolvers. Refers to the mechanisms defined in this document.¶

Designated Resolver:

A resolver, presumably an Encrypted Resolver, designated by another resolver for use in its own place. This designation can be authenticated with TLS certificates.¶

Encrypted Resolver:

A DNS resolver using any encrypted DNS transport. This includes current mechanisms such as DoH and DoT as well as future mechanisms.¶

Unencrypted Resolver:

A DNS resolver using TCP or UDP port 53.¶

3. DNS Service Binding Records

DNS resolvers can advertise one or more Designated Resolvers that may offer support over encrypted channels and are controlled by the same entity.¶

When a client discovers Designated Resolvers, it learns information such as the supported protocols, ports, and server name to use in certificate validation. This information is provided in Service Binding (SVCB) records for DNS Servers, defined by [I-D.schwartz-svcb-dns].¶

The following is an example of an SVCB record describing a DoH server:¶

_dns.example.net  7200  IN SVCB 1 . (
     alpn=h2 dohpath=/dns-query{?dns} )

The following is an example of an SVCB record describing a DoT server:¶

_dns.example.net  7200  IN SVCB 1 dot.example.net (
     alpn=dot port=8530 )

If multiple Designated Resolvers are available, using one or more encrypted DNS protocols, the resolver deployment can indicate a preference using the priority fields in each SVCB record [I-D.ietf-dnsop-svcb-https].¶

This document focuses on discovering DoH and DoT Designated Resolvers. Other protocols can also use the format defined by [I-D.schwartz-svcb-dns]. However, if any protocol does not involve some form of certificate validation, new validation mechanisms will need to be defined to support validating designation as defined in Section 4.1.¶

4. Discovery Using Resolver IP Addresses

When a DNS client is configured with an Unencrypted Resolver IP address, it SHOULD query the resolver for SVCB records for "dns://resolver.arpa" before making other queries. Specifically, the client issues a query for _dns.resolver.arpa with the SVCB resource record type (64) [I-D.ietf-dnsop-svcb-https].¶

If the recursive resolver that receives this query has one or more Designated Resolvers, it will return the corresponding SVCB records. When responding to these special queries for "dns://resolver.arpa", the recursive resolver SHOULD include the A and AAAA records for the name of the Designated Resolver in the Additional Answers section. This will allow the DNS client to make queries over an encrypted connection without waiting to resolve the Encrypted Resolver name per [I-D.ietf-dnsop-svcb-https]. If no A/AAAA records or SVCB IP address hints are included, clients will be forced to delay use of the Encrypted Resolver until an additional DNS lookup for the A and AAAA records can be made to the Unencrypted Resolver (or some other resolver the DNS client has been configured to use).¶

If the recursive resolver that receives this query has no Designated Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa" SUDN.¶

5. Discovery Using Resolver Names

A DNS client that already knows the name of an Encrypted Resolver can use DDR to discover details about all supported encrypted DNS protocols. This situation can arise if a client has been configured to use a given Encrypted Resolver, or if a network provisioning protocol (such as DHCP or IPv6 Router Advertisements) provides a name for an Encrypted Resolver alongside the resolver IP address.¶

For these cases, the client simply sends a DNS SVCB query using the known name of the resolver. This query can be issued to the named Encrypted Resolver itself or to any other resolver. Unlike the case of bootstrapping from an Unencrypted Resolver (Section 4), these records SHOULD be available in the public DNS.¶

For example, if the client already knows about a DoT server resolver.example.com, it can issue an SVCB query for _dns.resolver.example.com to discover if there are other encrypted DNS protocols available. In the following example, the SVCB answers indicate that resolver.example.com supports both DoH and DoT, and that the DoH server indicates a higher priority than the DoT server.¶

_dns.resolver.example.com  7200  IN SVCB 1 . (
     alpn=h2 dohpath=/dns-query{?dns} )
_dns.resolver.example.com  7200  IN SVCB 2 . (
     alpn=dot )

Often, the various supported encrypted DNS protocols will be accessible using the same hostname. In the example above, both DoH and DoT use the name resolver.example.com for their TLS certificates. If a deployment uses a different hostname for one protocol, but still wants clients to treat both DNS servers as designated, the TLS certificates MUST include both names in the SubjectAlternativeName fields. Note that this name verification is not related to the DNS resolver that provided the SVCB answer.¶

For example, being able to discover a Designated Resolver for a known Encrypted Resolver is useful when a client has a DoT configuration for foo.resolver.example.com but is on a network that blocks DoT traffic. The client can still send a query to any other accessible resolver (either the local network resolver or an accessible DoH server) to discover if there is a designated DoH server for foo.resolver.example.com.¶

6. Deployment Considerations

Resolver deployments that support DDR are advised to consider the following points.¶

7. Security Considerations

Since client can receive DNS SVCB answers over unencrypted DNS, on-path attackers can prevent successful discovery by dropping SVCB packets. Clients should be aware that it might not be possible to distinguish between resolvers that do not have any Designated Resolver and such an active attack.¶

While the IP address of the Unencrypted Resolver is often provisioned over insecure mechanisms, it can also be provisioned securely, such as via manual configuration, a VPN, or on a network with protections like RA guard [RFC6105]. An attacker might try to direct Encrypted DNS traffic to itself by causing the client to think that a discovered Designated Resolver uses a different IP address from the Unencrypted Resolver. Such an Encrypted Resolver might have a valid certificate, but be operated by an attacker that is trying to observe or modify user queries without the knowledge of the client or network.¶

If the IP address of a Designated Resolver differs from that of an Unencrypted Resolver, clients MUST validate that the IP address of the Unencrypted Resolver is covered by the SubjectAlternativeName of the Encrypted Resolver's TLS certificate (Section 4.1).¶

Opportunistic use of Encrypted Resolvers MUST be limited to cases where the Unencrypted Resolver and Designated Resolver have the same IP address (Section 4.2).¶

8. IANA Considerations

Appendix A. Rationale for using SVCB records

This mechanism uses SVCB/HTTPS resource records [I-D.ietf-dnsop-svcb-https] to communicate that a given domain designates a particular Designated Resolver for clients to use in place of an Unencrypted Resolver (using a SUDN) or another Encrypted Resolver (using its domain name).¶

There are various other proposals for how to provide similar functionality. There are several reasons that this mechanism has chosen SVCB records:¶

• Discovering encrypted resolver using DNS records keeps client logic for DNS self-contained and allows a DNS resolver operator to define which resolver names and IP addresses are related to one another.¶
• Using DNS records also does not rely on bootstrapping with higher-level application operations (such as [I-D.schinazi-httpbis-doh-preference-hints]).¶
• SVCB records are extensible and allow definition of parameter keys. This makes them a superior mechanism for extensibility as compared to approaches such as overloading TXT records. The same keys can be used for discovering Designated Resolvers of different transport types as well as those advertised by Unencrypted Resolvers or another Encrypted Resolver.¶
• Clients and servers that are interested in privacy of names will already need to support SVCB records in order to use Encrypted TLS Client Hello [I-D.ietf-tls-esni]. Without encrypting names in TLS, the value of encrypting DNS is reduced, so pairing the solutions provides the largest benefit.¶
• Clients that support SVCB will generally send out three queries when accessing web content on a dual-stack network: A, AAAA, and HTTPS queries. Discovering a Designated Resolver as part of one of these queries, without having to add yet another query, minimizes the total number of queries clients send. While [RFC5507] recommends adding new RRTypes for new functionality, SVCB provides an extension mechanism that simplifies client behavior.¶

Authors' Addresses