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], DNS-over-QUIC (DoQ) [RFC9250], or DNS-over-HTTPS (DoH) [RFC8484], they can require additional information beyond the IP address of the DNS server, such as the resolver's hostname, alternate IP addresses, non-standard ports, or URI templates. However, common configuration mechanisms only provide the resolver's IP address during configuration. Such mechanisms include network provisioning protocols like DHCP [RFC2132] [RFC8415] and IPv6 Router Advertisement (RA) options [RFC8106], as well as manual configuration.¶

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

1. When only an IP address of an Unencrypted DNS Resolver is known, the client queries a special use domain name (SUDN) [RFC6761] to discover DNS SVCB records associated with one or more Encrypted DNS Resolvers the Unencrypted DNS Resolver has designated for use when support for DNS encryption is requested (Section 4).¶
2. When the hostname of an Encrypted DNS Resolver 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 DNS 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 contains the IP address for the original designating resolver.¶

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 DNS Resolver, designated by another resolver for use in its own place. This designation can be verified with TLS certificates.¶

Encrypted DNS Resolver:

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

Unencrypted DNS Resolver:

A DNS resolver using a transport without encryption, historically 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 and ports. This information is provided in ServiceMode Service Binding (SVCB) records for DNS Servers, although AliasMode SVCB records can be used to direct clients to the needed ServiceMode SVCB record per [I-D.ietf-dnsop-svcb-https]. The formatting of these records, including the DNS-unique parameters such as "dohpath", are defined by [I-D.ietf-add-svcb-dns].¶

The following is an example of an SVCB record describing a DoH server discovered by querying for _dns.example.net:¶

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

The following is an example of an SVCB record describing a DoT server discovered by querying for _dns.example.net:¶

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

The following is an example of an SVCB record describing a DoQ server discovered by querying for _dns.example.net:¶

_dns.example.net.  7200  IN SVCB 1 doq.example.net (
     alpn=doq 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].¶

If the client encounters a mandatory parameter in an SVCB record it does not understand, it MUST NOT use that record to discover a Designated Resolver, in accordance with Section 8 of [I-D.ietf-dnsop-svcb-https]. The client can still use other records in the same response if the client can understand all of their mandatory parameters. This allows future encrypted deployments to simultaneously support protocols even if a given client is not aware of all those protocols. For example, if the Unencrypted DNS Resolver returns three SVCB records, one for DoH, one for DoT, and one for a yet-to-exist protocol, a client which only supports DoH and DoT should be able to use those records while safely ignoring the third record.¶

To avoid name lookup deadlock, clients that use Designated Resolvers need to ensure that a specific Encrypted Resolver is not used for any queries that are needed to resolve the name of the resolver itself or to perform certificate revocation checks for the resolver, as described in Section 10 of [RFC8484]. Designated Resolvers need to ensure this deadlock is avoidable as described in Section 10 of [RFC8484].¶

This document focuses on discovering DoH, DoT, and DoQ Designated Resolvers. Other protocols can also use the format defined by [I-D.ietf-add-svcb-dns]. However, if any such 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 4.2.¶

4. Discovery Using Resolver IP Addresses

When a DNS client is configured with an Unencrypted DNS Resolver IP address, it SHOULD query the resolver for SVCB records of a service with a scheme of "dns" and an Authority of "resolver.arpa" before making other queries. This allows the client to switch to using Encrypted DNS for all other queries, if possible. Specifically, the client issues a query for _dns.resolver.arpa. with the SVCB resource record type (64) [I-D.ietf-dnsop-svcb-https].¶

Responses to the SVCB query for the "resolver.arpa" SUDN describe Designated Resolvers. To ensure that different Designated Resolver configurations can be correctly distinguished and associated with A and AAAA records for the resolver, ServiceMode SVCB responses to these queries MUST NOT use the "." or "resolver.arpa" value for the TargetName. Similarly, clients MUST NOT perform A or AAAA queries for "resolver.arpa".¶

The following is an example of an SVCB record describing a DoH server discovered by querying for _dns.resolver.arpa:¶

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

The following is an example of an SVCB record describing a DoT server discovered by querying for _dns.resolver.arpa:¶

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

The following is an example of an SVCB record describing a DoQ server discovered by querying for _dns.resolver.arpa:¶

_dns.resolver.arpa.  7200  IN SVCB 1 doq.example.net (
     alpn=doq port=8530 )

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 "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 save the DNS client an additional round trip to retrieve the address of the designated resolver; see Section 5 of [I-D.ietf-dnsop-svcb-https].¶

Designated Resolvers SHOULD be accessible using the IP address families that are supported by their associated Unencrypted DNS Resolvers. If an Unencrypted DNS Resolver is accessible using an IPv4 address, it ought to provide an A record for an IPv4 address of the Designated Resolver; similarly, if it is accessible using an IPv6 address, it ought to provide a AAAA record for an IPv6 address of the Designated Resolver. The Designated Resolver MAY support more address families than the Unencrypted DNS Resolver, but it SHOULD NOT support fewer. If this is not done, clients that only have connectivity over one address family might not be able to access the Designated Resolver.¶

If the recursive resolver that receives this query has no Designated Resolvers, it SHOULD return NODATA for queries to the "resolver.arpa" zone, to provide a consistent and accurate signal to clients that it does not have a Designated Resolver.¶

5. Discovery Using Resolver Names

A DNS client that already knows the name of an Encrypted DNS 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 DNS Resolver, or if a network provisioning protocol (such as DHCP or IPv6 Router Advertisements) provides a name for an Encrypted DNS Resolver alongside the resolver IP address, such as by using Discovery of Network Resolvers (DNR) [I-D.ietf-add-dnr].¶

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 DNS Resolver itself or to any other resolver. Unlike the case of bootstrapping from an Unencrypted DNS Resolver (Section 4), these records SHOULD be available in the public DNS if the same domain name's A or AAAA records are available in the public DNS to allow using any resolver to discover another resolver's Designated Resolvers. When the name can only be resolved in private namespaces, these records SHOULD be available to the same audience as the A and AAAA records.¶

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

Clients MUST validate that for any Encrypted DNS Resolver discovered using a known resolver name, the TLS certificate of the resolver contains the known name in a subjectAltName extension. In the example above, this means that both servers need to have certificates that cover the name resolver.example.com. Often, the various supported encrypted DNS protocols will be specified such that the SVCB TargetName matches the known name, as is true in the example above. However, even when the TargetName is different (for example, if the DoH server had a TargetName of doh.example.com), the clients still check for the original known resolver name in the certificate.¶

Note that this resolver validation is not related to the DNS resolver that provided the SVCB answer.¶

As another example, being able to discover a Designated Resolver for a known Encrypted DNS 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 clients can receive DNS SVCB answers over unencrypted DNS, on-path attackers can prevent successful discovery by dropping SVCB queries or answers, and thus prevent clients from switching to use encrypted DNS. 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. To limit the impact of discovery queries being dropped either maliciously or unintentionally, clients can re-send their SVCB queries periodically.¶

Section 8.2 of [I-D.ietf-add-svcb-dns] describes a second downgrade attack where an attacker can block connections to the encrypted DNS server. For DDR, clients need to validate a Designated Resolver using a connection to the server before trusting it, so attackers that can block these connections can prevent clients from switching to use encrypted DNS.¶

Encrypted DNS Resolvers that allow discovery using DNS SVCB answers over unencrypted DNS MUST NOT provide differentiated behavior based solely on metadata in the SVCB record, such as the HTTP path or alternate port number, which are parameters that an attacker could modify. For example, if a DoH resolver provides a filtering service for one URI path, and a non-filtered service for another URI path, an attacker could select which of these services is used by modifying the "dohpath" parameter. These attacks can be mitigated by providing separate resolver IP addresses or hostnames.¶

While the IP address of the Unencrypted DNS 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 DNS Resolver. Such a Designated 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 DNS Resolver, clients applying Verified Discovery (Section 4.2) MUST validate that the IP address of the Unencrypted DNS Resolver is covered by the SubjectAlternativeName of the Designated Resolver's TLS certificate. If that validation fails, the client MUST NOT automatically use the discovered Designated Resolver.¶

Clients using Opportunistic Discovery (Section 4.3) MUST be limited to cases where the Unencrypted DNS Resolver and Designated Resolver have the same IP address, which SHOULD be a private or local IP address. Clients which do not follow Opportunistic Discovery (Section 4.3) 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 control of the Unencrypted DNS Resolver.¶

The constraints on the use of Designated Resolvers specified here apply specifically to the automatic discovery mechanisms defined in this document, which are referred to as Verified Discovery and Opportunistic Discovery. Clients MAY use some other mechanism to verify and use Designated Resolvers discovered using the DNS SVCB record. However, use of such an alternate mechanism needs to take into account the attack scenarios detailed here.¶

8. IANA Considerations

Appendix A. Rationale for using a Special Use Domain Name

The "resolver.arpa" SUDN is similar to "ipv4only.arpa" in that the querying client is not interested in an answer from the authoritative "arpa" name servers. The intent of the SUDN is to allow clients to communicate with the Unencrypted DNS Resolver much like "ipv4only.arpa" allows for client-to-middlebox communication. For more context, see the rationale behind "ipv4only.arpa" in [RFC8880].¶

Appendix B. 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 DNS Resolver (using a SUDN) or another Encrypted DNS 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 DNS resolvers 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 DNS Resolvers or another Encrypted DNS 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.¶

Authors' Addresses