| Internet-Draft | DDR | April 2022 |
| Pauly, et al. | Expires 6 October 2022 | [Page] |
This document defines Discovery of Designated Resolvers (DDR), a mechanism for DNS clients to use DNS records to discover a resolver's encrypted DNS configuration. This mechanism can be used to move from unencrypted DNS to encrypted DNS when only the IP address of a resolver is known. This mechanism is designed to be limited to cases where unencrypted resolvers and their designated resolvers are operated by the same entity or cooperating entities. It can also be used to discover support for encrypted DNS protocols when the name of an encrypted resolver is known.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Adaptive DNS Discovery Working Group mailing list (add@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/add/.¶
Source for this draft and an issue tracker can be found at https://github.com/ietf-wg-add/draft-ietf-add-ddr.¶
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Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 6 October 2022.¶
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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:¶
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 the IP address for the original designating resolver.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document defines the following terms:¶
Discovery of Designated Resolvers. Refers to the mechanisms defined in this document.¶
A resolver, presumably an Encrypted Resolver, designated by another resolver for use in its own place. This designation can be verified with TLS certificates.¶
A DNS resolver using any encrypted DNS transport. This includes current mechanisms such as DoH and DoT as well as future mechanisms.¶
A DNS resolver using TCP or UDP port 53.¶
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 )
¶
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. The client can still use others 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 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, Designated Resolvers SHOULD follow the guidance in Section 10 of [RFC8484] regarding the avoidance of DNS-based references that block the completion of the TLS handshake.¶
This document focuses on discovering DoH and DoT Designated Resolvers. Other protocols can also use the format defined by [I-D.ietf-add-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 4.2.¶
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].¶
Because this query is for an SUDN, which no entity can claim ownership over, the ServiceMode SVCB response MUST NOT use the "." value for the TargetName. Instead, the domain name used for DoT or used to construct the DoH template MUST be provided.¶
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 )
¶
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.¶
When a client discovers Designated Resolvers from an Unencrypted Resolver IP address, it can choose to use these Designated Resolvers either automatically, or based on some other policy, heuristic, or user choice.¶
This document defines two preferred methods to automatically use Designated Resolvers:¶
A client MAY additionally use a discovered Designated Resolver without either of these methods, based on implementation-specific policy or user input. Details of such policy are out of scope of this document. Clients SHOULD NOT automatically use a Designated Resolver without some sort of validation, such as the two methods defined in this document or a future mechanism.¶
A client MUST NOT use a Designated Resolver designated by one Unencrypted Resolver in place of another Unencrypted Resolver. As these are known only by IP address, this means each unique IP address used for unencrypted DNS requires its own designation discovery. This ensures queries are being sent to a party designated by the resolver originally being used.¶
Generally, clients also SHOULD NOT reuse the Designated Resolver discovered from an Unencrypted Resolver over one network connection in place of the same Unencrypted Resolver on another network connection. Instead, clients SHOULD repeat the discovery process on the other network connection.¶
However, if a given Unencrypted Resolver designates a Designated Resolver that uses a public IP address and can be verified using the mechanism described in Section 4.2, it MAY be used on different network connections so long as the subsequent connections over other networks can also be successfully verified using the mechanism described in Section 4.2. This is a tradeoff between performance (by having no delay in establishing an encrypted DNS connection on the new network) and functionality (if the Unencrypted Resolver intends to designate different Designated Resolvers based on the network from which clients connect).¶
Verified Discovery is a mechanism that allows automatic use of a Designated Resolver that supports DNS encryption that performs a TLS handshake.¶
In order to be considered a verified Designated Resolver, the TLS certificate presented by the Designated Resolver MUST contain the IP address of the designating Unencrypted Resolver in a subjectAltName extension. If the certificate can be validated, the client SHOULD use the discovered Designated Resolver for any cases in which it would have otherwise used the Unencrypted Resolver. If the Designated Resolver has a different IP address than the Unencrypted Resolver and the TLS certificate does not cover the Unencrypted Resolver address, the client MUST NOT automatically use the discovered Designated Resolver. Additionally, the client SHOULD suppress any further queries for Designated Resolvers using this Unencrypted Resolver for the length of time indicated by the SVCB record's Time to Live (TTL).¶
If the Designated Resolver and the Unencrypted Resolver share an IP address, clients MAY choose to opportunistically use the Designated Resolver even without this certificate check (Section 4.3).¶
If resolving the name of a Designated Resolver from an SVCB record 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.¶
There are situations where Verified Discovery of encrypted DNS configuration over unencrypted DNS is not possible. This includes Unencrypted Resolvers on non-public IP addresses such as those defined in [RFC1918] whose identity cannot be confirmed using TLS certificates.¶
Opportunistic Privacy is defined for DoT in Section 4.1 of [RFC7858] as a mode in which clients do not validate the name of the resolver presented in the certificate. A client MAY use information from the SVCB record for "dns://resolver.arpa" with this "opportunistic" approach (not validating the names presented in the SubjectAlternativeName field of the certificate) as long as the IP address of the Encrypted Resolver does not differ from the IP address of the Unencrypted Resolver. Clients SHOULD use this mode only for resolvers using non-public IP addresses. This approach can be used for any encrypted DNS protocol that uses TLS.¶
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 resolver.example.com. (
alpn=h2 dohpath=/dns-query{?dns} )
_dns.resolver.example.com. 7200 IN SVCB 1 resolver.example.com. (
alpn=dot )
¶
Clients MUST validate that for any Encrypted 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 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.¶
Resolver deployments that support DDR are advised to consider the following points.¶
A DNS forwarder SHOULD NOT forward queries for "resolver.arpa" upstream. This prevents a client from receiving an SVCB record that will fail to authenticate because the forwarder's IP address is not in the upstream resolver's Designated Resolver's TLS certificate SAN field. A DNS forwarder which already acts as a completely blind forwarder MAY choose to forward these queries when the operator expects that this does not apply, either because the operator knows the upstream resolver does have the forwarder's IP address in its TLS certificate's SAN field or that the operator expects clients of the unencrypted resolver to use the SVCB information opportunistically.¶
Operators who choose to forward queries for "resolver.arpa" upstream should note that client behavior is never guaranteed and use of DDR by a resolver does not communicate a requirement for clients to use the SVCB record when it cannot be verified.¶
Resolver owners that support Verified Discovery will need to list valid referring IP addresses in their TLS certificates. This may pose challenges for resolvers with a large number of referring IP addresses.¶
Clients MUST NOT use "resolver.arpa" as the server name either in the TLS Server Name Indication (SNI) ([RFC8446]) for DoT or DoH connections, or in the URI host for DoH requests.¶
When performing discovery using resolver IP addresses, clients MUST use the IP address as the URI host for DoH requests.¶
Note that since IP addresses are not supported by default in the TLS SNI, resolvers that support discovery using IP addresses will need to be configured to present the appropriate TLS certificate when no SNI is present for both DoT and DoH.¶
DNS resolvers that support DDR by responding to queries for _dns.resolver.arpa SHOULD treat resolver.arpa as a locally served zone per [RFC6303]. In practice, this means that resolvers SHOULD respond to queries of any type other than SVCB for _dns.resolver.arpa with NODATA and queries of any type for any domain name under resolver.arpa with NODATA.¶
Discovery of network-designated resolvers (DNR, [I-D.ietf-add-dnr]) allows a network to provide designation of resolvers directly through DHCP [RFC2132] [RFC8415] and IPv6 Router Advertisement (RA) [RFC4861] options. When such indications are present, clients can suppress queries for "resolver.arpa" to the unencrypted DNS server indicated by the network over DHCP or RAs, and the DNR indications SHOULD take precedence over those discovered using "resolver.arpa" for the same resolver if there is a conflict.¶
The designated resolver information in DNR might not contain a full set of SvcParams needed to connect to an encrypted resolver. In such a case, the client can use an SVCB query using a resolver name, as described in Section 5, to the authentication-domain-name (ADN).¶
Since clients 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. To limit the impact of discovery queries being dropped either maliciously or unintentionally, clients can re-send their SVCB queries periodically.¶
DoH resolvers that allow discovery using DNS SVCB answers over unencrypted DNS MUST NOT provide differentiated behavior based on the HTTP path alone, since an attacker could modify the "dohpath" parameter.¶
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 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 Resolver, clients applying Verified Discovery (Section 4.2) MUST validate that the IP address of the Unencrypted Resolver is covered by the SubjectAlternativeName of the Designated Resolver's TLS certificate.¶
Clients using Opportunistic Discovery (Section 4.3) MUST be limited to cases where the Unencrypted Resolver and Designated Resolver have the same IP address.¶
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.¶
This document calls for the addition of "resolver.arpa" to the Special-Use Domain Names (SUDN) registry established by [RFC6761]. This will allow resolvers to respond to queries directed at themselves rather than a specific domain name. While this document uses "resolver.arpa" to return SVCB records indicating designated encrypted capability, the name is generic enough to allow future reuse for other purposes where the resolver wishes to provide information about itself to the client.¶
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 Resolver much like "ipv4only.arpa" allows for client-to-middlebox communication. For more context, see the rationale behind "ipv4only.arpa" in [RFC8880].¶
IANA is requested to add an entry in "Transport-Independent Locally-Served DNS Zones" registry for 'resolver.arpa.' with the description "DNS Resolver Special-Use Domain", listing this document as the reference.¶
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:¶