| Internet-Draft | DEER | November 2020 |
| Pauly, et al. | Expires 6 May 2021 | [Page] |
This document defines Discovery of Equivalent Encrypted Resolvers (DEER), 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 an encrypted resolver is known. It can also be used to discover support for encrypted DNS protocols when the name of an encrypted resolver is known. This mechanism is designed to be limited to cases where equivalent encrypted and unencrypted resolvers are operated by the same entity.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 6 May 2021.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
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 equivalent 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 equivalent to the originally provisioned resolver. "Equivalence" in this context means that the resolvers are operated by the same entity; for example, the resolvers are accessible on the same IP address, or there is a certificate that claims ownership over both resolvers.¶
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:¶
DNS resolvers can advertise one or more Equivalent Encrypted Resolvers that offer equivalent services over encrypted channels and are controlled by the same entity.¶
When a client discovers Equivalent Encrypted 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} ipv4hint=x.y.z.w )
¶
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 ipv4hint=x.y.z.w )
¶
If multiple Equivalent Encrypted 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 Equivalent Encrypted 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 equivalence as defined in Section 4.1.¶
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 Equivalent Encrypted Resolvers, it will return the corresponding SVCB records. When responding to these special queries for "dns://resolver.arpa", the SVCB records SHOULD contain at least one "ipv4hint" and/or "ipv6hint" keys. These address hints indicate the address on which the corresponding Encrypted Resolver can be reached and avoid additional DNS lookup for the A and AAAA records of the Encrypted Resolver name.¶
In order to be considered an authenticated Equivalent Encrypted Resolver, the TLS certificate presented by the Encrypted Resolver MUST contain both the domain name (from the SVCB answer) and the IP address of its equivalent Unencrypted Resolver within the SubjectAlternativeName certificate field. The client MUST check the SubjectAlternativeName field for both the Unencrypted Resolver's IP address and the advertised name of the Equivalent Encrypted Resolver. If the certificate can be validated, the client SHOULD use the discovered Equivalent Encrypted Resolver for any cases in which it would have otherwise used the Unencrypted Resolver. If the Equivalent Encrypted 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 use the discovered Encrypted Resolver. Additionally, the client SHOULD suppress any further queries for Equivalent Encrypted Resolvers using this Unencrypted Resolver for the length of time indicated by the SVCB record's Time to Live (TTL).¶
If the Equivalent Encrypted Resolver and the Unencrypted Resolver share an IP address, clients MAY choose to opportunistically use the Encrypted Resolver even without this certificate check (Section 4.2).¶
There are situations where authenticated discovery of encrypted DNS configuration over unencrypted DNS is not possible. This includes Unencrypted Resolvers on non-public IP addresses 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, and that IP address is a private address (such as those defined in [RFC1918]). This approach can be used for DoT or DoH.¶
If the IP addresses of the Encrypted and Unencrypted Resolvers are not the same, or the shared IP address is not a private IP address, the client MUST NOT use the Encrypted Resolver opportunistically.¶
A DNS client that already knows the name of an Encrypted Resolver can use DEER 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 certficates. If a deployment uses a
different hostname for one protocol, but still wants clients to treat the DNS
servers as equivalent, 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 an Equivalent Encrypted 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 an
equivalent DoH server for foo.resolver.example.com.¶
Resolver deployments that support DEER are advised to consider the following points.¶
If a caching forwarder consults multiple resolvers, it may be possible for it to
cache records for the "resolver.arpa" Special Use Domain Name (SUDN) for
multiple resolvers. This may result in clients sending queries intended to
discover Equivalent Encrypted Resolvers for resolver foo and receiving answers
for resolvers foo and bar.¶
A client will successfully reject unintended connections because the authenticated discovery will fail or the resolver addresses do not match. Clients that attempt unauthenticated connections to resolvers discovered through SVCB queries run the risk of connecting to the wrong server in this scenario.¶
To prevent unnecessary traffic from clients to incorrect resolvers, DNS caching resolvers SHOULD NOT cache results for the "resolver.arpa" SUDN other than for Equivalent Encrypted Resolvers under their control.¶
Resolver owners that support authenticated 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.¶
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 Equivalent Encrypted 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 Equivalent Encrypted 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 an Equivalent Encrypted 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 Equivalent Encrypted Resolver have the same IP address (Section 4.2).¶
This document calls for the creation of the "resolver.arpa" SUDN. 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 equivalent 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.¶
This mechanism uses SVCB/HTTPS resource records [I-D.ietf-dnsop-svcb-https] to communicate that a given domain designates a particular Equivalent Encrypted 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:¶