Internet Key Exchange Protocol Version 2 (IKEv2) Configuration for Encrypted DNS

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Table of Contents

1. Introduction

This document specifies encrypted DNS configuration for an Internet Key Exchange Protocol Version 2 (IKEv2) [RFC7296] initiator, particularly the Authentication Domain Name (ADN) of DNS servers that support encrypted DNS protocols such as DNS-over-HTTPS (DoH) [RFC8484], DNS-over-TLS (DoT) [RFC7858], or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic].¶

This document introduces new IKEv2 Configuration Payload Attribute Types (Section 3) for the support of encrypted DNS servers. These attributes can be used to provision ADNs, a list of IP addresses, and a set of service parameters.¶

Sample use cases are described in Appendix A. The Configuration Payload Attribute Types defined in this document are not specific to these deployments, but can also be used in other deployment contexts. It is out of the scope of this document to provide a comprehensive list of deployment contexts.¶

The encrypted DNS server hosted by a VPN provider can get a domain-validate certificate from a public Certificate Authority (CA). The VPN client does not need to be provisioned with the root certificate of a private CA to authenticate the certificate of the encrypted DNS server. The encrypted DNS server can run on private IP addresses and its access can be restricted to clients connected to the VPN.¶

Note that, for many years, typical designs have often considered that the DNS server was usually located inside the protected domain, but could be located outside of it. With encrypted DNS, the latter option becomes plausible.¶

2. Terminology

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 uses of the terms defined in [RFC8499].¶

Also, this document uses of the terms defined in [RFC7296]. In particular, readers should be familiar with "initiator" and "responder" terms used in that document.¶

This document makes use of the following terms:¶

Do53:

refers to unencrypted DNS.¶

Encrypted DNS:

refers to a scheme where DNS messages are sent over an encrypted channel. Examples of encrypted DNS are DoT, DoH, and DoQ.¶

ENCDNS_IP*:

refers to any IKEv2 Configuration Payload Attribute Types defined in Section 3.1.¶

3. IKEv2 Configuration Payload Attribute Types for Encrypted DNS

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-----------------------------+-------------------------------+
|R|         Attribute Type      |            Length             |
+-+-----------------------------+---------------+---------------+
|       Service Priority        | Num Addresses |  ADN Length   |
+-------------------------------+---------------+---------------+
~                         IP Addresses                          ~
+---------------------------------------------------------------+
~                  Authentication Domain Name                   ~
+---------------------------------------------------------------+
~                 Service Parameters (SvcParams)                ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-----------------------------+-------------------------------+
|R|         Attribute Type      |            Length             |
+-+-----------------------------+---------------+---------------+
|                    RESERVED                   |  ADN Length   |
+-----------------------------------------------+---------------+
~                  Authentication Domain Name                   ~
+---------------------------------------------------------------+
~                  Hash Algorithm Identifiers                   ~
+---------------------------------------------------------------+
~                     Certificate Digest                        ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. IKEv2 Protocol Exchange

This section describes how an initiator can be configured with an encrypted DNS server (e.g., DoH, DoT) using IKEv2.¶

Initiators indicate the support of an encrypted DNS in the CFG_REQUEST payloads by including one or two ENCDNS_IP* attributes, while responders supply the encrypted DNS configuration in the CFG_REPLY payloads. Concretely:¶

• If the initiator supports encrypted DNS, it includes one or two ENCDNS_IP* attributes in the CFG_REQUEST. For each IP address family the initiator MUST include exactly one attribute with the Length field set to 0 if no specific DNS server is requested. The initiator MAY include the ENCDNS_DIGEST_INFO attribute with a list of hash algorithms that are supported by the encrypted DNS client.¶
• For each ENCDNS_IP* attribute from the CFG_REQUEST, if the responder supports the corresponding address family, and absent any policy, the responder sends back ENCDNS_IP* attribute(s) in the CFG_REPLY with an appropriate list of IP addresses, service parameters, and an ADN. The list of IP addresses MUST include at least one IP address. The responder may ignore suggested values (if any). Multiple instances of the same ENCDNS_IP* attribute MAY be returned if distinct ADNs or service parameters are to be returned by the responder. The same or distinct IP addresses can be returned in such instances. These instances SHOULD be processed following their service priority (i.e., smaller service priority indicates a higher preference).¶
• In addition, the responder MAY return the ENCDNS_DIGEST_INFO attribute to convey a digest of the certificate of the encrypted DNS and the identifier of the hash algorithm that is used to generate the digest.¶
• If the CFG_REQUEST includes an ENCDNS_IP* attribute but the CFG_REPLY does not include an ENCDNS_IP* matching the requested address family, this is an indication that requested address family is not supported by the responder or the responder is not configured to provide corresponding server addresses.¶
• If the initiator receives both ENCDNS_IP* and INTERNAL_IP6_DNS (or INTERNAL_IP4_DNS) attributes, it is RECOMMENDED that the initiator uses the encrypted DNS servers.¶

The DNS client establishes an encrypted DNS session (e.g., DoT, DoH, DoQ) with the address(es) conveyed in ENCDNS_IP* and uses the mechanism discussed in Section 8 of [RFC8310] to authenticate the DNS server certificate using the authentication domain name conveyed in ENCDNS_IP*.¶

If the CFG_REPLY includes an ENCDNS_DIGEST_INFO attribute, the DNS client has to create a digest of the DNS server certificate received in the TLS handshake using the negotiated hash algorithm in the ENCDNS_DIGEST_INFO attribute. If the computed digest for an ADN matches the one sent in the ENCDNS_DIGEST_INFO attribute, the encrypted DNS server certificate is successfully validated. If so, the client continues with the TLS connection as normal. Otherwise, the client MUST treat the server certificate validation failure as a non-recoverable error. This approach is similar to certificate usage PKIX-EE(1) defined in [RFC7671].¶

If the IPsec connection is a split-tunnel configuration and the initiator negotiated INTERNAL_DNS_DOMAIN as per [RFC8598], the DNS client resolves the internal names using ENCDNS_IP* DNS servers.¶

• Note: [RFC8598] requires INTERNAL_IP6_DNS (or INTERNAL_IP4_DNS) attribute to be mandatory present when INTERNAL_DNS_DOMAIN is included. This specification relaxes that constraint in the presence of ENCDNS_IP* attributes. That is, if ENCDNS_IP* attributes are supplied, it is allowed to include INTERNAL_DNS_DOMAIN even in the absence of INTERNAL_IP6_DNS (or INTERNAL_IP4_DNS) attributes.¶

5. Security Considerations

This document adheres to the security considerations defined in [RFC7296]. In particular, this document does not alter the trust on the DNS configuration provided by a responder.¶

Networks are susceptible to internal attacks as discussed in Section 3.2 of [I-D.arkko-farrell-arch-model-t]. Hosting encrypted DNS servers even in case of split-VPN configuration minimizes the attack vector (e.g., a compromised network device cannot monitor/modify DNS traffic). This specification describes a mechanism to restrict access to the DNS messages to only the parties that need to know.¶

The initiator may trust the encrypted DNS servers supplied by means of IKEv2 from a trusted responder more than the locally provided DNS servers, especially in the case of connecting to unknown or untrusted networks (e.g., coffee shops or hotel networks).¶

If the IKEv2 responder has used NULL Authentication method [RFC7619] to authenticate itself, the initiator MUST NOT use returned ENCDNS_IP* servers configuration unless it is pre-configured, e.g., in the OS or the browser.¶

This specification does not extend the scope of accepting DNSSEC trust anchors beyond the usage guidelines defined in Section 6 of [RFC8598].¶

6. IANA Considerations

                               Multi-
Value  Attribute Type          Valued  Length     Reference
------ ------------------       -----  ---------  ---------
 TBA1  ENCDNS_IP4                YES   0 or more  RFC XXXX
 TBA2  ENCDNS_IP6                YES   0 or more  RFC XXXX
 TBA3  ENCDNS_ENCDNS_DIGEST_INFO YES   0 or more  RFC XXXX

Appendix A. Sample Deployment Scenarios

Authors' Addresses