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ipsecme This document specifies new Internet Key Exchange Protocol Version 2 (IKEv2) Configuration Payload Attribute Types for encrypted DNS protocols such as DNS-over-HTTPS (DoH), DNS-over-TLS (DoT), and DNS-over-QUIC (DoQ).

This document specifies encrypted DNS configuration for an Internet Key Exchange Protocol Version 2 (IKEv2) initiator, particularly the Authentication Domain Name (ADN) of DNS servers that support encrypted DNS protocols such as DNS-over-HTTPS (DoH) , DNS-over-TLS (DoT) , or DNS-over-QUIC (DoQ) . This document introduces new IKEv2 Configuration Payload Attribute Types () 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 discussed in . 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.

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 when, and only when, they appear in all capitals, as shown here. This document uses of the terms defined in . Also, this document uses of the terms defined in . In particular, readers should be familiar with "initiator" and "responder" terms used in that document. This document makes use of the following terms: refers to unencrypted DNS. refers to a scheme where DNS messages are sent over an encrypted channel. Examples of encrypted DNS are DoT, DoH, and DoQ. refers to any IKEv2 Configuration Payload Attribute Types defined in .

The ENCDNS_IP* IKEv2 Configuration Payload Attribute Types are used to configure encrypted DNS servers to an initiator. All these attributes share the format that is shown in .

The description of the fields of the attribute shown in is as follows: R (Reserved, 1 bit) - This bit MUST be set to zero and MUST be ignored on receipt (see Section 3.15.1 of for details). Attribute Type (15 bits) - Identifier for Configuration Attribute Type; is set to TBA1 or TBA2 values listed in . Length (2 octets, unsigned integer) - Length of the data in octets. In particular, this field is set to: 0 if the Configuration payload has types CFG_REQUEST (if no specific DNS server is requested) or CFG_ACK. (4 + Length of the ADN + N * 4 + Length of SvcParams) for ENCDNS_IP4 attributes if the Configuration payload has types CFG_REQUEST or CFG_REPLY or CFG_SET; N being the number of included IPv4 addresses ('Num addresses'). (4 + Length of the ADN + N * 16 + Length of SvcParams) for ENCDNS_IP6 attributes if the Configuration payload has types CFG_REQUEST or CFG_REPLY or CFG_SET; N being the number of included IPv6 addresses ('Num addresses'). Service Priority (2 octets) - The priority of this attribute compared to other ENCDNS_IP* instances. This field is encoded following the rules specified in Section 2.4.1 of . AliasMode (Section 2.4.2 of ) is not supported because such a mode will trigger additional Do53 queries while the data can be supplied directly in the IKE response. As such, this field MUST NOT be set to 0. Num Addresses (1 octet) - Indicates the number of enclosed IPv4 (for ENCDNS_IP4 attribute type) or IPv6 (for ENCDNS_IP6 attribute type) addresses. It MUST NOT be set to 0 if the Configuration payload has types CFG_REPLY or CFG_SET. ADN Length (1 octet) - Indicates the length of the authentication-domain-name field in octets. IP Address(es) (variable) - One or more IPv4 or IPv6 addresses to be used to reach the encrypted DNS server that is identified by the name in the Authentication Domain Name. Authentication Domain Name (variable) - A fully qualified domain name of the encrypted DNS server following the syntax defined in . The name MUST NOT contain any terminators (e.g., NULL, CR). An example of a valid ADN for DoH server is "doh1.example.com". Service Parameters (SvcParams) (variable) - Specifies a set of service parameters that are encoded following the rules in Section 2.1 of . Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. The service parameters MUST NOT include "ipv4hint" or "ipv6hint" SvcParams as they are superseded by the included IP addresses. If no port service parameter is included, this indicates that default port numbers should be used. As a reminder, the default port number is 853 for DoT, 443 for DoH, and 853 for DoQ. The service parameters apply to all IP addresses in the ENCDNS_IP* Configuration Payload Attribute.

The format of ENCDNS_DIGEST_INFO configuration payload attribute is shown in .

R (Reserved, 1 bit) - This bit MUST be set to zero and MUST be ignored on receipt (see Section 3.15.1 of for details). Attribute Type (15 bits) - Identifier for Configuration Attribute Type; is set to TBA3 value listed in . Length (2 octets, unsigned integer) - Length of the data in octets. RESERVED (3 octets) - These bits are reserved for future use. These bits MUST be set to zero by the sender and MUST be ignored by the receiver. ADN Length (1 octet) - Indicates the length of the authentication-domain-name field in octets. When set to '0', this means that the digest applies on the ADN conveyed in the ENCDNS_IP* Configuration Payload Attribute(s). Authentication Domain Name (variable) - A fully qualified domain name of the encrypted DNS server following the syntax defined in . The name MUST NOT contain any terminators (e.g., NULL, CR). A name is included only when multiple ADNs are included in the ENCDNS_IP* Configuration Payload Attributes. Hash Algorithm Identifiers (variable) - In a request, this field specifies a list of 16-bit hash algorithm identifiers that are supported by the encrypted DNS client. In a response, this field specifies the 16-bit hash algorithm identifier selected by the server to generate the digest of its certificate. The values of this field are taken from the Hash Algorithm Identifiers of IANA's "Internet Key Exchange Version 2 (IKEv2) Parameters" registry . There is no padding between the hash algorithm identifiers. Note that SHA2-256 is mandatory to implement. Certificate Digest (variable) - MUST only be present in a response. This field includes the digest of the encrypted DNS server certificate using the algorthm identified in the 'Hash Algorithm Identifiers' field.

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

This document adheres to the security considerations defined in . 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 . Hosting encrypted DNS server 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 IKEv2 responder has used NULL Authentication method to authenticate itself, the initiator MUST NOT use returned ENCDNS_IP* servers configuration unless it is pre-configured 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 .

This document requests IANA to assign the following new IKEv2 Configuration Payload Attribute Types from the "IKEv2 Configuration Payload Attribute Types" namespace available at https://www.iana.org/assignments/ikev2-parameters/ikev2-parameters.xhtml#ikev2-parameters-21.

Many thanks to Yoav Nir, Christian Jacquenet, Paul Wouters, and Tommy Pauly for the review and comments. Yoav and Paul suggested the use of one single attribute carrying both the name and an IP address instead of depending on the existing INTERNAL_IP6_DNS and INTERNAL_IP4_DNS attributes. Christian Huitema suggested to return a port number in the attributes.

In this Enterprise scenario (Section 1.1.3 of ), a roaming user connects to the Enterprise network through an IPsec tunnel. The split-tunnel Virtual Private Network (VPN) configuration allows the endpoint to access hosts that resides in the Enterprise network using that tunnel; other traffic not destined to the Enterprise does not traverse the tunnel. In contrast, a non-split-tunnel VPN configuration causes all traffic to traverse the tunnel into the enterprise. For both split- and non-split-tunnel configurations, the use of encrypted DNS instead of Do53 provides privacy and integrity protection along the entire path (rather than just to the VPN termination device) and can communicate the encrypted DNS server policies. For split-tunnel VPN configurations, the endpoint uses the Enterprise-provided encrypted DNS server to resolve internal-only domain names. For non-split-tunnel VPN configurations, the endpoint uses the Enterprise-provided encrypted DNS server to resolve both internal and external domain names. Enterprise networks are susceptible to internal and external attacks. To minimize that risk all enterprise traffic is encrypted (Section 2.1 of ).

Legacy VPN service providers usually preserve end-users' data confidentiality by sending all communication traffic through an encrypted tunnel. A VPN service provider can also provide guarantees about the security of the VPN network by filtering malware and phishing domains. Browsers and OSes support DoH/DoT; VPN providers may no longer expect DNS clients to fallback to Do53 just because it is a closed network. The encrypted DNS server hosted by the VPN service provider can be securely discovered by the endpoint using the IKEv2 Configuration Payload Attribute Type.

VPN service providers typically allow split-tunnel VPN configuration in which users can choose applications that can be excluded from the tunnel. For example, users may exclude applications that restrict VPN access. The encrypted DNS server hosted by the VPN service provider can be securely discovered by the endpoint using the IKEv2 Configuration Payload Attribute Type.