DHCP and Router Advertisement Options for the Discovery of Network-designated Resolvers (DNR)

This document focuses on the support of encrypted DNS such as DNS-over-HTTPS (DoH) , DNS-over-TLS (DoT) , or DNS-over-QUIC (DoQ) in local networks. In particular, the document specifies how a local encrypted DNS server can be discovered by connected hosts by means of DHCPv4 , DHCPv6 , and IPv6 Router Advertisement (RA) options. These options are designed to convey the following information: the DNS Authentication Domain Name (ADN), a list of IP addresses, and a set of service parameters. This procedure is called Discovery of Network-designated Resolvers (DNR). The options defined in this document can be deployed in a variety of deployments (e.g., local networks with Customer Premises Equipment (CPEs) that may or may not be managed by an Internet Service Provider (ISP), local networks with or without DNS forwarders). It is out of the scope of this document to provide an inventory of such deployments.

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 makes use of the terms defined in . The following additional terms are used: refers to unencrypted DNS. refers to the Discovery of Network-designated Resolvers procedure. refers to a scheme where DNS exchanges are transported over an encrypted channel. Examples of encrypted DNS are DoT, DoH, or DoQ. refers to the options defined in Sections , , and . refers to both DHCPv4 and DHCPv6.

This document describes how a DNS client can discover local encrypted DNS servers using DHCP (Sections and ) and Neighbor Discovery protocol (): Encrypted DNS options. These options configure an authentication domain name, a list of IPv6 addresses, and a set of service parameters of the encrypted DNS server. More information about the design of these options is provided in the following subsections.

In order to allow for PKIX-based authentication between a DNS client and an encrypted DNS server, the Encrypted DNS options are designed to include an authentication domain name. This ADN is presented as a reference identifier for DNS authentication purposes. This design accommodates the current best practices for issuing certificates as per Section 1.7.2 of :

To avoid adding a dependency on another server to resolve the ADN, the Encrypted DNS options return the IP address(es) to locate the encrypted DNS server. These encrypted DNS servers may be hosted on the same or distinct IP addresses. Such a decision is deployment specific. In order to optimize the size of discovery messages when all DNS servers terminate on the same IP address, early versions of this document considered relying upon the discovery mechanisms specified in to retrieve a list of IP addresses to reach their DNS servers. Nevertheless, this approach requires a client that supports more than one encrypted DNS protocol (e.g., DoH and DoT) to probe that list of IP addresses. To avoid such a probing, the options defined in Sections , , and associate an IP address with an encrypted DNS protocol. No probing is required in such a design. A list of IP addresses to reach an encrypted DNS server may be returned in an Encrypted DNS option to accommodate current deployments relying upon primary and backup servers. Whether one or more IP addresses are returned in an Encrypted DNS option is deployment specific. For example, a router embedding a recursive server or a forwarder has to include one single IP address pointing to one of its LAN-facing interfaces. This IP address can be a private IPv4 address, a link-local address, a Unique Local IPv6 unicast Address (ULA), or a Global Unicast Address (GUA). If multiple IP addresses are to be returned in an Encrypted DNS option, these addresses are ordered in the preference for use by the client. Because distinct encrypted DNS protocols may be provisioned by a network (e.g., DoT, DoH, and DoQ) and that some of these protocols may make use of customized port numbers instead of default ones, the Encrypted DNS options are designed to return a set of service parameters. These parameters are encoded following the same rules for encoding SvcParams in Section 2.1 of . This encoding approach may increase the size of the options but it has the merit relying upon an existing IANA registry and, thus, accommodating new encrypted DNS protocols and service parameters that may be defined in the future. At least the following service parameters are RECOMMENDED to be supported by a DNR implementation: Used to indicate the set of supported protocols (Section 7.1 of ). Used to indicate the target port number for the encrypted DNS connection (Section 7.2 of ). Used to enable Encrypted ClientHello (ECH) (Section 7.3 of ). Used to supply a relative DoH URI Template (Section 5.1 of ). A single option is used to convey both the ADN and IP addresses because otherwise means to correlate an IP address conveyed in an option with an ADN conveyed in another option will be required if, for example, more than one ADN is supported by the network. The DHCP options defined in Sections and follow the option ordering guidelines in Section 17 of . Likewise, the RA option () adheres to the recommendations in Section 9 of . ServiceMode (Section 2.4.3 of ) SHOULD be used because the Encrypted DNS options are self-contained and do not require any additional DNS queries. The reader may refer to for an overview of advanced capabilities that are supported by DHCP servers to populate configuration data (e.g., issue DNS queries). In contexts where putting additional complexity on requesting hosts is acceptable, returning an ADN only can be considered. The supplied ADN will be processed by a host following the procedure in Section 5 of . Note that this mode may be subject to active attacks, which can be mitigated by DNSSEC. Other mechanisms may be considered in other contexts (e.g., secure discovery) for the provisioning of encrypted DNS servers. It is RECOMMENDED that at least the following DNR information is made available to a requesting host: A service priority whenever the discovery mechanism does not rely on implicit ordering if multiple instances of the encrypted DNS are used. An authentication domain name. A list of IP addresses to locate the encrypted DNS server. A set of service parameters.

If the encrypted DNS is discovered by a host using both RA and DHCP, the rules discussed in Section 5.3.1 of MUST be followed. DHCP/RA options to discover encrypted DNS servers (including, DoH URI Templates) takes precedence over Discovery of Designated Resolvers (DDR) since DDR uses Do53 to an external DNS resolver, which is susceptible to both internal and external attacks whereas DHCP/RA is typically protected using the mechanisms discussed in .

If the local DNS client supports one of the discovered Encrypted DNS protocols identified by Application Layer Protocol Negotiation (ALPN) protocol identifiers, the DNS client establishes an encrypted DNS session following the order of the discovered servers. The client follows the mechanism discussed in Section 8 of to authenticate the DNS server certificate using the authentication domain name conveyed in the Encrypted DNS options. ALPN-related considerations can be found in Section 6.1 of .

Devices may be connected to multiple networks; each providing their own DNS configuration using the discovery mechanisms specified in this document. Nevertheless, it is out of the scope of this specification to discuss DNS selection of multi-interface devices. The reader may refer to for a discussion of issues and an example of DNS server selection for multi-interfaced devices.

The format of the DHCPv6 Encrypted DNS option is shown in .

The fields of the option shown in are as follows: OPTION_V6_DNR (TBA1, see ) Length of the enclosed data in octets. The option length is ('ADN Length' + 4) when only an ADN is included in the option. The priority of this OPTION_V6_DNR instance compared to other instances. This field is encoded following the rules specified in Section 2.4.1 of . Length of the authentication-domain-name field in octets. A fully qualified domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of .An example of the authentication-domain-name encoding is shown in . This example conveys the FQDN "doh1.example.com.", and the resulting Option-length field is 18.

Length of enclosed IPv6 addresses in octets. It MUST be a multiple of 16 for ServiceMode. Indicates one or more IPv6 addresses to reach the encrypted DNS server. An address can be link-local, ULA, or GUA. The format of this field is shown in .

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 length of this field is ('Option-length' - 6 - 'ADN Length' - 'Addr Length').

To discover an encrypted DNS server, the DHCPv6 client MUST include OPTION_V6_DNR in an Option Request Option (ORO), as in Sections 18.2.1, 18.2.2, 18.2.4, 18.2.5, 18.2.6, and 21.7 of . The DHCPv6 client MUST be prepared to receive multiple instances of the OPTION_V6_DNR option; each option is to be treated as a separate encrypted DNS server. These instances SHOULD be processed following their service priority (i.e., smaller service priority indicates a higher preference). The DHCPv6 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V6_DNR.

The format of the DHCPv4 Encrypted DNS option is illustrated in .

The fields of the option shown in are as follows: OPTION_V4_DNR (TBA2, see ). Indicates the length of the enclosed data in octets. The option length is ('ADN Length' + 3) when only an ADN is included in the option. The priority of this OPTION_V4_DNR instance compared to other instances. This field is encoded following the rules specified in Section 2.4.1 of . Indicates the length of the authentication-domain-name in octets. Includes the authentication domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of . The format of this field is shown in . The values s1, s2, s3, etc. represent the domain name labels in the domain name encoding.

Indicates the length of included IPv4 addresses in octets. It MUST be a multiple of 4 for ServiceMode. Indicates one or more IPv4 addresses to reach the encrypted DNS server. Both private and public IPv4 addresses can be included in this field. The format of this field is shown in . This format assumes that an IPv4 address is encoded as a1.a2.a3.a4.

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. The length of this field is ('Option-length' - 4 - 'ADN Length' - 'Addr Length'). OPTION_V4_DNR is a concatenation-requiring option. As such, the mechanism specified in MUST be used if OPTION_V4_DNR exceeds the maximum DHCPv4 option size of 255 octets.

To discover an encrypted DNS server, the DHCPv4 client requests the Encrypted DNS server by including OPTION_V4_DNR in a Parameter Request List option . The DHCPv4 client MUST be prepared to receive multiple instances of the OPTION_V4_DNR option; each option is to be treated as a separate encrypted DNS server. These instances SHOULD be processed following their service priority (i.e., smaller service priority indicates a higher preference). The DHCPv4 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V4_DNR.

This section defines a new Neighbor Discovery option : IPv6 RA Encrypted DNS option. This option is useful in contexts similar to those discussed in Section 1.1 of . The format of the IPv6 RA Encrypted DNS option is illustrated in .

The fields of the option shown in are as follows: 8-bit identifier of the Encrypted DNS option as assigned by IANA (TBA3, see ). 8-bit unsigned integer. The length of the option (including the Type and Length fields) is in units of 8 octets. The priority of this Encrypted DNS option instance compared to other instances. This field is encoded following the rules specified in Section 2.4.1 of . 32-bit unsigned integer. The maximum time in seconds (relative to the time the packet is received) over which the discovered Authentication Domain Name is valid. The value of Lifetime SHOULD by default be at least 3 * MaxRtrAdvInterval, where MaxRtrAdvInterval is the maximum RA interval as defined in . A value of all one bits (0xffffffff) represents infinity. A value of zero means that this Authentication Domain Name MUST no longer be used. 16-bit unsigned integer. This field indicates the length of the authentication-domain-name field in octets. The domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of . 16-bit unsigned integer. This field indicates the length of enclosed IPv6 addresses in octets. It MUST be a multiple of 16 for ServiceMode. One or more IPv6 addresses of the encrypted DNS server. An address can be link-local, ULA, or GUA. All of the addresses share the same Lifetime value. Similar to , if it is desirable to have different Lifetime values per IP address, multiple Encrypted DNS options may be used.The format of this field is shown in . 16-bit unsigned integer. This field indicates the length of the Service Parameters field in octets. 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. The option MUST be padded with zeros so that the full enclosed data is a multiple of 8 octets (Section 4.6 of ).

The procedure for DNS configuration is the same as it is with any other Neighbor Discovery option . In addition, the host follows the procedure described in Section 5.3.1 of with the formatting requirements in substituted for the length validation. The host MUST be prepared to receive multiple Encrypted DNS options in RAs. These instances SHOULD be processed following their service priority (i.e., smaller service priority indicates a higher preference). The host MUST silently discard multicast and host loopback addresses conveyed in the Encrypted DNS options.

DHCP/RA messages are not encrypted or protected against modification within the LAN. Unless mitigated (described below), the content of DHCP and RA messages can be spoofed or modified by active attackers, such as compromised devices within the local network. An active attacker (Section 3.3 of ) can spoof the DHCP/RA response to provide the attacker's Encrypted DNS server. Note that such an attacker can launch other attacks as discussed in Section 22 of . The attacker can get a domain name with a domain-validated public certificate from a CA and host an Encrypted DNS server. Attacks of spoofed or modified DHCP responses and RA messages by attackers within the local network may be mitigated by making use of the following mechanisms: DHCPv6-Shield described in , the router (e.g., a border router, a CPE) discards DHCP response messages received from any local endpoint. RA-Guard described in , the router discards RAs messages received from any local endpoint. Source Address Validation Improvement (SAVI) solution for DHCP described in , the router filters packets with forged source IP addresses. The above mechanisms would ensure that the endpoint receives the correct configuration information of the encrypted DNS servers selected by the DHCP server (or RA sender), but cannot provide any information about the DHCP server or the entity hosting the DHCP server (or RA sender) . Encrypted DNS sessions with rogue servers that spoof the IP address of a DNS server will fail because the DNS client will fail to authenticate that rogue server based upon PKIX authentication , particularly the authentication domain name in the Encrypted DNS Option. DNS clients that ignore authentication failures and accept spoofed certificates will be subject to attacks (e.g., redirect to malicious servers, intercept sensitive data). Encrypted DNS connections received from outside the local network MUST be discarded by the encrypted DNS forwarder in the CPE. This behavior adheres to REQ#8 in ; it MUST apply for both IPv4 and IPv6.

If the DHCP responses or RAs are dropped by the attacker, the client can fallback to use a preconfigured encrypted DNS server. However, the use of policies to select servers is out of the scope of this document. Note that deletion attack is not specific to DHCP/RA.

A passive attacker (Section 3.2 of ) can identify a host is using DHCP/RA to discover an encrypted DNS server and can infer that host is capable of using DoH/DoT/DoQ to encrypt DNS messages. However, a passive attacker cannot spoof or modify DHCP/RA messages.

Wireless LAN (WLAN) as frequently deployed in local networks (e.g., home networks) is vulnerable to various attacks (e.g., , , ). Because of these attacks, only cryptographically authenticated communications are trusted on WLANs. This means that an information (e.g., NTP server, DNS server, domain search list) provided by such networks via DHCP, DHCPv6, or RA are untrusted because DHCP and RA messages are not authenticated. If the pre-shared key is the same for all clients that connect to the same WLAN, the shared key will be available to all nodes, including attackers. As such, it is possible to mount an active on-path attack. Man-in-the-middle attacks are possible within local networks because such WLAN authentication lacks peer entity authentication. This leads to the need for provisioning unique credentials for different clients. Endpoints can be provisioned with unique credentials (username and password, typically) provided by the local network administrator to mutually authenticate to the local WLAN Access Point (e.g., 802.1x Wireless User Authentication on OpenWRT , EAP-pwd ). Not all endpoint devices (e.g., IoT devices) support 802.1x supplicant and need an alternate mechanism to connect to the local network. To address this limitation, unique pre-shared keys can be created for each such device and WPA-PSK is used (e.g., ).

IANA is requested to assign the following new DHCPv6 Option Code in the registry maintained in . Value Description Client ORO Singleton Option Reference TBA1 OPTION_V6_DNR Yes No [ThisDocument]

IANA is requested to assign the following new DHCP Option Code in the registry maintained in .

IANA is requested to assign the following new IPv6 Neighbor Discovery Option type in the "IPv6 Neighbor Discovery Option Formats" sub-registry under the "Internet Control Message Protocol version 6 (ICMPv6) Parameters" registry maintained in . Type Description Reference TBA3 DNS Encrypted DNS Option [ThisDocument]

Many thanks to Christian Jacquenet and Michael Richardson for the review. Thanks to Stephen Farrell, Martin Thomson, Vittorio Bertola, Stephane Bortzmeyer, Ben Schwartz, Iain Sharp, and Chris Box for the comments. Thanks to Mark Nottingham for the feedback on HTTP redirection that was discussed in previous versions of this specification. The use of DHCP to retrieve an authentication domain name was discussed in Section 7.3.1 of and . Thanks to Bernie Volz for the review of the DHCP part. Thanks to Andrew Campling for the Shepherd review.