Orange

Rennes 35000 France mohamed.boucadair@orange.com

Nokia

India kondtir@gmail.com

Cloud Software Group Holdings, Inc.

United States of America dwing-ietf@fuggles.com

Open-Xchange

United Kingdom neil.cook@noware.co.uk

Microsoft

United States of America tojens@microsoft.com

int add DNS service resolution encryption service discovery service provider network provider automation DoH DoT DoQ This document specifies new DHCP and IPv6 Router Advertisement options to discover encrypted DNS resolvers (e.g., DNS over HTTPS, DNS over TLS, and DNS over QUIC). Particularly, it allows a host to learn an Authentication Domain Name together with a list of IP addresses and a set of service parameters to reach such encrypted DNS resolvers.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2023 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 () 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

Table of Contents

• .  Introduction
• .  Terminology
• .  Overview
  • .  Configuration Data for Encrypted DNS
    • .  ADN as Reference Identifier for DNS Authentication
    • .  Avoiding Dependency on External Resolvers
    • .  Single vs. Multiple IP Addresses
    • .  Why Not Separate Options for the ADN and IP Addresses?
    • .  Service Parameters
    • .  ADN-Only Mode
    • .  Ordering of Encrypted DNS Options
    • .  DNR Validation Checks
    • .  DNR Information Using Other Provisioning Mechanisms
  • .  Handling Configuration Data Conflicts
  • .  Validating Discovered Resolvers
  • .  Multihoming Considerations
• .  DHCPv6 Encrypted DNS Option
  • .  Option Format
  • .  DHCPv6 Client Behavior
• .  DHCPv4 Encrypted DNS Option
  • .  Option Format
  • .  DHCPv4 Client Behavior
• .  IPv6 RA Encrypted DNS Option
  • .  Option Format
  • .  IPv6 Host Behavior
• .  Security Considerations
  • .  Spoofing Attacks
  • .  Deletion Attacks
  • .  Passive Attacks
  • .  Wireless Security - Authentication Attacks
• .  Privacy Considerations
• .  IANA Considerations
  • .  DHCPv6 Option
  • .  DHCPv4 Option
  • .  Neighbor Discovery Option
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgments
• Contributors
• Authors' Addresses

Introduction This document focuses on the discovery of encrypted DNS resolvers that are using protocols such as DNS over HTTPS (DoH) , DNS over TLS (DoT) , or DNS over QUIC (DoQ) in local networks. In particular, this document specifies how a local encrypted DNS resolver 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), or local networks with or without DNS forwarders). Providing an inventory of such deployments is beyond the scope of this document. Resolver selection considerations are out of scope. Likewise, policies (including any interactions with users) are out of scope.

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 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:

Authentication Domain Name (ADN):

Refers to a domain name that is used by a DNS client to authenticate a DNS resolver.

ADN-only mode:

Refers to a DNS discovery mode where only the ADN of the DNS resolver is retrieved. See .

Do53:

Refers to unencrypted DNS.

DNR:

Refers to the procedure called Discovery of Network-designated Resolvers.

Encrypted DNS:

Refers to a scheme where DNS exchanges are transported over an encrypted channel. Examples include DoT, DoH, and DoQ.

Encrypted DNS resolver:

Refers to a DNS resolver that supports any encrypted DNS scheme.

Encrypted DNS options:

Refers to the options defined in Sections , , and .

DHCP:

Refers to both DHCPv4 and DHCPv6.

Overview This document describes how a DNS client can discover local encrypted DNS resolvers using DHCP (Sections  and ) and Neighbor Discovery protocol () Encrypted DNS options. These options configure an ADN, a list of IP addresses, and a set of service parameters of the encrypted DNS resolver. More information about the design of these options is provided in the following subsections.

Configuration Data for Encrypted DNS

ADN as Reference Identifier for DNS Authentication In order to allow for a PKIX-based authentication of the encrypted DNS resolver to the DNS client, the Encrypted DNS options are designed to always include an ADN. This ADN is presented as a reference identifier for DNS authentication purposes. This design accommodates the current best practices for issuing certificates as per :

Avoiding Dependency on External Resolvers To avoid adding a dependency on another server to resolve the ADN, the Encrypted DNS options return the IP address(es) to locate an encrypted DNS resolver. These encrypted DNS resolvers may be hosted on the same IP address or distinct IP addresses. Such a decision is deployment specific. In order to optimize the size of discovery messages when all DNS resolvers terminate on the same IP address, early draft versions of this document considered relying upon the discovery mechanisms specified in , , and to retrieve a list of IP addresses to reach their DNS resolvers. 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 probing, the options defined in Sections , , and associate an encrypted DNS protocol with an IP address. No probing is required in such a design.

Single vs. Multiple IP Addresses A list of IP addresses to reach an encrypted DNS resolver may be returned in an Encrypted DNS option to accommodate current deployments relying upon primary and backup resolvers. Also, DNR can be used in contexts where other DNS redundancy schemes (e.g., anycast as discussed in BCP 126) are used. 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. Typically, this IP address can be a private IPv4 address, a Link-Local address, an IPv6 Unique Local Address (ULA), or a Global Unicast Address (GUA). If multiple IP addresses are to be returned in an Encrypted DNS option, these addresses are returned, ordered by preference, for use by the client.

Why Not Separate Options for the ADN and IP Addresses? A single option is used to convey both the ADN and IP addresses. Otherwise, a 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.

Service Parameters Because distinct encrypted DNS protocols (e.g., DoT, DoH, and DoQ) may be provisioned by a network and some of these protocols may make use of customized port numbers instead of default port numbers, the Encrypted DNS options are designed to return a set of service parameters. These parameters are encoded following the same rules for encoding SvcParams using the wire format specified in . This encoding approach may increase the size of the options, but it has the merit of relying upon an existing IANA registry and, thus, accommodating new encrypted DNS protocols and service parameters that may be defined in the future. The following service parameters MUST be supported by a DNR implementation:

alpn:

Used to indicate the set of supported protocols ().

port:

Used to indicate the target port number for the encrypted DNS connection ().

In addition, the following service parameter is RECOMMENDED to be supported by a DNR implementation:

dohpath:

Used to supply a relative DoH URI Template ().

ADN-Only Mode The provisioning mode in which an ADN, a list of IP addresses, and a set of service parameters of the encrypted DNS resolver are supplied to a host 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 passed to a local resolution library (a DNS client, typically), which will then issue Service Binding (SVCB) queries . These SVCB queries can be sent to the discovered encrypted DNS resolver itself or to the network-designated Do53 resolver. Note that this mode may be subject to active attacks, which can be mitigated by DNSSEC.

Ordering of Encrypted DNS Options The DHCP options defined in Sections  and follow the option ordering guidelines in . Likewise, the RA option () adheres to the recommendations in .

DNR Validation Checks On receipt of an Encrypted DNS option, the DHCP client (or IPv6 host) makes the following validation checks:

• The ADN is present and encoded as per .
• If additional data is supplied:
  • The service parameters are encoded following the rules specified in .
  • The option includes at least one valid IP address.
  • The service parameters do not include "ipv4hint" or "ipv6hint" parameters.

If any of the checks fail, the receiver discards the received Encrypted DNS option.

DNR Information Using Other Provisioning Mechanisms The provisioning mechanisms specified in this document may not be available in specific networks (e.g., some cellular networks exclusively use Protocol Configuration Options (PCOs) ) or may not be suitable in some contexts (e.g., where secure discovery is needed). Other mechanisms may be considered in these contexts for the provisioning of encrypted DNS resolvers. It is RECOMMENDED that at least the following DNR information be 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 ADN. This parameter is mandatory.
• A list of IP addresses to locate the encrypted DNS resolver.
• A set of service parameters.

Handling Configuration Data Conflicts If encrypted DNS resolvers are discovered by a host using both RA and DHCP, the rules discussed in MUST be followed. DHCP/RA options to discover encrypted DNS resolvers (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 a client learns both Do53 and encrypted DNS resolvers from the same network, and absent explicit configuration otherwise, it is RECOMMENDED that the client use the encrypted DNS resolvers for that network. If the client cannot establish an authenticated and encrypted connection with the encrypted DNS resolver, it may fall back to using the Do53 resolver.

Validating Discovered Resolvers This section describes a set of validation checks to confirm that an encrypted DNS resolver matches what is provided using DNR (e.g., DHCP or RA). Such validation checks do not intend to validate the security of the DNR provisioning mechanisms or the user's trust relationship to the network. If the local DNS client supports one of the discovered encrypted DNS protocols identified by Application-Layer Protocol Negotiation (ALPN) protocol identifiers (or another service parameter that indicates some other protocol disambiguation mechanism), the DNS client establishes an encrypted DNS session following the service priority of the discovered encrypted resolvers. The DNS client verifies the connection based on PKIX validation of the DNS resolver certificate and uses the validation techniques as described in to compare the ADN conveyed in the Encrypted DNS options to the certificate provided (see for more details). The DNS client uses the default system or application PKI trust anchors unless configured otherwise to use explicit trust anchors. ALPN-related considerations can be found in . Operational considerations related to checking the revocation status of the certificate of an encrypted DNS resolver are discussed in .

Multihoming Considerations Devices may be connected to multiple networks, each providing their own DNS configuration using the discovery mechanisms specified in this document. Nevertheless, discussing DNS selection of multi-interfaced devices is beyond the scope of this specification. Such considerations fall under the generic issue of handling multiple provisioning sources and should not be processed in each option separately, as per the recommendation in . The reader may refer to for a discussion of DNS selection issues and an example of DNS resolver selection for multi-interfaced devices. Also, the reader may refer to for a discussion on how DNR and Provisioning Domain (PvD) key "dnsZones" () can be used in "split DNS" environments ().

DHCPv6 Encrypted DNS Option

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

DHCPv6 Encrypted DNS Option 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION_V6_DNR | Option-length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Priority | ADN Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ authentication-domain-name ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Addr Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ ipv6-address(es) ~ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ Service Parameters (SvcParams) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields of the option shown in are as follows:

Option-code:

OPTION_V6_DNR (144; see ).

Option-length:

Length of the enclosed data in octets. The option length is ('ADN Length' + 4) when only an ADN is included in the option.

Service Priority:

The priority of this OPTION_V6_DNR instance compared to other instances. This 16-bit unsigned integer is interpreted following the rules specified in .

ADN Length:

Length of the authentication-domain-name field in octets.

authentication-domain-name (variable length):

A Fully Qualified Domain Name (FQDN) of the encrypted DNS resolver. This field is formatted as specified in . An example of the authentication-domain-name encoding is shown in . This example conveys the FQDN "doh1.example.com.", and the resulting ADN Length field is 18.

An Example of the DNS authentication-domain-name Encoding +------+------+------+------+------+------+------+------+------+ | 0x04 | d | o | h | 1 | 0x07 | e | x | a | +------+------+------+------+------+------+------+------+------+ | m | p | l | e | 0x03 | c | o | m | 0x00 | +------+------+------+------+------+------+------+------+------+

Addr Length:

Length of enclosed IPv6 addresses in octets. When present, it MUST be a multiple of 16.

ipv6-address(es) (variable length):

Indicates one or more IPv6 addresses to reach the encrypted DNS resolver. An address can be a Link-Local address, a ULA, or a GUA. The format of this field is shown in .

Format of the ipv6-address(es) Field 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ipv6-address | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Service Parameters (SvcParams) (variable length):

Specifies a set of service parameters that are encoded following the rules in . Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. This field SHOULD include at least the "alpn" SvcParam. The "alpn" SvcParam may not be required in contexts such as a variant of DNS over the Constrained Application Protocol (CoAP) where messages are encrypted using Object Security for Constrained RESTful Environments (OSCORE) . 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').

Note that the "Addr Length", "ipv6-address(es)", and "Service Parameters (SvcParams)" fields are not present if the ADN-only mode is used ().

DHCPv6 Client Behavior To discover an encrypted DNS resolver, the DHCPv6 client MUST include OPTION_V6_DNR in an Option Request Option (ORO), per Sections , , , , , and 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 resolver. These instances MUST be processed following their service priority (i.e., a smaller service priority value indicates a higher preference). The DHCPv6 client MUST silently discard any OPTION_V6_DNR that fails to pass the validation steps defined in . The DHCPv6 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V6_DNR.

DHCPv4 Encrypted DNS Option

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

DHCPv4 Encrypted DNS Option 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION_V4_DNR | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ DNR Instance Data #1 ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- . ... . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ optional ~ DNR Instance Data #n ~ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

The fields of the option shown in are as follows:

Code:

OPTION_V4_DNR (162; see ).

Length:

Indicates the length of the enclosed data in octets.

DNR Instance Data:

Includes the configuration data of an encrypted DNS resolver. The format of this field is shown in .

DNR Instance Data Format 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DNR Instance Data Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Service Priority | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ADN Length | | +-+-+-+-+-+-+-+-+ | ~ authentication-domain-name ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Addr Length | | +-+-+-+-+-+-+-+-+ | ~ IPv4 Address(es) ~ | +-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+ | ~Service Parameters (SvcParams) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

When several encrypted DNS resolvers are to be included, the "DNR Instance Data" field is repeated.

The fields shown in are as follows:

DNR Instance Data Length:

Length of all following data in octets. This field is set to ('ADN Length' + 3) when only an ADN is provided for a DNR instance.

Service Priority:

The priority of this instance compared to other DNR instances. This 16-bit unsigned integer is interpreted following the rules specified in .

ADN Length:

Length of the authentication-domain-name field in octets.

authentication-domain-name (variable length):

The ADN of the encrypted DNS resolver. This field is formatted as specified in . An example is provided in .

Addr Length:

Length of included IPv4 addresses in octets. When present, it MUST be a multiple of 4.

IPv4 Address(es) (variable length):

Indicates one or more IPv4 addresses to reach the encrypted DNS resolver. 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.

Format of the IPv4 Address(es) Field 0 8 16 24 32 40 48 +-----+-----+-----+-----+-----+-----+-- | a1 | a2 | a3 | a4 | a1 | a2 | ... +-----+-----+-----+-----+-----+-----+-- IPv4 Address 1 IPv4 Address 2 ...

Service Parameters (SvcParams) (variable length):

Specifies a set of service parameters that are encoded following the rules in . Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. This field SHOULD include at least the "alpn" SvcParam. The "alpn" SvcParam may not be required in contexts such as a variant of DNS over CoAP where messages are encrypted using OSCORE. 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 ('DNR Instance Data Length' - 4 - 'ADN Length' - 'Addr Length').

Note that the "Addr Length", "IPv4 Address(es)", and "Service Parameters (SvcParams)" fields are not present if the ADN-only mode is used (). 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.

DHCPv4 Client Behavior To discover an encrypted DNS resolver, the DHCPv4 client requests the encrypted DNS resolver by including OPTION_V4_DNR in a Parameter Request List option . The DHCPv4 client MUST be prepared to receive multiple "DNR Instance Data" field entries in the OPTION_V4_DNR option; each instance is to be treated as a separate encrypted DNS resolver. These instances MUST be processed following their service priority (i.e., a smaller service priority value indicates a higher preference). The DHCPv4 client MUST silently discard any OPTION_V4_DNR that fails to pass the validation steps defined in . The DHCPv4 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V4_DNR.

IPv6 RA Encrypted DNS Option

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

RA Encrypted DNS Option 0 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Service Priority | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ADN Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ authentication-domain-name ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Addr Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ ipv6-address(es) ~ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | SvcParams Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Service Parameters (SvcParams) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields of the option shown in are as follows:

Type:

8-bit identifier of the Encrypted DNS option as assigned by IANA (144; see ).

Length:

8-bit unsigned integer. The length of the option (including the Type and Length fields) is in units of 8 octets.

Service Priority:

16-bit unsigned integer. The priority of this Encrypted DNS option instance compared to other instances. This field is interpreted following the rules specified in .

Lifetime:

32-bit unsigned integer. This represents the maximum time in seconds (relative to the time the packet is received) over which the discovered ADN 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 ADN MUST no longer be used.

ADN Length:

16-bit unsigned integer. This field indicates the length of the authentication-domain-name field in octets.

authentication-domain-name (variable length):

The ADN of the encrypted DNS resolver. This field is formatted as specified in .

Addr Length:

16-bit unsigned integer. This field indicates the length of enclosed IPv6 addresses in octets. When present, it MUST be a multiple of 16.

ipv6-address(es) (variable length):

One or more IPv6 addresses of the encrypted DNS resolver. An address can be a Link-Local address, a ULA, or a GUA. All of the addresses share the same Lifetime value. As also discussed in , 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 .

SvcParams Length:

16-bit unsigned integer. This field indicates the length of the "Service Parameters (SvcParams)" field in octets.

Service Parameters (SvcParams) (variable length):

Specifies a set of service parameters that are encoded following the rules in . Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. This field SHOULD include at least the "alpn" SvcParam. The "alpn" SvcParam may not be required in contexts such as a variant of DNS over CoAP where messages are encrypted using OSCORE. 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.

Note that the "Addr Length", "ipv6-address(es)", and "Service Parameters (SvcParams)" fields are not present if the ADN-only mode is used (). The option MUST be padded with zeros so that the full enclosed data is a multiple of 8 octets ().

IPv6 Host Behavior The procedure for DNS configuration is the same as it is with any other Neighbor Discovery option . In addition, the host follows the same procedure as the procedure described in for processing received Encrypted DNS options, with the formatting requirements listed in and the validation checks listed in substituted for length and field validations. The host MUST be prepared to receive multiple Encrypted DNS options in RAs. These instances MUST be processed following their service priority (i.e., a smaller service priority value indicates a higher preference). The host MUST silently discard multicast and host loopback addresses conveyed in the Encrypted DNS options.

Security Considerations

Spoofing Attacks DHCP/RA messages are not encrypted or protected against modification within the LAN. Unless spoofing attacks are mitigated as 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 () can spoof the DHCP/RA response to provide the attacker's encrypted DNS resolver. Note that such an attacker can launch other attacks as discussed in . The attacker can get a domain name with a domain-validated public certificate from a Certificate Authority (CA) and host an encrypted DNS resolver. 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 :

The network access node (e.g., a border router, a CPE, an Access Point (AP)) discards DHCP response messages received from any local endpoint.

RA-Guard :

The network access node discards RA messages received from any local endpoint.

Source Address Validation Improvement (SAVI) solution for DHCP :

The network access node filters packets with forged source IP addresses.

The above mechanisms would ensure that the endpoint receives the correct configuration information of the encrypted DNS resolvers selected by the DHCP server (or RA sender), but these mechanisms cannot provide any information about the DHCP server or the entity hosting the DHCP server (or RA sender). Encrypted DNS sessions with rogue resolvers that spoof the IP address of a DNS resolver will fail because the DNS client will fail to authenticate that rogue resolver based upon PKIX authentication , particularly the ADN in the Encrypted DNS option. DNS clients that ignore authentication failures and accept spoofed certificates will be subject to attacks (e.g., attacks that redirect to malicious resolvers or intercept sensitive data).

Deletion Attacks If the DHCP responses or RAs are dropped by the attacker, the client can fall back to using a preconfigured encrypted DNS resolver. However, the use of policies to select resolvers is beyond the scope of this document. Note that deletion attacks are not specific to DHCP/RA.

Passive Attacks A passive attacker () can determine that a host is using DHCP/RA to discover an encrypted DNS resolver and can infer that the host is capable of using DoH/DoT/DoQ to encrypt DNS messages. However, a passive attacker cannot spoof or modify DHCP/RA messages.

Wireless Security - Authentication Attacks Wireless LANs (WLANs), frequently deployed in local networks (e.g., home networks), are vulnerable to various attacks (e.g., , , ). Because of these attacks, only cryptographically authenticated communications are trusted on WLANs. This means that any information (e.g., regarding NTP servers, DNS resolvers, or domain search lists) provided by such networks via DHCP, DHCPv6, or RA is untrusted because DHCP and RA messages are not authenticated. If the pre-shared key (PSK) is the same for all clients that connect to the same WLAN (e.g., Wi-Fi Protected Access Pre-Shared Key (WPA-PSK)), the shared key will be available to all nodes, including attackers. As such, it is possible to mount an active on-path attack. On-path attacks are possible within local networks because this form of 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 AP (e.g., 802.1x Wireless User Authentication on OpenWrt , EAP-pwd ("EAP" stands for "Extensible Authentication Protocol")). Not all endpoint devices (e.g., Internet of Things (IoT) devices) support 802.1x supplicants and need an alternate mechanism to connect to the local network. To address this limitation, unique PSKs can be created for each such device and WPA-PSK is used (e.g., ).

Privacy Considerations Privacy considerations that are also specific to DNR provisioning mechanisms are discussed in and in . Anonymity profiles for DHCP clients are discussed in . The mechanisms defined in this document can be used to infer that a DHCP client or IPv6 host supports Encrypted DNS options, but these mechanisms do not explicitly reveal whether local DNS clients are able to consume these options or infer their encryption capabilities. Other than that, this document does not expose more privacy information compared to Do53 discovery options. As discussed in , the use of encrypted DNS does not reduce the data available in the DNS resolver. For example, the reader may refer to or for a discussion on specific privacy considerations for encrypted DNS.

IANA Considerations

DHCPv6 Option IANA has assigned the following new DHCPv6 Option Code in the "Option Codes" registry maintained at . DHCPv6 Encrypted DNS Option

Value	Description	Client ORO	Singleton Option	Reference
144	OPTION_V6_DNR	Yes	No	RFC 9463

DHCPv4 Option IANA has assigned the following new DHCP Option Code in the "BOOTP Vendor Extensions and DHCP Options" registry maintained at . DHCPv4 Encrypted DNS Option

Tag	Name	Data Length	Meaning	Reference
162	OPTION_V4_DNR	N	Encrypted DNS Server	RFC 9463

Neighbor Discovery Option IANA has assigned the following new IPv6 Neighbor Discovery Option type in the "IPv6 Neighbor Discovery Option Formats" subregistry under the "Internet Control Message Protocol version 6 (ICMPv6) Parameters" registry maintained at . Neighbor Discovery Encrypted DNS Option

Type	Description	Reference
144	Encrypted DNS Option	RFC 9463

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies the current set of DHCP options. Future options will be specified in separate RFCs. The current list of valid options is also available in ftp://ftp.isi.edu/in-notes/iana/assignments. [STANDARDS-TRACK] This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK] This document specifies IPv6 Router Advertisement (RA) options (called "DNS RA options") to allow IPv6 routers to advertise a list of DNS Recursive Server Addresses and a DNS Search List to IPv6 hosts. This document, which obsoletes RFC 6106, defines a higher default value of the lifetime of the DNS RA options to reduce the likelihood of expiry of the options on links with a relatively high rate of packet loss. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes the Dynamic Host Configuration Protocol for IPv6 (DHCPv6): an extensible mechanism for configuring nodes with network configuration parameters, IP addresses, and prefixes. Parameters can be provided statelessly, or in combination with stateful assignment of one or more IPv6 addresses and/or IPv6 prefixes. DHCPv6 can operate either in place of or in addition to stateless address autoconfiguration (SLAAC). This document updates the text from RFC 3315 (the original DHCPv6 specification) and incorporates prefix delegation (RFC 3633), stateless DHCPv6 (RFC 3736), an option to specify an upper bound for how long a client should wait before refreshing information (RFC 4242), a mechanism for throttling DHCPv6 clients when DHCPv6 service is not available (RFC 7083), and relay agent handling of unknown messages (RFC 7283). In addition, this document clarifies the interactions between models of operation (RFC 7550). As such, this document obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550. Google LLC Informative References IANA IANA Unaffiliated NLnet Labs This draft provides a series of DHCPv6 options for a DHCPv6 client to request from a DHCPv6 server to aid in configuring DNS servers that support private requests/responses. Work in Progress OpenWrt 2020 IEEE Symposium on Security and Privacy (SP), San Francisco, pp. 517-533 Wikipedia Cisco CCS '17: Proceedings of the 2017 ACM SIGSAC Conference on Computer and Communications Security, pp. 1313-1328 Work in Progress IANA All RFCs are required to have a Security Considerations section. Historically, such sections have been relatively weak. This document provides guidelines to RFC authors on how to write a good Security Considerations section. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes Dynamic Host Configuration Protocol for IPv6 (DHCPv6) options for passing a list of available DNS recursive name servers and a domain search list to a client. As the Internet has grown, and as systems and networked services within enterprises have become more pervasive, many services with high availability requirements have emerged. These requirements have increased the demands on the reliability of the infrastructure on which those services rely. Various techniques have been employed to increase the availability of services deployed on the Internet. This document presents commentary and recommendations for distribution of services using anycast. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] Many application technologies enable secure communication between two entities by means of Internet Public Key Infrastructure Using X.509 (PKIX) certificates in the context of Transport Layer Security (TLS). This document specifies procedures for representing and verifying the identity of application services in such interactions. [STANDARDS-TRACK] A multi-interfaced node is connected to multiple networks, some of which might be utilizing private DNS namespaces. A node commonly receives recursive DNS server configuration information from all connected networks. Some of the recursive DNS servers might have information about namespaces other servers do not have. When a multi-interfaced node needs to utilize DNS, the node has to choose which of the recursive DNS servers to use. This document describes DHCPv4 and DHCPv6 options that can be used to configure nodes with information required to perform informed recursive DNS server selection decisions. [STANDARDS-TRACK] The IPv6 Router Advertisement Guard (RA-Guard) mechanism is commonly employed to mitigate attack vectors based on forged ICMPv6 Router Advertisement messages. Many existing IPv6 deployments rely on RA-Guard as the first line of defense against the aforementioned attack vectors. However, some implementations of RA-Guard have been found to be prone to circumvention by employing IPv6 Extension Headers. This document describes the evasion techniques that affect the aforementioned implementations and formally updates RFC 6105, such that the aforementioned RA-Guard evasion vectors are eliminated. This document provides guidance to prospective DHCPv6 option developers to help them create option formats that are easily adoptable by existing DHCPv6 software. It also provides guidelines for expert reviewers to evaluate new registrations. This document updates RFC 3315. This document specifies the procedure for creating a binding between a DHCPv4/DHCPv6-assigned IP address and a binding anchor on a Source Address Validation Improvement (SAVI) device. The bindings set up by this procedure are used to filter packets with forged source IP addresses. This mechanism complements BCP 38 (RFC 2827) ingress filtering, providing finer-grained source IP address validation. This document specifies a mechanism for protecting hosts connected to a switched network against rogue DHCPv6 servers. It is based on DHCPv6 packet filtering at the layer 2 device at which the packets are received. A similar mechanism has been widely deployed in IPv4 networks ('DHCP snooping'); hence, it is desirable that similar functionality be provided for IPv6 networks. This document specifies a Best Current Practice for the implementation of DHCPv6-Shield. DHCPv6 is a protocol that is used to provide addressing and configuration information to IPv6 hosts. This document describes the privacy issues associated with the use of DHCPv6 by Internet users. It is intended to be an analysis of the present situation and does not propose any solutions. Some DHCP options carry unique identifiers. These identifiers can enable device tracking even if the device administrator takes care of randomizing other potential identifications like link-layer addresses or IPv6 addresses. The anonymity profiles are designed for clients that wish to remain anonymous to the visited network. The profiles provide guidelines on the composition of DHCP or DHCPv6 messages, designed to minimize disclosure of identifying information. This document describes the use of Transport Layer Security (TLS) to provide privacy for DNS. Encryption provided by TLS eliminates opportunities for eavesdropping and on-path tampering with DNS queries in the network, such as discussed in RFC 7626. In addition, this document specifies two usage profiles for DNS over TLS and provides advice on performance considerations to minimize overhead from using TCP and TLS with DNS. This document focuses on securing stub-to-recursive traffic, as per the charter of the DPRIVE Working Group. It does not prevent future applications of the protocol to recursive-to-authoritative traffic. DHCP servers have evolved over the years to provide significant functionality beyond that described in the DHCP base specifications. One aspect of this functionality is support for context-specific configuration information. This memo describes some such features and explains their operation. EAP-pwd is an Extensible Authentication Protocol (EAP) method that utilizes a shared password for authentication using a technique that is resistant to dictionary attacks. It includes support for raw keys and double hashing of a password in the style of Microsoft Challenge Handshake Authentication Protocol version 2 (MSCHAPv2), but it does not include support for salted passwords. There are many existing databases of salted passwords, and it is desirable to allow their use with EAP-pwd. This document discusses usage profiles, based on one or more authentication mechanisms, which can be used for DNS over Transport Layer Security (TLS) or Datagram TLS (DTLS). These profiles can increase the privacy of DNS transactions compared to using only cleartext DNS. This document also specifies new authentication mechanisms -- it describes several ways that a DNS client can use an authentication domain name to authenticate a (D)TLS connection to a DNS server. Additionally, it defines (D)TLS protocol profiles for DNS clients and servers implementing DNS over (D)TLS. This document updates RFC 7858. This document defines a protocol for sending DNS queries and getting DNS responses over HTTPS. Each DNS query-response pair is mapped into an HTTP exchange. The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has sometimes changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document. This document obsoletes RFC 7719 and updates RFC 2308. This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. Provisioning Domains (PvDs) are defined as consistent sets of network configuration information. PvDs allows hosts to manage connections to multiple networks and interfaces simultaneously, such as when a home router provides connectivity through both a broadband and cellular network provider. This document defines a mechanism for explicitly identifying PvDs through a Router Advertisement (RA) option. This RA option announces a PvD identifier, which hosts can compare to differentiate between PvDs. The option can directly carry some information about a PvD and can optionally point to PvD Additional Information that can be retrieved using HTTP over TLS. This document describes the privacy issues associated with the use of the DNS by Internet users. It provides general observations about typical current privacy practices. It is intended to be an analysis of the present situation and does not prescribe solutions. This document obsoletes RFC 7626. This document describes the use of QUIC to provide transport confidentiality for DNS. The encryption provided by QUIC has similar properties to those provided by TLS, while QUIC transport eliminates the head-of-line blocking issues inherent with TCP and provides more efficient packet-loss recovery than UDP. DNS over QUIC (DoQ) has privacy properties similar to DNS over TLS (DoT) specified in RFC 7858, and latency characteristics similar to classic DNS over UDP. This specification describes the use of DoQ as a general-purpose transport for DNS and includes the use of DoQ for stub to recursive, recursive to authoritative, and zone transfer scenarios. Apple Inc. Apple Inc. Cloudflare Fastly Microsoft 3GPP version 18.4.0

Acknowledgments Many thanks to and for their reviews. Thanks to , , , , , , and for their comments. Thanks to for the feedback on HTTP redirection that was discussed in previous draft versions of this specification. The use of DHCP as a candidate protocol to retrieve an ADN was mentioned in and in an Internet-Draft authored by and . Thanks to for the review of the DHCP part. reported a case where the ALPN service parameter cannot be used. Thanks to for the Shepherd review and for the AD review. Thanks to for the secdir reviews, for the opsdir review, for the artart review, and for the dnsdir review. Thanks to , , , , , , and for the IESG review.

Contributors Deutsche Telekom

Germany n.leymann@telekom.de

CNNIC

No.4 South 4th Street, Zhongguancun Beijing 100190 China yan@cnnic.cn

Authors' Addresses Orange

Rennes 35000 France mohamed.boucadair@orange.com

Nokia

India kondtir@gmail.com

Cloud Software Group Holdings, Inc.

United States of America dwing-ietf@fuggles.com

Open-Xchange

United Kingdom neil.cook@noware.co.uk

Microsoft

United States of America tojens@microsoft.com