ADD M. Boucadair, Ed. Internet-Draft Orange Intended status: Standards Track T. Reddy, Ed. Expires: November 5, 2021 McAfee D. Wing Citrix N. Cook Open-Xchange T. Jensen Microsoft May 4, 2021 DHCP and Router Advertisement Options for the Discovery of Network- designated Resolvers (DNR) draft-ietf-add-dnr-01 Abstract The document specifies new DHCP and IPv6 Router Advertisement options to discover encrypted DNS servers (e.g., DNS-over-HTTPS, DNS-over- TLS, DNS-over-QUIC). Particularly, it allows to learn an authentication domain name together with a list of IP addresses and a set of service parameters to reach such encrypted DNS servers. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 5, 2021. Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. Boucadair, et al. Expires November 5, 2021 [Page 1] Internet-Draft Discovery of Network-designated Resolvers May 2021 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Configuration Data for Encrypted DNS . . . . . . . . . . 4 3.2. Handling Configuration Data Conflicts . . . . . . . . . . 5 3.3. Connection Establishment . . . . . . . . . . . . . . . . 6 3.4. Multihoming Considerations . . . . . . . . . . . . . . . 6 4. DHCPv6 Encrypted DNS Option . . . . . . . . . . . . . . . . . 6 4.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 6 4.2. DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . 8 5. DHCPv4 Encrypted DNS Option . . . . . . . . . . . . . . . . . 8 5.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 8 5.2. DHCPv4 Client Behavior . . . . . . . . . . . . . . . . . 10 6. IPv6 RA Encrypted DNS Option . . . . . . . . . . . . . . . . 11 6.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 11 6.2. IPv6 Host Behavior . . . . . . . . . . . . . . . . . . . 12 7. Hosting Encrypted DNS Forwarder in Local Networks . . . . . . 13 7.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 13 7.1.1. DNS Forwarders . . . . . . . . . . . . . . . . . . . 13 7.1.2. ACME . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1.3. Auto-Upgrade Based on Domains and their Subdomains . 13 7.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 14 8. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8.1. Spoofing Attacks . . . . . . . . . . . . . . . . . . . . 15 8.2. Deletion Attacks . . . . . . . . . . . . . . . . . . . . 16 8.3. Passive Attacks . . . . . . . . . . . . . . . . . . . . . 16 8.4. Wireless Security - Authentication Attacks . . . . . . . 16 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 9.1. DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . . 17 9.2. DHCPv4 Option . . . . . . . . . . . . . . . . . . . . . . 17 9.3. Neighbor Discovery Option . . . . . . . . . . . . . . . . 17 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 11. Contributing Authors . . . . . . . . . . . . . . . . . . . . 18 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 12.1. Normative References . . . . . . . . . . . . . . . . . . 18 12.2. Informative References . . . . . . . . . . . . . . . . . 19 Boucadair, et al. Expires November 5, 2021 [Page 2] Internet-Draft Discovery of Network-designated Resolvers May 2021 Appendix A. Sample Target Deployment Scenarios . . . . . . . . . 23 A.1. Managed CPEs . . . . . . . . . . . . . . . . . . . . . . 24 A.1.1. Direct DNS . . . . . . . . . . . . . . . . . . . . . 24 A.1.2. Proxied DNS . . . . . . . . . . . . . . . . . . . . . 26 A.2. Unmanaged CPEs . . . . . . . . . . . . . . . . . . . . . 27 A.2.1. ISP-facing Unmanaged CPEs . . . . . . . . . . . . . . 27 A.2.2. Internal Unmanaged CPEs . . . . . . . . . . . . . . . 27 Appendix B. Make Use of Discovered Encrypted DNS Servers . . . . 28 Appendix C. Legacy CPEs . . . . . . . . . . . . . . . . . . . . 29 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 1. Introduction This document focuses on the support of encrypted DNS such as DNS- over-HTTPS (DoH) [RFC8484], DNS-over-TLS (DoT) [RFC7858], or DNS- over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic] in local networks. In particular, the document specifies how a local encrypted DNS server can be discovered by connected hosts by means of DHCP [RFC2132], DHCPv6 [RFC8415], and IPv6 Router Advertisement (RA) [RFC4861] 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. Sample target deployment scenarios are discussed in Appendix A; both managed and unmanaged Customer Premises Equipment (CPEs) are covered. It is out of the scope of this document to provide an exhaustive inventory of deployments where Encrypted DNS options (Sections 4, 5, and 6) can be used. Considerations related to hosting a DNS forwarder in a local network are described in Section 7. 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 makes use of the terms defined in [RFC8499]. The following additional terms are used: Do53: refers to unencrypted DNS. Encrypted DNS: refers to a scheme where DNS exchanges are transported over an encrypted channel. Examples of encrypted DNS Boucadair, et al. Expires November 5, 2021 [Page 3] Internet-Draft Discovery of Network-designated Resolvers May 2021 are DNS-over-TLS (DoT) [RFC7858], DNS-over-HTTPS (DoH) [RFC8484], or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic]. Encrypted DNS options: refers to the options defined in Sections 4, 5, and 6. Managed CPE: refers to a CPE that is managed by an Internet Service Provider (ISP). Unmanaged CPE: refers to a CPE that is not managed by an ISP. DHCP: refers to both DHCPv4 and DHCPv6. 3. Overview This document describes how a DNS client can discover local encrypted DNS servers using DHCP (Sections 4 and 5) and Neighbor Discovery protocol (Section 6): 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. 3.1. Configuration Data for Encrypted DNS 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 [RFC6125]: | Some certification authorities issue server certificates based on | IP addresses, but preliminary evidence indicates that such | certificates are a very small percentage (less than 1%) of issued | certificates. 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. In the various scenarios sketched in Appendix A, encrypted DNS servers may terminate on the same IP address or distinct IP addresses. Terminating encrypted DNS servers on the same or distinct IP addresses is deployment specific. In order to optimize the size of discovery messages when all servers terminate on the same IP address, early versions of this document considered relying upon the discovery mechanisms specified in Boucadair, et al. Expires November 5, 2021 [Page 4] Internet-Draft Discovery of Network-designated Resolvers May 2021 [RFC2132][RFC3646][RFC8106] 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 to probe that list of IP addresses. To avoid such probing, the options defined in the following sections associate an IP address with an encrypted DNS type. No probing is required in such a design. A list of IP addresses to reach an encrypted DNS server may be returned in the Encrypted DNS options to accommodate current deployments relying upon primary and backup servers. Whether one IP address or more are returned in an Encrypted DNS option is deployment specific. For example, a router embedding a recursive server or forwarder has to include one single IP address pointing to one of its LAN-facing interfaces. This 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 more than one IP address 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 [I-D.ietf-dnsop-svcb-https]. This encoding approach may increase the size of the options but it has the merit to rely upon an existing IANA registry and thus to accommodate new Encrypted DNS protocols and service parameters that may be defined in the future. For example, "dohpath" service parameter (Section 5.1 of [I-D.schwartz-svcb-dns]) supplies a relative DoH URI Template. A single option is used to convey both the ADN and IP addresses because otherwise means to correlate an IP address with an ADN will be required if, for example, more than one ADN is supported by the network. 3.2. Handling Configuration Data Conflicts If the encrypted DNS is discovered by a host using both RA and DHCP, the rules discussed in Section 5.3.1 of [RFC8106] MUST be followed. DHCP/RA options to discover encrypted DNS servers (including, DoH URI Templates) takes precedence over DDR [I-D.ietf-add-ddr] since DDR uses unencrypted DNS to an external DNS resolver, which is Boucadair, et al. Expires November 5, 2021 [Page 5] Internet-Draft Discovery of Network-designated Resolvers May 2021 susceptible to both internal and external attacks whereas DHCP/RA is typically protected using the mechanisms discussed in Section 8.1. 3.3. Connection Establishment 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 [RFC8310] 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 [I-D.ietf-dnsop-svcb-https]. 3.4. Multihoming Considerations 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 [RFC6731] for a discussion of issues and an example of DNS server selection for multi-interfaced devices. 4. DHCPv6 Encrypted DNS Option 4.1. Option Format The format of the DHCPv6 Encrypted DNS option is shown in Figure 1. 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ADN Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ authentication-domain-name ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Addr Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ ipv6-address(es) ~ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ Service Parameters (SvcParams) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: DHCPv6 Encrypted DNS Option Boucadair, et al. Expires November 5, 2021 [Page 6] Internet-Draft Discovery of Network-designated Resolvers May 2021 The fields of the option shown in Figure 1 are as follows: Option-code: OPTION_V6_DNR (TBA1, see Section 9.1) Option-length: Length of the enclosed data in octets. ADN Length: Length of the authentication-domain-name field in octets. authentication-domain-name (variable length): A fully qualified domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of [RFC8415]. An example of the authentication-domain-name encoding is shown in Figure 2. This example conveys the FQDN "doh1.example.com.", and the resulting Option-length field is 18. +------+------+------+------+------+------+------+------+------+ | 0x04 | d | o | h | 1 | 0x07 | e | x | a | +------+------+------+------+------+------+------+------+------+ | m | p | l | e | 0x03 | c | o | m | 0x00 | +------+------+------+------+------+------+------+------+------+ Figure 2: An Example of the DNS authentication-domain-name Encoding Addr Length: Length of enclosed IPv6 addresses in octets. It MUST be a multiple of 16. ipv6-address(es) (variable length): 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 Figure 3. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | ipv6-address | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Format of the IPv6 Addresses Field Service Parameters (SvcParams) (variable length): Specifies a set of service parameters that are encoded following the rules in Section 2.1 of [I-D.ietf-dnsop-svcb-https]. Service parameters Boucadair, et al. Expires November 5, 2021 [Page 7] Internet-Draft Discovery of Network-designated Resolvers May 2021 may include, for example, a list of ALPN protocol identifiers or alternate port numbers. 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 and 443 for DoH. The length of this field is ('Option-length' - 4 - 'ADN Length' - 'Addr Length'). Multiple instances of OPTION_V6_DNR may be returned to a DHCPv6 client; each pointing to a distinct encrypted DNS server. These instances are ordered in the preference for use by the client. 4.2. DHCPv6 Client Behavior 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 [RFC8415]. The DHCP client MUST be prepared to receive multiple OPTION_V6_DNR options; each option is to be treated as a separate encrypted DNS server. The DHCPv6 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V6_DNR. 5. DHCPv4 Encrypted DNS Option 5.1. Option Format The format of the DHCPv4 Encrypted DNS option is illustrated in Figure 4. Boucadair, et al. Expires November 5, 2021 [Page 8] Internet-Draft Discovery of Network-designated Resolvers May 2021 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA2 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ADN Length | | +-+-+-+-+-+-+-+-+ | ~ authentication-domain-name ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Addr Length | | +-+-+-+-+-+-+-+-+ + ~ IPv4 Address(es) ~ | +-+-+-+-+-+-+-+-+ | | | +-+-+-+-+-+-+-+-+ | ~Service Parameters (SvcParams) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: DHCPv4 Encrypted DNS Option The fields of the option shown in Figure 4 are as follows: Code: OPTION_V4_DNR (TBA2, see Section 9.2). Length: Indicates the length of the enclosed data in octets. ADN Length: Indicates the length of the authentication-domain-name in octets. authentication-domain-name (variable length): Includes the authentication domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of [RFC8415]. The format of this field is shown in Figure 5. The values s1, s2, s3, etc. represent the domain name labels in the domain name encoding. +-----+-----+-----+-----+-----+-- | s1 | s2 | s3 | s4 | s5 | ... +-----+-----+-----+-----+-----+-- authentication-domain-name Figure 5: Format of the Authentication Domain Name Field Addr Length: Indicates the length of included IPv4 addresses in octets. It MUST be a multiple of 4. IPv4 Address(es) (variable length): 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 Boucadair, et al. Expires November 5, 2021 [Page 9] Internet-Draft Discovery of Network-designated Resolvers May 2021 of this field is shown in Figure 6. This format assumes that an IPv4 address is encoded as a1.a2.a3.a4. 0 8 16 24 32 40 48 +-----+-----+-----+-----+-----+-----+-- | a1 | a2 | a3 | a4 | a1 | a2 | ... +-----+-----+-----+-----+-----+-----+-- IPv4 Address 1 IPv4 Address 2 ... Figure 6: Format of the IPv4 Addresses Field Service Paramters (SvcParams) (variable length): Specifies a set of service parameters that are encoded following the rules in Section 2.1 of [I-D.ietf-dnsop-svcb-https]. Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. If no port service parameter is included, this indicates that default port numbers should be used. The length of this field is ('Option-length' - 2 - 'ADN Length' - 'Addr Length'). OPTION_V4_DNR is a concatenation-requiring option. As such, the mechanism specified in [RFC3396] MUST be used if OPTION_V4_DNR exceeds the maximum DHCPv4 option size of 255 octets. Multiple instances of OPTION_V4_DNR may be returned to a DHCPv4 client; each pointing to a distinct encrypted DNS server. These instances are ordered in the preference for use by the client. 5.2. DHCPv4 Client Behavior 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 [RFC2132]. The DHCPv4 client MUST be prepared to receive multiple DHCP OPTION_V4_DNR options; each option is to be treated as a separate encrypted DNS server. The DHCPv4 client MUST silently discard multicast and host loopback addresses conveyed in OPTION_V4_DNR. Boucadair, et al. Expires November 5, 2021 [Page 10] Internet-Draft Discovery of Network-designated Resolvers May 2021 6. IPv6 RA Encrypted DNS Option 6.1. Option Format This section defines a new Neighbor Discovery option [RFC4861]: IPv6 RA Encrypted DNS option. This option is useful in contexts similar to those discussed in Section 1.1 of [RFC8106]. The format of the IPv6 RA Encrypted DNS option is illustrated in Figure 7. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TBA3 | Length | ADN Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ authentication-domain-name ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Addr Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ~ ipv6-address(es) ~ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | SvcParams Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Service Parameters (SvcParams) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 7: RA Encrypted DNS Option The fields of the option shown in Figure 7 are as follows: Type: 8-bit identifier of the Encrypted DNS Option as assigned by IANA (TBA3, see Section 9.3). Length: 8-bit unsigned integer. The length of the option (including the Type and Length fields) is in units of 8 octets. Lifetime: 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 [RFC4861]. A value of all one bits (0xffffffff) represents infinity. Boucadair, et al. Expires November 5, 2021 [Page 11] Internet-Draft Discovery of Network-designated Resolvers May 2021 A value of zero means that this Authentication Domain Name 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 domain name of the encrypted DNS server. This field is formatted as specified in Section 10 of [RFC8415]. Addr Length: 16-bit unsigned integer. This field indicates the length of enclosed IPv6 addresses in octets. It MUST be a multiple of 16. ipv6-address(es) (variable length): 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 [RFC8106], 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 Figure 3. SvcParams Length: 16-bit unsigned integer. This field indicates the length of the Service Parameters field in octets. Service Paramters (SvcParams) (variable length): Specifies a set of service parameters that are encoded following the rules in Section 2.1 of [I-D.ietf-dnsop-svcb-https]. Service parameters may include, for example, a list of ALPN protocol identifiers or alternate port numbers. 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 [RFC4861]). 6.2. IPv6 Host Behavior The procedure for DNS configuration is the same as it is with any other Neighbor Discovery option [RFC4861]. In addition, the host follows the procedure described in Section 5.3.1 of [RFC8106]. The host MUST silently discard multicast and host loopback addresses conveyed in the Encrypted DNS options. Boucadair, et al. Expires November 5, 2021 [Page 12] Internet-Draft Discovery of Network-designated Resolvers May 2021 7. Hosting Encrypted DNS Forwarder in Local Networks This section discusses some deployment considerations to host an encrypted DNS forwarder within a local network. 7.1. Managed CPEs The section discusses mechanisms that can be used to host an encrypted DNS forwarder in a managed CPE (Appendix A.1). 7.1.1. DNS Forwarders The managed CPE should support a configuration parameter to instruct the CPE whether it has to relay the encrypted DNS server received from the ISP's network or has to announce itself as a forwarder within the local network. The default behavior of the CPE is to supply the encrypted DNS server received from the ISP's network. 7.1.2. ACME The ISP can assign a unique FQDN (e.g., "cpe1.example.com") and a domain-validated public certificate to the encrypted DNS forwarder hosted on the CPE. Automatic Certificate Management Environment (ACME) [RFC8555] can be used by the ISP to automate certificate management functions such as domain validation procedure, certificate issuance and certificate revocation. 7.1.3. Auto-Upgrade Based on Domains and their Subdomains If the ADN conveyed in DHCP/RA (Sections 4, 5, and 6) is preconfigured in popular OSes or browsers as a verified resolver and the auto-upgrade (Appendix B) is allowed for both the preconfigured ADN and its sub-domains, the encrypted DNS client will learn the local encrypted DNS forwarder using DHCP/RA and auto-upgrade because the preconfigured ADN would match the subjectAltName value in the server certificate. For example, if the preconfigured ADN is "*.example.com" and the discovered encrypted DNS forwarder is "cpe1.example.com", auto-upgrade will take place. In this case, the CPE can communicate the ADN of the local DoH forwarder (Section 7.1.2) to internal hosts using DHCP/RA (Sections 4, 5, and 6). Let's suppose that "*.example.net" is preconfigured as a verified resolved in the browser or OS. If the encrypted DNS client discovers a local forwarder "cpe1-internal.example.net", the encrypted DNS client will auto-upgrade because the preconfigured ADN would match subjectAltName value "cpe1-internal.example.net" of type dNSName. As Boucadair, et al. Expires November 5, 2021 [Page 13] Internet-Draft Discovery of Network-designated Resolvers May 2021 shown in Figure 8, the auto-upgrade to a rogue server advertising "rs.example.org" will fail because it does not match "*.example.net". Encrypted DNS CPE capable client (@i) | | |<=================DHCP===============| | {ADN=cpe1-internal.example.net, @i} | | | | Rogue Server | | (@rs) | | | | X<===========DHCP=========| | |{ADN=rs.example.org, @rs}| | | | | | | |<=================DoH===============>| | | Legend: * @i: internal IP address of the CPE * @rs: IP address of a rogue server Figure 8: A Simplified Example of Auto-upgrade based on Subdomains 7.2. Unmanaged CPEs The approach specified in Section 7.1 does not apply for hosting a DNS forwarder in an unmanaged CPE. The unmanaged CPE administrator can host an encrypted DNS forwarder on the unmanaged CPE. This assumes the following: o The encrypted DNS server certificate is managed by the entity in- charge of hosting the encrypted DNS forwarder. Alternatively, a security service provider can assign a unique FQDN to the CPE. The encrypted DNS forwarder will act like a private encrypted DNS server only be accessible from within the local network. o The encrypted DNS forwarder will either be configured to use the ISP's or a 3rd party encrypted DNS server. o The unmanaged CPE will advertise the encrypted DNS forwarder ADN using DHCP/RA to internal hosts. Boucadair, et al. Expires November 5, 2021 [Page 14] Internet-Draft Discovery of Network-designated Resolvers May 2021 Figure 9 illustrates an example of an unmanaged CPE hosting a forwarder which connects to a 3rd party encrypted DNS server. In this example, the DNS information received from the managed CPE (and therefore from the ISP) is ignored by the Internal CPE hosting the forwarder. ,--,--,--. ,--, ,' Internal Managed ,-' '- 3rd Party Host--( Network#A CPE--------CPE------( ISP )--- DNS Server | `. ,-'| | `-. -' | | `-'--'--' | |<==DHCP==>|`--' | | |<==DHCP==>| | | |<======DHCP=======>| | | | {RI, @i} | | |<==Encrypted DNS==>|<==========Encrypted DNS==========>| Legend: * @i: IP address of the DNS forwarder hosted in the Internal CPE. Figure 9: Example of an Internal CPE Hosting a Forwarder 8. Security Considerations 8.1. Spoofing Attacks 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 [RFC3552]) 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 [RFC8415]. The attacker can get a domain name with a domain- validated public certificate from a CA and host an Encrypted DNS server. Also, an attacker can use a public IP address and get an 'IP address'-validated public certificate from a CA to 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: o DHCPv6-Shield described in [RFC7610], the CPE discards DHCP response messages received from any local endpoint. o RA-Guard described in [RFC7113], the CPE discards RAs messages received from any local endpoint. Boucadair, et al. Expires November 5, 2021 [Page 15] Internet-Draft Discovery of Network-designated Resolvers May 2021 o Source Address Validation Improvement (SAVI) solution for DHCP described in [RFC7513], the CPE filters packets with forged source IP addresses. 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 [RFC6125], 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 [RFC6092]; it MUST apply for both IPv4 and IPv6. 8.2. Deletion Attacks 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. 8.3. Passive Attacks A passive attacker (Section 3.2 of [RFC3552]) 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. 8.4. Wireless Security - Authentication Attacks Wireless LAN (WLAN) as frequently deployed in local networks (e.g., home networks) is vulnerable to various attacks (e.g., [Evil-Twin], [Krack], [Dragonblood]). Because of these attacks, only cryptographically authenticated communications are trusted on WLANs. This means that an information (e.g., NTP server, DNS server, default domain) 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. Boucadair, et al. Expires November 5, 2021 [Page 16] Internet-Draft Discovery of Network-designated Resolvers May 2021 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 [dot1x], EAP-pwd [RFC8146]). 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., [PSK]). 9. IANA Considerations 9.1. DHCPv6 Option IANA is requested to assign the following new DHCPv6 Option Code in the registry maintained in [DHCPV6]. +-------+---------------+-----------+--------------+----------------+ | Value | Description | Client | Singleton | Reference | | | | ORO | Option | | +-------+---------------+-----------+--------------+----------------+ | TBA1 | OPTION_V6_DNR | Yes | No | [ThisDocument] | +-------+---------------+-----------+--------------+----------------+ 9.2. DHCPv4 Option IANA is requested to assign the following new DHCP Option Code in the registry maintained in [BOOTP]. +------+------------------+-------+----------------+----------------+ | Tag | Name | Data | Meaning | Reference | | | | Length| | | +------+------------------+-------+----------------+----------------+ | TBA2 | OPTION_V4_DNR | N | Encrypted DNS | [ThisDocument] | | | | | Server | | +------+------------------+-------+----------------+----------------+ 9.3. Neighbor Discovery Option 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 [ND]. Boucadair, et al. Expires November 5, 2021 [Page 17] Internet-Draft Discovery of Network-designated Resolvers May 2021 +------+--------------------------+----------------+ | Type | Description | Reference | +------+--------------------------+----------------+ | TBA3 | DNS Encrypted DNS Option | [ThisDocument] | +------+--------------------------+----------------+ 10. Acknowledgements Many thanks to Christian Jacquenet and Michael Richardson for the review. Thanks to Stephen Farrell, Martin Thomson, Vittorio Bertola, Stephane Bortzmeyer, Ben Schwartz, and Iain Sharp for the comments. Thanks to Mark Nottingham for the feedback on HTTP redirection. The use of DHCP to retrieve an authentication domain name was discussed in Section 7.3.1 of [RFC8310] and [I-D.pusateri-dhc-dns-driu]. Thanks to Bernie Volz for the review of the DHCP part. 11. Contributing Authors Nicolai Leymann Deutsche Telekom Germany Email: n.leymann@telekom.de Zhiwei Yan CNNIC No.4 South 4th Street, Zhongguancun Beijing 100190 China EMail: yan@cnnic.cn 12. References 12.1. Normative References [I-D.ietf-dnsop-svcb-https] Schwartz, B., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)", draft-ietf-dnsop-svcb-https-05 (work in progress), April 2021. Boucadair, et al. Expires November 5, 2021 [Page 18] Internet-Draft Discovery of Network-designated Resolvers May 2021 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, . [RFC3396] Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396, DOI 10.17487/RFC3396, November 2002, . [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, . [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 Router Advertisement Options for DNS Configuration", RFC 8106, DOI 10.17487/RFC8106, March 2017, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, November 2018, . 12.2. Informative References [Auto-upgrade] The Unicode Consortium, "DoH providers: criteria, process for Chrome", . Boucadair, et al. Expires November 5, 2021 [Page 19] Internet-Draft Discovery of Network-designated Resolvers May 2021 [BOOTP] "BOOTP Vendor Extensions and DHCP Options", . [DHCPV6] "DHCPv6 Option Codes", . [dot1x] Cisco, "Basic 802.1x Wireless User Authentication", . [Dragonblood] The Unicode Consortium, "Dragonblood: Analyzing the Dragonfly Handshake of WPA3 and EAP-pwd", . [Evil-Twin] The Unicode Consortium, "Evil twin (wireless networks)", . [I-D.ietf-add-ddr] Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T. Jensen, "Discovery of Designated Resolvers", draft-ietf- add-ddr-00 (work in progress), February 2021. [I-D.ietf-dprive-dnsoquic] Huitema, C., Mankin, A., and S. Dickinson, "Specification of DNS over Dedicated QUIC Connections", draft-ietf- dprive-dnsoquic-02 (work in progress), February 2021. [I-D.ietf-v6ops-rfc7084-bis] Martinez, J. P., "Basic Requirements for IPv6 Customer Edge Routers", draft-ietf-v6ops-rfc7084-bis-04 (work in progress), June 2017. [I-D.pusateri-dhc-dns-driu] Pusateri, T. and W. Toorop, "DHCPv6 Options for private DNS Discovery", draft-pusateri-dhc-dns-driu-00 (work in progress), July 2018. [I-D.schwartz-svcb-dns] Schwartz, B., "Service Binding Mapping for DNS Servers", draft-schwartz-svcb-dns-03 (work in progress), April 2021. [Krack] The Unicode Consortium, "Key Reinstallation Attacks", 2017, . Boucadair, et al. Expires November 5, 2021 [Page 20] Internet-Draft Discovery of Network-designated Resolvers May 2021 [ND] "IPv6 Neighbor Discovery Option Formats", . [PSK] Cisco, "Identity PSK Feature Deployment Guide", . [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, . [RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, DOI 10.17487/RFC3646, December 2003, . [RFC6092] Woodyatt, J., Ed., "Recommended Simple Security Capabilities in Customer Premises Equipment (CPE) for Providing Residential IPv6 Internet Service", RFC 6092, DOI 10.17487/RFC6092, January 2011, . [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011, . [RFC6731] Savolainen, T., Kato, J., and T. Lemon, "Improved Recursive DNS Server Selection for Multi-Interfaced Nodes", RFC 6731, DOI 10.17487/RFC6731, December 2012, . [RFC7113] Gont, F., "Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)", RFC 7113, DOI 10.17487/RFC7113, February 2014, . [RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address Validation Improvement (SAVI) Solution for DHCP", RFC 7513, DOI 10.17487/RFC7513, May 2015, . Boucadair, et al. Expires November 5, 2021 [Page 21] Internet-Draft Discovery of Network-designated Resolvers May 2021 [RFC7610] Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers", BCP 199, RFC 7610, DOI 10.17487/RFC7610, August 2015, . [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 2016, . [RFC8146] Harkins, D., "Adding Support for Salted Password Databases to EAP-pwd", RFC 8146, DOI 10.17487/RFC8146, April 2017, . [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles for DNS over TLS and DNS over DTLS", RFC 8310, DOI 10.17487/RFC8310, March 2018, . [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, . [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, January 2019, . [RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage Description Specification", RFC 8520, DOI 10.17487/RFC8520, March 2019, . [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, . [TR-069] The Broadband Forum, "CPE WAN Management Protocol", December 2018, . [TS.24008] 3GPP, "Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 (Release 16)", December 2019, . Boucadair, et al. Expires November 5, 2021 [Page 22] Internet-Draft Discovery of Network-designated Resolvers May 2021 Appendix A. Sample Target Deployment Scenarios ISPs traditionally provide DNS resolvers to their customers. To that aim, ISPs deploy the following mechanisms to advertise a list of DNS Recursive DNS server(s) to their customers: o Protocol Configuration Options in cellular networks [TS.24008]. o DHCPv4 [RFC2132] (Domain Name Server Option) or DHCPv6 [RFC8415][RFC3646] (OPTION_DNS_SERVERS). o IPv6 Router Advertisement [RFC4861][RFC8106] (Type 25 (Recursive DNS Server Option)). The communication between a customer's device (possibly via Customer Premises Equipment (CPE)) and an ISP-supplied DNS resolver takes place by using cleartext DNS messages (Do53). Some examples are depicted in Figure 10. In the case of cellular networks, the cellular network will provide connectivity directly to a host (e.g., smartphone, tablet) or via a CPE. Do53 mechanisms used within the Local Area Network (LAN) are similar in both fixed and cellular CPE- based broadband service offerings. Some ISPs rely upon external resolvers (e.g., outsourced service or public resolvers); these ISPs provide their customers with the IP addresses of these resolvers. These addresses are typically configured on CPEs using dedicated management tools. Likewise, users can modify the default DNS configuration of their CPEs (e.g., supplied by their ISP) to configure their favorite DNS servers. This document permits such deployments. Boucadair, et al. Expires November 5, 2021 [Page 23] Internet-Draft Discovery of Network-designated Resolvers May 2021 (a) Fixed Networks ,--,--,--. +-+ LAN +---+ ,-' `-. |H+--------------+CPE+---+ ISP ) +-+ +---+ `-. ,-' | `--'--'--' | | |<=============Do53============>| | | (b) Cellular Networks | | |<=============Do53============>| | | | ,--,--,-. +-+ LAN +---+ ,-' . |H+--------------+CPE+---+ \ +-+ +---+ ,' ISP `-. ( ) +-----+-. ,-' +-+ | `--'--'--' |H+----------------+ | +-+ | | | |<=============Do53============>| | | Legend: * H: refers to a host. Figure 10: Sample Legacy Deployments A.1. Managed CPEs This section focuses on CPEs that are managed by ISPs. A.1.1. Direct DNS ISPs have developed an expertise in managing service-specific configuration information (e.g., CPE WAN Management Protocol [TR-069]). For example, these tools may be used to provision the DNS server's ADN to managed CPEs if an encrypted DNS is supported by a local network similar to what is depicted in Figure 11. For example, DoH-capable (or DoT) clients establish the DoH (or DoT) session with the discovered DoH (or DoT) server. Boucadair, et al. Expires November 5, 2021 [Page 24] Internet-Draft Discovery of Network-designated Resolvers May 2021 The DNS client discovers whether the DNS server in the local network supports DoH/DoT/DoQ by using a dedicated field in the discovery message: Encrypted DNS Types (Sections 4, 5, and 6) . (a) Fixed Networks ,--,--,--. +-+ LAN +---+ ,-' `-. |H+--------------+CPE+---+ ISP ) +-+ +---+ `-. ,-' | `--'--'--' | | |<========Encrypted DNS========>| | | (b) Cellular Networks | | |<========Encrypted DNS========>| | | | ,--,--,-. +-+ LAN +---+ ,-' . |H+--------------+CPE+---+ \ +-+ +---+ ,' ISP `-. ( ) +-----+-. ,-' +-+ | `--'--'--' |H+----------------+ | +-+ | | | |<========Encrypted DNS========>| | | Figure 11: Encrypted DNS in the WAN Figure 11 shows the scenario where the CPE relays the list of encrypted DNS servers it learns for the network by using mechanisms like DHCP or a specific Router Advertisement message. In such context, direct encrypted DNS sessions will be established between a host serviced by a CPE and an ISP-supplied encrypted DNS server (see the example depicted in Figure 12 for a DoH/DoT-capable host). Boucadair, et al. Expires November 5, 2021 [Page 25] Internet-Draft Discovery of Network-designated Resolvers May 2021 ,--,--,--. ,--,--,--. ,-' `-. ,-' ISP `-. Host---( LAN CPE----( DNS Server ) | `-. ,-' `-. ,-' | `--'--'--' `--'--'--' | | |<=========Encrypted DNS===========>| Figure 12: Direct Encrypted DNS Sessions A.1.2. Proxied DNS Figure 13 shows a deployment where the CPE embeds a caching DNS forwarder. The CPE advertises itself as the default DNS server to the hosts it serves. The CPE relies upon DHCP or RA to advertise itself to internal hosts as the default DoT/DoH/Do53 server. When receiving a DNS request it cannot handle locally, the CPE forwards the request to an upstream DoH/DoT/Do53 resolver. Such deployment is required for IPv4 service continuity purposes (e.g., Section 5.4.1 of [I-D.ietf-v6ops-rfc7084-bis]) or for supporting advanced services within a local network (e.g., malware filtering, parental control, Manufacturer Usage Description (MUD) [RFC8520] to only allow intended communications to and from an IoT device). When the CPE behaves as a DNS forwarder, DNS communications can be decomposed into two legs: o The leg between an internal host and the CPE. o The leg between the CPE and an upstream DNS resolver. An ISP that offers encrypted DNS to its customers may enable encrypted DNS in one or both legs as shown in Figure 13. Additional considerations related to this deployment are discussed in Section 7. Boucadair, et al. Expires November 5, 2021 [Page 26] Internet-Draft Discovery of Network-designated Resolvers May 2021 (a) ,--,--,--. ,--,--,--. ,-' `-. ,-' ISP `-. Host---( LAN CPE----( DNS Server ) | `-. ,-'| `-. ,-' | `--'--'--' | `--'--'--' | | | |<=====Encrypted=====>|<=Encrypted=>| | DNS | DNS | (b) ,--,--,--. ,--,--,--. Legacy ,-' `-. ,-' ISP `-. Host---( LAN CPE----( DNS Server ) | `-. ,-'| `-. ,-' | `--'--'--' | `--'--'--' | | | |<=======Do53========>|<=Encrypted=>| | | DNS | Figure 13: Proxied Encrypted DNS Sessions A.2. Unmanaged CPEs A.2.1. ISP-facing Unmanaged CPEs Customers may decide to deploy unmanaged CPEs (assuming the CPE is compliant with the network access technical specification that is usually published by ISPs). Upon attachment to the network, an unmanaged CPE receives from the network its service configuration (including the DNS information) by means of, e.g., DHCP. That DNS information is shared within the LAN following the same mechanisms as those discussed in Appendix A.1. A host can thus establish DoH/DoT session with a DoH/DoT server similar to what is depicted in Figure 12 or Figure 13. A.2.2. Internal Unmanaged CPEs Customers may also decide to deploy internal routers (called hereafter, Internal CPEs) for a variety of reasons that are not detailed here. Absent any explicit configuration on the internal CPE to override the DNS configuration it receives from the ISP-supplied CPE, an Internal CPE relays the DNS information it receives via DHCP/ RA from the ISP-supplied CPE to connected hosts. Encrypted DNS sessions can be established by a host with the DNS servers of the ISP (see Figure 14). Boucadair, et al. Expires November 5, 2021 [Page 27] Internet-Draft Discovery of Network-designated Resolvers May 2021 ,--,--,--. ,--,--,--. ,-' Internal ,-' ISP `-. Host--( Network#A CPE----CPE---( DNS Server ) | `-. ,-' `-. ,-' | `--'--'--' `--'--'--' | | |<==============Encrypted DNS=============>| Figure 14: Direct Encrypted DNS Sessions with the ISP DNS Resolver (Internal CPE) Similar to managed CPEs, a user may modify the default DNS configuration of an unmanaged CPE to use his/her favorite DNS servers instead. Encrypted DNS sessions can be established directly between a host and a 3rd Party DNS server (see Figure 15). ,--,--,--. ,--, ,' Internal ,-' '- 3rd Party Host--( Network#A CPE----CPE---( ISP )--- DNS Server | `. ,-' `-. -' | | `-'--'--' `--' | | | |<=================Encrypted DNS==================>| Figure 15: Direct Encrypted DNS Sessions with a Third Party DNS Resolver Section 7.2 discusses considerations related to hosting a forwarder in the Internal CPE. Appendix B. Make Use of Discovered Encrypted DNS Servers Even if the use of a discovered encrypted DNS server is beyond the discovery process and falls under encrypted server selection, the following discusses typical conditions under which discovered encrypted DNS server can be used. o If the DNS server's IP address discovered by using DHCP/RA is preconfigured in the OS or Browser as a verified resolver (e.g., part of an auto-upgrade program such as [Auto-upgrade]), the DNS client auto-upgrades to use the preconfigured encrypted DNS server tied to the discovered DNS server IP address. In such a case the DNS client will perform additional checks out of band, such as confirming that the Do53 IP address and the encrypted DNS server are owned and operated by the same organisation. o Similarly, if the ADN conveyed in DHCP/RA (Sections 4, 5, and 6) is preconfigured in the OS or browser as a verified resolver, the Boucadair, et al. Expires November 5, 2021 [Page 28] Internet-Draft Discovery of Network-designated Resolvers May 2021 DNS client auto-upgrades to establish an encrypted a DoH/DoT/DoQ session with the ADN. In such case, the DNS client matches the domain name in the Encrypted DNS DHCP/RA option with the 'DNS-ID' identifier type within subjectAltName entry in the server certificate conveyed in the TLS handshake. Appendix C. Legacy CPEs Hosts serviced by legacy CPEs that can't be upgraded to support the options defined in Sections 4, 5, and 6 won't be able to learn the encrypted DNS server hosted by the ISP, in particular. If the ADN is not discovered using DHCP/RA, such hosts will have to fallback to use discovery using the resolver IP address as defined in Section 4 of [I-D.ietf-add-ddr] to discover the designated resolvers. The guidance in Sections 4.1 and 4.2 of [I-D.ietf-add-ddr] related to the designated resolver verification has to be followed in such case. Authors' Addresses Mohamed Boucadair (editor) Orange Rennes 35000 France Email: mohamed.boucadair@orange.com Tirumaleswar Reddy (editor) McAfee, Inc. Embassy Golf Link Business Park Bangalore, Karnataka 560071 India Email: TirumaleswarReddy_Konda@McAfee.com Dan Wing Citrix Systems, Inc. USA Email: dwing-ietf@fuggles.com Boucadair, et al. Expires November 5, 2021 [Page 29] Internet-Draft Discovery of Network-designated Resolvers May 2021 Neil Cook Open-Xchange UK Email: neil.cook@noware.co.uk Tommy Jensen Microsoft USA Email: tojens@microsoft.com Boucadair, et al. Expires November 5, 2021 [Page 30]