DNS Resolver Discovery Protocol (DRDP)
draft-mglt-add-rdp-01
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draft-mglt-add-rdp-01
add D. Migault Internet-Draft Ericsson Intended status: Informational March 24, 2020 Expires: September 25, 2020 DNS Resolver Discovery Protocol (DRDP) draft-mglt-add-rdp-01 Abstract This document describes the DNS Resolver Discovery Protocol (DRDP) that enables a DNS client to discover various available local and global resolving service. The discovery is primarily initiated by a DNS client, but a resolver may also inform the DNS client other resolving services are available and eventually preferred. 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 September 25, 2020. Copyright Notice Copyright (c) 2020 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 (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. Migault Expires September 25, 2020 [Page 1] Internet-Draft DRDP March 2020 Table of Contents 1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. DRDP Requirements . . . . . . . . . . . . . . . . . . . . . . 3 5. DRDP outputs . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. Resolving Service Identity . . . . . . . . . . . . . . . 4 5.2. DNS Transport related information . . . . . . . . . . . . 5 5.3. DNS Service related Information . . . . . . . . . . . . . 5 6. Architecture Overview . . . . . . . . . . . . . . . . . . . . 5 7. Domain Discovery with DRDP . . . . . . . . . . . . . . . . . 6 7.1. Global Domain . . . . . . . . . . . . . . . . . . . . . . 6 7.2. Local Domain . . . . . . . . . . . . . . . . . . . . . . 7 8. Resolving Service Discovery . . . . . . . . . . . . . . . . 8 8.1. Discovery of all service instances . . . . . . . . . . . 8 8.2. Discovery of specific service instances . . . . . . . . . 9 9. Resolver advertising other service sub type . . . . . . . . . 10 10. Migration to service sub types . . . . . . . . . . . . . . . 10 11. Security Considerations . . . . . . . . . . . . . . . . . . . 10 11.1. Use of protected channel is RECOMMENDED . . . . . . . . 11 11.2. DNSSEC is RECOMMENDED . . . . . . . . . . . . . . . . . 11 11.3. TLSA is RECOMMENDED . . . . . . . . . . . . . . . . . . 12 12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 15.1. Normative References . . . . . . . . . . . . . . . . . . 14 15.2. Informative References . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 1. Requirements Notation 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. 2. Introduction A DNS client can proceed to DNS resolution using various resolving services. These services can be local, global and can use a wide range of DNS transport protocols such as, for example, standard DNS [RFC1035], DNS over TLS[RFC7858] or DNS over HTTPS [RFC8484]. The purpose of the DNS Resolving service Protocol (DRDP) is to discover these resolving services available to the DNS client so a Migault Expires September 25, 2020 [Page 2] Internet-Draft DRDP March 2020 selection process can be applied. The information returned by DRDP typically includes information related to the identity of the resolving service, the transport ( IP addresses, the transport protocols, TLS parameters, HTTP version) as well as to characteristics of the resolving service (filtering, associated authoritative domains). The pieces of information can be extended to meet future usage. How the selection is performed is out of scope of this document. 3. Terminology DNS client the client that sends DNS queries fro resolution. In this document the DNS client designates also the end entity that is collecting information about the available Resolving Services and then proceed to the selection of a subset them. The selection is processed according to the DNS client's policy. Resolving Service designates a service that receives DNS queries from a DNS client and resolves them. A Resolving Service is implemented by one or multiple resolvers. Resolver: A resolver designates the software or hardware handling the DNS exchange. See [RFC7719] for more details. DNS transport designates the necessary parameters a DNS client needs to establish a session with a Resolving Service. Resolving Domain a DNS domain that hosts one or multiple resolving services. 4. DRDP Requirements This section lists the DRDP requirements. REQ 1: DRDP MAY be used by a DNS client (Do53, DoT, DoH, ...) to discover resolving service or by a resolver to advertise other resolving services are available. REQ 2: DRDP MUST be able to list dynamically locally and globally resolving services available to the DNS client. The necessary inputs for the resolving service instances may be of various form. Not all of them are expected to be in the scope of DRDP and DRDP limits its scope to parameters that are inherent to the resolving service. For example, an end user may simply willing to know which DNS resolver provides the fastest resolution. Such inputs are not inherent to a specific resolver and are out of scope of Migault Expires September 25, 2020 [Page 3] Internet-Draft DRDP March 2020 DRDP.Another example could be the activation of some services such as parental protection. REQ 3: DRDP MUST at least return DNS transport parameters associated of the resolving services and MAY be extended with additional parameters. The selection of the resolving service MAY take various form between fully automated to fully manual. This, in particularly includes interaction with the end user on a subset of the selection parameters as well as the ability for a resolving service operator to indicate a preference toward a resolving service instance. REQ 4: DRDP MUST return selection parameters in a standard format to ease automation. REQ 5: DRDP MUST return selection parameters that can be displayed to an end user either as a simple notification of when user interaction is involved in the selection process. REQ 6: DRDP MUST enable a resolving service provider to indicate a preference between multiple provided resolving services. The resolving service selection MAY be performed over a subset of the available resolvers as opposed as the full set of available resolvers. REQ 7: DRDP SHOULD be able to narrow down the discovery to a subset of resolving services. REQ 8: DRDP MUST provide authenticated information REQ 9: DRDP deployment MUST NOT be disruptive for the legacy DNS client or infrastructure and legacy client SHOULD be able to incrementally include DRDP. 5. DRDP outputs 5.1. Resolving Service Identity The identity of the resolving service is an important selection parameter as it usually reflects the trust an end user puts into this service. In addition, trust can hardly be inferred automatically and is likely to require an interaction with the end user, unless explicitly provided by the end user. This document considers the resolver's FQDN resolver.example.com as its identifier. example.com designates the resolver domain and "resolver" represents hostname. Migault Expires September 25, 2020 [Page 4] Internet-Draft DRDP March 2020 a) The resolving domain is expected to be the part that will mostly be used by the end user as a way to select trust as these are expected to represent the brand or legal entity of the institution the end user sends its data to. The resolving domain follows some DNS encoding rules and as such may not be believed to be so user friendly. Typically, it might be ericsson.com or ericsson which is different from Ericsson (with appropriated police character and color) which is probably what would be more meaningful for the end user. On the other hand, the end user may also be familiar with that format and the use or a standardize format helps automation in the selection. As a result, this document will assume that the rdns domain will reflect the legal entity administrating the resolver to the user. Note that a user interface may also use the rdns domain to derive more user friendly and additional specific information that will be presented to the user. This could include for example additional RDAP queries, favicons of web sites that are shown to the end users. What is presented to the end user is out of scope of this document, but the rdns domain can be used as the key. b) The hostname part is only meaningful within the resolving domain. While, it may carry some information that may be interpreted to the end user, the constraint provided by the DNS format may be too restricting. As a result, it is expected that a more user friendly string might be associated with the hostname and that the hostname remain reserved for networking administrators. 5.2. DNS Transport related information Parameters associated to the DNS transport are the type of transport that is DNS, DoT or DoH as well as the necessary parameters to establish the session. This may include specific TLS parameters for DoT and DoH as well as specific HTTP versions and URI template for DoH. These parameters are expected to be identified in a standard way. 5.3. DNS Service related Information Parameters associated to the resolving service include for example, the presence of filtering services, the associated authoritative domains. 6. Architecture Overview DRDP can be used by a resolver or a DNS client (REQ1), which share DNS as a common protocol. In addition, the ability to deploy incrementally DRDP over legacy DNS client (REQ9) makes DNS a good candidate for DRDP. Migault Expires September 25, 2020 [Page 5] Internet-Draft DRDP March 2020 DNS based Service Discovery (DNS-SD) [RFC6763] is a discovery protocol for services based on DNS messages. DNS-SD provides the ability to display user-friendly names in UTF-8 and uses a combination of DNS RRsets of type PTR, SRV and TXT. The current document is largely inspired from this long time and already existing protocol. However, DRDP differs from DNS SD in that DNS-SD discovers services within a specific domain (such as .local or .home.arpa for example) while DRDP needs to discover the resolving domain as well as the resolving services associated to this domain. In addition, DRDP is taking advantage of the latests development of SRVCB RRsets [I-D.ietf-dnsop-svcb-httpssvc] which, among other things, enables to combine the SRV and TXT Rsets. While nothing prevents DRDP to use SRV and TXT RRsets, DRDP uses instead SVCB RRset as web browser are more likely to implement SVBC. The overall procedure is performed as described below: 1. Discovery of the global and local available resolving domains 2. Discovery of the resolving services within a resolving domain. 7. Domain Discovery with DRDP 7.1. Global Domain The mechanism involves the creation of a special domain name rdns.arpa that lists the various resolving domains. This mechanism remains valid as long as the list of resolving domain name remains relatively limited. The number of resolving domain that can fit into a payload will depend on the length of the various resolving domain. That said, a UDP packet of 4096 bytes is expected to contain a significant amount of resolvers. The number of open resolver is not expected to reach that limit and if so the list can be retrieved through TCP. The zone file below is inspired from DNS-SD where b indicates a browsing domain, _dns indicates the DNS resolving service and rdns.arpa. indicates the special domain. dns domain_0, nds_domain_n indicates the various resolving domains. The order of the resolving domains is irrelevant, and the zone administrator SHOULD regularly reorder them. The RRsets MUST be signed with DNSSEC. b._dns.rdns.arpa PTR <resolving_domain_0> [...] b._dns.rdns.arpa PTR <resolving_domain_n> Migault Expires September 25, 2020 [Page 6] Internet-Draft DRDP March 2020 7.2. Local Domain The resolving domains that are local needs to be provisioned or advertised by the network. With that resolving domain the DNS client could proceed to its resolving service selection. Resolving service are currently configured or advertised via IP addresses rather than a FQDN as a DNS resolution would be needed to resolve the IP address. More specifically, networks usually advertise the resolving service via a Recursive Name Server option [RFC3646] that contains an IP address. Similarly application usually configures their resolving services with IP addresses (8.8.8.8, 1.1.1.1, 9.9.9.9,...). As a result, this section indicates a mechanism that would enable a DNS client to derive a resolving domain of a resolver from an IP address of an advertised resolver. The mechanism described here is expected to be used as an hint. The resolving domain will be derived from the IP address by: 1. performing a reverse resolution 2. assume the resulting FQDN is composed of a hostname appended to the resolving domain. For example, if resolver.example.com is the resulting FQDN from the reverse resolution, then the rdns domain will be example.com. In most cases local resolving services uses global IP address which does not limit the reverse resolution to an associated local resolver. However the zone associated to the resolving domain might not be available globally and instead be restricted to the local network. As a result, DNS client SHOULD perform DNS resolution associated to the local resolving domain using the local resolver, and resolver operator SHOULD publish the resolving domain zone to the global Internet. Legacy DNS client will not be impacted. Upon receiving the IP address they will send their DNS queries to that IP address. DRDP aware DNS client will derive the resolving domain and attempt to perform a discovery within the resolving domain. If other mechanisms as used to advertise the resolving domains such as those described in [I-D.btw-add-home], and the resolving domain are different, the DNS client should perform DRDP with both resolving domains. Migault Expires September 25, 2020 [Page 7] Internet-Draft DRDP March 2020 8. Resolving Service Discovery 8.1. Discovery of all service instances Given a resolving domain example.com, a DNS client MAY request all possible resolving service instances with a query of type SVCB with the service _dns. The example below presents the use of an AliasForm followed by a ServiceForm which allows an indirection. The Alias form is not madatory and instead only ServiceForm associated to _dn.example.com could have been used instead. The SvcFieldPriority indicates the preference of the resolving service instance. The SvcParamKey alpn MUST be present when TLS is used as its presence and value indicates the DNS transport. The absence of the alpn SvcParamKey indicates Do53, alpn set to dot indicates DoT is served while h* indicates DoH is served. Note that the port value (53, 853, 443) is not used to determine teh DNS transport as non standard port MAY be used. The example below uses an non standard port 5353 for illustrative purpose. The SvcParamField ux is optional is provides an UTF-8 string that is expected to be displayed to the end user if needed. The RRsets MUST be protected with DNSSEC and when alpn is provided a TLSA RRset SHOULD be present. These RRsets have been omitted for clarity. ## Discovery of all service instances _dns.example.com. 7200 IN SVCB 0 svc.example.com. svc.example.com. 7200 IN SVCB 12 ( svc0.example.net. port="5353" ux="Legacy Resolver" ) svc.example.com. 7200 IN SVCB 1 ( svc1.example.net. alpn="dot" port="5353" esniconfig="..." ux="Preferred Example's Choice" ) svc.example.com. 7200 IN SVCB 3 ( svc2.example.net. alpn="h2" port="5353" esniconfig="..." ux= ) svc.example.com. 7200 IN SVCB 2 ( svc3.example.net. alpn="h3" port="5353" esniconfig="..." ux= ) Migault Expires September 25, 2020 [Page 8] Internet-Draft DRDP March 2020 8.2. Discovery of specific service instances To reduce the size of the messages, the DNS client MAY also prefer to query information of resolvers using a specific transport (DNS, DoT, DoH) that are designated as sub sets. A DNS client MAY list the the different subsets of that resolving domain with a PTR query. This document defines the following subsets _53._dns for DNS, _853._dns for DoT and _443.__dns for DoH. Other subsets MAY be defined in the future. A DNS client that does not understand a subset SHOULD ignore it and maybe proceed to the discovery as defined in Section 8.1. All subsets MUST share the same resolving domain and be listed with a PTR RRsets. The DNS client MAY NOT performed a DNS query of type PTR, for example, if it has a previous knowledge of the existence of the subset or if indicated by its policy. In this it MAY directly proceed to the SRVCB resolution. The same restrictions as defined in section Section 8.1 apply. Note that while the SvcFieldPriority indicates the priority within a subservice, this field MUST have a coherence across subservices. The priority provided SHOULD be coherent with the case of a _dns SRVCB query of section Section 8.1. The figure below illustrates an example of zone file. RRSIG and TLSA have been omited for the purpose of clarity. ### Definition of the resolving service subsets _dns.example.com PTR _53._dns.example.com _dns.example.com PTR _853._dns.example.com _dns.example.com PTR _443._dns.example.com ### services instances per service subset _53._dns.example.com. 7200 IN SVCB 0 svc0.example.com. svc0.example.com. 7200 IN SVCB 12 ( svc0.example.net. port="5353" ux="Legacy Resolver" ) _853._dns.example.com. 7200 IN SVCB 0 svc1.example.com. svc1.example.com. 7200 IN SVCB 1 ( svc1.example.net. alpn="dot" port="5353" esniconfig="..." ux="Preferred Example's Choice" ) _443_dns.example.com. 7200 IN SVCB 0 svc4.example.net. svc4.example.com. 7200 IN SVCB 3 ( svc2.example.net. alpn="h2" port="5353" esniconfig="..." ux= ) svc4.example.com. 7200 IN SVCB 2 ( svc3.example.net. alpn="h3" port="5353" esniconfig="..." ux="Testing QUIC") Migault Expires September 25, 2020 [Page 9] Internet-Draft DRDP March 2020 Some notes: 1. _domain uses SVCB but does not have TLS. While SVCB has been created essentially for TLS based service, this does not appear to be mandatory. 2. Should we have some constraints regarding the SvcDomainName and QNAME ? 9. Resolver advertising other service sub type A resolver receiving a DNS request over a service sub type MAY be willing to advertise the DNS client that other sub service type are available. This is especially useful, when, for example, a resolver wants that the DNS resolver switches to other service sub types that are more secure. In order to do so the resolver MAY provide in the additional data field the _dns SRVCB of ServiceForm. 10. Migration to service sub types The principle of the discovery mechanism is that the resolver indicates the available service sub types and let the DNS client chose which sub type it prefers. On the other hand, the resolver MAY also indicate a preference using the priority and weight fields. However, there is no mechanisms that could permit an indirection from one service sub type to another service sub type. This document specifies that weight needs to be considered across sub types. Redirection MAY especially be needed when a DNS client is using the Do53 and the resolver would like to upgrade the DNS client session to a more secure session. This MAY require a specific ERROR code that will request the DNS client to perform service discovery. It is expected that DRDP MUST always be available via Do53. However, this does not mean that a resolver is expected to implement the Do53 sub type service for a resolving service. If a resolving service provider chooses not to provide a resolving service using Do53, that service MUST NOT be pointed by the _53._dns.example.com search and MUST NOT provide _dns.example.com SRVCB RRsets with no SvcParamKey alpn. 11. Security Considerations Migault Expires September 25, 2020 [Page 10] Internet-Draft DRDP March 2020 11.1. Use of protected channel is RECOMMENDED When available, it is recommended to chose a protected version of the rdns service. More specifically, the use of end-to-end protection ensures that the DNS client is connected to the expected platform and that its traffic cannot be intercepted on path. Typically, the selection of resolver on the Internet (and not on your ISP network) and the use of a non protected channel enables an attacker to monitor your DNS traffic. The similar observation remains true if you are connected to the resolver of your ISP. It is commonly believed that trusting your ISP (that is your first hop) makes encryption unecessary. Trusting your ISP is mandatory in any case, but the associated level of trust with an protected channel is restricted to the operation of the DNS platform. With non protected channel the trust is extended to any segment between the DNS client and the resolver, which is consequently larger. The use of a protected channel is recommended as it will prevent anyone on path to monitor your traffic. 11.2. DNSSEC is RECOMMENDED The exchanges SHOULD be protected with DNSSEC to ensure integrity of the information between the authoritative servers and the DNS client. Without DNSSEC protection, DNS messages may be tampered typically when they are transmitted over an unprotected channel either between the DNS client and the resolver or between the resolver and the authoritative servers. The messages may be tampered by an online attacker intercepting the messages or by the intermediary devices. It is important to realize that protection provided by TLS is limited to the channel between the DNS client and the resolver. There are a number of cases were the trust in the resolver is not sufficient which justify the generalization of the use of DNSSEC. The following examples are illustrative and are intended to be exhaustive. First, the discovery exchanges may happen over an unprotected channel, in which case, the messages exchanged may be tampered by anyone on-path between the DNS client and the resolver as well as between the resolver and the authoritative servers - including the resolver. When TLS is used between the DNS client and the resolver, this does not necessarily mean the DNS client trusts the resolver. Typically, the TLS session may be established with a self-signed certificate in which case the session is basically protected by a proof-of-ownership. In other cases, the session may be established based on Certificate Authorities (CA) that have been configured into the TLS client, but that are not necessarily trusted by the DNS client. In such cases, the connected resolver may be used to discover resolvers from another domain. In this case, the resolver Migault Expires September 25, 2020 [Page 11] Internet-Draft DRDP March 2020 is probably interacting with authoritative servers using untrusted and unprotected channels. Integrity protection relies on DNSSEC. 11.3. TLSA is RECOMMENDED When TLS is used to protect the DNS exchanges, certificates or fingerprint SHOULD be provided to implement trust into the communication between the DNS client and the resolver. The TLS session and the association of the private key to a specific identity can be based on two different trust model. The Web PKI that will rely on CA provisioned in the TLS library or the TA provided to the DNS client. A DNS client SHOULD be able to validate the trust of a TLS session based on the DNSSEC trust model using DANE. When the DNS client is protecting its session to the resolver via TLS, the DNS client may initiate an TLS session that is not validated by a CA or a TLSA RRsets. The DNS client MUST proceed to the discovery process and validate the certificate match the TLSA RRset. In case of mismatch the DNS client MUST abort the session. 12. Privacy Considerations When the discovery protocol is performed using a resolver that belongs to one domain for another domain, or over an unprotected channel, the DNS client must be conscious that its is revealing to the resolver its intention to use another resolver. More specifically, suppose an resolver is complying some legal requirements that DNS traffic must be unencrypted. Using this resolver to perform a resolver discovery reveals the intention of potentially using alternative resolvers. Alternatively, narrowing down the discovery over a specific sub type of resolver (DoT, or DoH) may reveal to that resolver the type of communication. As result, when performing a discovery over a domain that differs to the domain the resolver belongs to, it is RECOMMENDED to request the SRV RRsets associated to all different sub type of proposed services. The absence of traffic that results from switching completely to a newly discovered resolver right after the discovery process provides an indication to the resolver the DNS client is switching to. It is hard to make that switch unnoticed to the initial resolver and the DNS resolver MUST assume this will be noticed. The information of switching may be limited by sharing the traffic between different resolvers, however, the traffic pattern associated to each resolver may also reveal the switch. In addition, when the initial resolver is provided by the ISP, the ISP is also able to monitor the IP traffic and infer the switch. As a result, the DNS client SHOULD assume the switch will be detected. Migault Expires September 25, 2020 [Page 12] Internet-Draft DRDP March 2020 With DoT or DoH, the selection of port 443 will make the traffic indistinguishable from HTTPS traffic. This means that an observer will not be able to tell whether the traffic carries web traffic or DNS traffic. Note that it presents an interest if the server offers both a web service as well as a resolution service. Note that many resolvers have a dedicated IP address for the resolution service, in which case, the information will be inferred from the IP address. Note also that traffic analysis may infer this as well. Typically suppose an IP address hosts one or multiple web sites that are not popular as well as a resolving service. If this IP address is associated frequent short size exchanges, it is likely that these exchanges will be DNS exchanges rather than Web traffic. The size of the packet may also be used as well as many other patterns. As a result, the use port 443 to hide the DNS traffic over web traffic should be considered as providing limited privacy. 13. IANA Considerations This document requests the IANA the creation of the following underscored node names in the Underscored and Globally Scoped DNS Node Names registry https://www.iana.org/assignments/dns-parameters/ dns-parameters.xhtml#dns-parameters-14 RR Type | _NODE NAME | Reference --------+------------+---------- SRVCB | _dns | RFC-TBD SvcParamKey | NAME | Meaning | Reference ------------+--------------+-----------------------------+----------- 7 | user-display | User friendly string (UTF8) | RFC-TBD | | to represent the resolver | | uri_template | URI template | | auth_domain | Domains the resolving | | | service is authoritative | | filetring | Filetring services provided | SvcParamValue for filetring 14. Acknowledgments We would like thank Mirja Kuehlewind as well as the GSMA IG for their comments. We also thank Ted Hardie and Paul Hoffman for their feed backs regarding the dns schemes for DoT and DoH. Migault Expires September 25, 2020 [Page 13] Internet-Draft DRDP March 2020 15. References 15.1. Normative References [I-D.ietf-dnsop-svcb-httpssvc] Schwartz, B., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPSSVC)", draft-ietf-dnsop-svcb-httpssvc-02 (work in progress), March 2020. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <https://www.rfc-editor.org/info/rfc1035>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC3646] Droms, R., Ed., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, DOI 10.17487/RFC3646, December 2003, <https://www.rfc-editor.org/info/rfc3646>. [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, <https://www.rfc-editor.org/info/rfc6763>. [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, <https://www.rfc-editor.org/info/rfc7858>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, <https://www.rfc-editor.org/info/rfc8484>. 15.2. Informative References [I-D.btw-add-home] Boucadair, M., Reddy.K, T., Wing, D., and N. Cook, "DNS- over-HTTPS and DNS-over-TLS Server Discovery and Deployment Considerations for Home and Mobile Networks", draft-btw-add-home-04 (work in progress), March 2020. Migault Expires September 25, 2020 [Page 14] Internet-Draft DRDP March 2020 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", RFC 7719, DOI 10.17487/RFC7719, December 2015, <https://www.rfc-editor.org/info/rfc7719>. Author's Address Daniel Migault Ericsson 8275 Trans Canada Route Saint Laurent, QC 4S 0B6 Canada EMail: daniel.migault@ericsson.com Migault Expires September 25, 2020 [Page 15]