Network Working Group P. van Dijk Internet-Draft PowerDNS Intended status: Experimental P. Hoffman Expires: 3 November 2021 ICANN 2 May 2021 Common Features for Encrypted Recursive to Authoritative DNS draft-pp-dprive-common-features-00 Abstract Encryption between recursive and authoritative DNS servers is currently being defined in two modes: unauthenticated and fully- authenticated. These two modes have some features in common, and this document defines those common features so that the documents defining the modes do not need to point to each other. 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/. 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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. van Dijk & Hoffman Expires 3 November 2021 [Page 1] Internet-Draft Common Features May 2021 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Discovery of Authoritative Server Encryption . . . . . . . . 3 2.1. DNS SVCB Records in the Parent Zone . . . . . . . . . . . 3 3. Processing Discovery Responses . . . . . . . . . . . . . . . 3 3.1. Resolver Process as Pseudocode . . . . . . . . . . . . . 4 4. TLS Requirements for Encrypting Resolver to Authoritative Server Sessions . . . . . . . . . . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . 6 8.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction The DPRIVE Working Group in the IETF is working on standardizing methods for encrypted communication between DNS recursive resolvers and authoritative servers. At the time of this writing, [UNAUTH] is a work item in the working group, and [FULL-AUTH] has been widely discussed. The working group expressed a desire that the modes share as much design as possible to simplify the working group's process of evaluating the security and operational aspects of the methods. If the DPRIVE Working Group later adopts other modes, those modes should be considered in this document. This document lists the major technical features that are shared by [UNAUTH] and [FULL-AUTH]. Differences from the common features in this document are listed in the respective method documents. The following are the features in common between and [UNAUTH] and [FULL-AUTH]: * Discovery of an authoritative server's encryption support (Section 2) * Order of processing discovered authoritative servers (Section 3) * TLS requirements (Section 4) Other topics might be added as the working group discusses [UNAUTH] and [FULL-AUTH] (and maybe other methods). van Dijk & Hoffman Expires 3 November 2021 [Page 2] Internet-Draft Common Features May 2021 2. Discovery of Authoritative Server Encryption An authoritative server that supports DNS with encryption makes itself discoverable by publishing one or more DNS SVCB records that contain "alpn" parameter keys. SVCB records are defined in [SVCB], and the DNS extension to those records are define in [DNS-SVCB]. A recursive resolver discovers whether an authoritative server supports DNS with encryption by looking for cached SVCB records for the name of the authoritative server with a positive answer. A cached DNS SVCB record with a negative answer indicates that the authoritative server does not support any encrypted transport. If the cache has no positive or negative answers for any DNS SVCB record for any of a zone's authoritative servers, the resolver needs to send queries for the DNS SVCB records for some or all of the zone's authoritative servers. Because some authoritative servers or middleboxes are misconfigured, requests for unknown RRtypes might be ignored by them. Resolvers should be ready to deal with timeouts or other bad responses to their SVCB queries. 2.1. DNS SVCB Records in the Parent Zone DNS SVCB records act as advisory information for resolvers about the encrypted protocols that are supported. They can be thought of as similar to NS records on the parent side of a zone cut: advisory enough to act on, but not authoritative. Given this, authoritative servers that know the DNS SCVB records associated with NS records for any child zones MAY include those DNS SCVB records in the Additional section of responses to queries to a parent authoritative server. (( Before this is published for real, it would be useful to check whether any resolvers freak out or fall over when they receive SVCB records in the Additional section. )) 3. Processing Discovery Responses After a resolver has DNS SCVB records in its cache (possibly due to having just queried for them), it needs to use those records to try to find an authoritative server that uses DNS with encryption. This section describes how the resolver can make that selection. A resolver MUST NOT attempt encryption for a server that has a negative response in its cache for the associated DNS SVCB record. van Dijk & Hoffman Expires 3 November 2021 [Page 3] Internet-Draft Common Features May 2021 After sending out all requests for SVCB records for the authoritative servers in the NS RRset for a name, if all of the SVCB records for those authoritative servers in the cache are negative responses, the resolver MUST use classic (unencrypted) DNS instead of encryption. Similarly, if none of the DNS SVCB records for the authoritative servers in the cache have supported "alpn" parameters, the resolver MUST use classic (unencrypted) DNS instead of encryption. If there are any DNS SVCB records in the cache for the authoritative servers for a zone with supported "alpn" parameters, the resolver MUST try each indicated authoritative server using DNS with encryption until it successfully sets up a connection. The resolver only attempts to use the encrypted transports that are in the associated SVCB record for the authoritative server. (( Note that this completely prohibits "simple port 853 probing" even though that is what some operators are currently doing. Does the WG want to be this strict? )) A resolver SHOULD keep a DNS with encryption session to a particular server open if it expects to send additional queries to that server in a short period of time. [DNS-OVER-TCP] says "both clients and servers SHOULD support connection reuse" for TCP connections, and that advice could apply as well for DNS with encryption, especially as DNS with encryption has far greater overhead for re-establishing a connection. If the server closes the DNS with encryption session, the resolver can possibly re-establish a DNS with encryption session using encrypted session resumption. 3.1. Resolver Process as Pseudocode This section is meant as an informal clarification of the protocol, and is not normative. The pseudocode here is designed to show the intent of the protocol, so it is not optimized for things like intersection of sets and other shortcuts. In this code, "signal_rrset(name)" means an "SVCB" query for the "'_dns'" prefix of "this_name". The "Query over secure transport until successful" section ignores differences in name server selection and retry behaviour in different resolvers. The pseudocode was written to roughly cover the shared behaviour between [UNAUTH] and [FULL-AUTH]. Specifically, whether an implementation waits for the resolution of "queue a query" would differ between the two. van Dijk & Hoffman Expires 3 November 2021 [Page 4] Internet-Draft Common Features May 2021 # Inputs ns_names = List of NS Rdatas from the NS RRset for the queried name can_do_secure = List of secure transports supported by resolver secure_names_and_transports = Empty list, filled in below # Fill secure_names_and_transports with (name, transport) tuples for this_name in ns_names: if signal_rrset(this_name) is in the resolver cache: if signal_rrset(this_name) positively does not exist: continue for this_transport in signal_rrset(this_name): if this_transport in can_do_secure: add (this_name, this_transport) to secure_names_and_transports else: # signal_rrset(this_name) is not in the resolver cache queue a query for signal_rrset(this_name) for later caching # Query over secure transport until successful for (this_name, this_transport) tuple in secure_names_and_transports: query using this_transport on this_name if successful: finished # Got here if no this_name/this_transport query was successful # or if secure_names_and_transports was empty query using classic DNS on any/all ns_names; finished 4. TLS Requirements for Encrypting Resolver to Authoritative Server Sessions All protocols for DNS with encryption rely on TLS. This section defines requirements for the TLS use of DNS with encryption clients and servers. For any DNS with encryption protocols, TLS version 1.3 [TLS-13] or later MUST be used. (( There are other requirements, surely? )) 5. IANA Considerations This document contains no changes to IANA registries. 6. Security Considerations (( Talk about requiring TLS 1.3 )) van Dijk & Hoffman Expires 3 November 2021 [Page 5] Internet-Draft Common Features May 2021 7. Acknowledgements The use of SVCB records for discovering whether an authoritative server supports encryption was first described by the authors of [FULL-AUTH]. 8. References 8.1. Normative References [DNS-SVCB] Schwartz, B., "Service Binding Mapping for DNS Servers", Work in Progress, Internet-Draft, draft-schwartz-svcb-dns- 03, 19 April 2021, . [FULL-AUTH] Pauly, T., Rescorla, E., Schinazi, D., and C. A. Wood, "Signaling Authoritative DNS Encryption", Work in Progress, Internet-Draft, draft-rescorla-dprive-adox- latest-00, 26 February 2021, . [SVCB] Schwartz, B., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf- dnsop-svcb-https-05, 21 April 2021, . [TLS-13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [UNAUTH] Hoffman, P. and P. V. Dijk, "Recursive to Authoritative DNS with Unauthenticated Encryption", Work in Progress, Internet-Draft, draft-ietf-dprive-unauth-to-authoritative- 00, 12 April 2021, . 8.2. Informative References [DNS-OVER-TCP] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and D. Wessels, "DNS Transport over TCP - Implementation Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, . van Dijk & Hoffman Expires 3 November 2021 [Page 6] Internet-Draft Common Features May 2021 Authors' Addresses Peter van Dijk PowerDNS Email: peter.van.dijk@powerdns.com Paul Hoffman ICANN Email: paul.hoffman@icann.org van Dijk & Hoffman Expires 3 November 2021 [Page 7]