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Hoffman 3 Internet-Draft ICANN 4 Intended status: Standards Track 29 October 2020 5 Expires: 2 May 2021 7 Recursive to Authoritative DNS with Opportunistic Encryption 8 draft-pp-recursive-authoritative-opportunistic-01 10 Abstract 12 This document describes a use case and a method for a DNS recursive 13 resolver to use opportunistic encryption when communicating with 14 authoritative servers. A motivating use case for this method is that 15 more encryption on the Internet is better, and opportunistic 16 encryption is better than no encryption at all. The method here is 17 optional for both the recursive resolver and the authoritative 18 server. Nothing in this method prevents use cases and methods that 19 require authenticated encryption. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at https://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on 2 May 2021. 38 Copyright Notice 40 Copyright (c) 2020 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 45 license-info) in effect on the date of publication of this document. 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. Code Components 48 extracted from this document must include Simplified BSD License text 49 as described in Section 4.e of the Trust Legal Provisions and are 50 provided without warranty as described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 1.1. Use Case . . . . . . . . . . . . . . . . . . . . . . . . 2 56 1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Method for Opportunistic Encryption . . . . . . . . . . . . . 3 58 3. Transport Caches . . . . . . . . . . . . . . . . . . . . . . 4 59 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 60 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 61 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 6.1. Normative References . . . . . . . . . . . . . . . . . . 5 63 6.2. Informative References . . . . . . . . . . . . . . . . . 6 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 66 1. Introduction 68 A recursive resolver using traditional DNS over port 53 may wish 69 instead to use encrypted communication with authoritative servers in 70 order to prevent passive snooping of its DNS traffic. The recursive 71 resolver can use opportunistic encryption (defined in [RFC7435] to 72 achieve this goal. 74 This document describes a use case and a method for recursive 75 resolvers to use opportunistic encryption. The use case is described 76 in Section 1.1. The method uses DNS-over-TLS [RFC7858] with 77 authoritative servers in an efficient manner. 79 1.1. Use Case 81 The use case in this document is recursive resolver operators who are 82 happy to use TLS [RFC8446] encryption with authoritative servers if 83 doing so doesn't significantly slow down getting answers, and 84 authoritative server operators that are happy to use encryption with 85 recursive resolvers if it doesn't cost much. 87 Both parties understand that using encryption costs something, but 88 are willing to absorb the costs for the benefit of more Internet 89 traffic being encrypted. The extra costs (compared to using 90 traditional DNS on port 53) include: 92 * Extra round trips to establish TCP for every session 94 * Extra round trips for TLS establishment 96 * Greater CPU use for TLS establishment 98 * Greater CPU use for encryption after TLS establishment 100 * Greater memory use for holding TLS state 102 1.2. Definitions 104 The terms "recursive resolver" and "authoritative server" are defined 105 in [RFC8499]. 107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 109 "OPTIONAL" in this document are to be interpreted as described in BCP 110 14 [RFC2119] [RFC8174] when, and only when, they appear in all 111 capitals, as shown here. 113 2. Method for Opportunistic Encryption 115 [RFC7435] defines opportunistic encryption. The method described 116 here uses DNS-over-TLS [RFC7858] between resolvers and authoritative 117 servers. 119 In this document, the only difference between normal TLS session 120 establishment and opportunistic encryption is that the the TLS client 121 (the recursive resolver) optionally authenticates the server. In 122 normal TLS, the client is required to authenticate the server and the 123 TLS connection fails if the authentication is not successful. 125 In the opportunistic encryption described here, there is no need for 126 the recursive resolver to authenticate the authoritative server 127 because any authentication failure does not cause the TLS session 128 from being set up. If it is easier programmatically for the 129 recursive resolver to authenticate the authoritative server and then 130 ignore the result than to just not authenticate, the recursive 131 resolver MAY do that. The recursive resolver MAY note the 132 authentication failure and act on it (such as by logging it or noting 133 it in the cache), as long as the failure does not prevent the TLS 134 session from completing. 136 Note that later protocols for encrypted resolver-to-authoritative 137 communication might to require normal TLS authentication. Because of 138 this, authoritative servers SHOULD use TLS certificates that can be 139 used in authenticated TLS authentication, such as those issued by 140 trusted third parties or self-issued certificates that can be 141 authenticated with DANE [RFC6698] records. However, if an 142 authoritative server does not care about the use cases for such 143 future protocols, it MAY use self-issued certificates that cannot be 144 authenticated. 146 3. Transport Caches 148 A recursive resolver that attempted to use encrypted transport every 149 time it connected to any authoritative server would inherently be 150 slower than one that did not. Similarly, a recursive resolver that 151 made an external lookup of what secure transports every authoritative 152 server supports each time it connected would also inherently be 153 slower than one that did not. Recursive resolver operators desire to 154 give answers to stub resolvers as quickly as possible, so neither of 155 these two strategies would make sense. 157 Instead, recursive resolvers following the method described in this 158 document MUST keep a cache of what they know about how DNS-over-TLS 159 is supported by authoritative servers. This is called a "transport 160 cache" in this document. 162 This document only DNS-over-TLS for encryption. Thus, a recursive 163 resolver can test whether an authoritative server supports DNS-over- 164 TLS by attempting to open a TLS session on port 853, and can cached 165 information that it discovers in its transport cache. Future 166 specifications might describe how to use other secure DNS transports 167 for encryption, and thus would also have to describe ways that a 168 resolver could discover whether an authoritative server supports 169 them. 171 The recursive resolver MUST look in its transport cache before 172 sending DNS queries to an authoritative server. If there is no entry 173 for an authoritative server in its transport cache, the recursive 174 resolver MUST use plain, unencrypted DNS over port 53. 176 This document explicitly does not mandate the contents of the 177 transport cache. Different recursive resolver implementers are 178 likely to have different cache structures based on many factors, such 179 as research results, active measurements, secure protocols supported, 180 and customer feedback, There will likely be different strategies for 181 the time-to-live for parts of the transport cache, such as how often 182 to refresh the data in the cache, how often to refresh negative data, 183 whether to prioritize refreshing certain zones or types of zones, and 184 so on. 186 This document also explicitly doesn't mandate how the strategy for 187 filling transport caches. Some strategies might include one or more 188 of "try to send a refresh query over DoT", "use external data", 189 "trust a third-party service for filling the transport cache", and so 190 on. 192 There are no interoperability issues with different implementors 193 making different choices for the contents and fill strategies of 194 their transport caches, and having many different options available 195 will likely cause the cache designs to get better over time. 197 4. Security Considerations 199 The method described in this document explicitly allows a stub to 200 perform DNS communications over traditional unencrypted, 201 unauthenticated DNS on port 53. 203 The method described in this document explicitly allows a stub to 204 choose to allow unauthenticated TLS. In this case, the resulting 205 communication will be susceptible to obvious and well-understood 206 attacks from an attacker in the path of the communications. 208 5. Acknowledgements 210 Puneet Sood contributed many ideas to early drafts of this document. 212 6. References 214 6.1. Normative References 216 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 217 Requirement Levels", BCP 14, RFC 2119, 218 DOI 10.17487/RFC2119, March 1997, 219 . 221 [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection 222 Most of the Time", RFC 7435, DOI 10.17487/RFC7435, 223 December 2014, . 225 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 226 and P. Hoffman, "Specification for DNS over Transport 227 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 228 2016, . 230 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 231 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 232 May 2017, . 234 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 235 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 236 . 238 [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 239 Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, 240 January 2019, . 242 6.2. Informative References 244 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication 245 of Named Entities (DANE) Transport Layer Security (TLS) 246 Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 247 2012, . 249 Author's Address 251 Paul Hoffman 252 ICANN 254 Email: paul.hoffman@icann.org