DNSOP G. Huston Internet-Draft J. Damas Intended status: Standards Track APNIC Expires: June 14, 2018 W. Kumari Google December 11, 2017 A Sentinel for Detecting Trusted Keys in DNSSEC draft-ietf-dnsop-kskroll-sentinel-00.txt Abstract The DNS Security Extensions (DNSSEC) were developed to provide origin authentication and integrity protection for DNS data by using digital signatures. These digital signatures can be verified by building a chain of trust starting from a trust anchor and proceeding down to a particular node in the DNS. This document specifies a mechanism that will allow an end user to determine the trusted key state of the resolvers that handle the user's DNS queries. 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 June 14, 2018. Copyright Notice Copyright (c) 2017 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 Huston, et al. Expires June 14, 2018 [Page 1] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Sentinel Mechanism . . . . . . . . . . . . . . . . . . . . . 3 3. Sentinel Processing . . . . . . . . . . . . . . . . . . . . . 4 4. Sentinel Test Result Considerations . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 8.2. Informative References . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and [RFC4035] were developed to provide origin authentication and integrity protection for DNS data by using digital signatures. DNSSEC uses Key Tags to efficiently match signatures to the keys from which they are generated. The Key Tag is a 16-bit value computed from the RDATA portion of a DNSKEY RR using a formula not unlike a ones-complement checksum. RRSIG RRs contain a Key Tag field whose value is equal to the Key Tag of the DNSKEY RR that validates the signature. This document specifies how validating resolvers can respond to certain queries in a manner that allows a querier to deduce whether a particular key has been loaded into that resolver's trusted key store. In particular, this response mechanism can be used to determine whether a certain Root Zone KSK is ready to be used as a trusted key within the context of a key roll by this resolver. This new mechanism is OPTIONAL to implement and use, although for reasons of supporting broad-based measurement techniques, it is strongly preferred if configurations of DNSSEC-validating resolvers enabled this mechanism by default, allowing for local configuration directives to disable this mechanism if desired. Huston, et al. Expires June 14, 2018 [Page 2] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. 2. Sentinel Mechanism DNSSEC-Validating resolvers that implement this mechanism MUST be performing validation of responses in accordance with the DNSSEC response validation specification [RFC4035]. This sentinel mechanism makes use of 2 special labels, "_is-ta-." (intended to be used in a query where the response can answer the question: Is this the key tag a trust anchor which the validating DNS resolver is currently trusting?) and "_not-ta-." (intended to be used in a query where the response can answer the question: Is this the key tag of a key that is NOT in the resolver's current trust store?). The use of the positive question and its inverse allows for queries to detect whether resolvers support this sentinel mechanism. If the outcome of the DNSSEC validation process on the response RRset indicates that the response RRset is authentic, and if the left-most label of the original query name matches the template "_is-ta-.", then the following rule should be applied to the response: If the resolver has placed a Root Zone Key Signing Key with tag index value matching the value specified in the query into the local resolver's store of trusted keys, then the resolver should return a response indicating that the response contains authenticated data according to section 5.8 of [RFC6840]. Otherwise, the resolver MUST return RCODE 2 (server failure). Note that the is specified in the DNS label using hexadecimal notation. If the outcome of the DNSSEC validation process aplied to the response RRset indicates that the response RRset is authentic, and if the left-most label of the original query name matches the template "_not-ta-.", then the following rule should be applied to the response: If the resolver has not placed a Root Zone Key Signing Key with tag index value matching the value specified in the query into the local resolver's store of trusted keys, then the resolver should return a response indicating that the response contains authenticated data according to section 5.8 of [RFC6840]. Otherwise, the resolver MUST return RCODE 2 (server failure). Note that the is specified in the DNS label using hexadecimal notation. In all other cases the resolver MUST NOT alter the outcome of the DNS response validation process. Huston, et al. Expires June 14, 2018 [Page 3] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 This mechanism is to be applied only by resolvers that are performing DNSSEC validation, and applies only to RRset responses to an A or AAAA query (Query Type value 1 or 28) where the resolver has authenticated the response RRset according to the DNSSEC validation process and where the query name contains either of the labels described in this section as its left-most label. In this case, the resolver is to perform an additional test following the conventional validation function, as described in this section. The result of this additional test determines whether the resolver will alter its response that would've indicated that the RRset is authentic to a response that indicates DNSSEC validation failure via the use of RCODE 2. 3. Sentinel Processing This proposed test that uses the sentinel detection mechanism described in this document is based on the use of three DNS names that have three distinct DNS resolution behaviours. The test is intended to allow a user to determine the state of their DNS resolution system, and, in particular, whether or not they are using validating DNS resolvers that have picked up an incoming trust anchor as a trusted key in a root zone KSK roll scenario. The name format can be defined in a number of ways, and no name form is intrinsically better than any other in terms of the test itself. The critical aspect of the DNS names used in any such test is that they contain the specified label for either the positive and negative test as the left-most label in the query name. The sentinel detection process is envisaged to use a test with three query names: a. a query name containing the left-most label "_is-ta-.". This corresponds to a a validly-signed RRset in the zone, so that responses associated with queried names in this zone can be authenticated by a DNSSEC-validating resolver. Any validly-signed DNS zone can be used for this test. b. a query name containing the left-most label "_not-ta-.". This is also a validly-signed name. Any validly- signed DNS zone can be used for this test. c. a third query name that is signed with a DNSSEC signature that cannot be validated (i.e. the corresponding RRset is not signed with a valid RRSIG record). The responses received from queries to resolve each of these names would allow us to infer a trust key state of the resolution Huston, et al. Expires June 14, 2018 [Page 4] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 environment. To describe this process of classification, we can classify resolvers into four distinct behavior types, for which we will use the labels: "Vnew", "Vold", "Vleg", and "nonV". These labels correspond to resolver behaviour types as follows: o Vnew: A DNSSEC-Validating resolver that is configured to implement this mechanism has loaded the nominated key into its local trusted key store will respond with an A or AAAA RRset response for "_is- ta" queries, SERVFAIL for "_not-ta" queries and SERVFAIL for the invalidly signed name queries. o Vold: A DNSSEC-Validating resolver that is configured to implement this mechanism that has not loaded the nominated key into its local trusted key store will respond with an SERVFAIL for "_is-ta" queries, an A or AAAA RRset response for "_not-ta" queries and SERVFAIL for the invalidly signed name queries. o Vleg: A DNSSEC-Validating resolver that does not implement this mechanism will respond with an A or AAAA RRSET response for "_is- ta", an A record response for "_not-ta" and SERVFAIL for the invalid name. o nonV: A non-DNSSEC-Validating resolver will respond with an A record response for "_is-ta", an A record response for "_not-ta" and an A record response for the invalid name. Given the clear delineation amongst these three cases, if a client directs these three queries to a simple resolver, the variation in response to the three queries should allow the client to determine the category of the resolver, and if it supports this mechanism, whether or not it has loaded a particular key into its local trusted key stash. +-------------+----------+-----------+------------+ | Type\Query | _is-ta | _not-ta | invalid | +-------------+----------+-----------+------------+ | Vnew | A | SERVFAIL | SERVFAIL | | Vold | SERVFAIL | A | SERVFAIL | | Vleg | A | A | SERVFAIL | | nonV | A | A | A | +-------------+----------+-----------+------------+ A "Vnew" response pattern says that the nominated key is trusted by the resolver and has been loaded into its local trusted key stash. A "Vold" response pattern says that the nominated key is not yet trusted by the resolver in its own right. A "Vleg" response pattern Huston, et al. Expires June 14, 2018 [Page 5] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 is indeterminate, and a "nonV" response pattern indicates that the resolver does not perform DNSSEC validation. 4. Sentinel Test Result Considerations The description in the previous section describes a simple situation where the test queries were being passed to a single recursive resolver that directly queried authoritative name servers, including the root servers. There is also the common case where the end client is configured to use multiple resolvers. In these cases the SERVFAIL responses from one resolver will prompt the end client to repeat the query against one of the other configured resolvers. If any of the client's resolvers are non-validating resolvers, the tests will result in the client reporting that it has a non- validating DNS environment ("nonV"), which is effectively the case. If all of the client resolvers are DNSSEC-validating resolvers, but some do not support this trusted key mechanism, then the result will be indeterminate with respect to trusted key status ("Vleg"). Simlarly, if all the client's resolvers support this mechanism, but some have loaded the key into the trusted key stash and some have not, then the result is indeterminate ("Vleg"). There is also the common case of a recursive resolver using a forwarder. If the resolver is non-validating, and it has a single forwarder clause, then the resolver will presumably mirror the capabilities of the forwarder target resolver. If this non-validating resolver it has multiple forwarders, then the above considerations will apply. If the validating resolver has a forwarding configuration, and uses the CD flag on all forwarded queries, then this resolver is acting in a manner that is identical to a standalone resolver. The same consideration applies if any one one of the forwarder targets is a non-validating resolver. Similarly, if all the forwarder targets do not apply this trusted key mechanism, the same considerations apply. A more complex case is where the following conditions all hold: both the validating resolver and the forwarder target resolver support this trusted key sentinel mechanism, and the local resolver's queries do not have the CD bit set, and Huston, et al. Expires June 14, 2018 [Page 6] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 the trusted key state differs between the forwarding resolver and the forwarder target resolver then either the outcome is indeterminate validating ("Vleg"), or a case of mixed signals (SERVFAIL in all three responses), which is similarly an indeterminate response with respect to the trusted key state. 5. Security Considerations This document describes a mechanism to allow users to determine the trust state of root zone key signing keys in the DNS resolution system that they use. The mechanism does not require resolvers to set otherwise unauthenticated responses to be marked as authenticated, and does not alter the security properties of DNSSEC with respect to the interpretation of the authenticity of responses that are so marked. The mechanism does not require any further significant processing of DNS responses, and queries of the form described in this document do not impose any additional load that could be exploited in an attack over the the normal DNSSEC validation processing load. 6. IANA Considerations [Note to IANA, to be removed prior to publication: there are no IANA considerations stated in this version of the document.] 7. Acknowledgements This document has borrowed extensively from [RFC8145] for the introductory text, and the authors would like to acknowledge and thank the authors of that document both for some text excerpts and for the more general stimulation of thoughts about monitoring the progress of a roll of the Key Signing Key of the Root Zone of the DNS. 8. References 8.1. Normative References [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005, . Huston, et al. Expires June 14, 2018 [Page 7] Internet-Draft DNSSEC Trusted Key Sentinel December 2017 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005, . [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, . [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and Implementation Notes for DNS Security (DNSSEC)", RFC 6840, DOI 10.17487/RFC6840, February 2013, . 8.2. Informative References [RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust Anchor Knowledge in DNS Security Extensions (DNSSEC)", RFC 8145, DOI 10.17487/RFC8145, April 2017, . Authors' Addresses Geoff Huston Email: gih@apnic.net URI: http://www.apnic.net Joao Silva Damas Email: joao@apnic.net URI: http://www.apnic.net Warren Kumari Email: warren@kumari.net Huston, et al. Expires June 14, 2018 [Page 8]