Network Working Group S. Weiler Internet-Draft SPARTA, Inc. Updates: 4033, 4034, 4035, 5155 D. Blacka (if approved) VeriSign, Inc. Intended status: Standards Track September 5, 2009 Expires: March 9, 2010 Clarifications and Implementation Notes for DNSSECbis draft-ietf-dnsext-dnssec-bis-updates-09 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 9, 2010. Copyright Notice Copyright (c) 2009 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document is a collection of technical clarifications to the Weiler & Blacka Expires March 9, 2010 [Page 1]
Internet-Draft DNSSECbis Implementation Notes September 2009 DNSSECbis document set. It is meant to serve as a resource to implementors as well as a repository of DNSSECbis errata. Table of Contents 1. Introduction and Terminology . . . . . . . . . . . . . . . . . 3 1.1. Structure of this Document . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Important Additions to DNSSSECbis . . . . . . . . . . . . . . 3 2.1. NSEC3 Support . . . . . . . . . . . . . . . . . . . . . . 3 2.2. SHA-256 Support . . . . . . . . . . . . . . . . . . . . . 3 3. Security Concerns . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Clarifications on Non-Existence Proofs . . . . . . . . . . 4 3.2. Validating Responses to an ANY Query . . . . . . . . . . . 4 3.3. Check for CNAME . . . . . . . . . . . . . . . . . . . . . 5 3.4. Insecure Delegation Proofs . . . . . . . . . . . . . . . . 5 4. Interoperability Concerns . . . . . . . . . . . . . . . . . . 5 4.1. Errors in Canonical Form Type Code List . . . . . . . . . 5 4.2. Unknown DS Message Digest Algorithms . . . . . . . . . . . 5 4.3. Private Algorithms . . . . . . . . . . . . . . . . . . . . 6 4.4. Caution About Local Policy and Multiple RRSIGs . . . . . . 7 4.5. Key Tag Calculation . . . . . . . . . . . . . . . . . . . 7 4.6. Setting the DO Bit on Replies . . . . . . . . . . . . . . 7 4.7. Setting the AD bit on Replies . . . . . . . . . . . . . . 7 4.8. Setting the CD bit on Requests . . . . . . . . . . . . . . 8 4.9. Nested Trust Anchors . . . . . . . . . . . . . . . . . . . 8 5. Minor Corrections and Clarifications . . . . . . . . . . . . . 8 5.1. Finding Zone Cuts . . . . . . . . . . . . . . . . . . . . 8 5.2. Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 9 5.3. Errors in Examples . . . . . . . . . . . . . . . . . . . . 9 5.4. Errors in RFC 5155 . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . . 11 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Weiler & Blacka Expires March 9, 2010 [Page 2]
Internet-Draft DNSSECbis Implementation Notes September 2009 1. Introduction and Terminology This document lists some additions, clarifications and corrections to the core DNSSECbis specification, as originally described in [RFC4033], [RFC4034], and [RFC4035]. It is intended to serve as a resource for implementors and as a repository of items that need to be addressed when advancing the DNSSECbis documents from Proposed Standard to Draft Standard. 1.1. Structure of this Document The clarifications to DNSSECbis are sorted according to their importance, starting with ones which could, if ignored, lead to security problems and progressing down to clarifications that are expected to have little operational impact. 1.2. 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 [RFC2119]. 2. Important Additions to DNSSSECbis This section updates the set of core DNSSEC protocol documents originally specified in Section 10 of [RFC4033]. 2.1. NSEC3 Support [RFC5155] describes the use and behavior of the NSEC3 and NSEC3PARAM records for hashed denial of existence. Validator implementations are strongly encouraged to include support for NSEC3 because a number of highly visible zones are expected to use it. Validators that do not support validation of responses using NSEC3 will likely be hampered in validating large portions of the DNS space. [RFC5155] should be considered part of the DNS Security Document Family as described by [RFC4033], Section 10. 2.2. SHA-256 Support [RFC4509] describes the use of SHA-256 as a digest algorithm for use with Delegation Signer (DS) RRs. [I-D.ietf-dnsext-dnssec-rsasha256] describes the use of the RSASHA256 algorithm for use in DNSKEY and RRSIG RRs. Validator implementations are strongly encouraged to include support for this algorithm for DS, DNSKEY, and RRSIG records. Weiler & Blacka Expires March 9, 2010 [Page 3]
Internet-Draft DNSSECbis Implementation Notes September 2009 Both [RFC4509] and [I-D.ietf-dnsext-dnssec-rsasha256] should also be considered part of the DNS Security Document Family as described by [RFC4033], Section 10. 3. Security Concerns This section provides clarifications that, if overlooked, could lead to security issues. 3.1. Clarifications on Non-Existence Proofs [RFC4035] Section 5.4 under-specifies the algorithm for checking non- existence proofs. In particular, the algorithm as presented would incorrectly allow an NSEC or NSEC3 RR from an ancestor zone to prove the non-existence of RRs in the child zone. An "ancestor delegation" NSEC RR (or NSEC3 RR) is one with: o the NS bit set, o the SOA bit clear, and o a signer field that is shorter than the owner name of the NSEC RR, or the original owner name for the NSEC3 RR. Ancestor delegation NSEC or NSEC3 RRs MUST NOT be used to assume non- existence of any RRs below that zone cut, which include all RRs at that (original) owner name other than DS RRs, and all RRs below that owner name regardless of type. Similarly, the algorithm would also allow an NSEC RR at the same owner name as a DNAME RR, or an NSEC3 RR at the same original owner name as a DNAME, to prove the non-existence of names beneath that DNAME. An NSEC or NSEC3 RR with the DNAME bit set MUST NOT be used to assume the non-existence of any subdomain of that NSEC/NSEC3 RR's (original) owner name. 3.2. Validating Responses to an ANY Query [RFC4035] does not address how to validate responses when QTYPE=*. As described in Section 6.2.2 of [RFC1034], a proper response to QTYPE=* may include a subset of the RRsets at a given name. That is, it is not necessary to include all RRsets at the QNAME in the response. When validating a response to QTYPE=*, all received RRsets that match QNAME and QCLASS MUST be validated. If any of those RRsets fail validation, the answer is considered Bogus. If there are no RRsets matching QNAME and QCLASS, that fact MUST be validated according to Weiler & Blacka Expires March 9, 2010 [Page 4]
Internet-Draft DNSSECbis Implementation Notes September 2009 the rules in [RFC4035] Section 5.4 (as clarified in this document). To be clear, a validator must not expect to receive all records at the QNAME in response to QTYPE=*. 3.3. Check for CNAME Section 5 of [RFC4035] says little about validating responses based on (or that should be based on) CNAMEs. When validating a NOERROR/ NODATA response, validators MUST check the CNAME bit in the matching NSEC or NSEC3 RR's type bitmap in addition to the bit for the query type. Without this check, an attacker could successfully transform a positive CNAME response into a NOERROR/NODATA response. 3.4. Insecure Delegation Proofs [RFC4035] Section 5.2 specifies that a validator, when proving a delegation is not secure, needs to check for the absence of the DS and SOA bits in the NSEC (or NSEC3) type bitmap. The validator also needs to check for the presence of the NS bit in the matching NSEC (or NSEC3) RR (proving that there is, indeed, a delegation), or alternately make sure that the delegation is covered by an NSEC3 RR with the Opt-Out flag set. If this is not checked, spoofed unsigned delegations might be used to claim that an existing signed record is not signed. 4. Interoperability Concerns 4.1. Errors in Canonical Form Type Code List When canonicalizing DNS names, DNS names in the RDATA section of NSEC and RRSIG resource records are not downcased. [RFC4034] Section 6.2 item 3 has a list of resource record types for which DNS names in the RDATA are downcased for purposes of DNSSEC canonical form (for both ordering and signing). That list erroneously contains NSEC and RRSIG. According to [RFC3755], DNS names in the RDATA of NSEC and RRSIG should not be downcased. The same section also erroneously lists HINFO, and twice at that. Since HINFO records contain no domain names, they are not subject to downcasing. 4.2. Unknown DS Message Digest Algorithms Section 5.2 of [RFC4035] includes rules for how to handle delegations to zones that are signed with entirely unsupported public key algorithms, as indicated by the key algorithms shown in those zone's Weiler & Blacka Expires March 9, 2010 [Page 5]
Internet-Draft DNSSECbis Implementation Notes September 2009 DS RRsets. It does not explicitly address how to handle DS records that use unsupported message digest algorithms. In brief, DS records using unknown or unsupported message digest algorithms MUST be treated the same way as DS records referring to DNSKEY RRs of unknown or unsupported public key algorithms. The existing text says: If the validator does not support any of the algorithms listed in an authenticated DS RRset, then the resolver has no supported authentication path leading from the parent to the child. The resolver should treat this case as it would the case of an authenticated NSEC RRset proving that no DS RRset exists, as described above. To paraphrase the above, when determining the security status of a zone, a validator disregards any DS records listing unknown or unsupported algorithms. If none are left, the zone is treated as if it were unsigned. Modified to consider DS message digest algorithms, a validator also disregards any DS records using unknown or unsupported message digest algorithms. 4.3. Private Algorithms As discussed above, section 5.2 of [RFC4035] requires that validators make decisions about the security status of zones based on the public key algorithms shown in the DS records for those zones. In the case of private algorithms, as described in [RFC4034] Appendix A.1.1, the eight-bit algorithm field in the DS RR is not conclusive about what algorithm(s) is actually in use. If no private algorithms appear in the DS set or if any supported algorithm appears in the DS set, no special processing will be needed. In the remaining cases, the security status of the zone depends on whether or not the resolver supports any of the private algorithms in use (provided that these DS records use supported hash functions, as discussed in Section 4.2). In these cases, the resolver MUST retrieve the corresponding DNSKEY for each private algorithm DS record and examine the public key field to determine the algorithm in use. The security-aware resolver MUST ensure that the hash of the DNSKEY RR's owner name and RDATA matches the digest in the DS RR. If they do not match, and no other DS establishes that the zone is secure, the referral should be considered Bogus data, as discussed in [RFC4035]. This clarification facilitates the broader use of private algorithms, Weiler & Blacka Expires March 9, 2010 [Page 6]
Internet-Draft DNSSECbis Implementation Notes September 2009 as suggested by [RFC4955]. 4.4. Caution About Local Policy and Multiple RRSIGs When multiple RRSIGs cover a given RRset, [RFC4035] Section 5.3.3 suggests that "the local resolver security policy determines whether the resolver also has to test these RRSIG RRs and how to resolve conflicts if these RRSIG RRs lead to differing results." In most cases, a resolver would be well advised to accept any valid RRSIG as sufficient. If the first RRSIG tested fails validation, a resolver would be well advised to try others, giving a successful validation result if any can be validated and giving a failure only if all RRSIGs fail validation. If a resolver adopts a more restrictive policy, there's a danger that properly-signed data might unnecessarily fail validation, perhaps because of cache timing issues. Furthermore, certain zone management techniques, like the Double Signature Zone-signing Key Rollover method described in section 4.2.1.2 of [RFC4641] might not work reliably. 4.5. Key Tag Calculation [RFC4034] Appendix B.1 incorrectly defines the Key Tag field calculation for algorithm 1. It correctly says that the Key Tag is the most significant 16 of the least significant 24 bits of the public key modulus. However, [RFC4034] then goes on to incorrectly say that this is 4th to last and 3rd to last octets of the public key modulus. It is, in fact, the 3rd to last and 2nd to last octets. 4.6. Setting the DO Bit on Replies [RFC4035] does not provide any instructions to servers as to how to set the DO bit. Some authoritative server implementations have chosen to copy the DO bit settings from the incoming query to the outgoing response. Others have chosen to never set the DO bit in responses. Either behavior is permitted. To be clear, in replies to queries with the DO-bit set servers may or may not set the DO bit. 4.7. Setting the AD bit on Replies Section 3.2.3 of [RFC4035] describes under which conditions a validating resolver should set or clear the AD bit in a response. In order to protect legacy stub resolvers and middleboxes, validating resolvers SHOULD only set the AD bit when a response both meets the conditions listed in RFC 4035, section 3.2.3, and the request contained either a set DO bit or a set AD bit. Weiler & Blacka Expires March 9, 2010 [Page 7]
Internet-Draft DNSSECbis Implementation Notes September 2009 Note that the use of the AD bit in the query was previously undefined. This document defines it as a signal indicating that the requester understands and is interested in the value of the AD bit in the response. This allows a requestor to indicate that it understands the AD bit without also requesting DNSSEC data via the DO bit. 4.8. Setting the CD bit on Requests When processing a request with the CD bit set, the resolver MUST set the CD bit on its upstream queries. 4.9. Nested Trust Anchors A DNSSEC validator may be configured such that, for a given response, more than one trust anchor could be used to validate the chain of trust to the response zone. For example, imagine a validator configured with trust anchors for "example." and "zone.example." When the validator is asked to validate a response to "www.sub.zone.example.", either trust anchor could apply. When presented with this situation, DNSSEC validators SHOULD try all applicable trust anchors until one succeeds. There are some scenarios where different behaviors, such as choosing the trust anchor closest to the QNAME of the response, may be desired. A DNSSEC validator MAY enable such behaviors as configurable overrides. 5. Minor Corrections and Clarifications 5.1. Finding Zone Cuts Appendix C.8 of [RFC4035] discusses sending DS queries to the servers for a parent zone. To do that, a resolver may first need to apply special rules to discover what those servers are. As explained in Section 3.1.4.1 of [RFC4035], security-aware name servers need to apply special processing rules to handle the DS RR, and in some situations the resolver may also need to apply special rules to locate the name servers for the parent zone if the resolver does not already have the parent's NS RRset. Section 4.2 of [RFC4035] specifies a mechanism for doing that. Weiler & Blacka Expires March 9, 2010 [Page 8]
Internet-Draft DNSSECbis Implementation Notes September 2009 5.2. Clarifications on DNSKEY Usage Questions of the form "can I use a different DNSKEY for signing this RRset" have occasionally arisen. The short answer is "yes, absolutely". You can even use a different DNSKEY for each RRset in a zone, subject only to practical limits on the size of the DNSKEY RRset. However, be aware that there is no way to tell resolvers what a particularly DNSKEY is supposed to be used for -- any DNSKEY in the zone's signed DNSKEY RRset may be used to authenticate any RRset in the zone. For example, if a weaker or less trusted DNSKEY is being used to authenticate NSEC RRsets or all dynamically updated records, that same DNSKEY can also be used to sign any other RRsets from the zone. Furthermore, note that the SEP bit setting has no effect on how a DNSKEY may be used -- the validation process is specifically prohibited from using that bit by [RFC4034] section 2.1.2. It is possible to use a DNSKEY without the SEP bit set as the sole secure entry point to the zone, yet use a DNSKEY with the SEP bit set to sign all RRsets in the zone (other than the DNSKEY RRset). It's also possible to use a single DNSKEY, with or without the SEP bit set, to sign the entire zone, including the DNSKEY RRset itself. 5.3. Errors in Examples The text in [RFC4035] Section C.1 refers to the examples in B.1 as "x.w.example.com" while B.1 uses "x.w.example". This is painfully obvious in the second paragraph where it states that the RRSIG labels field value of 3 indicates that the answer was not the result of wildcard expansion. This is true for "x.w.example" but not for "x.w.example.com", which of course has a label count of 4 (antithetically, a label count of 3 would imply the answer was the result of a wildcard expansion). The first paragraph of [RFC4035] Section C.6 also has a minor error: the reference to "a.z.w.w.example" should instead be "a.z.w.example", as in the previous line. 5.4. Errors in RFC 5155 A NSEC3 record that matches an Empty Non-Terminal effectively has no type associated with it. This NSEC3 record has an empty type bit map. Section 3.2.1 of [RFC5155] contains the statement: Blocks with no types present MUST NOT be included. However, the same section contains a regular expression: Weiler & Blacka Expires March 9, 2010 [Page 9]
Internet-Draft DNSSECbis Implementation Notes September 2009 Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )+ The plus sign in the regular expression indicates that there is one or more of the preceding element. This means that there must be at least one window block. If this window block has no types, it contradicts with the first statement. Therefore, the correct text in RFC 5155 3.2.1 should be: Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )* 6. IANA Considerations This document specifies no IANA Actions. 7. Security Considerations This document adds two cryptographic features to the core DNSSEC protocol. Additionally, it addresses some ambiguities and omissions in the core DNSSEC documents that, if not recognized and addressed in implementations, could lead to security failures. In particular, the validation algorithm clarifications in Section 3 are critical for preserving the security properties DNSSEC offers. Furthermore, failure to address some of the interoperability concerns in Section 4 could limit the ability to later change or expand DNSSEC, including adding new algorithms. 8. References 8.1. Normative References [I-D.ietf-dnsext-dnssec-rsasha256] Jansen, J., "Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC", draft-ietf-dnsext-dnssec-rsasha256-14 (work in progress), June 2009. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", RFC 1034, STD 13, November 1987. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, BCP 14, March 1997. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. Weiler & Blacka Expires March 9, 2010 [Page 10]
Internet-Draft DNSSECbis Implementation Notes September 2009 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005. [RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs)", RFC 4509, May 2006. [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, March 2008. 8.2. Informative References [RFC3755] Weiler, S., "Legacy Resolver Compatibility for Delegation Signer (DS)", RFC 3755, May 2004. [RFC4641] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices", RFC 4641, September 2006. [RFC4955] Blacka, D., "DNS Security (DNSSEC) Experiments", RFC 4955, July 2007. Appendix A. Acknowledgments The editors would like the thank Rob Austein for his previous work as an editor of this document. The editors are extremely grateful to those who, in addition to finding errors and omissions in the DNSSECbis document set, have provided text suitable for inclusion in this document. The lack of specificity about handling private algorithms, as described in Section 4.3, and the lack of specificity in handling ANY queries, as described in Section 3.2, were discovered by David Blacka. The error in algorithm 1 key tag calculation, as described in Section 4.5, was found by Abhijit Hayatnagarkar. Donald Eastlake contributed text for Section 4.5. The bug relating to delegation NSEC RR's in Section 3.1 was found by Roy Badami. Roy Arends found the related problem with DNAME. Weiler & Blacka Expires March 9, 2010 [Page 11]
Internet-Draft DNSSECbis Implementation Notes September 2009 The errors in the [RFC4035] examples were found by Roy Arends, who also contributed text for Section 5.3 of this document. The editors would like to thank Ed Lewis, Danny Mayer, Olafur Gudmundsson, Suzanne Woolf, and Scott Rose for their substantive comments on the text of this document. Authors' Addresses Samuel Weiler SPARTA, Inc. 7110 Samuel Morse Drive Columbia, Maryland 21046 US Email: weiler@tislabs.com David Blacka VeriSign, Inc. 21345 Ridgetop Circle Dulles, VA 20166 US Email: davidb@verisign.com Weiler & Blacka Expires March 9, 2010 [Page 12]
