Clarifications and Implementation Notes for DNSSECbis
draft-ietf-dnsext-dnssec-bis-updates-09

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Abstract

This document is a collection of technical clarifications to the 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
    1.1.  Structure of this Document
    1.2.  Terminology
2.  Important Additions to DNSSSECbis
    2.1.  NSEC3 Support
    2.2.  SHA-256 Support
3.  Security Concerns
    3.1.  Clarifications on Non-Existence Proofs
    3.2.  Validating Responses to an ANY Query
    3.3.  Check for CNAME
    3.4.  Insecure Delegation Proofs
4.  Interoperability Concerns
    4.1.  Errors in Canonical Form Type Code List
    4.2.  Unknown DS Message Digest Algorithms
    4.3.  Private Algorithms
    4.4.  Caution About Local Policy and Multiple RRSIGs
    4.5.  Key Tag Calculation
    4.6.  Setting the DO Bit on Replies
    4.7.  Setting the AD bit on Replies
    4.8.  Setting the CD bit on Requests
    4.9.  Nested Trust Anchors
5.  Minor Corrections and Clarifications
    5.1.  Finding Zone Cuts
    5.2.  Clarifications on DNSKEY Usage
    5.3.  Errors in Examples
    5.4.  Errors in RFC 5155
6.  IANA Considerations
7.  Security Considerations
8.  References
    8.1.  Normative References
    8.2.  Informative References
Appendix A.  Acknowledgments
§  Authors' Addresses

1.  Introduction and Terminology

This document lists some additions, clarifications and corrections to the core DNSSECbis specification, as originally described in [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.), [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.), and [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Important Additions to DNSSSECbis

This section updates the set of core DNSSEC protocol documents originally specified in Section 10 of [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.).

2.1.  NSEC3 Support

[RFC5155] (Laurie, B., Sisson, G., Arends, R., and D. Blacka, “DNS Security (DNSSEC) Hashed Authenticated Denial of Existence,” March 2008.) 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] (Laurie, B., Sisson, G., Arends, R., and D. Blacka, “DNS Security (DNSSEC) Hashed Authenticated Denial of Existence,” March 2008.) should be considered part of the DNS Security Document Family as described by [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.), Section 10.

2.2.  SHA-256 Support

[RFC4509] (Hardaker, W., “Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs),” May 2006.) describes the use of SHA-256 as a digest algorithm for use with Delegation Signer (DS) RRs. [I‑D.ietf‑dnsext‑dnssec‑rsasha256] (Jansen, J., “Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC,” June 2009.) 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.

Both [RFC4509] (Hardaker, W., “Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs),” May 2006.) and [I‑D.ietf‑dnsext‑dnssec‑rsasha256] (Jansen, J., “Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC,” June 2009.) should also be considered part of the DNS Security Document Family as described by [RFC4033] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.), 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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:

• the NS bit set,
• the SOA bit clear, and
• 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) does not address how to validate responses when QTYPE=*. As described in Section 6.2.2 of [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.), 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 the rules in [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) 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] (Weiler, S., “Legacy Resolver Compatibility for Delegation Signer (DS),” May 2004.), 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) 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 (Unknown DS Message Digest Algorithms)). 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.).

This clarification facilitates the broader use of private algorithms, as suggested by [RFC4955] (Blacka, D., “DNS Security (DNSSEC) Experiments,” July 2007.).

4.4.  Caution About Local Policy and Multiple RRSIGs

When multiple RRSIGs cover a given RRset, [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Kolkman, O. and R. Gieben, “DNSSEC Operational Practices,” September 2006.) might not work reliably.

4.5.  Key Tag Calculation

[RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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.

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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.), 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) specifies a mechanism for doing that.

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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) 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] (Laurie, B., Sisson, G., Arends, R., and D. Blacka, “DNS Security (DNSSEC) Hashed Authenticated Denial of Existence,” March 2008.) contains the statement:

Blocks with no types present MUST NOT be included.

However, the same section contains a regular expression:

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 (Security Concerns) are critical for preserving the security properties DNSSEC offers. Furthermore, failure to address some of the interoperability concerns in Section 4 (Interoperability Concerns) could limit the ability to later change or expand DNSSEC, including adding new algorithms.

8.  References

8.1. Normative References

8.2. Informative References

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 (Private Algorithms), and the lack of specificity in handling ANY queries, as described in Section 3.2 (Validating Responses to an ANY Query), were discovered by David Blacka.

The error in algorithm 1 key tag calculation, as described in Section 4.5 (Key Tag Calculation), was found by Abhijit Hayatnagarkar. Donald Eastlake contributed text for Section 4.5 (Key Tag Calculation).

The bug relating to delegation NSEC RR's in Section 3.1 (Clarifications on Non-Existence Proofs) was found by Roy Badami. Roy Arends found the related problem with DNAME.

The errors in the [RFC4035] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Protocol Modifications for the DNS Security Extensions,” March 2005.) examples were found by Roy Arends, who also contributed text for Section 5.3 (Errors in Examples) 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