Definition and Use of DNSSEC Negative Trust Anchors
draft-livingood-negative-trust-anchors-07

Abstract

DNS Security Extensions (DNSSEC) is now entering widespread deployment. However, domain signing tools and processes are not yet as mature and reliable as is the case for non-DNSSEC-related domain administration tools and processes. One potential technique to mitigate this is to use a Negative Trust Anchor, which is defined in this document.

This document discusses Trust Anchors for DNSSEC and defines a Negative Trust Anchor, which is potentially useful during the transition to ubiquitous DNSSEC deployment. These are configured locally on a particular instance of a validating DNS recursive resolver and can shield end users of such a resolver from the DNSSEC-related authoritative name server operational errors that appear to be somewhat typical during the transition to ubiquitous DNSSEC deployment. Negative Trust Anchors are intended to be temporary, and should not be distributed by IANA or any other organization outside of the administrative boundary of the organization locally implementing a Negative Trust Anchor. Finally, Negative Trust Anchors pertain only to DNSSEC and not to Public Key Infrastructures (PKI) such ad X.509.

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Table of Contents

• 1. Introduction
• 2. Definition of a Negative Trust Anchor
• 3. Limited Time and Scope of Use
• 4. Domain Validation Failures
• 5. End User Reaction
• 6. Switching to a Non-Validating Resolver is Not Recommended
• 7. Responsibility for Failures
• 8. Use of a Negative Trust Anchor
• 9. Managing Negative Trust Anchors
• 10. Removal of a Negative Trust Anchor
• 11. Comparison to Other DNS Misconfigurations
• 12. Intentionally Broken Domains
• 13. Other Considerations
• 13.1. Security Considerations
• 13.2. Privacy Considerations
• 13.3. IANA Considerations
• 14. Acknowledgements
• 15. References
• 15.1. Normative References
• 15.2. Informative References
• Appendix A. Document Change Log
• Appendix B. Open Issues
• Authors' Addresses

1. Introduction

The Domain Name System (DNS), DNS Security Extensions (DNSSEC), and related operational practices are defined extensively [RFC1034] [RFC1035] [RFC4033] [RFC4034] [RFC4035] [RFC4398] [RFC4509] [RFC6781] [RFC5155].

This document discusses Trust Anchors for DNSSEC and defines a Negative Trust Anchor, which is potentially useful during the transition to ubiquitous DNSSEC deployment. These are configured locally on a particular instance of a validating DNS recursive resolver and can shield end users of such a resolver from the DNSSEC-related authoritative name server operational errors that appear to be somewhat typical during the transition to ubiquitous DNSSEC deployment. Negative Trust Anchors are intended to be temporary, and should not be distributed by IANA or any other organization outside of the administrative boundary of the organization locally implementing a Negative Trust Anchor. Finally, Negative Trust Anchors pertain only to DNSSEC and not to Public Key Infrastructures (PKI) such ad X.509. [REFERENCE NECESSARY?]

DNSSEC has now entered widespread deployment. However, domain signing tools and processes are not yet as mature and reliable as is the case for non-DNSSEC-related domain administration tools and processes. As a result, operators of DNS recursive resolvers, such as Internet Service Providers (ISPs), occasionally observe domains incorrectly managing DNSSEC-related resource records. This mismanagement triggers DNSSEC validation failures, and then causes large numbers of end users to be unable to reach a domain. Many end users tend interpret this as a failure of their DNS servers, and may switch to a non-validating resolver or contact their ISP to complain, rather than seeing this as a failure on the part of the domain they wanted to reach.

In the short-term, one potential way to address this is for DNS operators to use a Negative Trust Anchor to temporarily disable DNSSEC validation for a specific misconfigured domain name. This immediately restores access for end users while that domain's administrators fix their misconfiguration. While DNS operators likely prefer not to use this tool, during the global transition to DNSSEC it seems some tool is needed to reduce the negative impact on such operators.

A Negative Trust Anchor should be considered a transitional and temporary tactic which is not particularly scalable and should not be used in the long-term. Over time, however, the use of Negative Trust Anchors will become less necessary as DNSSEC-related domain administration becomes more resilient.

2. Definition of a Negative Trust Anchor

Trust Anchors are defined in [RFC5914]. A trust anchor should be used by a validating caching resolver as a starting point for building the authentication chain for a signed DNS response. The inverse of this is a Negative Trust Anchor, which creates a stopping point for a caching resolver to end validation of the authentication chain. This Negative Trust Anchor can potentially be placed at any level within the chain of trust and would stop validation at that point in the chain.

3. Limited Time and Scope of Use

As noted in Section 1, the use of Negative Trust Anchors should be temporary. These are key recommendations pertaining to this practice:

1. The general practice of using Negative Trust Anchors should be limited to the transition to widespread deployment of DNSSEC (including signing of domain names and validation in DNS recursive resolvers). Thus, the practice of using Negative Trust Anchors should not be permanent.
2. During this transition phase when Negative Trust Anchors may be useful, the use of a particular Negative Trust Anchor should be temporary and in most cases limited to no more than 1 day. Thus, the use of an individual Negative Trust Anchor should be strictly time limited and very short in duration.
3. So that the use of Negative Trust Anchors remains temporary and useful only during a transition to widespread DNSSEC deployment, the use and distribution of individual Negative Trust Anchors should not be centralized, beyond the borders of one organization's operational unit. Thus, no organization should endeavor to create and centrally distribute Negative Trust Anchors to other organizations as was the case with positive Trust Anchors prior to the signing of the root.
4. As noted in Section 12, organizations that utilize Negative Trust Anchors should not add a Negative Trust Anchor for any intentionally broken domain.
5. As noted in Section 8, use of a Negative Trust Anchor should not be automatic in any way, and must involve investigation by technical personnel trained in the operation of DNS servers.

4. Domain Validation Failures

A domain name can fail validation for two general reasons, a legitimate security failure such as due to an attack or compromise of some sort, or as a result of misconfiguration on the part of an domain administrator. As domains transition to DNSSEC the most likely reason for a validation failure will be due to misconfiguration. Thus, domain administrators should be sure to read [RFC6781] in full. They should also pay special attention to Section 4.2, pertaining to key rollovers, which appears to be the cause of many recent validation failures.

In one recent example [DNSSEC-Validation-Failure-Analysis], a specific domain name failed to validate. An investigation revealed that the domain's administrators performed a Key Signing Key (KSK) rollover by (1) generating a new key and (2) signing the domain with the new key. However, they did not use a double-signing procedure for the KSK and a pre-publish procedure for the ZSK. Double-signing refers to signing a zone with two KSKs and then updating the parent zone with the new DS record so that both keys are valid at the same time. This meant that the domain name was signed with the new KSK, but it was not double-signed with the old KSK. So, the new key was used for signing the zone but the old key was not. As a result, the domain could not be trusted and returned an error when trying to reach the domain. Thus, the domain was in a situation where the DNSSEC chain of trust was broken because the Delegation Signer (DS) record pointed to the old KSK, which was no longer used for signing the zone. (A DS record provides a link in the chain of trust for DNSSEC from the parent zone to the child zone - in this case between TLD and domain name.)

In addition, it is possible that some DNSSEC validation failures could arise due to differences in how different software developers interpret DNSSEC standards and/or how those developers choose to implement support for DNSSEC. For example, it is conceivable that some domain may be DNSSEC signed properly, and Unbound-based DNS recursive resolvers will validate the domain but those using BIND or Nominum's Vantio software may fail to validate a domain.

5. End User Reaction

End users generally do not know what DNSSEC is, nor should they be expected to at the current time (especially absent widespread integration of DNSSEC indicators in end user software such as web browsers). As a result, end users may incorrectly interpret the failure to reach a domain due to DNSSEC-related misconfiguration as their ISP purposely blocking access to the domain or as a performance failure on the part of their ISP (especially of the ISP's DNS servers). End users may feel less satisfied with their ISP's service, which may make them more likely to switch to a competing ISP. They may also contact their ISP to complain, which of course will incur cost for their ISP. In addition, they may use online tools and sites to complain of this problem, such as via a blog, web forum, or social media site, which may lead to dissatisfaction on the part of other end users or general criticism of an ISP or operator of a DNS recursive resolver.

As end users publicize these failures, others may recommend they switch from security-aware DNS resolvers to resolvers not performing DNSSEC validation. This is a shame since the ISP or other DNS recursive resolver operator is actually doing exactly what they are supposed to do in failing to resolve a domain name, as this is the expected result when a domain can no longer be validated, protecting end users from a potential security threat.

6. Switching to a Non-Validating Resolver is Not Recommended

As noted in Section 5 some people may consider switching to an alternative, non-validating resolver themselves, or may recommend that others do so. But if a domain fails DNSSEC validation and is inaccessible, this could very well be due to a security-related issue. In order to be as safe and secure as possible, end users should not change to DNS servers that do not perform DNSSEC validation as a workaround, and people should not recommend that others do so either. Even if a website in a domain seems to look "normal" and valid, according to the DNSSEC protocol, that domain is not secure. Domains that fail DNSSEC for legitimate reasons may be in control of hackers or there could be other significant security issues with the domain.

Thus, switching to a non-validating resolver to restore access to a domain that fails DNSSEC validation is not a recommended practice, is bad advice to others, is potentially harmful to end user security, and is potentially harmful to DNSSEC adoption.

7. Responsibility for Failures

A domain administrator is solely and completely responsible for managing their domain name(s) and DNS resource records. This includes complete responsibility for the correctness of those resource records, the proper functioning of their DNS authoritative servers, and the correctness of DNS records linking their domain to a top-level domain (TLD) or other higher level domain. Even in cases where some error may be introduced by a third party, whether that is due to an authoritative server software vendor, software tools vendor, domain name registrar, or other organization, these are all parties that the domain administrator has selected and is responsible for managing successfully.

There are some cases where the domain administrator is different than the domain owner. In those cases, a domain owner has delegated operational responsibility to the domain administrator. So no matter whether a domain owner is also the domain administrator or not, the domain administrator is nevertheless operationally responsible for the proper configuration operation of the domain.

So in the case of a domain name failing to successfully validate, when this is due to a misconfiguration of the domain, that is the sole responsibility of the domain administrator.

Any assistance or mitigation responses undertaken by other parties to mitigate the misconfiguration of a domain name by a domain administrator, especially operators of DNS recursive resolvers, are optional and at the pleasure of those parties.

8. Use of a Negative Trust Anchor

When a domain has been confirmed to fail DNSSEC validation due to a DNSSEC-related misconfiguration, an ISP or other DNS recursive resolver operator may in some cases use a Negative Trust Anchor for a domain or sub-domain. This instructs a DNS recursive resolver to temporarily NOT perform DNSSEC validation for a specific domain name. This immediately restores access to the domain for end users while the domain's administrator corrects the misconfiguration(s).

In the case of a validation failure due to misconfiguration of a TLD or popular domain name (such as a top 100 website), this could make content or services in the affected TLD or domain to be inaccessible for a large number of users. A Negative Trust Anchor can therefore be useful in the short-term when used on a targeted and time-limited basis. It does not and should not involve turning off validation more broadly, and helps during the transition to DNSSEC as organizations that are new to signing their domains are still maturing their DNSSEC operational practices, alleviating end user issues as noted in Section 5 and restoring end user access. However, use of a Negative Trust Anchor should not be automatic in any way, and must involve investigation by technical personnel trained in the operation of DNS servers.

Technical personnel should also confirm that the domain is not intentionally broken, such as for testing purposes as noted in Section 12. Such an investigation must confirm that a failure is due to misconfiguration, as a similar breakage could have occurred if an attacker gained access to a domain's authoritative servers and modified those records or had the domain pointed to their own rogue authoritative servers. In addition, personnel should make a reasonable attempt to contact a domain for which a Negative Trust Anchor may be used, and preferably prior to implementing it.

Furthermore, a Negative Trust Anchor MUST only be used for a short duration, such as for a day or less. Implementors SHOULD set an end time and date associated with any Negative Trust Anchor. Implementors SHOULD in most cases limit the maximum duration to one day, meaning the Negative Trust Anchor will be removed or invalidated from the point of implementation, plus 86,400 seconds. However, there may be corner cases where a Negative Trust Anchor is needed for a longer period of time. Optimally this time and date is set in a DNS recursive resolver's configuration, though in the short-term this may also be achieved via other systems or supporting processes.

Finally, a Negative Trust Anchor is used only in a specific domain or sub-domain and would not affect validation at other names up the authentication chain. For example, a Negative Trust Anchor for zone1.example.com would affect only names within zone1.example.com, and validation would still be performed on example.com, .com, and the root ("."). In another example, a Negative Trust Anchor for example.com would affect only names within example.com, and validation would still be performed on .com, and the root (".")

      Root (.)              <======
          |                       ||
          |                       ||<======>+----+----+     DNSSEC
          |                       ||        |Recursive|    Validation
      TLD (com)             <=====||        |Resolver | <==============>
          |                        +<------>+---------+
          |                        |                       DNS NTA
          |                        |                 (zone1.example.com)
  SUB TLD (example.com)     <------|                    <-------------->
          |                        |
          |                        |
          |                        |
          (zone1.example.com <-----|