Negative Caching of Looping NS records

Status of This Memo

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Table of Contents

1. Introduction

Loops are a well-known configuration error in DNS zones. CNAME loops were first documented in [RFC1034], and can occur when two domains point to each other. For example:¶

.org zone file:¶

     example.org CNAME example.com

.com zone file:¶

    example.com CNAME example.org

[RFC1536] states that "a set of servers might form a loop wherein A refers to B and B refers to A". Although RFC1536 did not explicitly define other types of loops, others can also occur using NS records, as shown in the example below:¶

.org zone file:¶

    example.org NS  ns1.example.com

    example.org NS  ns2.example.com

.com zone file:¶

    example.com NS  ns1.example.org

example.com NS  ns2.example.org

In both the CNAME and NS loop cases, recursive resolvers will not be able to resolve these domain names, or any child domains underneath these zones.¶

2. Past solutions

The first solution was proposed in RFC1034, which states that CNAME loops should be "signalled (sic) as an error" (Section 3.6.2). To avoid resolvers starting to loop infinitely in presence of configuration errors, RFC1034 also recommends that resolvers limit the number of queries it sends out when resolving an individual domain name. [RFC1035] stipulates that resolvers should use counters to implement these limits.¶

Later, [RFC1536] states that "a set of servers might form a loop wherein A refers to B and B refers to A". It does not, however, specify what type of records might create these loops. Additionally, it offers no new solutions beyond what RFC1034 and RFC1035 suggested.¶

In short, [RFC1034], [RFC1035] and [RFC1536] describe the problem and do provide guidance to resolver implementers to help avoid indefinite loops in the presence of misconfigured zone files with NS or CNAME loops. However, we continue to observe different forms of this problem and so here we seek to clarify that guidance.¶

3. Current Problem

Recent research[Moura21b] has shown how NS configuration loops can lead to significant increases in traffic: New Zealand's .nz country-code top-level domain (a ccTLD) experienced a 50% traffic surge when two domains were misconfigured with NS loops. Another anonymous European ccTLD saw its traffic grow by 10-fold when two subdomains were also miscofigured with NS loops. [Moura21b] also reproduced the experiments under multiple controlled scenarios.¶

If existing RFCs already provide solution for looping misconfiguration (Section 2), how come recent research [Moura21b] still showed that these loops exist in the wild and lead to such traffic surges?¶

        +-----+  +-----+  +-----+ +-----+
        | AT1 |  | AT2 |  | AT3 |   | AT4 |
        +-----+  +-----+  +-----+ +-----+
          ^         ^             ^        ^
          |            |             |           |
          |       +-----+        |           |
          +------| Re3 |----+|           |
                  +-----+                    |
                      ^                        |
                      |                          |
                 +----+   +----+         |
                 |Re1 |   |Re2 |---------+
                 +----+   +----+
                   ^          ^
                    |           |
                    | +------+
                      +-|stub|
                     +------+
                          ^
                           |
                    | +--------+
                   +-| client |
                      +--------+

4. New requirement

Besides following the recommendations from RFC1034, RFC1035 and RFC2181 for handling loops, this memo requires that recursive resolvers MUST detect loop and MUST cache these records (negative caching)[RFC2308]. Recursive resolvers need to refrain from forwarding queries from clients/stub/forwarders to misconfigured domain names when a negative answer can be answered from its cache.¶

How long these loops should be cached for is an implementation choice; however, recursive results MUST answer from it's cache for at least 15 minutes, given that most looping NS/CNAME record situations will require human intervention.¶

5. Operational considerations

TBD¶

6. Security considerations

TBD¶

7. Privacy Considerations

This document does not add any practical new privacy issues, aside from possible benefits in deploying longer TTLs which in turn requires less traffic to be sent and thus preserves privacy by query omission: longer TTLs may help preserve a user's privacy by reducing the number of requests that get transmitted in both the client-to-resolver and resolver-to-authoritative cases.¶

8. IANA considerations

This document has no IANA actions.¶

Appendix A. Acknowledgements

TBD¶

Appendix B. Current implemenations

The requirements in this document have been implemented and deployed by:¶

• Google Public DNS¶
• Cisco OpenDNS¶

Authors' Addresses