Serving Stale Data to Improve DNS Resiliency
RFC 8767
Internet Engineering Task Force (IETF) D. Lawrence
Request for Comments: 8767 Oracle
Updates: 1034, 1035, 2181 W. Kumari
Category: Standards Track P. Sood
ISSN: 2070-1721 Google
March 2020
Serving Stale Data to Improve DNS Resiliency
Abstract
This document defines a method (serve-stale) for recursive resolvers
to use stale DNS data to avoid outages when authoritative nameservers
cannot be reached to refresh expired data. One of the motivations
for serve-stale is to make the DNS more resilient to DoS attacks and
thereby make them less attractive as an attack vector. This document
updates the definitions of TTL from RFCs 1034 and 1035 so that data
can be kept in the cache beyond the TTL expiry; it also updates RFC
2181 by interpreting values with the high-order bit set as being
positive, rather than 0, and suggests a cap of 7 days.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8767.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Terminology
3. Background
4. Standards Action
5. Example Method
6. Implementation Considerations
7. Implementation Caveats
8. Implementation Status
9. EDNS Option
10. Security Considerations
11. Privacy Considerations
12. NAT Considerations
13. IANA Considerations
14. References
14.1. Normative References
14.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Traditionally, the Time To Live (TTL) of a DNS Resource Record (RR)
has been understood to represent the maximum number of seconds that a
record can be used before it must be discarded, based on its
description and usage in [RFC1035] and clarifications in [RFC2181].
This document expands the definition of the TTL to explicitly allow
for expired data to be used in the exceptional circumstance that a
recursive resolver is unable to refresh the information. It is
predicated on the observation that authoritative answer
unavailability can cause outages even when the underlying data those
servers would return is typically unchanged.
We describe a method below for this use of stale data, balancing the
competing needs of resiliency and freshness.
This document updates the definitions of TTL from [RFC1034] and
[RFC1035] so that data can be kept in the cache beyond the TTL
expiry; it also updates [RFC2181] by interpreting values with the
high-order bit set as being positive, rather than 0, and also
suggests a cap of 7 days.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
For a glossary of DNS terms, please see [RFC8499].
3. Background
There are a number of reasons why an authoritative server may become
unreachable, including Denial-of-Service (DoS) attacks, network
issues, and so on. If a recursive server is unable to contact the
authoritative servers for a query but still has relevant data that
has aged past its TTL, that information can still be useful for
generating an answer under the metaphorical assumption that "stale
bread is better than no bread."
[RFC1035], Section 3.2.1 says that the TTL "specifies the time
interval that the resource record may be cached before the source of
the information should again be consulted." [RFC1035], Section 4.1.3
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