draft-ietf-dnsop-serve-stale-05.txt   draft-ietf-dnsop-serve-stale-06.txt 
DNSOP Working Group D. Lawrence DNSOP Working Group D. Lawrence
Internet-Draft Oracle Internet-Draft Oracle
Updates: 1034, 1035 (if approved) W. Kumari Updates: 1034, 1035 (if approved) W. Kumari
Intended status: Standards Track P. Sood Intended status: Standards Track P. Sood
Expires: October 18, 2019 Google Expires: February 9, 2020 Google
April 16, 2019 August 08, 2019
Serving Stale Data to Improve DNS Resiliency Serving Stale Data to Improve DNS Resiliency
draft-ietf-dnsop-serve-stale-05 draft-ietf-dnsop-serve-stale-06
Abstract Abstract
This draft defines a method (serve-stale) for recursive resolvers to This draft defines a method (serve-stale) for recursive resolvers to
use stale DNS data to avoid outages when authoritative nameservers use stale DNS data to avoid outages when authoritative nameservers
cannot be reached to refresh expired data. It updates the definition cannot be reached to refresh expired data. It updates the definition
of TTL from [RFC1034], [RFC1035], and [RFC2181] to make it clear that of TTL from [RFC1034], [RFC1035], and [RFC2181] to make it clear that
data can be kept in the cache beyond the TTL expiry and used for data can be kept in the cache beyond the TTL expiry and used for
responses when a refreshed answer is not readily available. One of responses when a refreshed answer is not readily available. One of
the motivations for serve-stale is to make the DNS more resilient to the motivations for serve-stale is to make the DNS more resilient to
DoS attacks, and thereby make them less attractive as an attack DoS attacks, and thereby make them less attractive as an attack
vector. vector.
Ed note
Text inside square brackets ([]) is additional background
information, answers to frequently asked questions, general musings,
etc. They will be removed before publication. This document is
being collaborated on in GitHub at <https://github.com/vttale/serve-
stale>. The most recent version of the document, open issues, etc
should all be available here. The authors gratefully accept pull
requests.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 18, 2019.
This Internet-Draft will expire on February 9, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Standards Action . . . . . . . . . . . . . . . . . . . . . . 4 4. Standards Action . . . . . . . . . . . . . . . . . . . . . . 4
5. Example Method . . . . . . . . . . . . . . . . . . . . . . . 4 5. Example Method . . . . . . . . . . . . . . . . . . . . . . . 4
6. Implementation Considerations . . . . . . . . . . . . . . . . 6 6. Implementation Considerations . . . . . . . . . . . . . . . . 6
7. Implementation Caveats . . . . . . . . . . . . . . . . . . . 8 7. Implementation Caveats . . . . . . . . . . . . . . . . . . . 8
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
9. EDNS Option . . . . . . . . . . . . . . . . . . . . . . . . . 10 9. EDNS Option . . . . . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10 10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10 11. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10
12. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 11 12. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 10
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
15.1. Normative References . . . . . . . . . . . . . . . . . . 11 15.1. Normative References . . . . . . . . . . . . . . . . . . 11
15.2. Informative References . . . . . . . . . . . . . . . . . 12 15.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
Traditionally the Time To Live (TTL) of a DNS resource record has Traditionally the Time To Live (TTL) of a DNS resource record has
been understood to represent the maximum number of seconds that a been understood to represent the maximum number of seconds that a
record can be used before it must be discarded, based on its record can be used before it must be discarded, based on its
description and usage in [RFC1035] and clarifications in [RFC2181]. description and usage in [RFC1035] and clarifications in [RFC2181].
This document proposes that the definition of the TTL be explicitly This document proposes that the definition of the TTL be explicitly
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"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
For a comprehensive treatment of DNS terms, please see [RFC7719]. For a comprehensive treatment of DNS terms, please see [RFC7719].
3. Background 3. Background
There are a number of reasons why an authoritative server may become There are a number of reasons why an authoritative server may become
unreachable, including Denial of Service (DoS) attacks, network unreachable, including Denial of Service (DoS) attacks, network
issues, and so on. If the recursive server is unable to contact the issues, and so on. If a recursive server is unable to contact the
authoritative servers for a query but still has relevant data that authoritative servers for a query but still has relevant data that
has aged past its TTL, that information can still be useful for has aged past its TTL, that information can still be useful for
generating an answer under the metaphorical assumption that "stale generating an answer under the metaphorical assumption that "stale
bread is better than no bread." bread is better than no bread."
[RFC1035] Section 3.2.1 says that the TTL "specifies the time [RFC1035] Section 3.2.1 says that the TTL "specifies the time
interval that the resource record may be cached before the source of interval that the resource record may be cached before the source of
the information should again be consulted", and Section 4.1.3 further the information should again be consulted", and Section 4.1.3 further
says the TTL, "specifies the time interval (in seconds) that the says the TTL, "specifies the time interval (in seconds) that the
resource record may be cached before it should be discarded." resource record may be cached before it should be discarded."
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[RFC2181] aimed to provide "the precise definition of the Time to [RFC2181] aimed to provide "the precise definition of the Time to
Live", but in Section 8 was mostly concerned with the numeric range Live", but in Section 8 was mostly concerned with the numeric range
of values and the possibility that very large values should be of values and the possibility that very large values should be
capped. (It also has the curious suggestion that a value in the capped. (It also has the curious suggestion that a value in the
range 2147483648 to 4294967295 should be treated as zero.) It closes range 2147483648 to 4294967295 should be treated as zero.) It closes
that section by noting, "The TTL specifies a maximum time to live, that section by noting, "The TTL specifies a maximum time to live,
not a mandatory time to live." This is again not [RFC2119]-normative not a mandatory time to live." This is again not [RFC2119]-normative
language, but does convey the natural language connotation that data language, but does convey the natural language connotation that data
becomes unusable past TTL expiry. becomes unusable past TTL expiry.
Several major recursive resolver operators currently use stale data Several recursive resolver operators currently use stale data for
for answers in some way, including Akamai (in three different answers in some way, including Akamai. A number of recursive
resolver implementations), BIND, Knot, OpenDNS, and Unbound. Apple resolver packages (including BIND, Know, OpenDNS, Unbound) provide
MacOS can also use stale data as part of the Happy Eyeballs options to use stale data. Apple MacOS can also use stale data as
algorithms in mDNSResponder. The collective operational experience part of the Happy Eyeballs algorithms in mDNSResponder. The
is that it provides significant benefit with minimal downside. collective operational experience is that it provides significant
benefit with minimal downside.
4. Standards Action 4. Standards Action
The definition of TTL in [RFC1035] Sections 3.2.1 and 4.1.3 is The definition of TTL in [RFC1035] Sections 3.2.1 and 4.1.3 is
amended to read: amended to read:
TTL a 32-bit unsigned integer number of seconds that specifies the TTL a 32-bit unsigned integer number of seconds that specifies the
duration that the resource record MAY be cached before the source duration that the resource record MAY be cached before the source
of the information MUST again be consulted. Zero values are of the information MUST again be consulted. Zero values are
interpreted to mean that the RR can only be used for the interpreted to mean that the RR can only be used for the
transaction in progress, and should not be cached. Values SHOULD transaction in progress, and should not be cached. Values SHOULD
be capped on the orders of days to weeks, with a recommended cap be capped on the orders of days to weeks, with a recommended cap
of 604,800 seconds. If the data is unable to be authoritatively of 604,800 seconds (seven days). If the data is unable to be
refreshed when the TTL expires, the record MAY be used as though authoritatively refreshed when the TTL expires, the record MAY be
it is unexpired. used as though it is unexpired.
Interpreting values which have the high order bit set as being Interpreting values which have the high order bit set as being
positive, rather than 0, is a change from [RFC2181]. Suggesting a positive, rather than 0, is a change from [RFC2181]. Suggesting a
cap of seven days, rather than the 68 years allowed by [RFC2181], cap of seven days, rather than the 68 years allowed by [RFC2181],
reflects the current practice of major modern DNS resolvers. reflects the current practice of major modern DNS resolvers.
When returning a response containing stale records, the recursive When returning a response containing stale records, a recursive
resolver MUST set the TTL of each expired record in the message to a resolver MUST set the TTL of each expired record in the message to a
value greater than 0, with 30 seconds RECOMMENDED. value greater than 0, with 30 seconds RECOMMENDED.
Answers from authoritative servers that have a DNS Response Code of Answers from authoritative servers that have a DNS Response Code of
either 0 (NoError) or 3 (NXDomain) and the Authoritative Answers (AA) either 0 (NoError) or 3 (NXDomain) and the Authoritative Answers (AA)
bit set MUST be considered to have refreshed the data at the bit set MUST be considered to have refreshed the data at the
resolver. Answers from authoritative servers that have any other resolver. Answers from authoritative servers that have any other
response code SHOULD be considered a failure to refresh the data and response code SHOULD be considered a failure to refresh the data and
therefor leave any previous state intact. therefor leave any previous state intact.
5. Example Method 5. Example Method
There is conceivably more than one way a recursive resolver could There is more than one way a recursive resolver could responsibly
responsibly implement this resiliency feature while still respecting implement this resiliency feature while still respecting the intent
the intent of the TTL as a signal for when data is to be refreshed. of the TTL as a signal for when data is to be refreshed.
In this example method four notable timers drive considerations for In this example method four notable timers drive considerations for
the use of stale data, as follows: the use of stale data:
o A client response timer, which is the maximum amount of time a o A client response timer, which is the maximum amount of time a
recursive resolver should allow between the receipt of a recursive resolver should allow between the receipt of a
resolution request and sending its response. resolution request and sending its response.
o A query resolution timer, which caps the total amount of time a o A query resolution timer, which caps the total amount of time a
recursive resolver spends processing the query. recursive resolver spends processing the query.
o A failure recheck timer, which limits the frequency at which a o A failure recheck timer, which limits the frequency at which a
failed lookup will be attempted again. failed lookup will be attempted again.
o A maximum stale timer, which caps the amount of time that records o A maximum stale timer, which caps the amount of time that records
will be kept past their expiration. will be kept past their expiration.
Most recursive resolvers already have the query resolution timer, and Most recursive resolvers already have the query resolution timer, and
effectively some kind of failure recheck timer. The client response effectively some kind of failure recheck timer. The client response
timer and maximum stale timer are new concepts for this mechanism. timer and maximum stale timer are new concepts for this mechanism.
When a request is received by the recursive resolver, it should start When a request is received by a recursive resolver, it should start
the client response timer. This timer is used to avoid client the client response timer. This timer is used to avoid client
timeouts. It should be configurable, with a recommended value of 1.8 timeouts. It should be configurable, with a recommended value of 1.8
seconds as being just under a common timeout value of 2 seconds while seconds as being just under a common timeout value of 2 seconds while
still giving the resolver a fair shot at resolving the name. still giving the resolver a fair shot at resolving the name.
The resolver then checks its cache for any unexpired data that The resolver then checks its cache for any unexpired records that
satisfies the request and of course returns them if available. If it satisfy the request and returns them if available. If it finds no
finds no relevant unexpired data and the Recursion Desired flag is relevant unexpired data and the Recursion Desired flag is not set in
not set in the request, it should immediately return the response the request, it should immediately return the response without
without consulting the cache for expired records. Typically this consulting the cache for expired records. Typically this response
response would be a referral to authoritative nameservers covering would be a referral to authoritative nameservers covering the zone,
the zone, but the specifics are implementation dependent. but the specifics are implementation dependent.
If iterative lookups will be done, then the failure recheck timer is If iterative lookups will be done, then the failure recheck timer is
consulted. Attempts to refresh from non-responsive or otherwise consulted. Attempts to refresh from non-responsive or otherwise
failing authoritative nameservers are recommended to be done no more failing authoritative nameservers are recommended to be done no more
frequently than every 30 seconds. If this request was received frequently than every 30 seconds. If this request was received
within this period, the cache may be immediately consulted for stale within this period, the cache may be immediately consulted for stale
data to satisfy the request. data to satisfy the request.
Outside the period of the failure recheck timer, the resolver should Outside the period of the failure recheck timer, the resolver should
start the query resolution timer and begin the iterative resolution start the query resolution timer and begin the iterative resolution
process. This timer bounds the work done by the resolver when process. This timer bounds the work done by the resolver when
contacting external authorities, and is commonly around 10 to 30 contacting external authorities, and is commonly around 10 to 30
seconds. If this timer expires on an attempted lookup that is still seconds. If this timer expires on an attempted lookup that is still
being processed, the resolution effort is abandoned. being processed, the resolution effort is abandoned.
If the answer has not been completely determined by the time the If the answer has not been completely determined by the time the
client response timer has elapsed, the resolver should then check its client response timer has elapsed, the resolver should then check its
cache to see whether there is expired data that would satisfy the cache to see whether there is expired data that would satisfy the
request. If so, it adds that data to the response message with a TTL request. If so, it adds that data to the response message with a TTL
greater than 0 per Section 4. The response is then sent to the greater than 0 (as specified in Section 4). The response is then
client while the resolver continues its attempt to refresh the data. sent to the client while the resolver continues its attempt to
refresh the data.
When no authorities are able to be reached during a resolution When no authorities are able to be reached during a resolution
attempt, the resolver should attempt to refresh the delegation and attempt, the resolver should attempt to refresh the delegation and
restart the iterative lookup process with the remaining time on the restart the iterative lookup process with the remaining time on the
query resolution timer. This resumption should be done only once query resolution timer. This resumption should be done only once
during one resolution effort. during one resolution effort.
Outside the resolution process, the maximum stale timer is used for Outside the resolution process, the maximum stale timer is used for
cache management and is independent of the query resolution process. cache management and is independent of the query resolution process.
This timer is conceptually different from the maximum cache TTL that This timer is conceptually different from the maximum cache TTL that
exists in many resolvers, the latter being a clamp on the value of exists in many resolvers, the latter being a clamp on the value of
TTLs as received from authoritative servers and recommended to be 7 TTLs as received from authoritative servers and recommended to be
days in the TTL definition above. The maximum stale timer should be seven days in the TTL definition in Section 4. The maximum stale
configurable, and defines the length of time after a record expires timer should be configurable, and defines the length of time after a
that it should be retained in the cache. The suggested value is record expires that it should be retained in the cache. The
between 1 and 3 days. suggested value is between 1 and 3 days.
6. Implementation Considerations 6. Implementation Considerations
This document mainly describes the issues behind serving stale data This document mainly describes the issues behind serving stale data
and intentionally does not provide a formal algorithm. The concept and intentionally does not provide a formal algorithm. The concept
is not overly complex, and the details are best left to resolver is not overly complex, and the details are best left to resolver
authors to implement in their codebases. The processing of serve- authors to implement in their codebases. The processing of serve-
stale is a local operation, and consistent variables between stale is a local operation, and consistent variables between
deployments are not needed for interoperability. However, we would deployments are not needed for interoperability. However, we would
like to highlight the impact of various implementation choices, like to highlight the impact of various implementation choices,
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where a failed lookup (say, during pre-fetching) doesn't impact the where a failed lookup (say, during pre-fetching) doesn't impact the
existing cache state. Some authoritative servers operators have said existing cache state. Some authoritative servers operators have said
that they would prefer stale answers to be used in the event that that they would prefer stale answers to be used in the event that
their servers are responding with errors like ServFail instead of their servers are responding with errors like ServFail instead of
giving true authoritative answers. Implementers MAY decide to return giving true authoritative answers. Implementers MAY decide to return
stale answers in this situation. stale answers in this situation.
Since the goal of serve-stale is to provide resiliency for all Since the goal of serve-stale is to provide resiliency for all
obvious errors to refresh data, these other RCODEs are treated as obvious errors to refresh data, these other RCODEs are treated as
though they are equivalent to not getting an authoritative response. though they are equivalent to not getting an authoritative response.
Although NXDomain for a previously existing name might well be an Although NXDomain for a previously existing name might well be an
error, it is not handled that way because there is no effective way error, it is not handled that way because there is no effective way
to distinguish operator intent for legitimate cases versus error to distinguish operator intent for legitimate cases versus error
cases. cases.
During discussion in dnsop it was suggested that Refused from all During discussion in the IETF, it was suggested that, if all
authorities should be treated, from a serve-stale perspective, as authorities return responses with RCODE of Refused, it may be an
though it were equivalent to NXDomain because it represents an
explicit signal to take down the zone from servers that still have explicit signal to take down the zone from servers that still have
the zone's delegation pointed to them. Refused, however, is also the zone's delegation pointed to them. Refused, however, is also
overloaded to mean multiple possible failures which could represent overloaded to mean multiple possible failures which could represent
transient configuration failures. Operational experience has shown transient configuration failures. Operational experience has shown
that purposely returning Refused is a poor way to achieve an explicit that purposely returning Refused is a poor way to achieve an explicit
takedown of a zone compared to either updating the delegation or takedown of a zone compared to either updating the delegation or
returning NXDomain with a suitable SOA for extended negative caching. returning NXDomain with a suitable SOA for extended negative caching.
Implementers MAY nonetheless consider whether to treat all Implementers MAY nonetheless consider whether to treat all
authorities returning Refused as preempting the use of stale data. authorities returning Refused as preempting the use of stale data.
7. Implementation Caveats 7. Implementation Caveats
Stale data is used only when refreshing has failed in order to adhere Stale data is used only when refreshing has failed in order to adhere
to the original intent of the design of the DNS and the behaviour to the original intent of the design of the DNS and the behaviour
expected by operators. If stale data were to always be used expected by operators. If stale data were to always be used
immediately and then a cache refresh attempted after the client immediately and then a cache refresh attempted after the client
response has been sent, the resolver would frequently be sending data response has been sent, the resolver would frequently be sending data
that it would have had no trouble refreshing. As modern resolvers that it would have had no trouble refreshing. Because modern
use techniques like pre-fetching and request coalescing for resolvers use techniques like pre-fetching and request coalescing for
efficiency, it is not necessary that every client request needs to efficiency, it is not necessary that every client request needs to
trigger a new lookup flow in the presence of stale data, but rather trigger a new lookup flow in the presence of stale data, but rather
that a good-faith effort has been recently made to refresh the stale that a good-faith effort has been recently made to refresh the stale
data before it is delivered to any client. data before it is delivered to any client.
It is important to continue the resolution attempt after the stale It is important to continue the resolution attempt after the stale
response has been sent, until the query resolution timeout, because response has been sent, until the query resolution timeout, because
some pathological resolutions can take many seconds to succeed as some pathological resolutions can take many seconds to succeed as
they cope with unavailable servers, bad networks, and other problems. they cope with unavailable servers, bad networks, and other problems.
Stopping the resolution attempt when the response with expired data Stopping the resolution attempt when the response with expired data
has been sent would mean that answers in these pathological cases has been sent would mean that answers in these pathological cases
would never be refreshed. would never be refreshed.
The continuing prohibition against using data with a 0 second TTL The continuing prohibition against using data with a 0 second TTL
beyond the current transaction explicitly extends to it being beyond the current transaction explicitly extends to it being
unusable even for stale fallback, as it is not to be cached at all. unusable even for stale fallback, as it is not to be cached at all.
Be aware that Canonical Name (CNAME) records mingled in the expired Be aware that Canonical Name (CNAME) and DNAME [RFC6672] records
cache with other records at the same owner name can cause surprising mingled in the expired cache with other records at the same owner
results. This was observed with an initial implementation in BIND name can cause surprising results. This was observed with an initial
when a hostname changed from having an IPv4 Address (A) record to a implementation in BIND when a hostname changed from having an IPv4
CNAME. The version of BIND being used did not evict other types in Address (A) record to a CNAME. The version of BIND being used did
the cache when a CNAME was received, which in normal operations is not evict other types in the cache when a CNAME was received, which
not a significant issue. However, after both records expired and the in normal operations is not a significant issue. However, after both
authorities became unavailable, the fallback to stale answers records expired and the authorities became unavailable, the fallback
returned the older A instead of the newer CNAME. to stale answers returned the older A instead of the newer CNAME.
8. Implementation Status 8. Implementation Status
[RFC Editor: per RFC 6982 this section should be removed prior to [RFC Editor: per RFC 6982 this section should be removed prior to
publication.] publication.]
The algorithm described in Section 5 was originally implemented as a The algorithm described in Section 5 was originally implemented as a
patch to BIND 9.7.0. It has been in production on Akamai's patch to BIND 9.7.0. It has been in production on Akamai's
production network since 2011, and effectively smoothed over production network since 2011, and effectively smoothed over
transient failures and longer outages that would have resulted in transient failures and longer outages that would have resulted in
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In the research paper "When the Dike Breaks: Dissecting DNS Defenses In the research paper "When the Dike Breaks: Dissecting DNS Defenses
During DDoS" [DikeBreaks], the authors detected some use of stale During DDoS" [DikeBreaks], the authors detected some use of stale
answers by resolvers when authorities came under attack. Their answers by resolvers when authorities came under attack. Their
research results suggest that more widespread adoption of the research results suggest that more widespread adoption of the
technique would significantly improve resiliency for the large number technique would significantly improve resiliency for the large number
of requests that fail or experience abnormally long resolution times of requests that fail or experience abnormally long resolution times
during an attack. during an attack.
9. EDNS Option 9. EDNS Option
During the discussion of serve-stale in the IETF dnsop working group, During the discussion of serve-stale in the IETF, it was suggested
it was suggested that an EDNS option should be available to either that an EDNS option should be available to either explicitly opt-in
explicitly opt-in to getting data that is possibly stale, or at least to getting data that is possibly stale, or at least as a debugging
as a debugging tool to indicate when stale data has been used for a tool to indicate when stale data has been used for a response.
response.
The opt-in use case was rejected as the technique was meant to be The opt-in use case was rejected as the technique was meant to be
immediately useful in improving DNS resiliency for all clients. immediately useful in improving DNS resiliency for all clients.
The reporting case was ultimately also rejected as working group The reporting case was ultimately also rejected because even the
participants determined that even the simpler version of a proposed simpler version of a proposed option was still too much bother to
option was still too much bother to implement for too little implement for too little perceived value.
perceived value.
10. Security Considerations 10. Security Considerations
The most obvious security issue is the increased likelihood of DNSSEC The most obvious security issue is the increased likelihood of DNSSEC
validation failures when using stale data because signatures could be validation failures when using stale data because signatures could be
returned outside their validity period. This would only be an issue returned outside their validity period. This would only be an issue
if the authoritative servers are unreachable, the only time the if the authoritative servers are unreachable, the only time the
techniques in this document are used, and thus does not introduce a techniques in this document are used, and thus does not introduce a
new failure in place of what would have otherwise been success. new failure in place of what would have otherwise been success.
Additionally, bad actors have been known to use DNS caches to keep Additionally, bad actors have been known to use DNS caches to keep
records alive even after their authorities have gone away. This records alive even after their authorities have gone away. This
potentially makes that easier, although without introducing a new potentially makes that easier, although without introducing a new
risk. risk.
In [CloudStrife] it was demonstrated how stale DNS data, namely In [CloudStrife], it was demonstrated how stale DNS data, namely
hostnames pointing to addresses that are no longer in use by the hostnames pointing to addresses that are no longer in use by the
owner of the name, can be used to co-opt security such as to get owner of the name, can be used to co-opt security such as to get
domain-validated certificates fraudulently issued to an attacker. domain-validated certificates fraudulently issued to an attacker.
While this RFC does not create a new vulnerability in this area, it While this document does not create a new vulnerability in this area,
does potentially enlarge the window in which such an attack could be it does potentially enlarge the window in which such an attack could
made. An obvious mitigation is that not only should a certificate be made. A proposed mitigation is that certificate authorities
authority not use a resolver that has this feature enabled, it should should fully look up each name starting at the DNS root for every
probably not use a caching resolver at all and instead fully look up name lookup. Alternatively, CAs should use a resolver that is not
each name freshly from the root. serving stale data.
11. Privacy Considerations 11. Privacy Considerations
This document does not add any practical new privacy issues. This document does not add any practical new privacy issues.
12. NAT Considerations 12. NAT Considerations
The method described here is not affected by the use of NAT devices. The method described here is not affected by the use of NAT devices.
13. IANA Considerations 13. IANA Considerations
There are no IANA considerations. There are no IANA considerations.
14. Acknowledgements 14. Acknowledgements
The authors wish to thank Robert Edmonds, Tony Finch, Bob Harold, The authors wish to thank Robert Edmonds, Tony Finch, Bob Harold,
Tatuya Jinmei, Matti Klock, Jason Moreau, Giovane Moura, Jean Roy, Tatuya Jinmei, Matti Klock, Jason Moreau, Giovane Moura, Jean Roy,
Mukund Sivaraman, Davey Song, Paul Vixie, Ralf Weber and Paul Wouters Mukund Sivaraman, Davey Song, Paul Vixie, Ralf Weber and Paul Wouters
for their review and feedback. for their review and feedback.
Paul Hoffman deserves special thanks for submitting a number of Pull
Requests.
15. References 15. References
15.1. Normative References 15.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>. <https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
skipping to change at page 12, line 23 skipping to change at page 12, line 5
content/uploads/2018/02/ content/uploads/2018/02/
ndss2018_06A-4_Borgolte_paper.pdf>. ndss2018_06A-4_Borgolte_paper.pdf>.
[DikeBreaks] [DikeBreaks]
Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M. Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M.
Davids, "When the Dike Breaks: Dissecting DNS Defenses Davids, "When the Dike Breaks: Dissecting DNS Defenses
During DDos", ACM 2018 Internet Measurement Conference, During DDos", ACM 2018 Internet Measurement Conference,
DOI 10.1145/3278532.3278534, October 2018, DOI 10.1145/3278532.3278534, October 2018,
<https://www.isi.edu/~johnh/PAPERS/Moura18b.pdf>. <https://www.isi.edu/~johnh/PAPERS/Moura18b.pdf>.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<https://www.rfc-editor.org/info/rfc6672>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>. 2015, <https://www.rfc-editor.org/info/rfc7719>.
Authors' Addresses Authors' Addresses
David C Lawrence David C Lawrence
Oracle Oracle
Email: tale@dd.org Email: tale@dd.org
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