Internet DRAFT - draft-huque-dnsop-ns-revalidation
draft-huque-dnsop-ns-revalidation
Internet Engineering Task Force S. Huque
Internet-Draft Salesforce
Intended status: Standards Track P. Vixie
Expires: September 10, 2020 Farsight Security
R. Dolmans
NLnet Labs
March 9, 2020
Delegation Revalidation by DNS Resolvers
draft-huque-dnsop-ns-revalidation-01
Abstract
This document recommends improved DNS [RFC1034] [RFC1035] resolver
behavior with respect to the processing of Name Server (NS) resource
record sets (RRset) during iterative resolution. When following a
referral response from an authoritative server to a child zone, DNS
resolvers should explicitly query the authoritative NS RRset at the
apex of the child zone and cache this in preference to the NS RRset
on the parent side of the zone cut. Resolvers should also
periodically revalidate the child delegation by re-quering the parent
zone at the expiration of the TTL of the parent side NS RRset.
Status of This Memo
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This Internet-Draft will expire on September 10, 2020.
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Upgrading NS RRset Credibility . . . . . . . . . . . . . . . 4
4. Delegation Revalidation . . . . . . . . . . . . . . . . . . . 4
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source for this draft is maintained in GitHub at:
https://github.com/shuque/ns-revalidation
This document recommends improved DNS resolver behavior with respect
to the processing of NS record sets during iterative resolution. The
first recommendation is that resolvers, when following a referral
response from an authoritative server to a child zone, should
explicitly query the authoritative NS RRset at the apex of the child
zone and cache this in preference to the NS RRset on the parent side
of the zone cut. The second recommendation is to revalidate the
delegation by re-quering the parent zone at the expiration of the TTL
of the parent side NS RRset.
2. Motivation
The delegation NS RRset at the bottom of the parent zone and the apex
NS RRset in the child zone are unsynchronized in the DNS protocol.
[RFC1034] Section 4.2.2 says "The administrators of both zones should
insure that the NS and glue RRs which mark both sides of the cut are
consistent and remain so.". But for a variety of reasons they could
not be. Officially, a child zone's apex NS RRset is authoritative
and thus has a higher cache credibility than the parent's delegation
NS RRset, which is non-authoritative glue ([RFC2181], Section 5.4.1.
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Ranking data). Hence the NS RRset "below the zone cut" should
immediately replace the parent's delegating NS RRset in cache when an
iterative caching DNS resolver crosses a zone boundary. However,
this can only happen if (1) the resolver receives the authoritative
NS RRset in the Authority section of a response from the child zone,
which is not mandatory, or (2) if the resolver explicitly issues an
NS RRset query to the child zone as part of its iterative resolution
algorithm. In the absence of this, it is possible for an iterative
caching resolver to never learn the authoritative NS RRset for a
zone, unless a downstream client of the resolver explicitly issues
such an NS query, which is not something that normal enduser
applications do, and thus cannot be relied upon to occur with any
regularity.
Increasingly, there is a trend towards minimizing unnecessary data in
DNS responses. Several popular DNS implementations default to such a
configuration (see "minimal-responses" in BIND and Unbound). So,
they may never include the authoritative NS RRset in the Authority
section of their responses.
A common reason that zone owners want to ensure that resolvers place
the authoritative NS RRset preferentially in their cache is that the
TTLs may differ between the parent and child side of the zone cut.
Some DNS Top Level Domains (TLDs) only support long fixed TTLs in
their delegation NS sets, and this inhibits a child zone owner's
ability to make more rapid changes to their nameserver configuration
using a shorter TTL, if resolvers have no systematic mechanism to
observe and cache the child NS RRset.
A child zone's delegation still needs to be periodically revalidated
at the parent to make sure that the parent zone has not legitimately
re-delegated the zone to a different set of nameservers. Otherwise,
resolvers that refresh the TTL of a child NS RRset on subsequent
queries or due to pre-fetching, may cling to those nameservers long
after they have been re-delegated elsewhere. This leads to the
second recommendation in this document, "Delegation Revalidation".
Essentially, the resolver should record the TTL of the parent's
delegating NS RRset, and use it to trigger a revalidation action.
If both parent and child zone are DNSSEC [RFC4033] [RFC4034]
[RFC4035] signed with a corresponding secure delegation between them,
then expiration of the Delegation Signer (DS) record set will cause
revalidation of the current child zone's DNSKEY set. According to
RFC 4035, Section 2.4, "The TTL of a DS RRSet SHOULD match the TTL of
the delegating NS RRset", so this revalidation should be triggered on
the same time scale, and thus responses from the stale child
nameservers would no longer be trusted. However, delegation
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revalidation is still necessary to locate the current nameserver
addresses to which subsequent DNS queries should be directed.
3. Upgrading NS RRset Credibility
o When a delegation response is received during iteration, a
validation query should be sent in parallel with the resolution of
the triggering query, to the delegated nameservers for the newly
discovered zone cut. Note that validating resolvers today, when
following a secure delegation response, already need to dispatch a
query to the delegated nameservers for the DNSKEY RRset, so this
validation query could be sent in parallel with that DNSKEY query.
o A validation query consists of a query for the child's apex NS
RRset, sent to the newly discovered delegation's nameservers.
Normal iterative logic applies to the processing of responses to
validation queries, including storing the results in cache, trying
the next server on SERVFAIL or timeout, and so on. Positive
answers to this validation query will be cached with an
authoritative data ranking. Successive queries directed to the
same zone will be directed to the nameservers listed in the
child's apex, due to the ranking of this answer. If the
validation query fails, the parent NS RRset will remain the one
with the highest ranking and will be used for successive queries.
o Some resolvers may choose to delay the response to the triggering
query until both the triggering query and the validation query
have been answered. In practice, we expect many implementations
may answer the triggering query in advance of the validation query
for performance reasons. An additional reason is that there are
number of nameservers in the field that (incorrectly) fail to
answer explicit queries for NS records, and thus the revalidation
logic may need to be applied lazily and opportunistically to deal
with them.
o If the resolver chooses to delay the response, and there are no
nameserver names in common between the child's apex NS RRset and
the parent's delegation NS RRset, then the responses received from
forwarding the triggering query to the parent's delegated
nameservers should be discarded after validation, and this query
should be forwarded again to the child's apex nameservers.
4. Delegation Revalidation
This documents proposes two mechanisms to perform delegation
revalidation: an extensive and a simple mechanism. [TODO: in the
next revision of this draft, we would prefer to discard the extensive
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mechanism description and keep only the simple one, assuming
agreement.]
The extensive mechanism:
o The lowest TTL found in a parent zone's delegating NS RRset should
be stored in the cache and used to trigger delegation revalidation
as follows: Whenever a cached RRset is being considered for use in
a response, the cache should be walked upward toward the root,
looking for expired delegations. At the first expired delegation
encountered while walking upward toward the root, revalidation
should be triggered, putting the processing of dependent queries
on hold until validation is complete.
o To revalidate a delegation, the iterative caching DNS resolver
will forward the query that triggered revalidation to the
nameservers at the closest enclosing zone cut above the
revalidation point. While searching for these nameservers,
additional revalidations may occur, perhaps placing a chain of
dependent queries on hold, unwinding in downward order as
revalidations closer to the root must be complete before
revalidations further from the root can begin.
o If a delegation can be revalidated at the same node, then the old
apex NS RRset should be deleted from cache and then the new
delegating NS RRset should be stored in cache. The minimum TTL
from the new delegating NS RRset should also be stored in cache to
facilitate future revalidations. This order of operations ensures
that the RRset credibility rules do not prevent the new delegating
NS RRset from entering the cache. It is expected that the child's
apex NS RRset will rapidly replace the parent's delegating NS
RRset as soon as iteration restarts after the revalidation event.
o If the new delegating NS RRset cannot be found (RCODE=NXDOMAIN) or
if there is a new zone cut at some different level of the
hierarchy (insertion or deletion of a delegation point above the
revalidation point) or if the new RRset shares no nameserver names
in common with the old one (indicating some kind of redelegation,
which is rare) then the cache should be purged of all names and
RRsets at or below the revalidation point. This facilitates
redelegation or revocation of a zone by a parent zone
administrator, and also conserves cache storage by deleting
unreachable data.
The simple mechanism:
o Cap the time to cache the child NS RRset to the lower of child and
parent NS RRset TTL. The normal iterative resolution algorithm
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will then cause delegation revalidation to naturally occur at the
expiration of the capped child NS TTL, along with dispatching of
the validation query to upgrade NS RRset credibility.
5. Acknowledgements
Wouter Wijngaards proposed explicitly obtaining authoritative child
NS data in [I-D.wijngaards-dnsext-resolver-side-mitigation]. This
behavior has been implemented in the Unbound DNS resolver via the
"harden-referral-path" option. The combination of child NS fetch and
revalidating the child delegation was originally proposed in
[I-D.vixie-dnsext-resimprove], by Vixie, Joffe, and Neves.
6. IANA Considerations
This document includes no request to IANA.
7. Security Considerations
Upgrading NS RRset Credibility (Section 3) allows resolvers to cache
and utilize the authoritative child apex NS RRset in preference to
the non-authoriative parent NS RRset. However, it is important to
implement the steps described in Delegation Revalidation (Section 4)
at the expiration of the parent's delegating TTL. Otherwise, the
operator of a malicious child zone, originally delegated to, but
subsequently delegated away from, can cause resolvers that refresh
TTLs on subsequent NS set queries, or that pre-fetch NS queries, to
never learn of the redelegated zone. This problem has been seen in
the wild [include reference to Ghost Domains paper here].
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
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8.2. Informative References
[I-D.vixie-dnsext-resimprove]
Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
Resolvers for Resiliency, Robustness, and Responsiveness",
draft-vixie-dnsext-resimprove-00 (work in progress), June
2010.
[I-D.wijngaards-dnsext-resolver-side-mitigation]
Wijngaards, W., "Resolver side mitigations", draft-
wijngaards-dnsext-resolver-side-mitigation-01 (work in
progress), February 2009.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, DOI 10.17487/RFC4034, March 2005,
<https://www.rfc-editor.org/info/rfc4034>.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<https://www.rfc-editor.org/info/rfc4035>.
Authors' Addresses
Shumon Huque
Salesforce
Email: shuque@gmail.com
Paul Vixie
Farsight Security
Email: paul@redbarn.org
Ralph Dolmans
NLnet Labs
Email: ralph@nlnetlabs.nl
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