Keywords: dns security, trust anchors
Network Working Group S. Weiler
Request for Comments: 5074 SPARTA, Inc.
Category: Informational November 2007
DNSSEC Lookaside Validation (DLV)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
DNSSEC Lookaside Validation (DLV) is a mechanism for publishing DNS
Security (DNSSEC) trust anchors outside of the DNS delegation chain.
It allows validating resolvers to validate DNSSEC-signed data from
zones whose ancestors either aren't signed or don't publish
Delegation Signer (DS) records for their children.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. DLV Domains . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview of Validator Behavior . . . . . . . . . . . . . . . . 3
5. Details of Validator Behavior . . . . . . . . . . . . . . . . . 4
6. Aggressive Negative Caching . . . . . . . . . . . . . . . . . . 5
6.1. Implementation Notes . . . . . . . . . . . . . . . . . . . 5
7. Overlapping DLV Domains . . . . . . . . . . . . . . . . . . . . 6
8. Optimization . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
11.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . .10
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1. Introduction
DNSSEC [RFC4033] [RFC4034] [RFC4035] authenticates DNS data by
building public-key signature chains along the DNS delegation chain
from a trust anchor.
In the present world, with the DNS root and many key top level
domains unsigned, the only way for a validating resolver
("validator") to validate the many DNSSEC-signed zones is to maintain
a sizable collection of preconfigured trust anchors. Maintaining
multiple preconfigured trust anchors in each DNSSEC-aware validator
presents a significant management challenge.
This document describes a way to publish trust anchors in the DNS
outside of the normal delegation chain, as a way to easily configure
many validators within an organization or to "outsource" trust anchor
management.
Some design trade-offs leading to the mechanism presented here are
described in [INI1999-19].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Architecture
DNSSEC Lookaside Validation allows a set of domains, called "DLV
domains", to publish secure entry points for zones that are not their
own children.
With DNSSEC, validators may expect a zone to be secure when
validators have one of two things: a preconfigured trust anchor for
the zone or a validated Delegation Signer (DS) record for the zone in
the zone's parent (which presumes a preconfigured trust anchor for
the parent or another ancestor). DLV adds a third mechanism: a
validated entry in a DLV domain (which presumes a preconfigured trust
anchor for the DLV domain). Whenever a DLV domain contains a DLV
RRset for a zone, a validator may expect the named zone to be signed.
Absence of a DLV RRset for a zone does not necessarily mean that the
zone should be expected to be insecure; if the validator has another
reason to believe the zone should be secured, validation of that
zone's data should still be attempted.
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3. DLV Domains
A DLV domain includes trust statements about descendants of a single
zone, called the 'target' zone. For example, the DLV domain
trustbroker.example.com could target the org zone and the DLV domain
bar.example.com could target the root.
A DLV domain contains one or more DLV records [RFC4431] for each of
the target's descendant zones that have registered security
information with it. For a given zone, the corresponding name in the
DLV domain is formed by replacing the target zone name with the DLV
domain name.
For example, assuming the DLV domain trustbroker.example.com targets
the org zone, any DLV records corresponding to the zone example.org
can be found at example.trustbroker.example.com. DLV records
corresponding to the org zone can be found at the apex of
trustbroker.example.com.
As another example, assuming the DLV domain bar.example.com targets
the root zone, DLV records corresponding to the zone example.org can
be found at example.org.bar.example.com. DLV records corresponding
to the org zone can be found at org.bar.example.com, and DLV records
corresponding to the root zone itself can be found at the apex of
bar.example.com.
A DLV domain need not contain data other than DLV records,
appropriate DNSSEC records validating that data, the apex NS and SOA
records, and, optionally, delegations. In most cases, the operator
of a DLV domain will probably not want to include any other RR types
in the DLV domain.
To gain full benefit from aggressive negative caching, described in
Section 6, a DLV domain SHOULD NOT use minimally-covering NSEC
records, as described in [RFC4470], and it SHOULD NOT use NSEC3
records, as described in [NSEC3].
4. Overview of Validator Behavior
To minimize the load on the DLV domain's authoritative servers as
well as query response time, a validator SHOULD first attempt
validation using any applicable (non-DLV) trust anchors. If the
validation succeeds (with a result of Secure), DLV processing need
not occur.
When configured with a trust anchor for a DLV domain, a validator
SHOULD attempt to validate all responses at and below the target of
that DLV domain.
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To do validation using DLV, a validator looks for a (validated) DLV
RRset applicable to the query, as described in the following section,
and uses it as though it were a DS RRset to validate the answer using
the normal procedures in Section 5 of RFC 4035.
For each response, the validator attempts validation using the
"closest enclosing" DLV RRset in the DLV domain, which is the DLV
RRset with the longest name that matches the query or could be an
ancestor of the QNAME. For example, assuming the DLV domain
trustbroker.example.com targets the org zone, and there exist DLV
RRsets named trustbroker.example.com (applicable to org),
example.trustbroker.example.com (applicable to example.org), and
sub.example.trustbroker.example.com (applicable to sub.example.org),
a validator would use the sub.example.trustbroker.example.com DLV
RRset for validating responses to a query for sub.example.org.
The choice of which DLV record(s) to use has a significant impact on
the query load seen at DLV domains' authoritative servers. The
particular DLV selection rule described in this document results in a
higher query load than some other selection rules, but it has some
advantages in terms of the security policies that it can implement.
More detailed discussion of this DLV selection rule as well as
several alternatives that were considered along the way can be found
in [INI1999-19].
5. Details of Validator Behavior
As above, to minimize the load on the DLV domain's authoritative
servers as well as query response time, a validator SHOULD first
attempt validation using any applicable (non-DLV) trust anchors. If
the validation succeeds (with a result of Secure), DLV processing
need not occur.
To find the closest enclosing DLV RRset for a given query, the
validator starts by looking for a DLV RRset corresponding to the
QNAME. If it doesn't find a DLV RRset for that name (as confirmed by
the presence of a validated NSEC record) and that name is not the
apex of the DLV domain, the validator removes the leading label from
the name and tries again. This process is repeated until a DLV RRset
is found or it is proved that there is no enclosing DLV RRset
applicable to the QNAME. In all cases, a validator SHOULD check its
cache for the desired DLV RRset before issuing a query. Section 8
discusses a slight optimization to this strategy.
Having found the closest enclosing DLV RRset or received proof that
no applicable DLV RRset exists, the validator MUST validate the RRset
or non-existence proof using the normal procedures in Section 5 of
RFC 4035. In particular, any delegations within the DLV domain need
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to be followed, with normal DNSSEC validation applied. If validation
of the DLV RRset leads to a result of Bogus, then it MUST NOT be used
and the validation result for the original response SHOULD be Bogus,
also. If validation of the DLV RRset leads to a result of Insecure
(i.e., the DLV record is in an unsecured portion of the DLV domain),
then it MUST NOT be used and the validation result for the original
response SHOULD be Insecure, also. (It should be very odd, indeed,
to find part of a DLV domain marked as Insecure: this is likely to
happen only when there are delegations within the DLV domain and some
portions of that domain use different cryptographic signing
algorithms.) If the validation of the DLV RRset leads to a result of
Secure, the validator then treats that DLV RRset as though it were a
DS RRset for the applicable zone and attempts validation using the
procedures described in Section 5 of RFC 4035.
In the interest of limiting complexity, validators SHOULD NOT attempt
to use DLV to validate data from another DLV domain.
6. Aggressive Negative Caching
To minimize load on authoritative servers for DLV domains,
particularly those with few entries, DLV validators SHOULD implement
aggressive negative caching, as defined in this section.
Previously, cached negative responses were indexed by QNAME, QCLASS,
QTYPE, and the setting of the CD bit (see RFC 4035, Section 4.7), and
only queries matching the index key would be answered from the cache.
With aggressive negative caching, the validator, in addition to
checking to see if the answer is in its cache before sending a query,
checks to see whether any cached and validated NSEC record denies the
existence of the sought record(s).
Using aggressive negative caching, a validator will not make queries
for any name covered by a cached and validated NSEC record.
Furthermore, a validator answering queries from clients will
synthesize a negative answer whenever it has an applicable validated
NSEC in its cache unless the CD bit was set on the incoming query.
6.1. Implementation Notes
Implementing aggressive negative caching suggests that a validator
will need to build an ordered data structure of NSEC records in order
to efficiently find covering NSEC records. Only NSEC records from
DLV domains need to be included in this data structure.
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Also note that some DLV validator implementations do not synthesize
negative answers to insert into outgoing responses -- they only use
aggressive negative caching when looking up DLV RRs as part of their
internal DLV validation.
7. Overlapping DLV Domains
It is possible to have multiple DLV domains targeting overlapping
portions of the DNS hierarchy. For example, two DLV domains, perhaps
operated by different parties, might target the org zone, or one DLV
domain might target the root while another targets org.
If a validator supports multiple DLV domains, the choice of
precedence in case of overlap is left up to the implementation and
SHOULD be exposed as a configuration option to the user (as compared
to the choice of DLV records within each domain, a precedence for
which is clearly specified in this document). As a very simple
default, a validator could give precedence to the most specific DLV
domain.
Some other reasonable options include:
1. Searching all applicable DLV domains until an applicable DLV
record is found that results in a successful validation of the
response. In the case where no applicable DLV record is found in
any DLV domain, the answer will be treated as Unsecure.
2. Applying some sort of precedence to the DLV domains based on
their perceived trustworthiness.
3. Searching all applicable DLV domains for applicable DLV records
and using only the most specific of those DLV records.
4. If multiple DLV domains provide applicable DLV records, use a
threshold or scoring system (e.g., "best 2 out of 3") to
determine the validation result.
The above list is surely not complete, and it's possible for
validators to have different precedence rules and configuration
options for these cases. [INI1999-19] discusses different policies
for selecting from multiple DLV records within the same DLV domain.
That discussion may also be applicable to the question of which DLV
domain to use and may be of interest to implementers of validators
that support multiple DLV domains.
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8. Optimization
This section documents an optimization to further reduce query load
on DLV servers and improve validator response time.
Authoritative servers, when processing a query for a DLV RRset,
SHOULD include all DLV RRsets potentially applicable to a query
(specifically, all DLV RRsets applicable to the QNAME and any of its
ancestors) in the Additional section of the response as well as NSEC
records proving the non-existence of any other applicable DLV records
in the DLV domain. Authoritative servers need only include DLV
RRsets they're aware of -- RRsets in sub-zones may be omitted.
Validators still seek out of the closest enclosing DLV RRset first.
If they receive any data about other DLV RRsets in the zone, they MAY
cache and use it (assuming that it validates), thus avoiding further
round-trips to the DLV domain's authoritative servers.
9. Security Considerations
Validators MUST NOT use a DLV record unless it has been successfully
authenticated. Normally, validators will have a trust anchor for the
DLV domain and use DNSSEC to validate the data in it.
Aggressive negative caching increases the need for validators to do
some basic validation of incoming NSEC records before caching them.
In particular, the 'next name' field in the NSEC record MUST be
within the zone that generated (and signed) the NSEC. Otherwise, a
malicious zone operator could generate an NSEC that reaches out of
its zone -- into its ancestor zones, even up into the root zone --
and use that NSEC to spoof away any name that sorts after the name of
the NSEC. We call these overreaching NSECs. More insidiously, an
attacker could use an overreaching NSEC in combination with a signed
wildcard record to substitute a signed positive answer in place of
the real data. This checking is not a new requirement -- these
attacks are a risk even without aggressive negative caching.
However, aggressive negative caching makes the checking more
important. Before aggressive negative caching, NSECs were cached
only as metadata associated with a particular query. An overreaching
NSEC that resulted from a broken zone signing tool or some
misconfiguration would only be used by a cache for those queries that
it had specifically made before. Only an overreaching NSEC actively
served by an attacker could cause misbehavior. With aggressive
negative caching, an overreaching NSEC can cause broader problems
even in the absence of an active attacker. This threat can be easily
mitigated by checking the bounds on the NSEC.
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As a reminder, validators MUST NOT use the mere presence of an RRSIG
or apex DNSKEY RRset as a trigger for doing validation, whether
through the normal DNSSEC hierarchy or DLV. Otherwise, an attacker
might perpetrate a downgrade attack by stripping off those RRSIGs or
DNSKEYs.
Section 8 of RFC 4034 describes security considerations specific to
the DS RR. Those considerations are equally applicable to DLV RRs.
Of particular note, the key tag field is used to help select DNSKEY
RRs efficiently, but it does not uniquely identify a single DNSKEY
RR. It is possible for two distinct DNSKEY RRs to have the same
owner name, the same algorithm type, and the same key tag. An
implementation that uses only the key tag to select a DNSKEY RR might
select the wrong public key in some circumstances.
For further discussion of the security implications of DNSSEC, see
RFCs 4033, 4034, and 4035.
10. IANA Considerations
DLV makes use of the DLV resource record (RR type 32769) previously
assigned in [RFC4431].
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security
Extensions", RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4431] Andrews, M. and S. Weiler, "The DNSSEC Lookaside
Validation (DLV) DNS Resource Record", RFC 4431,
February 2006.
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11.2. Informative References
[INI1999-19] Weiler, S., "Deploying DNSSEC Without a Signed Root",
Technical Report 1999-19, Information Networking
Institute, Carnegie Mellon University, April 2004.
[NSEC3] Laurie, B., Sisson, G., Arends, R., and D. Blacka,
"DNSSEC Hashed Authenticated Denial of Existence", Work
in Progress, July 2007.
[RFC4470] Weiler, S. and J. Ihren, "Minimally Covering NSEC
Records and DNSSEC On-line Signing", RFC 4470,
April 2006.
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Appendix A. Acknowledgments
Johan Ihren, Paul Vixie, and Suzanne Woolf contributed significantly
to the exploration of possible validator algorithms that led to this
design. More about those explorations is documented in [INI1999-19].
Johan Ihren and the editor share the blame for aggressive negative
caching.
Thanks to David B. Johnson and Marvin Sirbu for their patient review
of [INI1999-19] which led to this specification being far more
complete.
Thanks to Mark Andrews, Rob Austein, David Blacka, Stephane
Bortzmeyer, Steve Crocker, Wes Hardaker, Alfred Hoenes, Russ Housley,
Peter Koch, Olaf Kolkman, Juergen Quittek, and Suzanne Woolf for
their valuable comments on this document.
Author's Address
Samuel Weiler
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, Maryland 21046
US
EMail: weiler@tislabs.com
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