Network Working Group E. Hunt
Internet-Draft ISC
Intended status: Standards Track P. van Dijk
Expires: July 15, 2018 PowerDNS
A. Eden
DNSimple
January 11, 2018
Address-specific DNS Name Redirection (ANAME)
draft-ietf-dnsop-aname-01
Abstract
This document defines the "ANAME" DNS RR type, to provide similar
functionality to CNAME, but only redirects type A and AAAA queries.
Unlike CNAME, an ANAME can coexist with other record types. The
ANAME RR allows zone owners to redirect queries for apex domain names
in a standards compliant manner.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on July 15, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. The ANAME Resource Record . . . . . . . . . . . . . . . . . . 4
3. Authoritative Server Behavior . . . . . . . . . . . . . . . . 4
3.1. Address records returned with ANAME . . . . . . . . . . . 5
3.2. Coexistence with other types . . . . . . . . . . . . . . 6
3.3. DNSSEC signing . . . . . . . . . . . . . . . . . . . . . 6
4. Recursive Server Behavior . . . . . . . . . . . . . . . . . . 8
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Operational Considerations . . . . . . . . . . . . . . . . . 9
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Websites hosted by content distribution networks are often served by
multiple IP addresses handling different geographic areas. In many
cases, an initial query for a domain name returns a CNAME record
whose <target> is a name served by the CDN, and which ultimately
resolves to a different final answer depending on the client's IP
address or subnet, geographic location, or other considerations.
It is common practice for websites to publish content at their
registered domain name (sometimes referred to as a "bare domain" or
"zone apex": for example, "example.com" rather than
"www.example.com"). However, [RFC1033] forbids the use of CNAME
records at the same node as any other record type. Zone apex nodes
always contain SOA and NS RRsets, and frequently contain other types
such as DNSKEY, MX, TXT/SPF, etc. Consequently, a CNAME record is
not permitted at zone apex nodes.
It should be noted that [RFC4034] relaxed this restriction by
allowing coexistence of CNAME with RRSIG and NSEC records, but such
exceptions are not applicable to other resource records. RRSIG and
NSEC exist to prove the integrity of the CNAME record; they are not
intended to associate arbitrary data with the domain name.
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DNAME [RFC6672] is also not a solution, as its function is to
redirect all names in the namespace below the DNAME <owner>, not the
DNAME <owner> itself.
Redirecting website lookups to an alternate domain name via SRV or
URI resource records would be an effective solution, but to date this
approach has not been accepted by browser implementations. In
addition, it is not possible to use SRV records with wildcard names.
As a result of the above, the only widely supported and standards-
compliant way to publish content at a zone apex is to to place A and/
or AAAA records at that node. The flexibility afforded by CNAME is
not available.
This document specifies a new RR type "ANAME", which provides similar
functionality to CNAME, but only for address queries (i.e., for type
A or AAAA). The ANAME record can be present at any DNS node, and can
coexist with most other RR types, enabling it to be present at a zone
apex, or any other place where the presence of other records prevents
the use of CNAME.
Authoritative servers configured with ANAME records will answer
address queries for the ANAME owner with addresses found at the
ANAME's target, and also with the ANAME itself. Recursive resolvers
which understand ANAME can re-query for the ANAME target, just as if
they had received a CNAME response. Recursive resolvers which do not
understand ANAME will ignore the ANAME and consume the provided A/
AAAA records directly.
Similar authoritative functionality has been implemented and deployed
by a number of DNS software vendors and service providers, using
names such as ALIAS, ANAME, apex CNAME, CNAME flattening, and top
level redirection. These approaches have all been standards-
noncompliant in one way or another, and none have provided a
mechanism for a recursive resolver to follow the redirection chain
itself.
1.1. Terminology
"Address type" refers to a DNS RR type that encodes a network
address. Currently the set of address types consists of A and AAAA.
(This is not an exclusive list; in the event that any new address
types are standardized in the future, they will be included.)
"Address query" refers to a DNS query for any address type.
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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 [RFC2119].
2. The ANAME Resource Record
This document defines the "ANAME" DNS resource record type, with RR
TYPE value [TBD].
The ANAME presentation format is identical to that of CNAME
[RFC1033]:
owner ttl class ANAME target
The wire format is also identical to CNAME, except that name
compression is not permitted in ANAME RDATA, per [RFC3597].
Only one ANAME <target> can be defined per <owner>. An ANAME RRset
MUST NOT contain more than one resource record.
3. Authoritative Server Behavior
When an ANAME record is present at a DNS node and a query is received
by an authoritative server for type A or AAAA, the authoritative
server returns the ANAME RR in the answer section.
Because not all querying resolvers understand ANAME, the
authoritative server MUST also return address records, as described
below. This is conceptually similar to the synthesized CNAME record
included with DNAME responses [RFC6672].
Authoritative servers implementing ANAME MUST be equipped to resolve
the ANAME <target> on the querying resolver's behalf, either by
sending queries to an external recursive resolver or by implementing
recursive resolution logic internally, so that address records can be
expanded when the ANAME <target> is in a separate zone from <owner>.
If a query for the ANAME <target> returns a chaining response (i.e.,
CNAME, DNAME, or another ANAME), then the authoritative server (or
the resolver tasked with resolving the ANAME <target> on its behalf)
MUST attempt to follow the chain until it is able to resolve a final
address response, or until resolution fails. Intermediate ANAMEs,
CNAMEs, and DNAMEs MUST be omitted from the response.
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3.1. Address records returned with ANAME
If the original query is for type A, and an RRset of type A exists at
the final ANAME <target>, then that A RRset (with <owner> changed to
match that of the ANAME RR), MUST be appended to the answer section
after the ANAME RRset. If an AAAA RRset is also known to exist at
the ANAME <target>, then the AAAA RRset MAY be appended to the
additional section (again, with <owner> changed to match that of the
ANAME RR).
Similarly, if the original query was for type AAAA, and an AAAA RRset
exists at the final ANAME <target>, then it is appended to the answer
section (with <owner> changed), and if an A RRset also exists at the
final ANAME <target> then it MAY be appended to the additional
section.
If the original query is for type ANAME, A and AAAA records MAY be
returned in the additional section.
If the original query is for type ANY and access to ANY query
processing is not restricted, then the answer section MUST contain
both the ANAME and the A and AAAA RRsets, if present and successfully
resolved at the ANAME <target>.
How and when an authoritative server resolves the A and AAAA
responses from the ANAME <target> (when it is not itself
authoritative for <target>) is unspecified. If the authoritative
server is capable of performing recursive resolution, then it MAY
resolve the query itself, or it MAY send address queries to an
external resolver. It MAY send address queries to the ANAME <target>
when loading the zone and cache the responses locally, or it MAY
delay resolution of the address records until a query is received for
the ANAME <owner>. In either case, for performance reasons, it is
RECOMMENDED that address records be cached locally by the
authoritative server.
Address records cached locally MUST have a limited TTL. The initial
TTL for locally-cached address records MUST be set to the minimum of
the ANAME TTL and the TTLs of the intermediate and address records
retrieved during ANAME <> resolution. The TTL of the cached address
records MUST count down, just as it would in a conventional resolver
cache. Address records with expired TTLs MUST NOT be used to answer
address queries until refreshed by sending a new query to the ANAME
<target>.
If configured to do so, then the authoritative server MAY, when
sending queries to the ANAME <target>, include an EDNS CLIENT-SUBNET
(ECS) option [RFC7871], either forwarding an ECS option that was sent
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to it by the querying resolver, or generating a new ECS option from
the querying resolver's address. If a response from the ANAME
<target> includes an ECS option with a SCOPE PREFIX-LENGTH greater
than zero, the response SHOULD be cached in such a way that it would
subsequently only be used in response to queries from the same client
subnet.
If resolution of the ANAME <target> yields no address records due to
NODATA or NXDOMAIN, then the authoritative server MUST return only
the ANAME record. If the query was for a specific address type, then
the response MUST also include the SOA as in a normal NODATA
response, along with NSEC or NSEC3 if applicable.
If resolution of the ANAME <target> yields no address records due to
some other failure, and the query was for a specific address type,
the response MUST include the ANAME record and set the RCODE to
SERVFAIL.
3.2. Coexistence with other types
If the zone is configured with an A or AAAA RRset at the same DNS
node as ANAME, then the ANAME is considered to have already been
expanded. If during query processing any address records are found
at the same node as an ANAME RR, then the ANAME RR MUST NOT be
further expanded by the authoritative server.
ANAME MUST NOT coexist with CNAME or any other RR type that restricts
the types with which it can itself coexist.
Like other types, ANAME MUST NOT exist below a DNAME, but it can
coexist at the same node; in fact, the two can be used cooperatively
to redirect both the owner name (via ANAME) and everything under it
(via DNAME).
ANAME can freely coexist at the same owner name with any other RR
type.
3.3. DNSSEC signing
If the zone in which the ANAME resides is DNSSEC-signed, and if the
server has access to its private zone-signing key, then the A and
AAAA RRsets MUST be signed, either in advance when populating the A/
AAAA answers for the ANAME records, or "on the fly" when responding
to a query.
If the server does not have access to the private zone-signing key
then it MAY return unsigned address records, but this is NOT
RECOMMENDED unless every resolver with access to the zone is known to
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support ANAME (as might be the case in a split-horizon deployment
where ANAME records are only served to an internal network with its
own resolvers).
Validating resolvers which do not yet implement ANAME will not be
able to validate the A and AAAA responses included with an ANAME
response unless those responses are validly signed by a DNSKEY at the
apex of the zone in which the ANAME resides. Passing along the
RRSIGs associated with the original A and AAAA RRsets from the ANAME
<target> will not be sufficient for DNSSEC validation.
Implementers MAY allow address records associated with the ANAME to
be populated and signed by the primary server, then sent along with
their RRSIGs to secondaries via zone transfer. In this case, the
master server MUST respect the TTLs of the address records, MUST
refresh the address records by re-resolving the ANAME <target> when
their TTLs expire, SHOULD respond to address queries with TTLs that
count down as they would when answering from a normal DNS cache, and
MUST inform secondary servers via DNS NOTIFY they need to refresh the
zone when address records have been updated. A secondary server
SHOULD store address records and associated RRSIGs supplied via zone
transfer in such a way that their TTLs will count down, as they would
in a normal DNS cache, and ultimately trigger a zone refresh query
upon reaching zero. When a secondary server is responding to an
address query, it SHOULD answer with the reduced TTL, but when
responding to a zone transfer request, it MUST answer with the
original TTL received from the primary.
If this address record expansion and signing during zone transfer is
not supported, then every authoritative server providing ANAME
responses in a signed zone SHOULD have access to the private zone-
signing key for that zone. Deployment of ANAME in signed zones where
address records cannot be signed due to lack of access to the private
zone-signing key is NOT RECOMMENDED.
When ANAME is present in a signed DNS node and address records exist
at the ANAME <target>, the type bit map in the NSEC [RFC4034] or
NSEC3 [RFC5155] record for that node MUST include bits for A and/or
AAAA as well as ANAME. This is for the benefit of validating
resolvers not implementing ANAME which may use a signed proof of
nonexistence for type A and AAAA to prevent address queries from
being resolved. The type bit map SHOULD only include address types
which are known to exist at the <target>.
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4. Recursive Server Behavior
When a recursive resolver sends a query of type A or AAAA and
receives a response with an ANAME RRset in the answer section, it
MUST re-query for the ANAME <target>. This is necessary because, in
some cases, the address received will be dependent on network
topology and other considerations, and the resolver may find a
different answer than the authoritative server did. (This
requirement MAY be relaxed if both the ANAME <owner> and <target> are
validly signed and provably in the same zone.)
If resolution fails -- for example, due to the local resolver being
nonfunctional or the ANAME <target> zone being unreachable -- then
the resolver MAY use the address records that were included in the
authoritative response as a fallback. Otherwise, these records MUST
NOT be cached or returned.
If configured to do so, the resolver MAY include an EDNS CLIENT-
SUBNET option [RFC7871] both when sending the initial query to the
ANAME <owner> and when re-querying for the ANAME <target>. If the
response includes a SCOPE PREFIX-LENGTH greater than zero, the
response SHOULD be cached in such a way that it would subsequently
only be used in response to queries from the same client subnet.
5. Examples
Given the following zone:
$ORIGIN example.com.
@ IN SOA example.com hostmaster.example.com 1 7200 600 1209600 60
@ IN NS ns1
@ IN ANAME example.com.my-cdn.example.net.
www IN CNAME example.com.my-cdn.example.net.
A query for example.com/A would return the following:
;; QUESTION SECTION:
;example.com. IN A
;; ANSWER SECTION:
example.com. 5 IN ANAME example.com.my-cdn.example.net.
example.com. 5 IN A 192.0.2.1
;; ADDITIONAL SECTION:
example.com. 5 IN AAAA 2001:db8::1
Similarly, for example.com/AAAA:
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;; QUESTION SECTION:
;example.com. IN AAAA
;; ANSWER SECTION:
example.com. 5 IN ANAME example.com.my-cdn.example.net.
example.com. 5 IN AAAA 2001:db8::1
;; ADDITIONAL SECTION:
example.com. 5 IN A 192.0.2.1
A query for example.com/AANME would receive only the ANAME in the
answer section, with the addresses for example.com.my-cdn.example.net
expanded in the additional section:
;; QUESTION SECTION:
;example.com. IN ANAME
;; ANSWER SECTION:
example.com. 5 IN ANAME example.com.my-cdn.example.net.
;; ADDITIONAL SECTION:
example.com.my-cdn.example.net. 5 IN A 192.0.2.1
example.com.my-cdn.example.net. 5 IN AAAA 2001:db8::1
Meanwhile, a query for a non-address type would be returned normally:
;; QUESTION SECTION:
;example.com. IN NS
;; ANSWER SECTION:
example.com. 5 IN NS ns1.example.com.
6. Operational Considerations
When a zone containing ANAME records is transferred to a secondary
server, the ANAME records are transferred, but the A or AAAA records
retrieved from the ANAME <target> may not be. If the primary server
implements ANAME but the secondary server does not, then the two will
return different answers for address queries. It is therefore
RECOMMENDED that ANAME not be deployed in a zone unless all of the
authoritative servers for that zone implement ANAME, or the primary
is able to expand the ANAME with the related address RRsets during
the zone transfer.
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7. Implementation Status
PowerDNS <https://powerdns.com> currently implements a similar
authoritative-only feature using "ALIAS" records, which are expanded
by the primary server and transfered as address records to
secondaries.
[TODO: Add discussion of DNSimple, DNS Made Easy, EasyDNS,
Cloudflare, Amazon, and Akamai.]
8. Security Considerations
An authoritative server which implements ANAME resolves address
queries on behalf of its clients, either internally or by querying an
external resolver. This resolution must be allowed to take place
regardless of whether the client would ordinarily have been permitted
by local policy to send recursive queries.
When a resolver that does not understand ANAME receives a response
containing A or AAAA records with <owner> rewritten to match that of
the ANAME RR, this may bypass security mechanisms based on local
policy limiting access to the original ANAME <target>. One possible
mitigation for this is to make sure the resolver being used during
ANAME resolution lives outside of such critical network sections.
If ANAME is used in a signed zone, validating resolvers that do not
understand ANAME will not be able to valudate the A and AAAA records
included in the response, unless the responding server has added
signatures for those records. Merely passing along signatures from
the <target> is not sufficient. An authoritative server hosting a
secure domain that includes ANAME SHOULD therefore have access to the
private zone-signing key for that domain; otherwise, the operator
must accept that validation failures will be common until ANAME is
widly deployed.
Both authoritative servers and resolvers that implement ANAME SHOULD
carefully check for loops and treat them as an error condition. One
possible approach is to implement a hop counter and stop resolution
when a maximum hop count is reached.
An authoritative resolver returning address records which were
obtained by resolving the ANAME <target> is supplying its own best
information to clients as to the correct answer. The response may be
signed by the authoritative server, but that is not a guarantee of
the actual correctness of the answer. This can have the effect of
promoting an insecure response from the ANAME <target> to a signed
response from the <owner>, which may then appear to clients to be
more trustworthy than it should. To mitigate harm from this, DNSSEC
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validation SHOULD be used when resolving the ANAME <target>.
Authoritative servers MAY refuse to expand ANAME records unless the
<target> node is both signed and validated.
9. IANA Considerations
IANA is requested to assign a DNS RR data type value for the ANAME RR
type under the "Resource Record (RR) TYPEs" subregistry under the
"Domain Name System (DNS) Parameters" registry.
IANA may wish to consider the creation of a registry of address
types; addition of new types to such a registry would then implicitly
update this specification.
10. Acknowledgments
Thanks to Mukund Sivaraman, Stephen Morris, Ray Bellis, Mark Andrews,
Richard Salts, Job Snijders, Richard Gibson, Hakan Lindqvist, Jan
Vcelak, Tatuya JINMEI, and Tony Finch for discussion and feedback.
11. References
11.1. Normative References
[RFC1033] Lottor, M., "Domain administrators operations guide",
RFC 1033, November 1987.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
11.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, June 2012.
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[RFC7871] Contavalli, C., van der Gaast, W., Lawrence, D., and W.
Kumari, "Client Subnet in DNS Queries", RFC 7871,
DOI 10.17487/RFC7871, May 2016,
<http://www.rfc-editor.org/info/rfc7871>.
Authors' Addresses
Evan Hunt
ISC
950 Charter St
Redwood City, CA 94063
USA
Email: each@isc.org
Peter van Dijk
PowerDNS.COM B.V.
Den Haag
The Netherlands
Email: peter.van.dijk@powerdns.com
Anthony Eden
DNSimple
Boston, MA
USA
Email: anthony.eden@dnsimple.com
URI: https://dnsimple.com/
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