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Versions: (draft-hunt-dnsop-aname) 00 01 02 03

DNS Operations                                                  T. Finch
Internet-Draft                                   University of Cambridge
Intended status: Standards Track                                 E. Hunt
Expires: October 17, 2019                                            ISC
                                                             P. van Dijk
                                                                 A. Eden
                                                              W. Mekking
                                                          April 15, 2019

                  Address-specific DNS aliases (ANAME)


   This document defines the "ANAME" DNS RR type, to provide similar
   functionality to CNAME, but only for type A and AAAA queries.  Unlike
   CNAME, an ANAME can coexist with other record types.  The ANAME RR
   allows zone owners to make an apex domain name into an alias 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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 17, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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
     1.1.  Overview
     1.2.  Terminology
   2.  The ANAME resource record
     2.1.  Presentation and wire format
     2.2.  Coexistence with other types
   3.  Additional section processing
     3.1.  Address queries
     3.2.  ANAME queries
   4.  Substituting ANAME sibling address records
   5.  ANAME processing by primary masters
     5.1.  Zone transfers
     5.2.  DNSSEC
     5.3.  TTLs
   6.  ANAME processing by resolvers
   7.  IANA considerations
   8.  Security considerations
   9.  Acknowledgments
   10. Changes since the last revision
     10.1.  Version -03
     10.2.  Version -02
   11. References
     11.1.  Normative References
     11.2.  Informative References
     11.3.  URIs
   Appendix A.  Implementation status
   Appendix B.  Historical note
   Appendix C.  On preserving TTLs
     C.1.  Query bunching
     C.2.  Upstream caches
     C.3.  ANAME chains
     C.4.  TTLs and zone transfers
   Appendix D.  Answer vs Additional sections
   Appendix E.  Alternative setups
   Authors' Addresses

1.  Introduction

   It can be desirable to provide web sites (and other services) at a
   bare domain name (such as "example.com") as well as a service-
   specific subdomain ("www.example.com").

   If the web site is hosted by a third-party provider, the ideal way to
   provision its name in the DNS is using a CNAME record, so that the
   third party provider retains control over the mapping from names to
   IP address(es).  It is now common for name-to-address mappings to be
   highly dynamic, dependent on client location, server load, etc.

   However, CNAME records cannot coexist with other records with the
   same owner name.  (The reason why is explored in Appendix B).  This
   restriction means they cannot appear at a zone apex (such as
   "example.com") because of the SOA, NS, and other records that have to
   be present there.  CNAME records can also conflict at subdomains, for
   example, if "department.example.edu" has separately hosted mail and
   web servers.

   Redirecting website lookups to an alternate domain name via SRV or
   URI resource records would be an effective solution from the DNS
   point of view, but to date, browser vendors have not accepted this

   As a result, the only widely supported and standards-compliant way to
   publish a web site at a bare domain is to place A and/or AAAA records
   at the zone apex.  The flexibility afforded by CNAME is not

   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 basic idea is that the address records next to an
   ANAME record are automatically copied from and kept in sync with the
   ANAME target's address records.  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 name where the presence of
   other records prevents the use of a CNAME record.

   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 mechanisms are proprietary, which hinders
   the ability of zone owners to have the same data served from multiple
   providers or to move from one provider to another.  None of these
   proprietary implementations includes a mechanism for resolvers to
   follow the redirection chain themselves.

1.1.  Overview

   The core functionality of this mechanism allows zone administrators
   to start using ANAME records unilaterally, without requiring
   secondary servers or resolvers to be upgraded.

   o  The resource record definition in Section 2 is intended to provide
      zone data portability between standards-compliant DNS servers and
      the common core functionality of existing proprietary ANAME-like

   o  The zone maintenance mechanism described in Section 5 keeps the
      ANAME's sibling address records in sync with the ANAME target.

   This definition is enough to be useful by itself.  However, it can be
   less than optimal in certain situations: for instance, when the ANAME
   target uses clever tricks to provide different answers to different
   clients to improve latency or load balancing.

   o  The Additional section processing rules in Section 3 inform
      resolvers that an ANAME record is in play.

   o  Resolvers can use this ANAME information as described in Section 6
      to obtain answers that are tailored to the resolver rather than to
      the zone's primary master.

   Resolver support for ANAME is not necessary, since ANAME-oblivious
   resolvers can get working answers from authoritative servers.  It's
   just an optimization that can be rolled out incrementally, and that
   will help ANAME to work better the more widely it is deployed.

1.2.  Terminology

   An "address record" is a DNS resource record whose type is A or AAAA.
   These are referred to as "address types".  "Address query" refers to
   a DNS query for any address type.

   When talking about "address records" we mean the entire RRset,
   including owner name and TTL.  We treat missing address records (i.e.
   NXDOMAIN or NODATA) the same successfully resolving as a set of zero
   address records, and distinct from "failure" which covers error
   responses such as SERVFAIL or REFUSED.

   The "sibling address records" of an ANAME record are the address
   records at the same owner name as the ANAME, which are subject to
   ANAME substitution.

   The "target address records" of an ANAME record are the address
   records obtained by resolving the ultimate target of the ANAME (see
   Section 4).

   Other DNS-related terminology can be found in [RFC8499].

   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].

2.1.  Presentation and wire format

   The ANAME presentation format is identical to that of CNAME

       owner ttl class ANAME target

   The wire format is also identical to CNAME [RFC1035], except that
   name compression is not permitted in ANAME RDATA, per [RFC3597].

2.2.  Coexistence with other types

   Only one ANAME <target> can be defined per <owner>.  An ANAME RRset
   MUST NOT contain more than one resource record.

   An ANAME's sibling address records are under the control of ANAME
   processing (see Section 5) and are not first-class records in their
   own right.  They MAY exist in zone files, but they can subsequently
   be altered by ANAME processing.

   ANAME records MAY freely coexist at the same owner name with other RR
   types, except they MUST NOT coexist with CNAME or any other RR type
   that restricts the types with which it can itself coexist.

   Like other types, ANAME records can coexist with DNAME records at the
   same owner name; in fact, the two can be used cooperatively to
   redirect both the owner name address records (via ANAME) and
   everything under it (via DNAME).

3.  Additional section processing

   The requirements in this section apply to both recursive and
   authoritative servers.

   An ANAME target MAY resolve to address records via a chain of CNAME
   and/or ANAME records; any CNAME/ANAME chain MUST be included when
   adding target address records to a response's Additional section.

3.1.  Address queries

   When a server receives an address query for a name that has an ANAME
   record, the response's Additional section:

   o  MUST contain the ANAME record;

   o  MAY contain the target address records that match the query type
      (or the corresponding proof of nonexistence), if they are
      available and the target address RDATA fields differ from the
      sibling address RRset.

   The ANAME record indicates to a client that it might wish to resolve
   the target address records itself.  The target address records might
   not be available if the server is authoritative and does not include
   out-of-zone or non-authoritative data in its answers, or if the
   server is recursive and the records are not in the cache.

3.2.  ANAME queries

   When a server receives an query for type ANAME, there are three

   o  The query resolved to an ANAME record, and the server has the
      target address records; any target address records SHOULD be added
      to the Additional section.

   o  The query resolved to an ANAME record, and the server does not
      have the target address records; any sibling address records
      SHOULD be added to the Additional section.

   o  The query did not resolve to an ANAME record; any address records
      with the same owner name SHOULD be added to the Additional section
      of the NOERROR response.

   When adding address records to the Additional section, if not all
   address types are present and the zone is signed, the server SHOULD
   include a DNSSEC proof of nonexistence for the missing address types.

4.  Substituting ANAME sibling address records

   This process is used by both primary masters (see Section 5) and
   resolvers (see Section 6), though they vary in how they apply the
   edit described in the final step.

   The following steps MUST be performed for each address type:

   1.  Starting at the ANAME owner, follow the chain of ANAME and/or
       CNAME records as far as possible to find the ultimate target.

   2.  If a loop is detected, continue with an empty RRset, otherwise
       get the ultimate target's address records.  (Ignore any sibling
       address records of intermediate ANAMEs.)

   3.  Stop if resolution failed.  (Note that NXDOMAIN and NODATA count
       as successfully resolving an empty RRset.)

   4.  Replace the owner of the target address records with the owner of
       the ANAME record.  Reduce the TTL to match the ANAME record if it
       is greater.  Drop any RRSIG records.

   5.  Stop if this modified RRset is the same as the sibling RRset
       (ignoring any RRSIG records).  The comparison MAY treat nearly-
       equal TTLs as the same.

   6.  Delete the sibling address RRset and replace it with the modified

   At this point, the substituted RRset is not signed.  A primary master
   will proceed to sign the substituted RRset, whereas resolvers can
   only use the substituted RRset when an unsigned answer is
   appropriate.  This is explained in more detail in the following

5.  ANAME processing by primary masters

   Each ANAME's sibling address records are kept up-to-date as if by the
   following process, for each address type:

   o  Perform ANAME sibling address record substitution as described in
      Section 4.  Any edit performed in the final step is applied to the
      ANAME's zone.  A primary server MAY use Dynamic Updates (DNS
      UPDATE) [RFC2136] to update the zone.

   o  If resolution failed, wait for a period before trying again.  This
      retry time SHOULD be configurable.

   o  Otherwise, wait until the target address record TTL has expired,
      then repeat.

   It may be more efficient to manage the polling per ANAME target
   rather than per ANAME as specified (for example if the same ANAME
   target is used by multiple zones).

   Sibling address records are committed to the zone and stored in
   nonvolatile storage.  This allows a server to restart without delays
   due to ANAME processing, use offline DNSSEC signing, and not
   implement special ANAME processing logic when handling a DNS query.

   Appendix E describes how ANAME would fit in different DNS
   architectures that use online signing or tailored responses.

5.1.  Zone transfers

   ANAME is no more special than any other RRtype and does not introduce
   any special processing related to zone transfers.

   A zone containing ANAME records that point to frequently-changing
   targets will itself change frequently, and may see an increased
   number of zone transfers.  Or if a very large number of zones are
   sharing the same ANAME target, and that changes address, that may
   cause a great volume of zone transfers.  Guidance on dealing with
   ANAME in large scale implementations is provided Appendix E.

   Secondary servers that rely on zone transfers to obtain sibling
   address records, just like the rest of the zone, and serve them in
   the usual way (with Section 3 Additional section processing if they
   support it).  A working DNS NOTIFY [RFC1996] setup is recommended to
   avoid extra delays propagating updated sibling address records when
   they change.

5.2.  DNSSEC

   A zone containing ANAME records that will update A and AAAA records
   has to do so before signing the zone with DNSSEC [RFC4033] [RFC4034]

   DNSSEC signatures on sibling address records are generated in the
   same way as for normal (dynamic) updates.

5.3.  TTLs

   Sibling address records are served from authoritative servers with a
   fixed TTL.  Normally this TTL is expected to be the same as the
   target address records' TTL (or the ANAME TTL if that is smaller);
   however the exact mechanism for obtaining the target is unspecified,
   so cache effects or deliberate policies might make the sibling TTL
   smaller.  There is a more extended discussion of TTL handling in

6.  ANAME processing by resolvers

   When a resolver makes an address query in the usual way, it might
   receive a response containing ANAME information in the additional
   section, as described in Section 3.  This informs the resolver that
   it MAY resolve the ANAME target address records to get answers that
   are tailored to the resolver rather than the ANAME's primary master.
   It SHOULD include the target address records in the Additional
   section of its responses as described in Section 3.

   In order to provide tailored answers to clients that are ANAME-
   oblivious, the resolver MAY perform sibling address record
   substitution in the following situations:

   o  The resolver's client queries with DO=0.  (As discussed in
      Section 8, if the resolver finds it would downgrade a secure
      answer to insecure, it MAY choose not to substitute the sibling
      address records.)

   o  The resolver's client queries with DO=1 and the ANAME and sibling
      address records are unsigned.  (Note that this situation does not
      apply when the records are signed but insecure: the resolver might
      not be able to validate them because of a broken chain of trust,
      but its client could have an extra trust anchor that does allow it
      to validate them; if the resolver substitutes the sibling address
      records they will become bogus.)

   In these first two cases, the resolver MAY perform ANAME sibling
   address record substitution as described in Section 4.  Any edit
   performed in the final step is applied to the Answer section of the
   response.  The resolver SHOULD then perform Additional section
   processing as described in Section 3.

   If the resolver's client is querying using an API such as
   "getaddrinfo" [RFC3493] that does not support DNSSEC validation, the
   resolver MAY perform ANAME sibling address record substitution as
   described in Section 4.  Any edits performed in the final step are
   applied to the addresses returned by the API.  (This case is for
   validating stub resolvers that query an upstream recursive server
   with DO=1, so they cannot rely on the recursive server to do ANAME
   substitution for them.)

7.  IANA considerations

   IANA is requested to assign a DNS RR TYPE value for ANAME resource
   records under the "Resource Record (RR) TYPEs" subregistry under the
   "Domain Name System (DNS) Parameters" registry.

   IANA might 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.

8.  Security considerations

   When a primary master updates an ANAME's sibling address records to
   match its target address records, it uses its own best information as
   to the correct answer.  The primary master might sign the updated
   records, but that is not a guarantee of the actual correctness of the
   answer.  This signing can have the effect of promoting an insecure
   response from the ANAME <target> to a signed response from the
   <owner>, which can then appear to clients to be more trustworthy than
   it should.  DNSSEC validation SHOULD be used when resolving the ANAME
   <target> to mitigate this possible harm.  Primary masters MAY refuse
   to substitute ANAME sibling address records unless the <target> node
   is both signed and validated.

   When a resolver substitutes an ANAME's sibling address records, it
   can find that the sibling address records are secure but the target
   address records are insecure.  Going ahead with the substitution will
   downgrade a secure answer to an insecure one.  However this is likely
   to be the counterpart of the situation described in the previous
   paragraph, so the resolver is downgrading an answer that the ANAME's
   primary master upgraded.  A resolver will only downgrade an answer in
   this way when its client is security-oblivious; however the client's
   path to the resolver is likely to be practically safer than the
   resolver's path to the ANAME target's servers.  Resolvers MAY choose
   not to substitute sibling address records when they are more secure
   than the target address records.

9.  Acknowledgments

   Thanks to Mark Andrews, Ray Bellis, Stefan Buehler, Paul Ebersman,
   Richard Gibson, Tatuya JINMEI, Hakan Lindqvist, Mattijs Mekking,
   Stephen Morris, Bjorn Mott, Richard Salts, Mukund Sivaraman, Job
   Snijders, Jan Vcelak, Paul Vixie, Duane Wessels, and Paul Wouters,
   Olli Vanhoja for discussion and feedback.

10.  Changes since the last revision

   [This section is to be removed before publication as an RFC.]

   The full history of this draft and its issue tracker can be found at
   https://github.com/each/draft-aname [1]

10.1.  Version -03

   o  Grammar improvements (Olli Vanhoja)

   o  Split up Implications section, clarify text on zone transfers and
      dynamic updates.

   o  Rewrite Alternative setup section and move to Appendix, add text
      on zone transfer scalibility concerns and GeoIP.

10.2.  Version -02

   Major revamp, so authoritative servers (other than primary masters)
   now do not do any special ANAME processing, just Additional section

11.  References

11.1.  Normative References

   [RFC1033]  Lottor, M., "Domain Administrators Operations Guide",
              RFC 1033, DOI 10.17487/RFC1033, November 1987,

   [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>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <https://www.rfc-editor.org/info/rfc3597>.

   [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,

   [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,

   [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,

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

11.2.  Informative References

   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, DOI 10.17487/RFC0882, November 1983,

   [RFC0973]  Mockapetris, P., "Domain system changes and observations",
              RFC 973, DOI 10.17487/RFC0973, January 1986,

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <https://www.rfc-editor.org/info/rfc1996>.

   [RFC2065]  Eastlake 3rd, D. and C. Kaufman, "Domain Name System
              Security Extensions", RFC 2065, DOI 10.17487/RFC2065,
              January 1997, <https://www.rfc-editor.org/info/rfc2065>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, DOI 10.17487/RFC3493, February 2003,

11.3.  URIs

   [1] https://github.com/each/draft-aname

Appendix A.  Implementation status

   PowerDNS 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, Dyn, and Akamai.]

Appendix B.  Historical note

   In the early DNS [RFC0882], CNAME records were allowed to coexist
   with other records.  However this led to coherency problems: if a
   resolver had no cache entries for a given name, it would resolve
   queries for un-cached records at that name in the usual way; once it
   had cached a CNAME record for a name, it would resolve queries for
   un-cached records using CNAME target instead.

   For example, given the zone contents below, the original CNAME
   behaviour meant that if you asked for "alias.example.com TXT" first,
   you would get the answer "owner", but if you asked for
   "alias.example.com A" then "alias.example.com TXT" you would get the
   answer "target".

      alias.example.com.      TXT    "owner"
      alias.example.com.      CNAME  canonical.example.com.
      canonical.example.com.  TXT    "target"
      canonical.example.com.  A

   This coherency problem was fixed in [RFC0973] which introduced the
   inconvenient rule that a CNAME acts as an alias for all other RR
   types at a name, which prevents the coexistence of CNAME with other

   A better fix might have been to improve the cache's awareness of
   which records do and do not coexist with a CNAME record.  However
   that would have required a negative cache mechanism which was not
   added to the DNS until later [RFC1034] [RFC2308].

   While [RFC2065] relaxed the restriction by allowing coexistence of
   CNAME with DNSSEC records, this exception is still 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.  DNSSEC records avoid interoperability
   problems by being largely invisible to security-oblivious resolvers.

   Now that the DNS has negative caching, it is tempting to amend the
   algorithm for resolving with CNAME records to allow them to coexist
   with other types.  Although an amended resolver will be compatible
   with the rest of the DNS, it will not be of much practical use
   because authoritative servers which rely on coexisting CNAMEs will
   not interoperate well with older resolvers.  Practical experiments
   show that the problems are particularly acute when CNAME and MX try
   to coexist.

Appendix C.  On preserving TTLs

   An ANAME's sibling address records are in an unusual situation: they
   are authoritative data in the owner's zone, so from that point of
   view the owner has the last say over what their TTL should be; on the
   other hand, ANAMEs are supposed to act as aliases, in which case the
   target should control the address record TTLs.

   However there are some technical constraints that make it difficult
   to preserve the target address record TTLs.

   The following subsections conclude that the end-to-end TTL (from the
   authoritative servers for the target address records to end-user DNS
   caches) should be set as the target address record TTL plus the
   sibling address record TTL.

   [MM: Discuss: I think it should be just the ANAME record TTL perhaps
   the minimum of ANAME and sibling address RRset TTL.  We should
   provide some guidance on TTL settings for ANAME).

   [TF: see issue #30]

C.1.  Query bunching

   If the times of end-user queries for a domain name are well
   distributed, then (typically) queries received by the authoritative
   servers for that domain are also well distributed.  If the domain is
   popular, a recursive server will re-query for it once every TTL
   seconds, but the periodic queries from all the various recursive
   servers will not be aligned, so the queries remain well distributed.

   However, imagine that the TTLs of an ANAME's sibling address records
   are decremented in the same way as cache entries in recursive
   servers.  Then all the recursive servers querying for the name would
   try to refresh their caches at the same time when the TTL reaches
   zero.  They would become synchronized, and all the queries for the
   domain would be bunched into periodic spikes.

   This specification says that ANAME sibling address records have a
   normal fixed TTL derived from (e.g. equal or nearly equal to) the
   target address records' original TTL.  There is no cache-like
   decrementing TTL, so there is no bunching of queries.

C.2.  Upstream caches

   There are two straightforward ways to get an RRset's original TTL:

   o  by directly querying an authoritative server;

   o  using the original TTL field from the RRset's RRGIG record(s).

   However, not all zones are signed, and a primary master might not be
   able to query other authoritative servers directly (e.g. if it is a
   hidden primary behind a strict firewall).  Instead it might have to
   obtain an ANAME's target address records via some other recursive

   Querying via a separate recursive server means the primary master
   cannot trivially obtain the target address records' original TTLs.
   Fortunately this is likely to be a self-correcting problem for
   similar reasons to the query-bunching discussed in the previous
   subsection.  The primary master re-checks the target address records
   just after the TTL expires when its upstream cache has just refreshed
   them, so the TTL will be nearly equal to the original TTL.

   A related consideration is that the primary master cannot in general
   refresh its copies of an ANAME's target address records more
   frequently than their TTL, without privileged control over its
   resolver cache.

   Combined with the requirement that sibling address records are served
   with a fixed TTL, this means that the end-to-end TTL will be the
   target address record TTL (which determines when the sibling address
   records are updated) plus the sibling address record TTL (which
   determines when end-user caches are updated).

C.3.  ANAME chains

   ANAME sibling address record substitution is made slightly more
   complicated by the requirement to follow chains of ANAME and/or CNAME
   records.  This stops the end-to-end TTL from being inflated by each
   ANAME in the chain.

C.4.  TTLs and zone transfers

   When things are working properly (with secondary name servers
   responding to NOTIFY messages promptly) the authoritative servers
   will follow changes to ANAME target address records according to
   their TTLs.  As a result the end-to-end TTL is unchanged from the
   previous subsection.

   If NOTIFY doesn't work, the TTLs can be stretched by the zone's SOA
   refresh timer.  More serious breakage can stretch them up to the zone
   expiry time.

Appendix D.  Answer vs Additional sections

   [MM: Discuss what should be in the additional section: ANAME makes
   sense, but differs from CNAME logic (where the CNAME is in the answer
   section).  Additional target records that match the query type in my
   opinion should go in the answer section.  Additional target address
   records that do not match the query type can go in the additional

   [TF: from experience with DNAME I think there's a risk of interop
   problems if we put unexpected records in the answer section, so I
   said everything should go in additional.  We'll expand this appendix
   to explain the rationale.]

Appendix E.  Alternative setups

   If you are a large scale DNS provider, ANAME may introduce some
   scalability concerns.  A frequently changing ANAME target, or a ANAME
   target that changes its address and is used for many zones, can lead
   to an increased number of zone transfers.  Such DNS architectures may
   want to consider a zone transfer mechanism outside the DNS.

   Another way to deal with zone transfer scalability is to move the
   ANAME processing (Section 4) inside the name server daemon.  This is
   not a requirement for ANAME to work, but may be a better solution in
   large scale implementations.  These implementations usually already
   rely on online DNSSEC signing for similar reasons.  If ANAME
   processing occurs inside the name server daemon, it MUST be done
   before any DNSSEC online signing happens.

   For example, some existing ANAME-like implementations are based on a
   DNS server architecture, in which a zone's published authoritative
   servers all perform the duties of a primary master in a distributed
   manner: provisioning records from a non-DNS back-end store,
   refreshing DNSSEC signatures, and so forth.  They don't use standard
   standard zone transfers, and already implement their ANAME-like
   processing inside the name server daemon, substituting ANAME sibling
   address records on demand.

   Also, some DNS providers will tailor responses based on information
   in the client request.  Such implementations will use the source IP
   address or EDNS Client Subnet information and use geographical data
   (GeoIP) or network latency measurements to decide what the best
   answer is for a given query.  Such setups won't work with traditional
   DNSSEC and provide DNSSEC support usually through online signing.
   Similar such setups should provide ANAME support through substituting
   ANAME sibling records on demand.

   The exact mechanism for obtaining the target address records in such
   setups is unspecified; typically they will be resolved in the DNS in
   the usual way, but if an ANAME implementation has special knowledge
   of the target it can short-cut the substitution process, or it can
   use clever tricks such as client-dependant answers.

Authors' Addresses

   Tony Finch
   University of Cambridge
   University Information Services
   Roger Needham Building
   7 JJ Thomson Avenue
   Cambridge  CB3 0RB

   Email: dot@dotat.at

   Evan Hunt
   950 Charter St
   Redwood City, CA  94063

   Email: each@isc.org

   Peter van Dijk
   PowerDNS.COM B.V.
   Den Haag
   The Netherlands

   Email: peter.van.dijk@powerdns.com

   Anthony Eden
   Boston, MA  USA

   Email: anthony.eden@dnsimple.com
   URI:   https://dnsimple.com/

   Matthijs Mekking
   950 Charter St
   Redwood City, CA  94063

   Email: matthijs@isc.org

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