DNS-Based Address Mappping Record (AMR) for IPv4/IPv6 Mapping in IPv6-only Networks

4.1. Record Name Construction

AMR records use a special domain name space under “in-addr-m.arpa”. For an IPv4 address block 192.0.2.0/24, the corresponding AMR record name is constructed by reversing the IPv4 prefix and appending ".in-addr-m.arpa". Example:¶

• IPv4 block: 192.0.2.0/24¶

• IPv6 mapping prefix: 2001:db8::/64¶

• The AMR record: 0.2.0.192.in-addr-m.arpa. IN AMR 2001:db8::/64¶

4.2. AMR Encoding

The encoding of the type is shown below：¶

    +----------------------------------------+
    |        Name (variable)                 |
    +----------------------------------------+
    |        Type (2 octets)                 |
    +----------------------------------------+
    |        Class (2 octets)                |
    +----------------------------------------+
    |        TTL (4 octets)                  |
    +----------------------------------------+
    |        Data Length (2 octets)          |
    +----------------------------------------+
    |        v4prefix-length (1 octet)       |
    +----------------------------------------+
    |        v6prefix-length (1 octet)       |
    +----------------------------------------+
    |        v6prefix (0...16 octets)        |
    +----------------------------------------+

            Figure 1: Format of AMR Record

• Name— the reverse IPv4 address, its format is IPv4 address block.in-addr-m.arpa;¶

• Type— the type of the record，AMR;¶

• Class— indicating that the record is for the Internet environment;¶

• TTL—time-to-live in cache;¶

• Data length— length of the data section;¶

• v4prefix-length— length of the IPv4 address block;¶

• v6prefix-length— length of the IPv6 mapping prefix;¶

• v6prefix— IPv6 mapping prefix.¶

5.1. Mapping Relationship Registration

PE1 router generate mappings between local IPv4 address blocks (served by their connected IPv4 networks) and their own IPv6 mapping prefixes (Pref6). Then, PE1 send these mappings to an authoritative DNS server, which stores them as AMR records in its database. The authoritative server validates the format of the mappings (e.g., correct prefix lengths, valid IPv6 prefixes) before adding them to the appropriate zone file under in-addr-m.arpa.¶

5.2. AMR Record Publication in Global DNS

Authoritative DNS servers publish AMR records within their managed DNS zones, making them available for query by local DNS servers. AMR records are assigned a TTL value (e.g., 3600 seconds) to control cache freshness. Local DNS servers cache AMR records and refresh them by querying the authoritative server once the TTL expires, ensuring up-to-date mappings.¶

5.3. Procedure for Ingress PE to Query IPv6 Mapping Prefix for a Specific IPv4 Address via DNS

When Ingress PE receives an IPv4 packet, it queries the DNS to retrieve the corresponding IPv6 mapping prefix. The procedure is as follows:¶

1. Extracts destination IPv4 address (e.g., 192.0.2.1).¶

2. Query Construction:¶

   Ingress PE constructs a DNS query using the reversed destination IPv4 address appended with .in-addr-m.arpa and specifies the AMR record type. For example, a destination IPv4 address of 192.0.2.1 (within 192.0.2.0/24) results in the query name 1.2.0.192.in-addr-m.arpa. IN AMR.¶

3. Query Resolution:¶

   1. Checks local DNS cache for existing AMR record:¶

      i.

      If valid cached record exists with unexpired TTL:¶

      —

      Uses the cached IPv6 prefix for packet conversion;¶

      —

      Skips further DNS queries.¶

      ii.

      If no valid cache entry exists:¶

      —

      Proceeds to step 3b.¶

   2. Queries configured local recursive resolver (if any):¶

      i.

      Local resolver checks its cache and zone data;¶

      ii.

      If record exists in local authoritative zones:¶

      —

      Returns AMR record with TTL;¶

      —

      PE updates local cache;¶

      —

      Proceeds to packet conversion.¶

      iii.

      If no local record exists:¶

      —

      Proceeds to step 3c¶

   3. Initiates standard DNS resolution via recursive lookup:¶

      i.

      Follows normal DNS resolution chain;¶

      ii.

      Queries authoritative servers for in-addr-m.arpa;¶

      iii.

      Validates DNSSEC signatures if present.¶

4. Cache Refresh:¶

   When the TTL of a cached AMR record expires, the local server re-queries the authoritative server to fetch the latest record, updating its cache accordingly.¶

9.1. 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>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC4026]

Andersson, L. and T. Madsen, "Provider Provisioned Virtual Private Network (VPN) Terminology", RFC 4026, DOI 10.17487/RFC4026, March 2005, <https://www.rfc-editor.org/info/rfc4026>.

[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>.

9.2. Iormative References

[I-D.ietf-v6ops-framework-md-ipv6only-underlay]

Xie, C., Ma, C., Li, X., Mishra, G. S., and T. Graf, "Framework of Multi-domain IPv6-only Underlay Network and IPv4-as-a-Service", Work in Progress, Internet-Draft, draft-ietf-v6ops-framework-md-ipv6only-underlay-11, 30 June 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-framework-md-ipv6only-underlay-11>.

[RFC6891]

Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013, <https://www.rfc-editor.org/info/rfc6891>.