Extension of DNS64 (RFC6147) for SPF (RFC7208)
klaus.frank@posteo.de
Application
email
This document describes interoperability issues and resolutions between DNS64 and SPF records for mail transfer agents.
This RFC also aims to simplify the IPv6 migration for mail transfer agent operators.
This document updates and .
Introduction
The DNS64 definition causes issues for mail transfer agent operators as it failed to
consider the existance of SPF records . Because of this when an SPF validator tries to
validate it'll fail because the originating NAT64 IP isn't within the SPF records
allow-/denylist...
Rewriting SPF records
The section 5.1 of gets ammended with another subsection (5.1.9):
5.1.9. Dealing with SPF records
If the DNS64 server receives a SPF-record (within either the TXT-RR or the SPF-RR )
containing the "ip4" mechanism, it MUST rewrites the ipv4 address according to the same rules as an A-RR and
synthesize a new SPF record within the response that contains it as an additional "ip6" entry.
If an ip4-cidr-length is present, it gets converted as well (adding 96 will generate the new ip6-cidr-length).
The original "ip4" mechanism MUST NOT be removed from the response.
If any "a" or "mx" mechanism contains a dual-cidr-length without an ip6-cidr-length, it also gets generated.
E.g.:
"v=spf1 a:a.example.com/24 mx:mx.example.com/24 ip4:192.0.0.1/32 -all"
becomes:
"v=spf1 a:a.example.com/24/120 mx:mx.example.com/24/120 ip4:192.0.0.1/32 ip6:64:ff9b::c000:1/128 -all"
NOTE: Everything else is done by the SPF validator (as already defined in the standard).
* When it checks a.example.com, it'll query the A-RR and AAAA-RR and thereby get a response containing
the synthesized AAAA-RR and validation will pass accordingly.
* When it checks the NAT64 generated IPv6 source address against the SPF, it'll find the "ip6" mechanism and
also pass.
* For any macro-string, the SPF validator will generate new DNS lookups, which will be rewritten according to
this RFC and therefore pass as expected.
SPF Mechanism Definitions
The section 5.7 of currently explicitely
ignores the presence of IPv6 and to future proofe it for IPv6-only it gets updated from
This mechanism is used to construct an arbitrary domain name that is
used for a DNS A record query.
to
This mechanism is used to construct an arbitrary domain name that is
used for a dual DNS A-RR and AAAA-RR query.
and from
The <domain-spec> is expanded as per Section 7. The resulting domain
name is used for a DNS A RR lookup (even when the connection type is
IPv6). If any A record is returned, this mechanism matches.
to
The <domain-spec> is expanded as per Section 7. The resulting domain
name is used for a DNS A-RR and AAAA-RR lookup, depending on when the host is single-stack IPv6
or IPv4. For dual-stack, an SPF resolver MUST query both.
If any A or AAAA record is returned, this mechanism matches.
Informative References