Description
SPFv3 will use exclusively the SPF RR (type 99).
Rationale
RFC 4408 used the TXT RR because the newer record type was not widely available. After several years, most DNS software as been modified introducing the SPF type. The current threats and the status of DNSSEC (or DNS over SCTP) prefigure that updated versions of the software will have been installed on most hosts by the time the new RFC will be published.
Backward compatibility
It is RECOMMENDED that a backward compatible TXT RR starting with v=spf1 be maintained until some SPF checks will be carried out by old software. (Users can check their DNS logs, looking for TXT requests after sending.)
Old rfc4408-software should discard new SPF RRs starting with v=spf3 according to step 1 of section 4.5, and then proceed with v=spf1 RRs, probably but not necessarily of type TXT, if any. New software SHOULD look up TXT RRs if it finds no SPF RR, and MAY accept v=spf1 for backward compatibility. Admins MUST NOT create RRs of type TXT that start with v=spf3.
