name {ttl} addr-class PX prefer 822-dom X.400-dom
*.ADMD-garr.X42D.it. IN PX 50 it. ADMD-garr.C-it.
*.infn.it. IN PX 50 infn.it. O.PRMD-infn.ADMD-garr.C-it.
*.it. IN PX 50 it. O-gate.PRMD-garr.ADMD-garr.C-it.
The PX records (Pointer to X.400/RFC822 mapping information) are used to specify address mapping rules between X.400 O/R addresses and RFC822 style (domain-style) mail addresses. For a detailed description of the mapping process please refer to RFC1327.
Mapping rules are of 3 different types:
1) mapping from X.400 to RFC822 (defined as "table 1 rules" in RFC1327)
2) mapping from RFC822 to X.400 (defined as "table 2 rules" in RFC1327)
3) encoding RFC822 into X.400 (defined as "gate table" in RFC1327)
All three types of mapping rules are specified using PX Resource Records in DNS, although the name value is different: for case 1, the name value is an X.400 domain in DNS syntax, whereas for cases 2 and 3 the name value is an RFC822 domain. Refer to RFC-1664 for details on specifying an X.400 domain in DNS syntax and for the use of the X42D keyword in it. Tools are available to convert from RFC1327 tables format into DNS files syntax. Preference is analogous to the MX RR Preference parameter: it is currently advised to use a fixed value of 50 for it. 822-dom gives the RFC822 part of the mapping rules, and X.400-dom gives the X.400 part of the mapping rule (in DNS syntax). It is currently advised always to use wildcarded name values, as the RFC1327 tables specifications permit wildcard specifications only. This is to keep compatibility with existing services using static RFC1327 tables instead of DNS PX information.
Specifications of mapping rules from X.400 to RFC822 syntax requires the creation of an appropriate X.400 domain tree into DNS, including thus specific SOA and NS records for the domain itself. Specification of mapping rules from RFC822 into X.400 can be embedded directly into the normal direct name tree. Again, refer to RFC1664 for details about organization of this structure.
Tools and library routines, based on the standard resolver ones, are available to retrieve from DNS the appropriate mapping rules in RFC1327 or DNS syntax.
Once again, refer to RFC1664 to use the PX resource record, and be careful in coordinating the mapping information you can specify in DNS with the same information specified into the RFC1327 static tables.
The PX record is still experimental; not all servers implement or recognize it.