If Allman's concerns are solely about complexity, then drop include:
redirect= and macros, or at the very least make them MAY rather than
MUST. I don't think A, MX, and PTR mechanisms are any more complex
than IP4.
If his concerns are about performance, that multiple DNS lookups need
to be performed in sequence, then we should do what some BIND
implementations do with MX records: send additional information in the
response that are going to be asked for anyway.
An MX lookup for 'wayforward.net' on my server returns:
wayforward.net preference = 40, mail exchanger = mail.wayforward.net
wayforward.net preference = 30, mail exchanger = mx1.wayforward.net
mail.wayforward.net internet address = 209.208.196.229
mx1.wayforward.net internet address = 69.55.226.139
All these entries are (presumably) cached, so when a mailserver does
the 'A' query for 'mx1.wayforward.net' the response is immediate.
So, when the SPF DNS resource type is supported, DNS server
implementations SHOULD send additional info: A records if the SPF
record contains an A mechanism, MX records if the SPF record contains
an MX mechanism, etc.
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