Dear All,
The proposed syntax for SPF strikes me as being useful beyond the original
problem domain of sender authentication. With the exception of the "v=spf1"
token, the rest of an SPF record can describe the set-membership constraints
for internet hosts, regardless of the application.
The analogy I am thinking of is 'Regular Expressions', which (simplified)
describe the set-membership constraints for strings in a very concise format.
The match constraints of an SPF record can be generalized as a 'HostSet
Expression', where the set of hosts being described just happens to be the
permitted email senders for a domain.
Another way of looking at it is by analogy with XHTML vs XML. XHTML is an
application of XML, and similarly SPF could be an application of 'HostSet
Expressions'.
This would involve simply splitting the spec into two separate documents. One
describes HostSet Expressions in an application-neutral way, whilst the other
(much shorter) SPF document simply describes it's application to sender
authentication.
Some use cases:
1. Program operator - like perl's /regexp/.
2. /etc/hosts.allow file entries.
3. Database matching/constraints.
4. Router configuration.
Best of all, it could be done without actually invalidating any of the SPF
records that have already been published.
- Dan
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