Stan Hoeppner:
> Noel Jones put forth on 8/24/2010 2:18 PM:
> > - This is specific for  Postfix needs a general mechanism. 
> > Other whitelists are not required to follow's 127.0.x.y
> > mechanism.
> Yeah, I used this example as dnswl is, afaik, the most "established" of
> the dns whitelists.  I haven't yet looked at the return codes the others
> use.
> > - what do you mean by "accept the message"; OK? suppress further rbl
> > lookups?

Postfix has smtpd_mumble_restrictions with a large number of
reject-like features and a smaller number of permit-like features.
A reject terminates evaluation for all smtpd_mumble_restrictions;
a permit terminates evaluation only within one smtpd_mumble_restriction.

If whitelisting were to be used as a permit-like feature (which
has dangerous failure modes as discussed next) then it will have
to behave like all other permit-like features without exception.

DNSWL as a permit-like feature increases the risk of becoming an
open relay.  I don't think we want Postfix to massively fail wide
open and become an open relay just because some DNSWL operator made
a bad decision. Besides, I am not convinced that DNSWL is best used
as an unconditional "permit" operation.

Alternatively, DNSWLs would be safe to use when scores from different
lists are added up, and mail is rejected when the total score
exceeds some threshold.  With DNSXL lookup implemented as a
reject-like feature, there is no danger of Postfix massively failing
wide open when the DNSWQL operator screws up.

Currently the smtpd configuration language does not yet have weighted
DNSXL lookups.  It would be easy enough to configure a global fixed
list with domains and weights. Just copy the code for the deprecated
maps_rbl_domains configuration parameter and the no longer documented
reject_maps_rbl restriction, and add some syntax to the maps_rbl_domains


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