I believe that the solution is to have the applications, themselves,
distinguish the cache they are using (or the containing library). A
blocklist app needs to use a different library/cache than a web
You could do it either way. Either you could adjust the MTAs to have
a new parameter for the address of the cache to use for DNSBLs vs. the
cache to use for MX, A, and AAAA, or you could tell the cache about
the list of DNSBLs that the local MTAs use (a list which in my
experience is rarely very long.)
Pick your poison, depends which software is older and cruddier.
Or there's the pessimal approach, add a new EDNS parameter so the MTA
can tell the cache what kind of lookup it's making, so you have to
upgrade and debug both of them!
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