Re: Mis-documented negated DNSBL bitmask matches?

Does Exim not have the ability to perform AND ('&'), OR ('|') and XOR 
('^') bitwise operators on DNSBL results?  If it does then users ought 
to be able to configure their way round this sort of issue...

Mike



On 26/01/2026 00:25, Chris Siebenmann via Exim-users wrote:
>> On 2026/01/13 3:18 PM, Chris Siebenmann via Exim-users wrote:
>>> Currently, the documentation says this about negated bitwise-and
>>> conditions:
>>>
>>>           Negation can also be used with a bitwise-and restriction. The
>>>           dnslists condition with only be trus if a result is returned by
>>>           the lookup which, anded with the restriction, is all zeroes. For
>>>           example:
>>> As I read it, the documentation appears to be saying that a '!&'
>>> bitwise-and works as:
>>> 	(ip & bitmask) == 0
>> If I'm reading right, you're thinking that "the restriction" in the docs text
>> is identical to the supplied bitmask.
>>
>> I think it instead means "the bitmask as adjusted by the negation".
>>
>> We could write something like
>> "anded with the inverted bitmask"
>>
>> or even
>> "The dnslists condition will only be true if, after clearing the bits in the
>> result indicated by the mask, the result is all zeroes."
>>
>> Would you find that more clear?
>> Happy to hear comments from anyone else, too.
> The problem is that inverting the bitmask is not correct either, either
> by the code or (I believe) by bitwise logic. Consider a DNSBL result of
> 127.255.255.10 and a '!&0.255.255.0' condition. If you invert
> 0.255.255.0 you get 255.0.0.255, and then if you bitwise-and this with
> 127.255.255.10 you get the non-zero result 127.0.0.10, which is not
> all-zeros, which should apparently be accepted by this when we want it
> to be rejected (and it is rejected by Exim today).
>
> I can't come up with any non-contorted '(ip & <something>) == 0'
> condition that works correctly here, and (as mentioned) it's not what
> the Exim code itself is doing.
>
> 	- cks
>


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