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I'm trying to setup my own custom dnsbl server for blocking spam. I have successfully created a server running rbldnsd but am curious about what DNS records I would need to have in place so that my content filter can locate and communicate with the dnsbl...

For example, lets say that my dnsbl server has an ip of and I want my content filter to be able to specify the dnsbl server as

Would I need to have an NS record pointing to the blocklist server? Or would I just need an A record in the zone pointing to

Lastly, are there any RFCs for dnsbls? I'd like to understand the play-by-play for how content filters are supposed to request this type of information.


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up vote 1 down vote accepted

The closest thing to a "standard" for DNS blacklists is RFC 5782.

You will indeed need NS records to point the sub-domain of your blacklist server at the dnsbl server. Note that those NS records will have to point to the hostname of that server, and of course that hostname will need A records pointing to the actual IP address.

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From a DNS perspective a dnsbl server is just like any other DNS zone, and DNSBL lookups operate by your mail server making a DNS lookup in a specified zone. If you want to delegate your DNSBL zone (e.g. to the server running rbldnsd) you need NS records pointing to the server you're delegating to.

Re: a DNSBL play-by-play, there are probably RFCs on the subject but I can't think of any off the top of my head. is probably a good starting point if you're looking to understand how DNS blackhole lists are supposed to work.

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