How Direct Query Access works:
SHORT ANSWER (for those in a hurry): You don’t even need to know. Typically, you just copy a particular host name (that we will assign you), paste that into a form field in your spam filter, hit 'save', and start enjoying better spam filtering. This only takes a few minutes. It is that fast and easy!
TECHNICAL DETAILS: Almost all spam filters have the ability to add 3rd party DNS blacklists. When adding high quality DNS blacklists, this can potentally greatly improve a spam filtering system (or NOT if the DNS blacklist is of poor quality!). DNS blacklists are a sort of (whitehat!) 'hack' on the Domain Name System ('DNS' system) whereby a reversed-IP address, or a host name, is appended to the beginning of the domain or host name of the anti-spam blacklist. Then, in response to a DNS query, that blacklist either returns an NXDOMAIN value (domain not found), or, if the particular domain or IP is in fact blacklisted, then it 'resolves' that DNS query to an IP, typically 127.0.0.2, or some variation of that.
But you don't even need to understand all of these technical details: The important part is that, to get this to work, you simply input a single host name into a field in the control panel for your spam filter (or into a configuration file) - hit 'save' - and it just works. From that point forward, your spam filter starts to 'consult' the DNSBL (which stands for "Domain Name System Black List'). This is generally 'set it and forget it' and it only take a few minutes to implement this! You simply copy and paste a host name into a field and hit save - and you are done! It is that fast and easy to implement!
How RSYNC Access to RBLDNSD-formatted files works:
Now that you know what a DNSBL is, and how it works (you did read the Direct Queries section first, correct?), here is how RSYNC Access works. RSYNC is similar to FTP, but is a MUCH more efficient way to transfer data files. When you sign up for RSYNC access, we provide scripts to help you to RSYNC rbldnsd-formatted data files to your server. These files are similar to BIND zone files, except they are designed for RBLDNSD, which is a dumbed-down DNS server that is build for hosting DNSBLs, due to its extremely fast speed and its low memory usage. After you RSYNC those data files, they then load up into the rbldnsd program, which then answers queries by listening on a particular LAN or LOCAL IP address. To 'glue' this up, generally you might put a conditional forwarder into your caching DNS server, telling it to 'ask' your rbldnsd program for answers to DNS queries when resolving these particular DNSBL queries. RBLDNSD and RSYNC are completely free for Linux, and there are low-cost distributions that have been ported to Windows. But the setup for RSYNC and RBLDNSD (for either Windows or Linux) is more involved and takes more time than Direct Queries. Whereas someone can be up and running with direct queries in just a few minutes, RSYNC and RBLDNSD can typically take an hour or more to implement. But the benefit of RBLDNSD is that it can answer potentially thousands of queries per second, which enables it to efficently handle vastly higher volumes!