rbldns-data updates data.cdb atomically, so you can use it safely while rbldns(8) is running. If anything goes wrong with the creation of data.cdb, rbldns-data stops and leaves the old data.cdb in place.
The IP address ip is on the list.
All IP addresses that share the first n bits of the IP address ip are on the list. For example, 126.96.36.199/24 means all 1.2.3.* addresses. n must be at least 8, and all bits past the nth in ip must be 0.
DNS queries for addresses on the list will receive an A record of a and a TXT record of txt. Typically a is 127.0.0.2 and txt is a pointer to a web page about the list.
If txt ends with $, rbldns(8) replaces the $ with the IP address.
Comment line. The line is ignored.