Editing Domain Name System blocklist (section)

=== DNSBL queries ===
When a mail server receives a connection from a client, and wishes to check that client against a DNSBL (let's say, ''dnsbl.example.net''), it does more or less the following:

# Take the client's IP address—say, ''192.168.42.23''—and reverse the order of octets, yielding ''23.42.168.192''.
# Append the DNSBL's domain name: ''23.42.168.192.dnsbl.example.net''.
# Look up this name in the DNS as a domain name ("A" record). This will return either an address, indicating that the client is listed; or an "NXDOMAIN" ("No such domain") code, indicating that the client is not.
# Optionally, if the client is listed, look up the name as a text record ("TXT" record). Most DNSBLs publish information about why a client is listed as TXT records.

Looking up an address in a DNSBL is thus similar to looking it up in reverse-DNS. The differences are that a DNSBL lookup uses the "A" rather than "PTR" record type, and uses a forward domain (such as ''dnsbl.example.net'' above) rather than the special reverse domain ''in-addr.arpa''.

There is an informal protocol for the addresses returned by DNSBL queries which match. Most DNSBLs return an address in the 127.0.0.0/8 IP [[loopback]] network. The address 127.0.0.2 indicates a generic listing. Other addresses in this block may indicate something specific about the listing—that it indicates an open relay, proxy, spammer-owned host, etc. For details see RFC 5782.