https://web.archive.org/web/20081226214006/http://www.netwidget.net/books/apress/dns/info/dnsbl.html

Black Lists are used in the context of email to define the IP addresses or netblocks of well known sources of SPAM. DNSBL defines a method of using standard DNS zone files to store such IP addresses. Standard DNS A RR queries are used to interrogate the black list which is organised as a reverse mapping zone file. Assuming the blacklist is held at the domain name blacklist.example.com, the process works as follows:

1. The receiving SMTP Agent extracts the IP address of the sending SMTP Agent, for example 192.168.2.135.
2. The address is reversed to create a label, in the above case this becomes 135.2.168.192
3. The reversed label is prepended to the DNSRBL name to give a domain name of 135.2.168.192.blacklist.example.com
4. A DNS A RR query is issued to the domain name of 135.2.168.192.blacklist.example.com
5. The responding name server either returns a valid A RR (confirming the IP address is in the black list) or an NXDOMAIN error (the IP address is not in the blacklist).
6. For those IP addresses which appear in the black list the DNSBL may optionally store a standard TXT RR at the same name giving some explanation for the black listing.
7. The fact that an A RR is returned confirms that the queried IP address does appear in the specific blacklist. The actual address returned is by convention in the loopback range 127/8 - each address may have a specific meaning - it is used as a return code - some of which are defined in the list below.

​## Example black list zone file

```text $TTL 2d # default RR TTL $ORIGIN blacklist.example.com IN SOA ns1.example.com. hostmaster.example.com.( 2003080800 ; se = serial number 3h ; ref = refresh 15m ; ret = update retry 3w ; ex = expiry 3h ; min = minimum ) IN NS ns1.example.com. IN NS ns2.example.com.

2.0.0.127 IN A 127.0.0.2

135.2.168.192 IN A 127.0.0.2 # or some specific result code address IN TXT "Optional - Some explanation for black listing"

… 135.17.168.192 IN A 127.0.0.2 # generic list … ```