DNSBL
A blocklist published in the DNS: a receiving server looks up the connecting IP (or a domain) in the list zone, and an answer means it is listed.
What it means
RFC 5782 documents how DNSBLs work. For an IPv4 list, the octets of the address are reversed and the list's domain appended: if 192.0.2.99 is listed in bad.example.com, the name 99.2.0.192.bad.example.com exists. Every entry has an A record, conventionally 127.0.0.2 (other 127.x values can encode the reason), and should have a TXT record explaining the listing, often reused as the text of the SMTP rejection. Lists of domains rather than IPs work the same way and are called RHSBLs or URI blocklists.
RFC 5782 recounts that DNS distribution quickly became more popular than the first list's original BGP feed. RFC 6471 sets out good practice for operators: published criteria, temporary listings and a direct way to request removal.
Query terms vary: Spamhaus requires commercial users to subscribe.
Example
$ dig +short 99.2.0.192.bad.example.com A
127.0.0.2
$ dig +short 99.2.0.192.bad.example.com TXT
"Listed: see https://bad.example.com/lookup"Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5782: DNS blacklists and whitelistsrfc-editor.org
- RFC 6471: Overview of best email DNSBL operational practicesrfc-editor.org
- Spamhaus: ZEN blocklistspamhaus.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes