A
DNS Blacklist, or
DNSBL, is a means by which an Internet site may publish a list of
IP addresses that some people may want to avoid and in a format which can be easily queried by computer programs on the Internet.
The technology is built on top of the Internet
Domain Name System, or DNS. DNSBLs are chiefly used to publish lists of addresses linked to spamming. Most mail transport agent (mail server) software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists.
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 bytes, 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.
This tool is intended can be used to lookup IP Addresses in the w3dt Blacklsit/Whitelist Database.
-----------------------------------------------------------------------------------------------------------
The w3dt (world wide web domain tools) ARBL Checker tool is based off C code by Steve Friedl.
