Using RBLs

By Joshua Erdman
Digital Foundation, inc.

As mentioned before RBL is a Realtime Black List. Being BlackListed is not necessarily fun and since the data on these blacklists is easily accessible, as soon as your mailserver gets blacklisted on one list, it can quickly spread to be listed on others.

How RBLs work

It is amazingly simple and is a modification of DNS. On a typical domain in DNS an organization would post name records that resolve to IP addresses. Typically www.company.com and mail.company.com and querying these domain names would provide the IP addresses to that company's web server and mail servers. As a modification to DNS if this company provided a blacklist you would configure your server to query this special list server but the queries would not be for actual hosts, rather they would be the IP addresses of any mail server in question.

RBL Query, step-by-step

Your mail server receives a request to be given mail from another SMTP server on the Internet. At that point your mail server has the sending mail server's IP address.
Before continuing any communication with the sending mail server, your mail server will query the blacklists that it is configured with. The query is actually just a DNS query containing the IP address with a specific domain appended to it. It would look something like:
63.114.25.24.sbl.spamhaus.org
The modified DNS server will then return an response code in the for of an IP address. Each address given starts with 127.0.0 the last number refers to the reason why it was blocked which varies by RBL provider. If an IP address is not provided then the mail server in question is not blacklisted.
Your mail server will then respond to the incoming mail server either allowing the session to continue or by dropping it.

RBL Lists

Spam Server List - sbl-xbl.spamhaus.org

Open Relay List - relays.ordb.org - Deactivated!
-ORDB has been deactivated as of 12/18/2006

There are many more lists but the more lists you have your server check, the slower the response times for incoming messages. If you are using a free service, please realize that these organizations are doing all of us a favor - so send an annual donation.

References:
Configuring your 2003 Exchange Server for Spam Filtering

Configuring Sendmail for Spam Filtering

Article last reviewed: 12/29/2006

Created by: Digital Foundation, inc.

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