A Realtime Blackhole List is a way to prevent SPAM by intentional loss of network connectivity.

The MAPS (Mail Abuse Prevention System) RBL works the following way:

  1. Everytime a Mail Transport Agent (MTA) ,such as sendmail, receives an E-Mail from a host, it checks if the host is listet in the RBL. If it is listed, the MTA rejects the E-Mail. The lookup is done via DNS. Example:
    An E-Mail comes from . The MTA makes a lookup for If the lookup is successful, your MTA knows that the host ist used by a spammer and will block the E-Mail.

  2. The MAPS has registered the AS 7777. This AS announces routes to the blocked hosts. You can configure your router to peer with this AS, but define something similar to /dev/null as the next hop, so all packets from or to spammers host will be thrown away.

    It's the hell for a postmaster if his mailserver is put on the RBL, because all of his users won't be able to send mail anymore because a lot of hosts block it.

The problem:
Spam. Because networks of either being friendly to spammers, or neutral to, spammers take advantage of this and send email from a particular network. End users don't like spam.

One solution:
After numerous accounts of spam email from a site and failure for some reason or another to reach an agreement with the site owners, that site may be put on the RBL. The reasons for being put on the RBL are numerous but single minded - stop unsolicited bulk email.

  • badly managed opt-out lists from large companies
  • an open SMTP relay
  • hosting services in support of unsolicited bulk email
    • hosting web pages promoted by UBE
    • providing email boxes and/or autoresponders promoted by UBE
    • providing resources such as DNS, banner ads, hit counters, script processing, form handlers to sites promoted by spam.
    • providing software or services for distributing spam
    • hosting web pages or providing connectivity to those who provide software or services for UBE.
    • providing credit card processing or other online payment services for goods and services promoted by UBE
    • providing adult verification services to sites promoted by spam
    • providing email address by any other means than opt-in.
    • continuing to provide service to customers providing spam support services after this has been brought to the provider's attention.
People who subscribe to this DNS services may block out the networks that are listed. This can be anything from a simple sendmail ruleset that drops mail from a site or network on the RBL to placing these sites and networks in the router and effectively removing them from your view of the network.


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