Tom Olzak

A Different Kind of Whitelist?

In Business Continuity, Cybercrime, Email, Phishing, Risk Management, Spam on September 30, 2010 at 13:45

During my years as a security director, one of the weekly challenges I faced was how to tell my peers in engineering that we have more items to add to the growing list of blocked domains or IP addresses.  This was not only a management headache; it also occasionally caused a backup of the email queue feeding our perimeter Barracuda devices. If only there was a better way…

Well, Spamhaus claims it has found the answer.  Using a tightly controlled whitelist–membership is possible upon invitation by another member–Spamhaus says it provides comprehensive email filtering, free and without all the management issues faced by many enterprises.

“Unlike traditional whitelists, the Spamhaus Whitelist is not a service to help bulk mail senders improve delivery rates. You can not whitelist an IP address or domain that is used for sending marketing or soliciting bulk email, or used for sending any email on behalf of third parties. This rule therefore automatically excludes makes not eligible for whitelisting Email Service Providers, ISP customer mail relays and mail servers used by third-parties, and all bulk mailing list servers and services,” the company said in its explanation of the service.

(Source: Spamhaus Debuts New Whitelist Service | threatpost.)

Setup is easy and well documented at the Spamhaus site. At a high level,

The Spamhaus Whitelist is actually made up of two whitelists: an IP address whitelist called the ‘SWL’ and a domain whitelist called the ‘DWL’. These are published as and respectively.

The SWL is both an IPv4 and IPv6 whitelist. It responds to queries of either IPv4 or IPv6 addresses. (Note: IPv6 handling is not yet active. Spamhaus estimates IPv6 service starting in 2011)

The DWL is a VBR (vouch-by-reference) domain whitelist designed to automate DKIM certification.

(Source:, 2010)

So what happens if a sender abuses their membership in the whitelist?  Since the new service is in beta, we really don’t have any examples of deviant behavior.  However,

Spamhaus is reserving the right to revoke whitelist status for any email etiquette transgressions, such as the distribution of bulk mail of any type. The whitelist will be maintained in both IP addresses and domain name forms as two separate, but matched, lists. Controls mean no domain or IP address that is on the Spamhaus Project blocklist can ever be whitelisted.

(Source: Spamhaus debuts whitelist service, The Register, 28 September 2010)

Note that this service uses DKIM, something Microsoft Exchange DOES NOT support.  There are third-party solutions (example) that make Exchange compatible.  But if you use Exchange, I recommend adding a front end solution, like Barracuda Spam Firewall, between the Internet and your mail servers.  Other DKIM-compatible solutions are listed at

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