The Spamhaus DNSBL Datafeed Query Service (DQS) is the DNS-based commercial service for entities that do not satisfy the requirements to use the public, free Spamhaus blocklists. Rather than using 'zen.spamhaus.org' as a blocklist name, the DQS service uses <code>.zen.dq.spamhaus.net where <code> is a 26-characters customer code individually assigned to each customer. To use the DQS, Kerio Connect users have to define a new Blocklist entry in the configuration, similar to the existing spamhaus.org entry but using this longer name instead.
I work at the Spamhaus Technology support, and we have been baffled to receive several reports of Kerio Connect users experiencing DNS lookup errors when trying to use the DQS. Or, simply, finding that no message is blocked. Only some users reported this, though, not all of them. For others, everything works fine.
Today I did some experimentation to understand this, and found that the problem occurs when the "Ask the blacklist DNS server directly (necessary for paid blacklists)" option is selected. That option must NOT be enabled when using the DQS (while it may be used when using the public free service, and it may actually be necessary when the server uses public DNS resolvers such as Google's).
I venture to guess that when this option is not selected, Kerio Connect sends all DNS traffic through the normal DNS resolution path defined on the server, while when it is enabled it first does a NS lookup on the BL name, collects a list of authoritative nameservers for the BL and subsequently sends all the BL queries directly to those servers. Unfortunately, for the DQS this logic fails because there is no NS record for <code>.zen.dq.spamhaus.net: we put the delegation one level above, at the level of zen.dq.spamhaus.net. This poses no problems for normal queries, as every query follows the delegations from right to left, but it breaks the assumption - apparently made by the Kerio Connect code - that there are NS records for the exact BL name inserted in the configuration.
I hope this helps!
Alex - Spamhaus Technology
[Updated on: Tue, 21 January 2014 23:41] by Moderator
- Pavel Dobry (Kerio)
You're right. Direct DNS queries are required by some paid DNS blacklist services, which verify clients based on their IP address (the server must ask their DNS servers directly). In general use, this option should not be used if it is not explicitly required by the service.
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