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[Asrg] RE: Wierd reverse DNS resolution might affect spam filters

2006-06-24 21:40:31
Bill Cole said:

> Basically, this is a non-issue. Reverse DNS has never really been trustworthy, and trusting an unverified rDNS result by itself for authentication is an unlikely error for any serious piece of modern software.

Well, I guess my milter was either ancient or nonserious, in spite of the fact that it shields my family and myself from several thousand spams per day. I made this mistake and was alerting anyone else who might have made a similar mistake. Because I never added the value "localhost" to either my white or blacklist, my milter never used the value "localhost" in making any of its decisions, so it indicated it was neutral on the spam. My milter is set on top of sendmail 8.13, and sendmail never logged a byte about this problem. Two other posters (Jeroen Massar and David Wilson) offered the only possible solution to fix the problem of verifying the hostname (thanks, guys...).

I'm not at all sure that this is a non-issue for those of us using sendmail (it's nice to know EXIM users aren't bothered by this). Neither sendmail nor libmilter do the necessary step to verify correctness of the rDNS result that they pass to my milter code as the "hostname" argument of xxfi_connect(). Either sendmail or libmilter (whichever of these provides the rDNS value passed to a milter) should be the ultimate place where any modifications to add verity go. Other milter makers besides myself may have made this mistake. Of course, it is possible that Red Hat is the culprit -- I'm using RHEL4's version of sendmail. I will do further research and lodge a change request with the appropriate people.

In the meantime, until test on my modifications are completed, I've temporarily added "localhost" and "localhost.localdomain" to my reject list; they join "business" (which is the rDNS offered by spamhosts at Since the IP is in the whitelist and the whitelist trumps the blacklist, my milter doesn't self-destruct local outbound emails.


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