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HFrom: $>ruleset and some sendmail ranting

2008/11/08

Due to a spam burst last night, I was forced to write a ruleset in the spirit of my previous post:

LOCAL_CONFIG:
HFrom: $>BlockFrom

LOCAL_RULESETS
R$* "NEWS Sensation" $*        $#discard $: discard
R$*        $#OK

Again since this is going straight into your .mc file, it is not advised to use this method frequently. Nice five-liner to stop a spam burst, right?

No! As I was applying this filter I thought that although most people think that sendmail‘s most serious trouble is its bug / security track record, its most serious problem is that decision rules on routing and filtering messages come from all over the place: text files like mailertable, virtusertable, genericstable, relay-domains, access, the sendmail.cf rulesets themselves like the example above and more importantly the milters, their defaults and their configuration files (if any).

Say for example that you do not want email directed to abuse@ to be filtered at all. Depending on your rules and how they are written, you may have to upgrade your rulesets, edit access and probably write an exception for every single milter that you are applying. One exception might need to be declared in five different places. Talk about complication and management nightmare! That is why I like running MIMEDefang: It gives me a Perl interpreter and the ability to implement most of the functionality of any other milter I wish to apply. Have I reached that point of being able to run only one milter? No, but I have set this as a goal. MIMEDefang is not my only choice to this path, j-chkmail seems like a good alternative.

However, I am not the only one who manages our global filters, and I want to make it easier for the other admins to add their own global filters in my absence. Oh how much easier it could have been if the libmilter API offered as a return value a variant of SMFIS_ACCEPT that would instruct sendmail to accept the message with no more milters applied to it. Currently SMFIS_ACCEPT instructs the current milter to accept the message. Which is why if you want to write a global whitelist exception, you may have to write it for all milters that are enabled.

Oh well, I think all I want and wish for is to minimize the number of files I have to edit in order to implement a certain policy (or have a tool that can read the policy from one place and edit n-files for me) and a change to the libmilter API (which I do not know whether it is trivial or not). Suggestions to switch to alternatives like Postfix, Qmail or even Exim are outside the scope of this rant. I prefer to take the “Sendmail Theory and Practice” route and write the whole .mc file by hand instead.

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