On Thu, 3 Jul 2008, Ron Echeverri wrote:
> syslog looks like this:
>
> Jul  3 11:14:08 plum postfix/smtpd[27824]: warning: milter
> inet:localhost:8891: can't read SMFIC_DATA reply packet header: Success

This somewhat cryptic error appears to mean the read() call in postfix got 
a premature EOF rather than a reply to the SMFIC_DATA message it sent to 
the filter.  The most common cause for this is the death of the filter 
process.  However your "ps" output and dkim-filter.conf together suggest 
the filter didn't crash, so that's not the problem.

Similar things have been reported by other postfix users.  Some debugging 
patches one person added to his libmilter implementation suggested that 
somehow the socket being used to link postfix to the filter was being 
prematurely and unexpectedly closed.  That's as far as we've gotten to 
date.

Someone once said that this problem doesn't happen if you use TCP sockets, 
only if UNIX domain sockets are in use.  That seems unlikely, but not 
impossible.

The interesting points to me are (a) no sendmail installation has yet 
reported this symptom, and (b) the forensics previously collected suggest 
that there's an errant close() being called by, or on behalf of, the 
dkim-filter process, but a fairly thorough code review didn't find the 
culprit.

I meant to download and review the relevant parts of the postfix code, but 
haven't had time to do so.

As the dkim-filter code itself doesn't have direct access to the 
descriptor involved, the bug has to be in libmilter, postfix, or somewhere 
in the underlying layers.  That this never seems to happen at sendmail 
installations seems to me to reduce the likelihood that libmilter is the 
culprit, but I could've missed something in my reviews of the code.

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