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Gray Listing with Sendmail

I decided to see if graylisting would reduce the (admittedly small) amount of spam I receive on my mail server. I decided to try graymilter from Jef Poskanzer, since it seemed a reasonable implementation that I could actually understand, and his instructions were FreeBSD friendly.

Building and installing graymilter was fairly easy. I had to change the startup script (graymilter.sh) to read /etc/rc.subr rather than /usr/local/etc/rc.subr. Also, graymilter.sh has to be copied manually to /usr/local/etc/rc.d. FreeBSD will it run automatically at startup.

I followed the installation instructions, with one small change. I set the socket pathname to /var/run/gray/graymilter.sock, so that the user I run graymilter as (nobody) could be given ownership of the directory.

The /etc/rc.conf settings for graymilter are therefore as follows:

graymilter_enable="YES"
graymilter_flags="-user nobody -initialwhitelist /usr/local/etc/whitelist"
graymilter_socket="unix:/var/run/gray/graymilter.sock"

with corresponding changes to the crimson.mc file.

I made a modification to the graymilter.c source code; I reduced the verbosity of graymilter by only issuing a whitelisting message when the number of addresses to whitelist was non-zero.

I let this run for a few days, and while some spam messages were blocked by the graylisting technique, others got through, presumably because they were being delivered by proper MTAs.

Therefore, in conjuction with graylisting, I configured sendmail to use real-time spam blacklists, by adding the following to the crimson.mc file:

FEATURE(`enhdnsbl', `bl.spamcop.net', `"Spam blocked see: http://spamcop.net/bl.shtml?"$&{client_addr}', `t')
FEATURE(`enhdnsbl', `sbl-xbl.spamhaus.org', `"Spam blocked see: http://www.spamhause.org', `t')

I've been running graymilter and blocking for about a week, and my spam rate has gone from an average of four a day to one a day. Those that get through are processed by SpamBayes, so spam levels are down to an acceptable level.

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