The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Tuesday, September 11, 2007

Note on a greylist implementation

For such a simple concept, greylisting has a lot of pitfalls. I managed purely by chance to see that Mark had sent me an email (I saw the tuple in the log files). Curious to see how long it took to be accepted, I was horrified to see that not only had it not been accepted by the greylist daemon, but that it had been kicking around the system for over 30 hours!

Like clockwork, Mark's email server was attempting to send the message every thirty minutes, on the dot, and thus, was never getting through the embargo time out. It all came down to this one piece of code:

if (difftime(req->now,stored->atime) < c_timeout_embargo)
  stored->atime = req->now;

If the last access time was less than the embargo timeout, update the access time and send back “try again later.” At the time I found this out, I simply added Mark's server IP to the whitelist and restarted the greylist daemon.

Later, at the weekly Company meeting, I mentioned some of the issues I've had over the week and after some discussion, I made two changes to the greylist daemon:

  1. cut the embargo timeout from one hour to 25 minutes
  2. use only the sender and recipient in the tuple, dropping the IP address (or rather, ignoring it)

To test these changes, I also removed a bunch of the whitelisted IP addresses, to test the effectiveness.

They weren't all that effective.

I had problems with BellSouth, trying to deliver an email for four hours (and, as always, well below the embargo threshhold). I restarted the greylist daemon with an extended whitelist of IP addresses.

In reading many pages on greylisting, I realized I may have mis-interpreted the original whitepaper:

With this data, we simply follow a basic rule, which is:

If we have never seen this triplet before, then refuse this delivery and any others that may come within a certain period of time with a temporary failure.

So instead of checking against the last access time, I should compare against the creation time of the record.

Off to make that change and see how it goes.

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