Re: [apps-discuss] I-D Action: draft-ietf-appsawg-greylisting-01.txt
"John Levine" <johnl@taugh.com> Sat, 21 January 2012 04:45 UTC
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Date: Sat, 21 Jan 2012 04:44:37 -0000
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From: John Levine <johnl@taugh.com>
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Subject: Re: [apps-discuss] I-D Action: draft-ietf-appsawg-greylisting-01.txt
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Based on my experience, here's some advice for how to make greylisting work. My goal is to use it to recognize broken mail clients that don't retry. Once a client has shown that it knows how to retry, there is no further benefit from greylisting it. (It may be sending spam, but you'll have to recognize it other ways.) * Greylist on the triple [IP, MAIL FROM, RCPT TO]. After a successful retry, whitelist all further mail from the IP. I don't feel strongly whether you should put just one RCPT TO in the triple or wait for DATA and put all of them. * The greylister should have a time window within which retries will be accepted. On mine, the minimum is one minute, the maximum is 24 hours. I think those are reasonable. You want a minimum because some spamware retries immediately, you want a maximum both to keep the retry table from filling up with junk and so two spams sent a week apart aren't interpreted as a retry. Some people set their minimum much longer, several hours, but I don't see any benefit to that unless you believe in the blacklist theory. * Whitelist entries should time out eventually. I delete them when there's been no mail from the IP for a week. * If you have multiple incoming servers at the same MX distance, they should share a single greylist database. This handles the case where the retry goes to a different server, and lets all the servers share the IP whitelist. I do this by making the greylister a standalone daemon, and use a very simple UDP protocol to query it. If you're Google or Yahoo and have multiple server pools all over the world, I suppose a database per pool would do. * Since some senders have multiple outgoing servers, allow approximate matches on the retry IP. I take anything in the same /24 which seems to work OK. I only whitelist the IP that retries, but I should probably whitelist both. * The greylister MUST have a manual whitelist. Some legit senders just don't work with greylisting for a variety of reasons, most of which are not in conflict with RFC 5321. R's, John
- [apps-discuss] I-D Action: draft-ietf-appsawg-gre… internet-drafts
- Re: [apps-discuss] I-D Action: draft-ietf-appsawg… Paul E. Jones
- Re: [apps-discuss] I-D Action: draft-ietf-appsawg… Murray S. Kucherawy
- Re: [apps-discuss] I-D Action: draft-ietf-appsawg… John Levine
- Re: [apps-discuss] I-D Action: draft-ietf-appsawg… Murray S. Kucherawy