Hello all,
Are there any reasons not to introduce greylisting (GLing) functionality in amavisd-new?
GLing as a technique for avoiding spam is described here: http://projects.puremagic.com/greylisting/whitepaper.html
Probably the main reason for introducing GLing into amavisd-new is to try to take the pressure off the expensive checks like antivirus or antispam, thus boosting the performace of mail system.
Obvious flaw of GLing (initial delay) could be avoided by using:
- Lightlists (whitelist of hosts never to be GLed)
- Darklists (blacklist of hosts always to be GLed)
- Adaptive greylisting (AGL)- Only hosts/netblocks known to be sources of spam/viruses are to be greylisted. List of those blocks could be generated by keeping statistics about number of detected viruses, number of mails detected as spam or cumulative spam score for a particular netblock over some time period (thus the word "adaptive").
If, let's say, one /24 block generates more than 1000 viruses per hour or cumulative spam score for all mails from that block raises over 1000, that block is to be greylisted in the next 24hrs.
Exact algorighm for generating list of hosts for AGLing definitely needs to be worked out but I think this mail might be a good starting point. This is also the main reason GL code should be present inside amavis, and not outside of it (as a separate mail filter placed before it).
If GLing, epsecially AGLing fits into general amavisd-new philosophy, but we don't have available resources for writing it from the scratch, we might adopt and adapt Evan Harris's relaydelay code to fit into amavisd-new adding just the statistics part and the AGL logic.
After all, this is all open source, right? :)
Comments? :)
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