http://projects.puremagic.com/greylisting/whitepaper.html

1. Have minimal impact on users
2. Limit spammers ability to circumvent the blocking
3. Require minimal maintenance at both the user and administrator level

1. It provides no notice to the senders of legitimate email that is falsely identified as spam.
2. It places most of the costs of processing the spam on the receivers side rather than the spammers side.
3. It provides no real disincentive to spammers to stop wasting our time and resources.

  #! /bin/sh
  # tcl  \
  exec tclsh "$0" ${1+"$@"}

  # Created 2004-06-09 by Steve Redler IV
  # greylisting engine based on the principles of
  # http://projects.puremagic.com/greylisting/whitepaper.html

  lappend auto_path lib
  package require sqlite

  #main test proc, returns PASS or FAIL to caller
  proc grey_test {ipadress sender recipient} {

    # Test if weve seen this triplet before
    # For all checks, we ignore records whose lifetime has expired
    # If we have not seen it, create a record describing it and
    # return a tempfail to the sending MTA.

    set curtriple "ipadress = '$ipadress' AND  sender = '$sender' AND recipient = '$recipient'"
    greydb eval "SELECT * FROM triples WHERE $curtriple" query {}

    if {! [info exists query(ipadress)]} {
      greydb eval "INSERT INTO triples VALUES('$ipadress', '$sender', \
        '$recipient', [clock seconds], [expr [clock seconds] + ($::blockmins * 60)], \
        [expr [clock seconds] + ($::expiremins * 60)], 1, 0)"
      return "FAIL new record    "
    }

    #update records whose lifetime has expired ie: inactive records

    if {$query(recexptime) < [clock seconds]} {
      incr query(blockedattempts)
      set recexptime [expr [clock seconds] + ($::expiremins * 60)]
      set blockexptime [expr [clock seconds] + ($::blockmins * 60)]
      greydb eval "UPDATE triples SET blockexptime = $blockexptime, blockedattempts = \
        $query(blockedattempts), recexptime = $recexptime WHERE $curtriple"
      return "FAIL expired record"
    }

    # If we have seen it, and the block is not expired ,
    # return a tempfail to the sending MTA.

    # Increment the failed count on the matching row.

    # If the sender is the special case of the null sender,
    # do not return a failure after RCPT, instead wait until
    # after the DATA phase.

    if {$query(blockexptime) > [clock seconds]} {
      incr query(blockedattempts)
      greydb eval "UPDATE triples SET blockedattempts = $query(blockedattempts) WHERE $curtriple"
      return "FAIL timed block   "
    }

    # If we have seen it, and the block has expired and lifetime
    # has expired, then pass the email.

    # Increment the passed count on the matching row.

    # Reset the expiration time of the record to be the standard
    # lifetime past the current time.

    incr query(msgspassed)
    set recexptime [expr [clock seconds] + ($::expiremins * 60)]
    greydb eval "UPDATE triples SET msgspassed = $query(msgspassed), recexptime = $recexptime WHERE $curtriple"
    return "PASS email passed  "
  }

  catch {source [file join $::confdir smtpd.conf]}
  catch {source smtpd.conf}

  if {! [info exists ::confdir]} {set ::confdir "."}

  #blockmins is suggested to be 60 minutes
  set ::blockmins 60
  #expiremins is suggested to be 240 minutes
  set ::expiremins 240

  #open and/or initialize the database
  sqlite greydb [file join $::confdir greylist.db]
  greydb timeout 60000
  if {[greydb eval {Select name from SQLITE_MASTER}] == ""} {
    puts "Initializing databse [file join $::confdir greylist.db]"
    greydb eval {CREATE TABLE triples(ipadress text, sender text, recipient text, createtime int, blockexptime int, recexptime int, blockedattempts int, msgspassed int)}
  }

  puts "greylisting module loaded"
  #end of module

  if 1 {
    # set some short time, we dont got all day!
    set ::blockmins 1
    set ::expiremins 2

    set run 0
    #here are some initial messages
    while {$run < 5} {
      puts [clock format [clock seconds] -format %T]
      puts "  [grey_test  192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"
      puts "  [grey_test  192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"
      puts "  [grey_test  192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"

      after 20000
      incr run
    }

    #wait for records to expire, see if they reset
    puts "pausing for 120 seconds"
    after 130000
    puts [clock format [clock seconds] -format %T]
    puts "  [grey_test 192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"
    puts "  [grey_test 192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"
    puts "  [grey_test 192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"

    #test again
    set run 0
    while {$run < 5} {
      puts [clock format [clock seconds] -format %T]
      puts "  [grey_test 192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"
      puts "  [grey_test 192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"
      puts "  [grey_test 192.168.0.1 [email protected] [email protected]] 192.168.0.1 [email protected] [email protected]"

      after 20000
      incr run
    }

    #done
  }

http://dasproxy.sourceforge.net

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