Milter-greylist

Hi,

I create a patch to support 'tarpit'.

Here is a description how to use 'tarpit' feature from
README:

   15 Using tarpit
   ===============

  'tarpit' is an anti-spam technique by lazy response.

    acl whitelist tarpit 65s

  This ACL means that clients that can wait a response in
  65s are whitelisted. If the clients access again, they are
  acceptted without lazy response because they are in
  auto-whitelist.

  If clients that couldn't wait a lazy response access again,
  the ACL doesn't match.

    acl whitelist tarpit 65s
    default greylist

  Those ACLs means that clients that can wait a lazy
  response or resend a message are acceptable.

    acl greylist tarpit 10s

  This ACL means that clients should wait a response in 10s
  then pass greylist.

  If clients that couldn't wait a lazy reponse access again,
  the ACL doesn't match.

    acl greylist tarpit 10s
    default blacklist

  Those ACLs means that clients should wait a lazy response
  and pass greylist. Otherwise they are rejected.

  There is a 'tarpit_accumulation_period' configuration
  parameter. It controls how to count tarpitted
  time. Available values are 'session' and 'action'. 'session'
  means that tarpitted time is counted in a SMTP session
  scope.  'action' means that tarpitted time is coutned in an
  action (request/response) scope. The default is 'session'.

    acl whitelist rcpt user1@... tarpit 10s
    acl whitelist rcpt user2@... tarpit 30s
    acl whitelist rcpt user3@... tarpit 15s

  It assumes that a client sends a mail to user1@...,
  user2@... and user3@... in a SMTP session
  when those ACLs are used.

  'session' case:
    milter-greylist waits to returns a response in
    10s for user1@.... Then milter-greylist waits to
    returns a response in 20s for user2@.... 20s is
    30s (tarpit time for user2@...) - 10s (tarpit time
    for user1@...). milter-greylist just wait 20s
    because milter-greylist had waited 10s. Then
    milter-greylist doesn't wait to returns a response for
    user3@... because total 30s had waited in this
    SMTP session.

      user1@...: tarpit 10s
      user2@...: tarpit 20s
      user3@...: not tarpitted

  'action' case:
    milter-greylist waits to returns a response in 10s for
    user1@.... Then milter-greylist waits to returns a
    response in 30s for user2@.... Waited time in the
    previous action are not counted. Then milter-greylist
    doesn't wait to returns a response for user3@...
    because over 10s had waited in other action.

      user1@...: tarpit 10s
      user2@...: tarpit 30s
      user3@...: not tarpitted


RFC: It seems that name of 'tarpit_accumulation_period' can
be improved but I don't have better idea. :<
Could someone give us your idea?


Thanks,
--
kou