greylist plugin

Basic greylisting plugin that follows common practices found on internets.

Principles of work

Notation

The so-called tuple consists of the following:

• First subdomain of rDNS is stripped off (but no shorter than the domain boundary). This is considered a hostid.
• Envelope sender is the sender.
• RCPT TO would supply the recipient.

hostid in above notation is chosen unless:

1. The connecting host has no PTR record, a.k.a. reverse DNS (rDNS). gl
2. The rDNS record contains the first two or last two octets of the IP address. fcrdns
3. The rDNS record contains the ‘short’, decimal, or hex representation of the full IP address. fcrdns gl
4. Multiple rDNS records are returned. gl
5. The rDNS record cannot be verified by forward confirmation (e.g. FCrDNS). fcrdns
6. The top-level-domain (TLD) used is not valid. gl

In other cases, it's set to be the remote party's IP address.

We define the following time periods:

• black: between first connect and start of gray. Defer.
• gray: between black and start of white. Allow. Host must re-try within this window.
• white: comes after gray. Allow up until the end of period, then let the record expire in case no connections were made.

Algorithm

The greylist algo is as following:

• Party connects. All FcrDNS & DNSWL checks are run by earlier plugins.
• Party sends recipient
  • If not already whitelisted
    • Check tuple color (compare current TS against record creation TS)
      • black?
        • Create if no record exists. Defer.
      • gray?
        • Allow. Promote record to white status.
      • white?
        • Allow. Update record TS.
• In special case, data hook runs above algo for all recipients. If any matched, all inherit the action.

DB schema

We store in Redis.

Key format for greylisting entries:

• grey:${hostid}:${sender}:${recipient} - grey record
• white:${hostid} - white record

For white:

{ first_connect: TS, whitelisted: TS, updated: TS, lifetime: TTL, tried: Integer, tried_when_greylisted: Integer }

Where first_connect: TS of first connection (sender) whitelisted: basically the TS of this entry creation updated: last update TS lifetime: seconds for this entry to exist (== TTL) tried: number of checks against this entry tried_when_greylisted: number of checks while the host was +grey+ (sender).

For grey:

{ created: TS, updated: TS, lifetime: TTL, tried: Integer }

Where created: TS of first connection (copied to first_connect of white after promotion) updated: last update TS lifetime: seconds for this entry to exist (== TTL) tried: number of checks against this entry (copied to tried_when_greylisted of white after promotion)

Whitelisting

It's possible to whitelist hosts using the following section in greylist.ini config file:

• ip_whitelist IP or subnet (prefix notation)
• envelope_whitelist MAIL FROM (email or domain)
• recipient_whitelist RCPT TO (email or domain)

List of known dynamic hosts, to use the IP instead of the domain:

• special_dynamic_domains Domain