[Dovecot] Custom auth process in dovecot 2

Hi,

I'm trying to upgrade from dovecot-1.1.x to 2.1.7.

We have our own custom auth server process (because we want to do our own password validation and for other reasons) that listens on a UNIX domain socket and speaks the dovecot auth protocol.

In dovecot 1.1 we could configure this with

auth external { socket connect { master { path = /var/run/dovecot/auth.sock } } }

as per http://wiki.dovecot.org/MainConfig

I haven't been able to figure out how to do this in 2.1.7, is it possible?