rp-pppoe-3.5.tar.gz contains the 'non-commercial' pppoe server, with plenty of documentation. (just follow my earlier link!!!)
/***********************************************************************
*
* pppoe-server.c
*
* Implementation of a user-space PPPoE server
*
* Copyright (C) 2000 Roaring Penguin Software Inc.
*
* This program may be distributed according to the terms of the GNU
* General Public License, version 2 or (at your option) any later version.
*
* $Id: pppoe-server.c,v 1.87 2002/06/13 14:20:47 dfs Exp $
*
* LIC: GPL
*
***********************************************************************/
You should have taken a closer look