OpenL2TP Plugins
================

An OpenL2TP plugin is a shared library which extends OpenL2TP
functionality. Multiple plugins may be loaded.

Plugins may call any public function within the OpenL2TP server
software and may have visibility of tunnel/session
create/delete/modify/up/down events.

The following OpenL2TP plugins are available:-

ppp_unix:       This plugin is provided to allow L2TP to interface
                with the standard UNIX pppd. It is loaded by
                default. To operate with other (perhaps commercial)
                PPP implementations, a different plugin would be
                needed. Note, however, that unless a commercial L2TP
                license is used, the plugin also falls under GPL.

ppp_null:       This plugin may be used instead of ppp_unix to disable
                the use of PPP over L2TP sessions. This may be useful
                for testing.

The default location for OpenL2TP plugins is /usr/lib/openl2tp/.

Plugins are loaded using -p <plugin> arguments when starting
openl2tpd.  If any -p argument is given, the ppp_unix plugin (which is
usually loaded by default) is not loaded.

The plugin may register with one or more hooks and has access to all
global variables of OpenL2TP. The hooks are

extern int (*l2tp_tunnel_created_hook)(uint16_t tunnel_id);
extern int (*l2tp_tunnel_deleted_hook)(uint16_t tunnel_id);
extern int (*l2tp_tunnel_modified_hook)(uint16_t tunnel_id);
extern int (*l2tp_tunnel_up_hook)(uint16_t tunnel_id, uint16_t peer_tunnel_id);
extern int (*l2tp_tunnel_down_hook)(uint16_t tunnel_id);
extern int (*l2tp_session_created_hook)(struct l2tp_session const *session, 
                                        uint16_t tunnel_id, uint16_t session_id);
extern int (*l2tp_session_deleted_hook)(struct l2tp_session const *session, 
                                        uint16_t tunnel_id, uint16_t session_id);
extern int (*l2tp_session_modified_hook)(struct l2tp_session const *session, 
                                         uint16_t tunnel_id, uint16_t session_id);
extern int (*l2tp_session_up_hook)(struct l2tp_session const *session, 
                                   uint16_t tunnel_id, uint16_t session_id, 
                                   uint16_t peer_tunnel_id, uint16_t peer_session_id);
extern int (*l2tp_session_down_hook)(struct l2tp_session const *session, 
                                     uint16_t tunnel_id, uint16_t session_id);
extern int (*l2tp_session_get_stats_hook)(struct l2tp_session const *session, 
                                          uint16_t tunnel_id, uint16_t session_id, 
                                          struct pppol2tp_ioc_stats *stats);

When tunnels and sessions are created and destroyed or change state,
OpenL2TP calls any registered hooks. It gurantees to call the created
hook before the up hook and the deleted hook after the down hook.

Plugins must provide the following global symbols:-

const char l2tp_plugin_version[];
int l2tp_plugin_init(void);
int l2tp_plugin_cleanup(void);