Starting with version 2.3.10, pppd includes support for `plugins' -
pieces of code which can be loaded into pppd at runtime and which can
affect its behaviour in various ways.  The idea of plugins is to
provide a way for people to customize the behaviour of pppd without
having to either apply local patches to each version or get their
patches accepted into the standard distribution.  My aim is that
plugins will be able to be used with successive versions of pppd
without needing to recompile the plugins.

A plugin is a standard shared library object, typically with a name
ending in .so.  They are loaded using the standard dlopen() library
call, so plugins are only supported on systems which support shared
libraries and the dlopen call.  At present pppd is compiled with
plugin support only under Linux and Solaris.

Plugins are loaded into pppd using the `plugin' option, which takes
one argument, the name of a shared object file.  The plugin option is
a privileged option.  I suggest that you give the full path name of
the shared object file; if you don't, it may be possible for
unscrupulous users to substitute another shared object file for the
one you mean to load, e.g. by setting the LD_LIBRARY_PATH variable.

Plugins are usually written in C and compiled and linked to a shared
object file in the appropriate manner for your platform.  Using gcc
under Linux, a plugin called `xyz' could be compiled and linked with
the following commands:

	gcc -c -O xyz.c
	gcc -shared -o xyz.so xyz.o

There are some example plugins in the pppd/plugins directory in the
ppp distribution.  Currently there is one example, minconn.c, which
implements a `minconnect' option, which specifies a minimum connect
time before the idle timeout applies.

Plugins can access global variables within pppd, so it is useful for
them to #include "pppd.h" from the pppd source directory.

Every plugin must contain a global procedure called `plugin_init'.
This procedure will get called (with no arguments) immediately after
the plugin is loaded.

Plugins can affect the behaviour of pppd in at least three ways:

1. They can add extra options which pppd will then recognize.  This is
   done by calling the add_options() procedure with a pointer to an
   array of option_t structures.  The last entry in the array must
   have its name field set to NULL.

2. Pppd contains `hook' variables which are procedure pointers.  If a
   given hook is not NULL, pppd will call the procedure it points to
   at the appropriate point in its processing.  The plugin can set any
   of these hooks to point to its own procedures.  See below for a
   description of the hooks which are currently implemented.

3. Plugin code can call any global procedures and access any global
   variables in pppd.

Here is a list of the currently implemented hooks in pppd.


int (*idle_time_hook)(struct ppp_idle *idlep);

The idle_time_hook is called when the link first comes up (i.e. when
the first network protocol comes up) and at intervals thereafter.  On
the first call, the idlep parameter is NULL, and the return value is
the number of seconds before pppd should check the link activity, or 0
if there is to be no idle timeout.

On subsequent calls, idlep points to a structure giving the number of
seconds since the last packets were sent and received.  If the return
value is > 0, pppd will wait that many seconds before checking again.
If it is <= 0, that indicates that the link should be terminated due
to lack of activity.


int (*holdoff_hook)(void);

The holdoff_hook is called when an attempt to bring up the link fails,
or the link is terminated, and the persist or demand option was used.
It returns the number of seconds that pppd should wait before trying
to reestablish the link (0 means immediately).


int (*pap_check_hook)(void);
int (*pap_passwd_hook)(char *user, char *passwd);
int (*pap_auth_hook)(char *user, int userlen,
		     char *passwd, int passlen,
		     char **msgp, int *msglenp,
		     struct wordlist **paddrs,
		     struct wordlist **popts);

These hooks are designed to allow a plugin to replace the normal PAP
password processing in pppd with something different (e.g. contacting
an external server).

The pap_check_hook is called to check whether there is any possibility
that the peer could authenticate itself to us.  If it returns 1, pppd
will ask the peer to authenticate itself.  If it returns 0, pppd will
not ask the peer to authenticate itself (but if authentication is
required, pppd may exit, or terminate the link before network protocol
negotiation).  If it returns -1, pppd will look in the pap-secrets
file as it would normally.

The pap_passwd_hook is called to determine what username and password
pppd should use in authenticating itself to the peer with PAP.  The
user string will already be initialized, by the `user' option, the
`name' option, or from the hostname, but can be changed if necessary.
MAXNAMELEN bytes of space are available at *user, and MAXSECRETLEN
bytes of space at *passwd.  If this hook returns 0, pppd will use the
values at *user and *passwd; if it returns -1, pppd will look in the
pap-secrets file, or use the value from the +ua or password option, as
it would normally.

The pap_auth_hook is called to determine whether the username and
password supplied by the peer are valid.  user and passwd point to
null-terminated strings containing the username and password supplied
by the peer, with non-printable characters converted to a printable
form.  The pap_auth_hook function should set msg to a string to be
returned to the peer and return 1 if the username/password was valid
and 0 if not.  If the hook returns -1, pppd will look in the
pap-secrets file as usual.

If the username/password was valid, the hook can set *paddrs to point
to a wordlist containing the IP address(es) which the peer is
permitted to use, formatted as in the pap-secrets file.  It can also
set *popts to a wordlist containing any extra options for this user
which pppd should apply at this point.


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