How to know and limit the freeradius-client

[Spacelee]

I have pptp + ppp, l2tp + ppp, openvpn  to connect to the same freeradius to
authenticate, and my question is how to meet this need:
I want to limit some user could just access pptp, some could just access
l2tp, and some could just access openvpn.
and is there any variable in freeradius to implement this?

-- 
*Space Lee*
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