Would you mind rephrasing what you target?
Is the idea to have Zentyal services like VoIP or VPN to rely on external Radius server?
I'm asking because I'm a bit confused with generic "Zentyal as Radius client". To me it has to be at least seen "per service", Zentyal being only service container (thus there is no Zentyal authentication).
Then when you write "
VPN authenticate from an external RADIUS server" do you rather mean "
VPN authenticate against (or "to") an external RADIUS server"
It's not nitpicking
but understanding how you would like to handle authentication is key and I don't for the time being.
Anyway, I don't think so far Zentyal provides such capability that is to use external Radius for services hosted on Zentyal box. However your question may raise some interesting debate because of some specific needs.