PPTP forward port per user

[Sascha Kiefer]

Hi,

i would like to be able to forward an internal ports of users through 
the VPN.
The idea is that a user picks 2-3 ports (or maybe just one) tcp port out 
of a given port-pool,
and when he connects to the VPN, this portforwarding is established for him.
Any idea how to do this? I'm using pptpd with freeradius + mysql.

thanks for a hint.
cheers,
--sascha