Most likely what's happening is that clients can access the VPN server itself, but can't use it as a gateway.
You will need to create/enable a NAT interface in RRAS so that the VPN server can act as a NAT gateway/router to the internet.
It's not entirely clear to me how you got PPTP working to the Windows XP server when you have forwarded all port 1723 traffic to the Debian Squeeze server. You likely can only "VPN" to the WinXP server from within your local LAN, which seems to be of limited usefulness.
Regardless, PPTP requires not only TCP port 1723 traffic, but also GRE protocol. Is your router capable of handling GRE tunnels correctly? If it's an ordinary consumer-grade router, then I suspect not. And even if it is, GRE is esoteric enough that finding help may be difficult.
In your case I recommend trying a VPN solution that uses only TCP and/or UDP transports, since those protocols are ubiquitous and well-known. OpenVPN is one such VPN solution, and it is available for all the major OSes (Win, Mac, Linux, *BSD).
Depending on what you're trying to accomplish, another possibility is to run sshd on the Debian server, e.g.:
apt-get install openssh-server
All the major OSes have free ssh clients capable of creating tunnels over an ssh connection.
Best Answer
I'm fairly sure that Debian uses the PoPToP
pptpd, which in turn usespppd. Thepptpdprocess forks for each new user connection so you should just be able tokillthe 'pppd' process related to the user you want to terminate.The process list doesn't show the username associated with the
pppdprocess for a given connection, so you'll probably have to use thelastcommand to figure out what IP address user the user logged-on from and then kill the appropriatepppdprocess.pptpd, by default, updates thewtmpfile with logons, solastshould show you from what ip address a user logged-on from. Then it's a matter of grepping the process list for thatpppdinstance.