paulsm4 thanks for advice but Barracuda wants $4,000 for VPN system. SonicWall as same as barracuda.
I can't pay that kind of money. I've dedicated server and I need software.
Note: Are you sure about openVPN is REALLY open source? I mean they sell license for $5/1 connection.
I can pay $5 for each connection but the problem is I can't install software on our computers so I have to find different solution.