Building a Secure VPN

Most VPN clients also let you set compulsory tunnels or disable split tunnels so that when the client has a VPN tunnel established, the client doesn't allow communications from outside channels. This restriction prevents an attacker who compromises the VPN client computer from leapfrogging from the Internet onto your network. These client measures aren't silver bullets, but they thwart all but the most serious attackers. Unfortunately, most software-based VPNs, including the XP and Win2K VPN clients, don't offer these protections.

Another solution for the really paranoid (and well funded) is to locate a second smaller firewall between your internal VPN concentrator and internal LAN, as Figure 1 shows. Then, if an attacker compromises a VPN host, he or she still must penetrate another firewall. You could open up a few common ports, but the firewall would still block ping scans, common worms, and other garbage. Of course, it wouldn't stop someone who's just looking around and it wouldn't work if VPN users need full access to the internal network, but it adds a second line of defense when security is paramount. . . .