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.
Secure External VPN Clients
One of the most important things to remember when building your VPN is that a VPN secures only the data transmissions between two machines—it doesn't protect the machines themselves. Some firms hand out VPN connections as though they were candy at Halloween—to anyone who asks for one and without regard to how secure those computers are. Remember, you're handing out the front-door keys to your network, and you shouldn't do that lightly. A virus can bypass network-based antivirus protection by coming in on an encrypted VPN connection. Like IDS systems, antivirus systems can't read encrypted data, so they have problems with VPN traffic. If an intruder takes over a remote VPN client, he or she has an encrypted tunnel right to the heart of your network, making discovery and surveillance of the intruder much more difficult than if the intruder entered over an unencrypted channel. So, you should protect your VPN clients even better than you protect your internal machines because they're typically at least partially exposed to the outside.
Make sure when allocating VPN connections that the remote computers meet the same security requirements as computers on your local LAN—stricter, if possible. At a minimum, all remote VPN clients should have antivirus software and firewall software to offer some minimal protection, although some personal firewall software can interfere with some VPN client software. Include VPN client systems, such as home computers, field laptops, and partner and vendor machines, in all security assessments or vulnerability scans that you perform. You can check them the same way you check your local machines by making sure your remote VPN clients are logged on when you do your security testing and including the VPN IP range in your tests. Just make sure you get permission before you scan any machines your company doesn't own. If you use Active Directory (AD), you can also push out a standard security policy to your Win2K or later VPN clients to make sure that they conform to the policy for machines on your network.
For a VPN that services telecommuters, consider using a vendor that offers a firewall with separate zones for work and home machines that share an Internet connection. As Figure 2 shows, the firewall's trusted zone gives the telecommuter's work PC access to the Internet and VPN access to the corporate LAN, and an untrusted zone allows a personal machine access to the Internet only. SonicWALL and WatchGuard currently offer such firewalls, which aren't much more expensive than home routers and eliminate worries about the other computers on your telecommuters' home LANs. However, multizone home firewalls don't eliminate the need to continually verify the security of remote VPN clients.
Adding VPN capabilities to your network isn't a decision to take lightly, although in this 24 x 7 day and age, you might find a VPN implementation impossible to avoid as users demand external access to your network. Just remember: A VPN adds access, not security, to your network. Think of a VPN as just another potential vector for intruders attempting to access your network or information. Done right, a VPN can improve your company's communications and still keep your network safe. So when you take the plunge, use the security checklist that Figure 3 shows to make sure you've done the research and preparation. That way, your VPN won't turn into a Virtual Public Network or your Very Personal Nightmare.
End of Article
Prev. page
1
2
3
[4]
next page -->
