Hardware vs. Software
You'll have to decide whether you want to base your VPN on a software implementation or a dedicated hardware device. Some of the protocols make the decision for you—for example, SSH is strictly a software implementation, at least for now. Software implementations tend to be cheaper, sometimes even free. Windows NT 4.0 has PPTP support built in, and XP and Win2K have PPTP and IPSec built-in support, as I mentioned earlier. A nice open-source implementation of IPSec called Linux FreeS/WAN is available at http://www.freeswan.org. Software VPNs tend to work best for server-to-server communication or for small groups.
For large-scale implementations, choose a hardware device such as a VPN concentrator or VPN-enabled network appliance. Hardware-based VPNs perform better for larger installations. Also, the security of a software-based VPN built on a host with an OS such as Windows, UNIX, or Linux depends on the underlying security of that OS. Thus, you must keep the OS patched as well as keep an eye on the VPN software.
Hardware-based VPNs tend to be less vulnerable than software implementations because their chip-based OSs are more lightweight (i.e., they have fewer features to exploit than general-purpose OSs). Also, because they don't sit on everyone's desktop, they're less used and understood, although exploits on them aren't unheard of. For example, security researchers recently discovered several security holes in Cisco's VPN concentrators. Make sure you subscribe to your VPN vendor's security update mailing list and promptly apply all security patches.
If you're considering a hardware VPN, ask vendors whether their solution has a dedicated processor for encryption. Some of the newer VPN appliances use dedicated application-specific integrated circuits (ASICs) to handle the encryption algorithms, which make encryption much faster, especially on busy networks. Also make sure that the box you purchase will handle the number of tunnels and the throughput that you need now and in the future. You don't want to have to replace the box in a year or two.
Nokia, Cisco, Nortel, Lucent, and others offer dedicated VPN boxes, although standalone VPN concentrators are becoming less common. Most firewalls, routers, and network appliances—such as those by WatchGuard Technologies, SonicWALL, and NetScreen—provide some VPN functionality. For a good list of IPSec-certified VPN devices, go to http://www.icsalabs.com/html/communities/ipsec/certification/certified_products/index.shtml.
Secure Implementation
After you choose your VPN, you must install and maintain it correctly to enjoy all the benefits a VPN can provide. In addition to using a sufficiently long key length, you must properly secure keys and access to VPN concentrators. If you store your keys in plaintext files on Internet-connected computers, all the bits of key length in the world won't help you if someone compromises those computers. You should also change your shared base keys on a regular basis, preferably every 3 months. This practice limits your exposure if a key is compromised.
If you require a high level of trust on the authentication process as well as the encryption, you might consider using digital certificates instead of the standard preshared secret key that most VPNs default to. Digital certificates guarantee that the person trying to connect is who he or she says he or she is. A separate digital certificate for each end connection can be expensive; however, some VPN vendors offer authentication services that provide a bulk discount on certificates.
Closely control access to your VPN box, whether it's a concentrator or Windows machine. In the case of a Windows server, put the machine on a separate domain and have only a few accounts on it. Use the strongest possible passwords, and store and swap them out appropriately. In the case of a hardware device, disable insecure protocols, such as FTP and Telnet, that pass your logon information in the clear. An insecure VPN concentrator box or unpatched Windows VPN server presents a much easier target than do VPN keys that must be brute-forced.
Many installations treat external VPN clients as fully trusted internal hosts. I recommend that you create a second class of VPN user that doesn't have the full privileges of a local host and that can access only the resources that a user of that type requires. Don't give these users access to printers or shares that they don't need for external work.
Prev. page
1
2
[3]
4
next page 
