The VPN concept has been around for almost 10 years. Technologies that use public data lines for private corporate traffic promise companies a cornucopia of benefits—from saving money on expensive leased lines to a workforce empowered to access the entire wealth of corporate IT resources from any kind of connection anywhere on the globe. But as with other overhyped and overmarketed technologies, the devil is in the details.
The cost of VPNs has dropped dramatically and implementation has gotten much easier, but the VPN market is still confusing and crowded. Without proper research, planning, and deployment, a VPN can become a nightmare installation and an unusable system or even reduce your network's security.
The well-publicized case of the intruder who cracked Microsoft's VPN, accessed the corporate network, and almost made away with the company's precious source code should be a warning. VPNs offer many benefits but also open a hole into your network, usually bypassing your firewall or going right through it. So, you need to carefully consider which VPN product to choose and how to install and run it.
No one-size-fits-all VPN exists. Ambiguity in the standards and differences in feature sets from vendor to vendor make the decision process fairly complex. Several factors, including your organization size, privacy requirements, and user sophistication, determine which VPN solution might suit your needs. The right product and operational procedures can securely open your network borders, increasing worker productivity while still letting you sleep at night. If you keep in mind these considerations when purchasing a VPN solution and follow a few recommendations about how to securely run it, you can achieve the Private in your Virtual Private Network without pulling out your hair in the process.
Accommodating NAT
If your VPN will primarily support remote users such as telecommuters and traveling employees and these users will access internal LAN resources that use a Network Address Translation (NAT) address rather than a routable IP address, you might have problems with some vendors' VPN products. NAT lets multiple internal network hosts use nonroutable IP addresses to access the Internet through one IP address on a firewall or router. This arrangement provides an additional level of security and lets a company be much more flexible with its address assignments than if it used real IP addresses for all its hosts.
However, NAT can interfere with some VPN implementations because it changes information in a packet's IP header to route the packet to the correct internal IP address. VPN protocols often check the integrity of the packet header and terminate the connection if they detect any changes that were made after the packet was encrypted. Vendors have devised a workaround for this problem: A technique called UDP Traversal encapsulates the IP Security (IPSec) packet in a UDP packet so that the IPSec header can arrive intact. Most vendors, including Microsoft, Nortel Networks, SSH Communications Security, NetScreen Technologies, SonicWALL, and Cisco Systems—in IOS Software 12.2(8) and later—support UDP Traversal. However, some low-end VPN appliances and software implementations might not. Alternatively, if you use IPSec, your router or firewall might support IPSec pass-through, which recognizes the IPSec protocol and lets IPSec packets pass through unaltered, eliminating the need for NAT traversal. You might also be able to work around NAT by turning off IPSec's Authentication Header (AH) element (which verifies the header information), if your VPN allows this level of detail in configuration. Be sure to check with your VPN vendor about NAT if you plan to support remote users through a network that uses NAT.
IDS Placement
If you use Intrusion Detection System (IDS) technology, you should know that if the IDS machine is between the Internet and the VPN concentrator that decrypts the encrypted packets (e.g., on a demilitarized zone—DMZ—network), it won't be able to detect intrusion activity that occurs between VPN-connected machines. Most IDS sensors match packet payloads to a database of intrusion signatures so that they know when to flag something as suspicious. If the packets are encrypted, they'll look like gibberish to the IDS machine. If you want your IDS machine to be able to monitor network traffic from VPN connections, make sure you place the IDS machine behind the VPN concentrator so that the IDS machine checks the traffic after the VPN concentrator decrypts it. You can't use an IDS on a software VPN, which operates directly from one VPN host to another.
Preferred Protocols
One of the most important choices you make when selecting VPN hardware or software is which VPN protocol to use. A VPN product might support multiple protocols or only one. A protocol that's weak or not widely supported could render your VPN unusable if someone exploits a vulnerability. A proprietary protocol could mean future compatibility problems. Although the practice has become less common, a few vendors still try to do their own thing cryptographically. Avoid these vendors' products like the plague. I strongly recommend that you stay away from products that use proprietary, nonstandard protocols and stick to one of the following major protocols.
IPSec. Probably the best supported and most widely used protocol, IPSec is rapidly becoming the standard for VPNs. IPSec, which the Internet Engineering Task Force (IETF) developed, consists of multiple subprotocols; each handles a different element of the process, and some are optional or interchangeable. IPSec is a broad specification, and vendors' IPSec implementations differ. Make sure you read the fine print to understand what parts of IPSec a product uses.
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