A VPN is a conglomerate of useful technologies that originally were assembled by hand. Now the networking companies and ISPs have realized the value of a VPN and are offering products that do the hard work for you. In addition, there is an assortment of free software available on the Internet (usually for Unix systems) that can be used to create a VPN. In this book, we’re going to look at some of the commercial and free solutions in detail. Which one you choose for your network will depend on the resources available to you, the platforms you run, your network topology, the time you wish to spend installing and configuring the software, and whether or not you want commercial-level support. We can’t cover every vendor and product in this book; they change too quickly. Instead, we offer guidelines you can use on all networks and details on a few stable products that were available when we were writing this edition—we don’t mean to imply that there’s anything less valuable about competing products.
VPN packages range from software solutions that run on or integrate with a network operating system (such as the AltaVista Tunnel or CheckPoint Firewall-1 on Windows NT or Unix), to hardware routers/firewalls (such as those from Cisco and Ascend), to integrated hardware solutions designed specifically for VPN functions (such as VPNet and the Bay Networks Extranet Switch). Some VPN protocols, like SSH or SSL, gained popularity for performing other functions, but have since become used for VPNs as well.
In addition to products, ISPs are also offering VPN services to their customers. The tunneling usually takes place on the ISP’s equipment. If both ends of the connection are through the same ISP, that ISP might offer a Service Level Agreement (SLA) guaranteeing a certain maximum amount of latency and uptime.
Running a virtual private network over the Internet raises an easily forgotten issue of reliability. Let’s face it: the Internet isn’t always the most reliable network, by nature. Tracing a packet from one point to another, you may pass through a half-dozen different networks of varying speeds, reliability, and utilization—each run by a different company. Any one of these networks could cause problems for a VPN.
The lack of reliability of the Internet, and the fact that no one entity controls it, makes troubleshooting VPN problems difficult for a network administrator. If a user can’t dial into a remote access server at the corporate headquarters, or there’s a problem with a leased line connection, the network administrator knows there are a limited number of possibilities for where the problem may occur: the machine or router on the far end, the telecommunications company providing the link, or the machine or router at the corporate headquarters. For a VPN over the Internet, the problem could be with the machine on the far end, with the ISP on the far end, with one of the networks in between, with the corporate headquarters’ ISP, or with the machine or router at the corporate headquarters itself. Although a few large ISPs are offering quality of service guarantees with their VPN service (if all parties involved are connected to their network), smaller ISPs can’t make such a guarantee—and there will always be times when the network administrator is left to her own resources. This book will help you isolate and identify the problem when something goes wrong on your VPN.