Choosing an ISP Managing and Maintaining Your VPN

160 There are also VPN services that ISPs are selling, including GTE, UUNET, and others. With these services, they operate and manage your VPN for you. Prices are variable, and are typically based on the number of sites and the total amount of bandwidth used.

10.2 Solving VPN Problems

There are numerous points of failure with VPNs. This makes tracking down the cause of a problem more difficult than it might be for a normal WAN or remote access connection. Among the possible problems are connectivity problems, authentication errors, and routing problems.

10.2.1 Connectivity Problems

Anyone familiar with maintaining or dialing into remote access servers—or into an ISP for that matter—is also familiar with the frustration of trying to pinpoint the problem of a bad connection. The main difficulty with connectivity problems is that they have so many causes. Here are a few possibilities: • Telco problems o Bad lines o Busy switch • ISP problems o Busy signals probably from a user-to-modem ratio thats too high o Bad modem or router • End-user problems o Bad modem or router o A modem or router thats incompatible with the ISPs o Configuration problem Besides these general communication problems, you may discover problems with port usage on firewalls. As youve seen, several VPN packages use specific TCP or UDP ports in order to communicate for example, PPTP uses TCP port 1723. If these ports arent open, you may not be able to make a VPN connection or transport data across the VPN. Its possible that these ports may be blocked at your ISP or on your own routers.

10.2.2 Authentication Errors

Authentication problems are common in the realm of dial-up connections, even when VPNs arent involved. Here are the two most common authentication problems: • A mismatched username or password, which occurs when either the connecting machine or the far end thinks that the username or password is something other than what it is. This is sometimes caused by a simple typographical error. Likewise, there could be mismatched keys in a public key system. • The connecting system and the destination are using different authentication methods. For instance, the connecting machine might be attempting PAP authentication, while the destination system is expecting CHAP. There is a third level of authentication problems involving public key infrastructures. Its important to use the same key exchange protocol. For example, some IPSec products allow 161 for a number of key exchange options: Manual, SKIP, or IPSec. In a public key infrastructure there could also be problems with certificate authorities and certificates.

10.2.3 Routing Problems

Routing problems occur when youre able to connect successfully to your ISP, but have trouble getting to certain hosts over the Internet, or getting out to the Internet at all. These problems are commonly due to configuration errors. Either the IP address, netmask, or gateway on your system is set incorrectly, or your ISP doesnt have a route for you. Chances are that any one of the problems discussed in this chapter—authentication, connectivity, or routing—is caused by a configuration mismatch on your equipment, the ISPs equipment, or the equipment on the far end of the connection. The routing problem, however, could also be due to any one of the numerous connection points on the Internet backbone between you and the destination. You, and your ISP, will have little control over these problems, but its nice to know where the problem is so that you can report it to the proper people. In Chapter 5 , in Section 5.6 , we mentioned two useful utilities for testing routes: ping and traceroute. Both of these tools can be used to troubleshoot problems on other VPNs as well. ping is a utility found on Unix, Windows 9598, and Windows NT systems. It sends packets to a given destination and awaits a return. It doesnt tell you what route the packets take, but it does tell you if they get there at all and if theres any packet loss. traceroute is a program on Unix systems. The Windows 9598NT equivalent is TRACERT. traceroute will actually show you the path packets take to their destination. This information can be useful to pinpoint exactly where a problem is occurring. traceroute information can sometimes be confusing. Be sure to read up on what the various latencies mean, as well as asterisks, exclamation marks, and other symbols. A good TCPIP networking book will explain how to read traceroutes output, as will its manual page on a Unix system. Also keep in mind that an ISP or company may be blocking traceroutes UDP packets at their firewall for security reasons, so you may want to contact them and find out if this is the case. If the problem appears to be with a backbone provider, the best thing to do is still to contact your ISP. They can then contact the backbone provider and see what the problem is.

10.2.4 Dealing with an ISP

Working with an ISP to solve a VPN problem may prove difficult, especially if the ISP doesnt support VPNs. As a network administrator, therefore, youll want to know your VPN product inside and out. The most important thing to remember when troubleshooting a problem with an ISP is to give them as much information as possible. At a minimum, give them this information: • What VPN product youre using. • What the IP address of your system should be. • What the IP address of the destination VPN server or router should be e.g., the address of your PPTP server. • The TCP or UDP ports that your VPN product uses, in case your ISP has those ports blocked at a firewall.