Connectivity Problems Solving VPN Problems

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.