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Virtual Private Networks, Second Edition by Paul Wolfe, Charlie Scott, Mike Erwin

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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.

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

    • Bad lines

    • Busy switch

  • ISP problems

    • Busy signals (probably from a user-to-modem ratio that’s too high)

    • Bad modem or router

  • End-user problems

    • Bad modem or router

    • A modem or router that’s incompatible with the ISP’s

    • Configuration problem

Besides these general communication problems, you may discover problems with port usage on firewalls. As you’ve seen, several VPN packages use specific TCP or UDP ports in order to communicate (for example, PPTP uses TCP port 1723). If these ports aren’t open, you may not be able to make a VPN connection or transport data across the VPN. It’s possible that these ports may be blocked at your ISP or on your own routers.

Authentication Errors

Authentication problems are common in the realm of dial-up connections, even when VPNs aren’t involved. Here are the two most common authentication problems:

  • A mismatched username or password, ...

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