Filtering caveats PPTP Filtering

75 HKEY_LOCAL_MACHINE\SYSTEM\Services\RASPPTPE\Parameters\Configuration Add a new Registry entry with the data type REG_DWORD. Call the entry AllowPacketsForLocalMachine and give it a value of 1, then enter the changes and close the Registry Editor. Youll have to reboot for the change to take effect. As weve said before, however, we dont recommend that you run services that can cause security breaches anonymous FTP or disrupt the workings of your internal network DHCP on your RAS server.

5.1.4 Filtering by IP Address

Another type of security allows you to specify the IP addresses from which the RAS server will allow PPTP connections. In order to implement this, your remote users will need to have fixed IP addresses assigned by their ISPs, and youll need to know these addresses. Used in combination with PPTP filtering, this can make the RAS server secure from both unauthenticated connections and connections from unauthorized hosts. Unfortunately, this cant be done simply from a graphical user interface, so you must go back into the Windows NT 4.0 Registry. Run the Registry Editor again and go to the following Registry key: HKEY_LOCAL_MACHINE\SYSTEM\Services\RASPPTPE\Parameters\Configuration Under this key, youll need to create a new entry of data type REG_DWORD. The new entry should be called AuthenticateIncomingCalls. Make it a decimal value and give it a value of 1. Under the same key, create a new entry of data type REG_MULTI_SZ. Give this entry the name of PeerClientIPAddresses. This is where youll want to enter the valid IP addresses of the hosts you wish to be able to connect to the RAS server using PPTP. They should all be separated by single blank spaces.

5.1.5 Configuring Users for Dial-up Access

A dial-up user under Windows NT RAS is essentially set up the same way as a regular Windows NT domain user: 1. Use the User Manager for Domains, found under the Administrative Tools menu, to add or modify the user. 2. When editing the User Properties, click the Dialin button on the lower left- hand side of the dialog box. 3. The Dialin Information dialog box will appear see Figure 5-4 . For a PPTP user, check Grant dialin permission to user, and set the Call Back radio button to No Call Back. 76 Figure 5-4. Configuring a user for remote access 4. Click OK to exit both the Dialin Information and User Properties dialog boxes.

5.2 Configuring PPTP for Dial-up Networking on a Windows NT Client

To dial into an ISP that supports PPTP using a Windows NT client, you simply configure dial-up networking as you normally would—with the addition of setting authentication and encryption options. In this section, well focus on setting up an NT client to use the PPTP protocol when youre connecting to a provider that doesnt support it. Below weve listed the steps for setting up a PPTP connection. Well assume that youve already configured Dial-Up Networking to call your ISP and set up a PPP connection. 1. Set up the PPTP protocol the same way you set it up for the RAS server in items 1 through 6 in Section 5.1.1 . 2. RAS configuration is also similar to the way it was done in Section 5.1.2 . This time, for item 3, click the Configure button in the Remote Access Setup dialog box. From there, select the Dial Out box, since we want to be able to dial out from the client rather than dial in. 3. From the start menu under Programs Accessories, select Dial-Up Networking. When the Dial-Up Networking dialog box appears, click the New button and enter the name you want to call your VPN connection on the Phonebook Entry dialog box. For our scenario, well use Central Office VPN. In the Phone Number field, enter the IP address of the PPTP RAS server to which youre connecting. For our example, its 2.1.1.60. In the Dial Using field, select one of the VPN ports you created while installing PPTP and RAS. They will have the name RASPPTPPM VPNn, where n is the number of the port. See Figure 5-5 .