Choosing your authentication method

73 If its available to all of your clients e.g., if theyre all Windows clients or youre using TunnelBuilder on your Macs, we suggest that you use MS-CHAP. Using it will give you the benefit of being able to turn on data encryption, so that the PPTP connection will be truly secure. Using the other methods is certainly possible if you dont have MS-CHAP-capable clients, but you run the risk of sending unencrypted data over the Internet, and unencrypted passwords in the case of PAP.

5.1.2.3 IP address negotiation using DHCP

The Dynamic Host Configuration Protocol DHCP is an ideal way to configure incoming PPTP clients with a dynamic IP address. Windows NT 4.0 comes with a DHCP server service that must be installed through the Network Control Panel. [1] Follow the instructions for installing RAS, but install the Microsoft DHCP Server service instead. Once the service is installed, a DHCP Manager program will also be installed under the Start menu in Administrative Tools. To configure DHCP, follow these steps: 1. Under the Start Programs Administrative Tools listing, open the DHCP Manager. The DHCP Manager dialog box will appear see Figure 5-3 . 2. Under the DHCP Servers column, select the Local Machine. Then go to the Scope menu item and select Create. 3. The Create Scope dialog box shown in Figure 5-3 will appear. Enter the Start Address and End Address for your assignments. In our case, well choose 2.1.1.129 for the starting address and 2.1.1.136 for the ending address. Figure 5-3. The Windows NT DHCP Manager 1 DHCP will not work on a Windows NT 4.0 RAS server that has two network cards with PPTP filtering enabled on one of them. Microsoft found the problem and issued a fix in Windows NT 4.0 Service Pack 2. We recommend having Service Pack 3 or later installed if you want to use DHCP with RAS. See other problems between DHCP and PPTP filtering in Section 5.1.3.2 later in this chapter. 74 4. Enter the Subnet Mask for the range. Since these addresses are part of the 2.1.1.0 range, well enter 255.255.255.0. 5. At this point, well leave the Exclusion Range addresses blank. We dont want to exclude any addresses from this range. 6. Well enter the Name of the scope as Dial-Up Address Range, then click OK. When a dialog asks you if you want to activate the scope, click Yes. If you have several RAS servers, youll probably want to use the DHCP Relay Agent NT service, also installed through the Network Control Panel. Using this service, the RAS server will forward a connecting clients request for an IP address to a DHCP server specified in the DHCP Relay Agent properties. This allows you to assign IP addresses from a single, central pool for every RAS server on your network, even across different LANs.

5.1.3 PPTP Filtering

We discussed what PPTP filtering does at the beginning of this chapter. To set up PPTP filtering, open the Network Control Panel, click on the Protocols tab and choose TCP IP, then click the Properties button. Finally, click the Advanced button on the TCP IP setup dialog box. At the bottom of the Advanced IP Addressing box is the Enable PPTP Filtering checkbox.

5.1.3.1 Outbound authentication using PPTP filtering

On multihomed hosts, PPTP filtering can also be used as a type of outbound firewall by enabling it on the network adapter connected to the LAN. Users on the internal network would dial the PPTP server as if they were coming over a PPP link, using the RAS servers IP address as the phone number. They would then be forced to authenticate at the RAS server in order to tunnel out to the Internet using the servers routing capabilities. This allows a network administrator to limit Internet access, monitor whos accessing the Internet and for how long, and limit the number of simultaneous Internet connections.

5.1.3.2 Filtering caveats

Enabling PPTP filtering on a Windows NT 4.0 system with only one network card can make other NT network services you might be running such as the DHCP server service and the FTP server service inaccessible to non-PPTP clients; the adapter will require PPTP authentication on any request it gets. There is a way to allow packets to reach the RAS server itself, without going to the network beyond. To do so you must install the Windows NT 4.0 Service Pack 3 or later version and add a particular entry to the Windows NT Registry. Editing the Windows NT Registry can be very dangerous. Invalid data entry could corrupt the Registry and cause numerous—possibly irreversible—system problems. You may be forced to reinstall Windows NT. As always, make sure you have a recent backup of your system. Run the Registry Editor by selecting the Run option under the Start menu and entering REGEDIT.EXE in the filename field. The parameter to add is under the following Registry key: