IP address negotiation using DHCP

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: 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.