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way to force a particular user to dial into a specific VPN port, you might as well set all of them to the same values. Experienced RAS administrators will recognize this dialog
box as the very same one used to configure a regular dial-up RAS connection.
Figure 5-1. Setting up a PPTP RAS device
5.1.2.1 Choosing the protocols to tunnel
From the Network Configuration dialog box, one of the things youll be able to select is which protocols to allow over a particular VPN port see
Figure 5-2 . The choices are IP, IPX, and
NetBEUI. For our example, we want to enable IP so Sara N. can get to the Internet mail server, and enable NetBEUI so she can attach to the shared drive of her desktop machine, but
disable the unused IPX protocol.
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Figure 5-2. The RAS Network Configuration dialog box
From the same dialog box, you can also limit the user to the RAS server, rather than give access to the entire network. For our scenario, Sara N. will have access to the entire network.
We dont recommend limiting access for several reasons:
•
First of all, its uninteresting. One of the exciting things about VPNs is that they give users secure remote access as if they were directly connected to the LAN. Limiting
them to the RAS server means that youre limiting what theyre allowed to do on the network to the services the RAS server is performing.
•
If youre limiting remote user access to the RAS server itself, this probably means that youre running other services on the RAS server, such as email or printing services, or
that youre using the RAS server as an application server. Unless you have four or fewer clients on your network, we dont recommend using the RAS server for anything
but RAS. Otherwise, it can get bogged down by acting as both a router and a server.
•
A PPTP RAS server, by its very nature, needs to be at least partially accessible from the Internet. Thus, it will also be open to attacks from the Net. If youre running
critical applications on or from the RAS server and it proves vulnerable to one of these attacks and crashes, your application will go down with it.
5.1.2.2 Choosing your authentication method
In Chapter 4
, we went over the authentication methods available in RAS: authentication with encryption CHAP, authentication with Microsoft-enhanced encryption MS-CHAP, and
clear text PAP. You can either require CHAP or MS-CHAP, or allow both encryption methods plus PAP. You make your choice in the Network Configuration dialog box.
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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.
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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:
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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.
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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 .
