RAS Client

RAS Client

All Microsoft Windows platforms feature a RAS client, which allows you to connect your computer from a remote location to another computer network featuring a remote access server component. Typically, a RAS client will do this by using a serial communication device such as a modem that connects to a telephone line and calls the remote network by dialing a telephone number. Because of this, the RAS client is sometimes referred to as a dial-up networking (DUN) client. RAS also supports connecting to a remote network by tunneling connections securely over an IP network such as the Internet, which is known as Virtual Private Networking (VPN).

On the remote network, you must have a RAS server awaiting your DUN or VPN connections. A RAS client is capable of establishing a communication link with several types of remote access servers. RAS does this by using industry standard serial framing and IP tunneling protocols. The following protocols are serial framing, where data communication proceeds over a serial device such as a modem:

• Point-to-Point Protocol (PPP) Can transmit IP, IPX, and NetBEUI communication protocols

• Serial Line Internet Protocol (SLIP) Can transmit the IP communication protocol only

• Asynchronous NetBEUI (Microsoft Windows NT 3.1, Microsoft Windows for Workgroups 3.11) Can transmit the NetBEUI communication protocol only

RAS uses the following IP tunneling protocols where data communication proceeds over an existing IP connection:

• Point-to-Point Tunneling Protocol (PPTP) Can securely transmit IP and IPX communication protocols

• Layer 2 Tunneling Protocol (L2TP) Can securely transmit IP and IPX communication protocol

The framing and tunneling protocols describe how data is transmitted over a RAS communication link and dictate which network communication protocols (such as TCP/IP or IPX) can communicate over the link. If a RAS server supports one of the framing protocols defined in the previous list, a RAS client can establish a connection. All Windows 95, Windows 98, Windows Me, and Windows NT platforms feature a RAS server component capable of supporting the serial framing protocols listed. Windows Me and all Windows NT platforms also support the IP tunneling protocols.

Once a connection between a RAS client and server is established, network protocol stacks (depending on the framing or tunneling protocol used) can communicate over the RAS connection to the remote network as if the computers were connected directly over a LAN. Of course, the data communication rate of most RAS connections is typically slower than for a direct LAN connection.

When a RAS server accepts a serial framing or an IP tunneling connection, it first establishes communication with your client by negotiating one of the framing or tunneling protocols in the previous list. Once the protocol connection is established, the RAS server attempts to authenticate the user making the connection. The RAS API functions this chapter describes allow a RAS client to specify a user name, a password, and domain logon credentials to the RAS server. When a Windows NT–based RAS server receives this information, it validates these logon credentials using Windows NT domain security access control. Note that the RAS server does not log your client on to a Windows NT domain; instead, it uses the client credentials to verify that a user is allowed to make a RAS connection. The RAS connection process is not the same as the Windows NT domain logon process. After a RAS connection is successfully established, your computer can log on to a Windows NT domain. On Windows 95, Windows 98, Windows Me, Windows XP, and Windows .NET Server, RAS can automatically log a machine on to a domain after a RAS connection is authenticated through options available in a phonebook entry, as we will discuss later in this chapter.

For serial communications, RAS relies on the Telephony Application Programming Interface (TAPI) to set up and control serial communication devices such as modems on your computer. TAPI controls the hardware settings of these dialing devices. When you set up a RAS connection using a modem, TAPI turns on the modem and sends dialing information from RAS to the modem. As a result, RAS views modems as simple TAPI modem ports that are capable of dialing and making a phone connection to a remote server. As you will see later in this chapter, some of the RAS API functions refer to TAPI modem ports when you set up RAS connection information.

This chapter will explain how you can use RAS programmatically to establish remote network communication. We will begin by describing the header and library files you need to build your application. Next, we will describe the basics of dialing—how you actually establish a remote connection over a serial device. Then we'll describe how you can set up RAS phonebook entries to define detailed communication properties of a RAS connection. Once we've explained the basics of setting up communication, we'll show you how to manage established connections. Finally, we'll describe how to set up a VPN connection.