Chapter 2 -- The Redirector

Chapter 2

Microsoft Windows offers applications the capability to communicate over a network using built-in file system services. This is sometimes referred to as the network operating system (NOS) capability. This chapter explores these networking capabilities using Windows file system components available in Windows 95, Windows 98, Windows NT, Windows 2000, and Windows CE. The purpose of this chapter is to provide an understanding of these capabilities as they relate to the mailslot and named pipe networking technologies. The mailslot and named pipe networking technologies will be covered in greater detail in Chapters 3 and 4, respectively.

When applications want to access files on a local system, they rely on the operating system to service I/O requests. This is typically referred to as local I/O. For example, when an application opens or closes a file, the operating system determines how to access a device that contains the contents of the specified file. Once the device is found, the I/O request is forwarded to a local device driver. The same operating principle is also available for accessing devices over a network. However, the I/O request must be forwarded over a network to the remote device. This is referred to as I/O redirection. For example, Windows allows you to map or redirect a local disk identifier—such as E:—to a directory share point on a remote computer. When applications reference E:, the operating system redirects the I/O to a device called a redirector. The redirector forms a communication channel to a remote computer to access the desired remote directory. This functionality allows applications to use common file system API functions, such as ReadFile and WriteFile, to access remote files across a network.

This chapter discusses the details of how the redirector is used to redirect I/O requests to remote devices. This is important information—it is the foundation for communication in the mailslot and named pipe technologies. First we will discover how files can be referenced over a network with the Universal Naming Convention (UNC) using the Multiple UNC Provider (MUP) resource locator. This is followed by an explanation of how MUP calls a network provider, which exposes a redirector to form communications among computers using the Server Message Block (SMB) protocol. Finally, we will describe network security considerations when accessing files over a network using basic file I/O operations.