Linux Networking

Networking refers to all aspects of data exchange within one computer or between two or more computers, ranging from the physical connection to the protocol for the actual data exchange. A network protocol is the method the sender and receiver agree upon for exchanging data across a network.

Different network protocols are used at different levels of the network. At the physical level—at which the data bits travel through a medium, such as a cable—Ethernet and Asynchronous Transfer Mode (ATM) are two commonly-used protocols. Application programs don’t really work at the physical level, however. Instead, they rely on protocols that operate on blocks of data. These protocols include Novell’s Internet Packet Exchange (IPX) and the well-known Transmission Control Protocol/Internet Protocol (TCP/IP).

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The different levels of network protocols can be represented by a networking model such as the seven-layer Open Systems Interconnection (OSI) reference model, developed by ISO. Chapter 6 includes a discussion of this model.

Standard network protocols such as TCP/IP have been key to the growth of interconnected computers, resulting in local area networks (LANs), as well as wide area networks (WANs). Protocols have enabled these smaller networks to communicate with each other, and we now have interconnected networks that form an internetwork: the Internet.

TCP/IP

The ability to network has been one of the strengths of UNIX since its early days. In particular, the well-known TCP/IP protocol suite has been an integral part of UNIX ever since TCP/IP appeared in BSD UNIX around 1982. By now, TCP/IP is the wide area and local area networking protocol of choice in the global Internet. TCP/IP does not depend on the physical communication media. This media independence enables TCP/IP to work in a wide variety of networks.

Linux supports the TCP/IP protocol suite and includes all common network applications such as Telnet, FTP, and sendmail. At the physical-network level, Linux includes drivers for many Ethernet cards. Token ring is also an integral part of the Linux kernel source; all you have to do is load the token ring driver to enable support for token ring.

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You might say that Linux’s support for TCP/IP—the dominant protocol suite of the Internet—comes naturally. The rapid development of Linux itself would not have been possible without the collaboration of so many developers from Europe, America, and other parts of the world. That collaboration, in turn, has been possible only because of the Internet. Chapters 13 through 19 show you how to set up TCP/IP networking and to use various servers to offer services such as Web, email, and domain name service.

Linux also includes the Berkeley Sockets programming interface (so named because the Sockets interface was introduced in Berkeley UNIX around 1982), a popular interface for network programming in TCP/IP networks. For those of you with C programming experience, the Sockets interface consists of several C header files and several C functions that you call to set up connections and to send and receive data. Chapter 6 describes sockets.

You can use the Berkeley Sockets programming interface to develop Internet tools such as Web browsers. Because most TCP/IP programs (including those available for free at various Internet sites) use the Sockets programming interface, it is easy to get these programs up and running on Linux because it includes the Sockets interface.

PPP and SLIP

Not everyone has an Ethernet connection to the Internet (although a growing number of us are beginning to have high-speed always-on connections to the Internet, thanks to cable modems or DSL). Most of us still connect to the Internet and communicate by using the TCP/IP protocol over a phone line and a modem. To do this, what you need is access to a server—a system that has an Internet connection and that accepts a dial-in connection from your system.

Commercial outfits known as Internet service providers (ISPs) offer this type of service for a fee. If you don’t want to pay for such a connection, find out whether a computer at your place of business provides this access. That option may not be unreasonable, especially if you are doing UNIX software development (for your company) on your Linux PC at home.

When you access the Internet through a server, the server runs the Point-to-Point Protocol (PPP), which works over any serial link, including dial-up conenctions.

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Linux supports PPP for dial-up Internet connections. You can also turn your Linux system into a PPP server so that other computers can dial in to your computer and establish a TCP/IP connection over the phone. Chapter 13 explains how to set up a PPP dial-up Internet connection on your Red Hat Linux system.

Cable/DSL and Wireless Networks

If you have high-speed Internet access through cable modem or DSL (Digital Subscriber Line), you can easily hook up a Ethernet-equipped Linux PC to the Internet. For the most part, the configuration of the Linux PC is the same as that for TCP/IP networking. However, for ISPs that use PPP over Ethernet (PPPoE), you may have to do some additional configuring. Linux includes support for PPPoE.

Linux also supports wireless Ethernet cards that you can use to connect laptop PCs to an existing wired Ethernet local area network (LAN). These wireless Ethernet cards conform to the IEEE 802.11b standard, also known as Wi-Fi. If your LAN connects to the Internet through a cable/DSL router and hub, you can extend the LAN by connecting a wireless access point to the hub. Then, any Wi-Fi-equipped laptop or desktop PC can connect to the Internet through the cable or DSL connection.

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Chapter 13 describes how to configure Linux for a wireless Ethernet network.

File Sharing with NFS

In the Microsoft Windows or Novell NetWare world, you may be familiar with the concept of a file server—a system that maintains important files and allows all other systems on the network to access those files. Storing files on a central server provides for better security and enables convenient backups. Essentially, all PCs on the network share one or more central disks. In Windows and Novell, users see the file server’s disk as just another drive, with its own drive letter (such as U). Typically in PC networks, you implement file sharing with Novell NetWare or Microsoft LAN Manager protocols.

File sharing exists in UNIX as well. The Network File System (NFS) provides a standard way for a system to access another system’s files over the network. To the user, the remote system’s files appear to be in a directory on the local system.

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NFS is available in Linux; you can share your Linux system’s directories with other systems that support NFS. The other systems that access your Linux system’s files via NFS do not necessarily have to run UNIX; NFS is available for DOS, Windows, OS/2, and NetWare as well. Therefore, you can use a Linux PC as the file server for a small workgroup of PCs that run DOS and/or Windows. Chapter 19 further explores the use of a Linux PC as a file server. Chapter 19 also explains how to use the Samba package to set up your Linux PC as a server in a Windows network.