Configuring the Networking Hardware

For you to use a network card, special functions have to be present in your Linux kernel that understand the particular way this device is accessed. The software that implements these functions is called a device driver.
Linux has device drivers for many different types of network interface cards: ISA, PCI, MCA, EISA, Parallel
port, PCMCIA, and more recently, USB.

But what do you mean when you say a driver handles a device? Let's consider an Ethernet card. The driver has to be able to communicate with the peripheral's on-card logic somehow: it has to send commands and data to the card, while the card should deliver any data received to the driver.

In IBM-style personal computers, this communication takes place through a cluster of I/O addresses that are mapped to registers on the card and/or through shared or direct memory transfers. All commands and data the kernel sends to the card have to go to these addresses. I/O and memory addresses are generally described by providing the starting or base address. Typical base addresses for ISA bus Ethernet cards are 0x280 or 0x300.
PCI bus network cards generally have their I/O address automatically assigned.

Usually you don't have to worry about any hardware issues such as the base address because the kernel makes an attempt at boot time to detect a card's location. This is called auto probing, which means that the kernel reads several memory or I/O locations and compares the data it reads there with what it would expect to see if a certain network card were installed at that location. However, there may be network cards it cannot detect automatically; this is sometimes the case with cheap network cards that are not-quite clones of standard cards from other manufacturers. Also, the kernel will normally attempt to detect only one network device when booting. If you're using more than one card, you have to tell the kernel about the other cards explicitly.

Another parameter that you might have to tell the kernel about is the interrupt request line. Hardware
components usually interrupt the kernel when they need to be taken care offor example, when data has arrived or a special condition occurs. In an ISA bus PC, interrupts may occur on one of 15 interrupt channels numbered 0, 1, and 3 through 15.

The interrupt number assigned to a hardware component is called its interrupt request number (IRQ).(IRQs 2 and 9 are the same because the IBM PC design has two cascaded interrupt processors with eight IRQs each; the secondary processor is connected to IRQ 2 of the primary one).

The kernel accesses a piece of network hardware through a software construct called an interface. Interfaces offer an abstract set of functions that are the same across all types of hardware, such as sending or receiving a datagram.

Interfaces are identified by means of names. In many other Unix-like operating systems, the network interface is implemented as a special device file in the /dev/ directory. If you type the ls -las /dev/ command, you will see what these device files look like. In the file permissions (second) column you will see that device files begin with a letter rather than the hyphen seen for normal files. This character indicates the device type. The most common device types are b, which indicates the device is a block device and handles whole blocks of data with each read and write, and c, which indicates the device is a character device and handles data one character at a time. Where you would normally see the file length in the ls output, you instead see two numbers, called the major and minor device numbers. These numbers indicate the actual device with which the device file is associated.

Each device driver registers a unique major number with the kernel. Each instance of that device registers a unique minor number for that major device. The tty interfaces, /dev/tty*, are a character mode device
indicated by the c, and each have a major number of 4, but /dev/tty1 has a minor number of 1, and
/dev/tty2 has a minor number of 2. Device files are very useful for many types of devices, but can be
clumsy to use when trying to find an unused device to open.

Linux interface names are defined internally in the kernel and are not device files in the /dev directory. Some typical device names are listed later in the section called A Tour of Linux Network Devices. The assignment of interfaces to devices usually depends on the order in which devices are configured. For instance, the first Ethernet card installed will become eth0, and the next will be eth1. SLIP interfaces are handled differently from others because they are assigned dynamically. Whenever a SLIP connection is established, an interface is assigned to the serial port.

When booting, the kernel displays the devices it detects and the interfaces it installs. The following is an excerpt from typical boot messages:

          .
          .   This processor honors the WP bit even when in supervisor mode./
              Good.
        Swansea University Computer Society NET3.035 for Linux 2.0
        NET3: Unix domain sockets 0.13 for Linux NET3.035.
        Swansea University Computer Society TCP/IP for NET3.034
        IP Protocols: IGMP,ICMP, UDP, TCP
        Swansea University Computer Society IPX 0.34 for NET3.035
        IPX Portions Copyright (c) 1995 Caldera, Inc.
        Serial driver version 4.13 with no serial options enabled
        tty00 at 0x03f8 (irq = 4) is a 16550A
        tty01 at 0x02f8 (irq = 3) is a 16550A
        CSLIP: code copyright 1989 Regents of the University of California
        PPP: Version 2.2.0 (dynamic channel allocation)
        PPP Dynamic channel allocation code copyright 1995 Caldera, Inc.
        PPP line discipline registered.
        eth0: 3c509 at 0x300 tag 1, 10baseT port, address 00 a0 24 0e e4 e0,/
              IRQ 10.
        3c509.c:1.12 6/4/97 This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
        Linux Version 2.0.32 (root@perf) (gcc Version 2.7.2.1)
        #1 Tue Oct 21 15:30:44 EST 1997
          .
          .

This example shows that the kernel has been compiled with TCP/IP enabled, and it includes drivers for SLIP, CSLIP, and PPP. The third line from the bottom says that a 3C509 Ethernet card was detected and installed as interface eth0. If you have some other type of network cardperhaps a D-Link pocket adaptor, for examplethe kernel will usually print a line starting with its device name dl0 in the D-Link examplefollowed by the type of card detected. If you have a network card installed but don't see any similar message, the kernel is unable to detect your card properly.

Kernel Configuration

Most Linux distributions are supplied with boot disks that work for all common types of PC hardware.
Generally, the supplied kernel is highly modularized and includes nearly every possible driver. This is a great
idea for boot disks, but is probably not what you'd want for long-term use. There isn't much point in having drivers cluttering up your disk that you will never use. Therefore, you will generally roll your own kernel and include only those drivers you actually need or want; that way you save a little disk space and reduce the time it takes to compile a new kernel.

In any case, when running a Linux system, you should be familiar with building a kernel. Think of it as a right of passage, an affirmation of the one thing that makes free software as powerful as it isyou have the source. It isn't a case of, I have to compile a kernel, rather it's a case of, I can compile a kernel. The basics of compiling a Linux kernel are explained in Matt Welsh's book, Running Linux (O'Reilly). Therefore, we will discuss only configuration options that affect networking in this section.

One important point that does bear repeating here is the way the kernel version numbering scheme works. Linux kernels are numbered in the following format: 2.2.14. The first digit indicates the major version number. This digit changes when there are large and significant changes to the kernel design. For example, the kernel changed from major 1 to 2 when the kernel obtained support for machines other than Intel machines. The second number is the minor version number. In many respects, this number is the most important number to look at. The Linux development community has adopted a standard at which even minor version numbers indicate production, or stable, kernels and odd minor version numbers indicate development, or unstable, kernels. The stable kernels are what you should use on a machine that is important to you, as they have been more thoroughly tested. The development kernels are what you should use if you are interested in experimenting with the newest features of Linux, but they may have problems that haven't yet been found and fixed. The third number is simply incremented for each release of a minor version( People should use development kernels and report bugs if they are found; this is a very useful thing to do if you have a machine you can use as a test machine. Instructions on how to report bugs are detailed in /usr/src/linux/REPORTING-BUGS in the Linux kernel source).

When running make menuconfig, you are presented with a text-based menu that offers lists of configuration questions, such as whether you want kernel math emulation. One of these queries asks you whether you want TCP/IP networking support. You must answer this with y to get a kernel capable of networking.