Linux Network Server Chapter 1

The Boot process

This chapter looks at what happens during a Linux boot. It examines the processes that take place

and the configuration files that are read. Booting is a critical part of the operation of a server. The

boot process brings all of the network hardware online and starts all of the network daemon

processes when the system is powered−up. If the server will not boot, it is unavailable to all of the

users and computers that depend on it. For this reason, it is essential that the administrator of a

network server understand the boot process and the configuration files involved in that process.

After all, you're the person who maintains those configuration files and who is responsible for

recovering the system when it won't boot.

The term boot comes from bootstrap loader, which in turn comes from the old saying "pull yourself

up by your bootstraps." The meaning of this expression is that you must accomplish everything on

your own without any outside help. This is an apt term for a system that must start from nothing and

finish running a full operating system. When the boot process starts, there is nothing in RAM—no

program to load the system. The loader that begins the process resides in non−volatile memory. On

PC systems, this means that the loader is part of the ROM BIOS.

Booting a Linux PC is a multistep procedure. It involves basic PC functions as well as Linux

processes. This complex process begins in the PC ROM BIOS; it starts with the ROM BIOS

program that loads the boot sector from the boot device. The boot sector either contains or loads a

Linux boot loader, which then loads the Linux kernel. Finally, the kernel starts the init process, which

loads all of the Linux services. The next few sections discuss this process in detail.

Note

coverage because it is the default for most Linux distributions. GRUB is covered because it is

the default loader for Red Hat Linux 7.2.

LoadinSectorg the Boot

The ROM BIOS is configured through the BIOS setup program. Setup programs vary among

different BIOS versions, but all of them allow the administrator to define which devices are used to

boot the system and the order in which those devices are checked. On some PC systems, the

floppy drive and the first hard drive are the boot devices, and they are checked in that order.

Systems that permit booting from the CD−ROM usually list the CD−ROM as the first boot device,

followed by the first hard drive.

For an operational Linux server, set the ROM BIOS to check the floppy first and then the hard drive,

even if you used a bootable CD−ROM for the initial installation. The reason for this is simple: The

floppy is used to reboot an operational system when the hard drive is corrupted; the CD−ROM is

only booted to install or upgrade the system software. During an installation, the system is offline,

and you have plenty of time to fiddle with a BIOS setup program. But during an outage of an

operational server, time is critical. You want to be able to reboot Linux and fix things as quickly as

possible.

The first 512 bytes of a disk contain a boot sector. The ROM BIOS loads the boot sector from the

boot device into memory, and transfers control to it. The bootstrap program from the boot sector

then loads the operating system.

Two Linux loaders, LILO and GRUB, are covered in this chapter. LILO is given the bulk of the