Managers Boot & Troubleshooting
Section 7 Part 7
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Overview of System Initialization
1. 2. 3.
4.
boot process & boot loaders /sbin/init and System Initialization rc initialization scripts system services
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Boot Process Overview
4 distinct stages of initialization BIOS
minimal I/O operating system located in the ROM of the system's motherboard
Bootloader
minimal application loaded from disk's MBR used to load and pass control to the Linux Kernel.
Kernel
initializes internal structures and device drivers, provides initial filesystem, and starts the first process
/sbin/init
Performs most of the work for initializing system processes
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BIOS
POST System Check Verifies core components including CPU and memory are present and functioning. Device Initialization BIOS core device drivers for the keyboard, video, serial ports Assigns appropriate system resources such as IRQ lines and I/O Ports (limited set of device drivers) Boot Device Selection Configurable through BIOS's interactive setup application Handing Over the Reigns BIOS loads the disk's MBR (first block = 512 bytes) into memory Passes execution to this region The first block of any bootable disk must contain a small executable, referred to as a bootloader 4
Bootloader
Usually, the bootloader performs the following three tasks. Compose a kernel command line Load an initial ramdisk (initrd), if necessary Load and pass control to the Linux kernel
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What is an Initial Ramdisk?
The initial ramdisk is a filesystem image which contains necessary kernel modules to make it possible to load the kernel. The bootloader loads the ramdisk image into memory, then the kernel can mount the filesystem and load any necessary modules. In SUSE Linux initrd's are called /boot/initrdkernelversion, and it is the bootloader's responsibility to load them. In SUSE 10, /boot/initrd is a symbolic link to the /boot/initrd-kernelversion file for the initial ramdisk
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Linux Bootloaders
LILO
traditional Linux bootloader (Linux Loader) designed to support ext2 file systems ELILO - newer version that supports 64-bit architectures
GRUB
Grand Unified Bootloader (GNU project) Designed to operate with multiple operating systems and file systems. Default bootloader for many versions of Linux including SUSE and RedHat.
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Bootloader Design
Stage 1
Generally small, designed to fit into a Master Boot Record (512 bytes) Sole task of the first stage bootloader is to locate, load, and pass control to the second stage bootloader Usually not an observable file in the filesystem
Stage 2
main bootloader runs some form of (often interactive) application at startup, and is able to read configuration information about default settings Usually exists as an identifiable binary file in /boot
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GRUB Configuration
/boot/grub/menu.lst Can be edited by hand with a text editor, such as vi. View the grub configuration file using:
cat /boot/grub/menu.lst
Structure of menu.lst
Global section Menuitem definitions
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GRUB Syntax - files
(drive,partnum) filepath drive
GRUB refers to drives using the following names: hd0: "BIOS DRIVE #1" hd1: "BIOS DRIVE #2" fd0: "FLOPPY DRIVE #1"
partnum an integer used to identify the partition on a drive. GRUB counts partitions starting with 0, while the Linux kernel counts partitions starting with 1. what GRUB calls partition number 3, The Linux kernel would call partition number 4. filepath Absolute path to the file, relative to the root directory of the boot filesystem GRUB only knows about the boot filesystem.
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Example
gfxmenu (hd0,0)/message Interpreting the file name involves the following 3 steps. Determine the Drive:
Because we know our system booted from the primary master IDE drive, we will assume that hd0, or "BIOS Drive #1", refers to it. (refers to BIOS boot order) What GRUB is calling hd0, the Linux kernel would call /dev/hda or /dev/sda.
Determine the Partition:
Locate the partition on the drive using the partnum Remember that we need to add 1 to compensate for GRUB's different starting point. The 0 above therefore specifies the first partition on the drive, /dev/hda1.
Determine the File:
GRUB references files relative to the filesystem's root directory. Determine where the partition /dev/hda1 is mounted in our directory tree. Use the df command to examine current mounts. /dev/hda1 is mounted to /boot, so /grub/splash.xpm.gz refers to the file /boot/grub/splash.xpm.gz
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GRUB Global Configuration
All lines that appear before the line that starts title
color foreground/background options default=0 default menuitem to load timeout=8 time in seconds #gfxmenu (hd0,0)/message uses a graphical boot menu password novell a password can be specified
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GRUB Menuitem Configuration
title linux root(hd0,0) kernel /vmlinuz root=/dev/sda1 vga=0x314 initrd /initrd kernel file commandline
/vmlinuz refers to /boot/vmlinuz Is a symlink to the vmlinuz-kernelversion file
initrd file
/initrd refers to /boot/initrd Is a symlink to the initrd-kernelversion file
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The Linux Kernel Initialization
The kernel can be viewed as performing 3 steps. Replace all BIOS device drivers with Linux device drivers.
Limitations of BIOS drivers no longer issue.
Mount the root partition read-only.
root=kernel command line parameter identifies which partition to mount as the root partition.
Start the first process
/sbin/init GRUB must have the root right partition in order to correctly locate /sbin/init file.
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Kernel Command Line: Boot Parameters
[root@station ~]# cat /proc/cmdline displays the contents of the kernel command line from the most recent boot. Tokens specified after the kernel's filename in the menu.lst configuration file are passed to the kernel, which handles them as follows. kernel parameter if so, the kernel applies the parameter. environment variable tokens of the form name=value are passed to the first process (typically /sbin/init) as environment variables anything else Passed to the first process (/sbin/init) as a command line argument.
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GRUB
/etc/grub.conf
info on how and where components of GRUB are supposed to be installed. (SUSE Linux)
GRUB has its own shell that can be used to boot system manually
grub command to start GRUB shell. quit or Ctrl-d to exit. Esc from graphical boot menu also goes to GRUB shell.
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Boot Process Troubleshooting (Extra)
Identify which stage of boot process is failing
BIOS
load first stage bootloader from MBR
Boot loader (GRUB)
(display gray on black text mode)
access /boot partition and load second stage bootloader file presents menu of available boot options (/boot/grub/menu.lst) load initrd and kernel for kernel selected from menu
Kernel
detects low-level devices mounts root file system read only and run first process (/sbin/init)
/sbin/init
reads /etc/inittab and runs /etc/init.d/rc (display color) /etc/init.d/rc performs fsck, remounts root file system as read-write, mounts other file systems in /etc/fstab. /etc/init.d/rc executes the S and K service scripts in the /etc/init.d/rc.d/rc# directory for the default run level specified in /etc/inittab init starts 6 virtual consoles, the X display manager, and displays prompt for 17 user login.
Diagnostic and Troubleshooting Run Levels
Run level 3 (virtual console mode)
Boots normally, but does not start X server Useful for resolving problems related to X server.
Single-user mode or run level 1
May bypass root password for logging in not true in SUSE Linux (/sbin/sulogin command) No network services enabled Root file system mounted (read-write) Append "1" to the boot menu command line to run RC initialization scripts. Append "s" to boot menu command line to bypass RC initialization scripts.
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Booting to the Installed OS
When to use
The boot loader or kernel file is corrupted The root partition is usable
How to use
Boot from Linux installation CD At the menu, specify "Installation" When prompted, specify language At the menu, select "boot to installed OS" This will boot the OS from your root partition and you can use the system normally to perform repairs. Reboot when finished to restart normally
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Linux Rescue Environment
Cases when GRUB is not useful for repairing systems The bootloader is corrupted or missing The kernel is corrupted or missing The root partition is unusable In these cases, other media must be used to boot the system boot disk can be created using the dd command only useful if it's created before you need to use it! Linux Rescue environment on the installatation DVD (1st CD). Provides its own bootloader, kernel and system image to install an OS image w...