3.4 Legacy GRUB & Alternative Boot Loaders (102.2)

3.4 Legacy GRUB & Alternative Boot Loaders (102.2)

Quick Summary: While modern Linux distributions utilize GRUB 2, legacy systems and specialized environments employ alternative boot mechanisms. Legacy GRUB (version 0.97) relies on a directly editable /boot/grub/menu.lst file with 0-indexed partitions. Modern lightweight UEFI systems frequently use systemd-boot (formerly gummiboot), embedded devices use Syslinux/ISOLINUX, and historical servers may run LILO. Understanding partition numbering schemes and bootloader architectures is a core LPIC-1 requirement.

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1. Legacy GRUB (GRUB 0.97) Architecture

Legacy GRUB (GRUB version 1 or 0.97) operates using a single, static configuration file. Unlike GRUB 2, there is no configuration generator (grub-mkconfig), and administrators edit the menu file directly.

Primary Configuration File Paths

• Debian / openSUSE / Ubuntu (Legacy): /boot/grub/menu.lst
• Red Hat / CentOS / Fedora (Legacy): /boot/grub/grub.conf (frequently symlinked from /etc/grub.conf and /boot/grub/menu.lst)

# Sample Legacy GRUB configuration (/boot/grub/menu.lst)
default=0
timeout=10
splashimage=(hd0,0)/boot/grub/splash.xpm.gz
hiddenmenu

title CentOS Linux (2.6.32-754.el6.x86_64)
    root (hd0,0)
    kernel /vmlinuz-2.6.32-754.el6.x86_64 ro root=/dev/sda2 quiet rhgb
    initrd /initramfs-2.6.32-754.el6.x86_64.img

title CentOS Linux (Rescue Mode)
    root (hd0,0)
    kernel /vmlinuz-2.6.32-754.el6.x86_64 ro root=/dev/sda2 single
    initrd /initramfs-2.6.32-754.el6.x86_64.img

Legacy GRUB Directive Syntax

• default=N: Specifies the default boot entry index (0-indexed; 0 boots the first title stanza).
• timeout=N: Wait time in seconds before automatically booting the default entry.
• title <name>: Defines a new boot menu entry and its display label.
• root (hdX,Y): Defines the GRUB root partition where the kernel and initrd images reside.
• kernel <path> <options>: Specifies the path to the kernel binary on the GRUB root filesystem, followed by kernel boot arguments.
• initrd <path>: Specifies the path to the initial RAM disk image.

⚠️ LPIC-1 Trap (Separate /boot Partition): If /boot resides on a separate partition (hd0,0), the kernel path is /vmlinuz-..., NOT /boot/vmlinuz-.... If /boot is part of the root partition (hd0,1), the path is /boot/vmlinuz-....

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2. Partition Numbering Comparison: Legacy GRUB vs. GRUB 2

A critical topic on the LPIC-1 exam is the fundamental difference in disk and partition numbering between Legacy GRUB, GRUB 2, and Linux kernel device nodes.

💡 LPIC-1 Rule of Thumb:

• Legacy GRUB: Hard drives start at 0, partitions start at 0.
• GRUB 2: Hard drives start at 0, partitions start at 1.
• Linux OS: Disks use letters (sda=1st, sdb=2nd), partitions use numbers starting at 1 (sda1).

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3. Alternative Bootloader: systemd-boot (gummiboot)

systemd-boot (formerly known as gummiboot) is a simple, lightweight UEFI bootloader integrated into systemd. It does not load filesystem drivers or complex configuration scripts; instead, it relies strictly on UEFI firmware services.

Configuration Structure

• Global Configuration: /boot/loader/loader.conf (or $ESP/loader/loader.conf)
• Boot Entries Directory: /boot/loader/entries/*.conf

# Global Loader Configuration (/boot/loader/loader.conf)
default arch.conf
timeout 4
console-mode max
editor no

# Drop-in Boot Entry (/boot/loader/entries/arch.conf)
title   Arch Linux
linux   /vmlinuz-linux
initrd  /initramfs-linux.img
options root=UUID=1a2b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d rw quiet

bootctl Management Commands

• bootctl status: Displays current firmware, ESP mountpoint, secure boot status, and configured bootloader entries.
• bootctl install: Installs systemd-boot binary into the EFI System Partition.
• bootctl update: Updates installed bootloader binaries.

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4. Alternative Bootloader: Syslinux Suite

The Syslinux project is a collection of lightweight bootloaders designed for booting Linux from FAT filesystems, optical discs, and network PXE environments.

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5. Alternative Bootloader: LILO (Linux Loader)

LILO is a classic Linux bootloader used extensively before GRUB. It relies on a single configuration file at /etc/lilo.conf.

# Sample /etc/lilo.conf
boot=/dev/sda
map=/boot/map
install=/boot/boot.b
prompt
timeout=50
default=linux

image=/boot/vmlinuz-2.6.32
    label=linux
    read-only
    root=/dev/sda1

The Critical LILO Mechanism

LILO does not contain filesystem drivers. When LILO boots, it loads kernel sectors directly from raw physical disk block addresses recorded in the boot sector.

⚠️ LPIC-1 Trap: Any time /etc/lilo.conf is edited OR a new kernel image is installed, the administrator MUST execute /sbin/lilo. Running lilo recalculates physical disk block pointers and writes them to the boot sector. If you fail to run lilo, the system will fail to boot on the next restart!