Topics - NIX Startup

“The secret of getting ahead is getting started. The secret of getting started is breaking your complex overwhelming tasks into small manageable tasks, and starting on the first one.” ― Mark Twain

Editor Note:

• The material below was cobbled together for personal use, from attributed sources, and endured some mild look/feel massage.
• Document Purpose: Conveniently scoped refresher on the listed Linux material.

Sources:

• LPIC2 Prep - Linux Academy
• Photo source by Tiago B on Unsplash

System Startup

Knowledge Targets

• Query and modify the behaviour of system services at various run levels.
• Understand the init structure and boot process.
• Interact with run levels.

202.1 Customizing a SysV-Init System Startup

LSB - Linux Standard Base Spec

Linux Foundation and Distro Maintainers

• An attempt at improving a standard experience in Linux, among fractured distros
• Version 5 was major version change, breaking.
• FHS - File Hierarchy Spec

SysVInit Boot Process

Big topic, Other approaches are becoming more popular - Systemd and Upstart Process starts with Init process. At boot.

• Used to start all other processes and services.
• ps -aux | grep init shows init, process id 1

Run Levels, Determine what state the system is booted to, what services are started as a result.

• There are some differences between Redhat variants and Debian variants.

Redhat and variants

• RL 0, stop system
• RL 1, single user mode. Special - like windows safe mode. Key services, Filesystem.
• 2, multi-user mode, but no networking and no gui. You can login, but no remote access, etc.
• 3, multi-mode with network, no gui. Network is generally available, as long as it is defined at RL3.
• 4, Not defined in RH. LSB specifies this as not defined.
• 5, Standard one for a desktop. Standard b/c is multi-user, network, and GUI.
• 6, Reboots the system. Called when you do a reboot or shutdown -r

Debian and variants

Contrasted to RH

• 0-1 matches
• 2,3,4,5 are all multi-user mode, network, and GUI.
• 6 matches.

Note: From an LSB perspective, there is RL 7-9. Most distros don’t define these by default.

Important Ones

• 1 - Troubleshooting
• 3 - Server run level
• 5 - Desktop mode

For sysv systems, what determines the default RL is /etc/initab

• Sets default
• Allows for writing hooks around a run level. Like do this blah thing at RL x.

Redhat

Specific config /etc/rc.d

• rc script
• rcX.d dirs
• rc.sysinit

Debian

They bring the rcX.d dirs up to /etc level. Ie /etc/rc0.d

echo runlevel - might work Inside an rcX.d dir

• S - Start scripts
• K - Kill script
• Number orders how scripts are ran

/etc/init.d and modifying runlevel script

Things that are specific to Debian or Redhat

• Debian is closer to LSB by implementation
• /etc/init.d/script control-command

Modifying runlevels and services

Debian

• update-rc.d
• update-rc.d apache2 start 10 2 3 4 5 . stop 90 0 1 6 .
• Apache2 will be started (as long as it isn’t already)
  • When the system enters RunLevel 2, 3, 4 or 5 with a priority of 10.
• It will then be asked to stop when the system enters RunLevel 0, 1 or 6 with a priority of 90.

Redhat

Chkconfig SSHD example. chkconfig 2345 55 25.

• Start in RL 2345.
• If it wasn’t in 2, you edit file, add 2 to commented chkconfig, and then chkconfig –add sshd
• chkconfig --list is a nice table view across RLs
• chkconfig --off - turn off across all RLs
• chkconfig --level 3 on - Now only turn on for RL 3.

Changing Runlevel

runlevel command

rmay@dev-vm:~/$ runlevel
N 5

init command init 6 would reboot system Pretty basic, not alot of flexibility

telinit Use to pass a time (like do this in 2 minutes). Could be used with a broadcast message to all users.

Systemd

Systemd is an init system similar to sysvinit, but with improvements.

• While sysvinit is sequential, based on rules.
  • Systemd can start in parallel, define dependencies, start services automatically.
• SVI uses startup scripts, systemd uses unit files.
  • Unit files contain minimum unit name and service desc. Most also contain install section.
    • Tells when to stop/start service.
  • Unit files can also manage sockets, devices, mounts, swap devices and timers, among other things.

Targets Is a unit that provides a syncronization point for other units when booting or changing states.

• Can be used to bring the system to another desired state.
• Example - SVI, gui is RL5. For sysd, multi-user.target. Create target
• /etc/systemd/customTarget.wants/ symlink in the services you desire from /lib/systemd/system

202.2 System Recovery

Knowledge Targets

• Manipulate a Linux system during both the boot process and during recovery mode
  • Both the init utility and init-related kernel options
• Determine the cause of errors in loading and usage of bootloaders.

Understanding the boot process

4 Phases

• BIOS/UEFI (unified extensible firmware interface)
  • UEFI is software for BIOS. Not considered firmware like BIOS.
  • UEFI can handle modern disk technology (GUIDs, LVM, etc)
• Boot Loader
  • Once complete, hands off to approriate kernel
  • Presents us with option to choose a kernel, boot options
  • Grub/Grub2, Grub was a replacement for LILO
  • BL is stored in MBR. First 215 bytes of any disk
• Kernel
  • Sets up things, hands off to post-kernel
    • init process, sysvinit/systemd/etc time.

Grub2

Improvement on GRUB

• G could not used uuids
• G2 can boot encrypted devices
• Small diff btwn RH and Debian
• Config dir
  • /boot/grub2 - Kernel info
  • grub.cfg - we won’t change this. Is a menu builder.
• /etc/default/grub - where you edit and set behaviors
  • timeout, cmdline_linux (options)
• /etc/grub.d/ - see files referenced/included from grub.cfg
  • grub.cfg is built by Makefile, it concats these files.
• Change 40_custom file and rebuild via grub2-mkconfig -o newgrub.conf (don’t clobber old file)
  • Backup original in /boot/ and drop in new one, renamed.
  • Reboot

fsck

Check and repair NIX file system

• Can’t run on mounted FS

Systemd rescue and emergency recovery

Single user mode

• rescue - Lauches a shell shortly after the root filesytem is mounted read/write
• emergency - Launches a shell before most filesytems are mounted
• last resort - If systemd is broken, it is possible to launch a root shell using init (init=/bin/sh)
• debug shell - Have systemd launch a root shell on tty9 on boot.

System Recovery, BIOS and UEFI

BIOS looks for the MBR on the first sector of the HD in order to physically locate the computers kernel.

• Size limit of first sector UEFI looks at a special disk partition, the EFI system partition (ESP) to store bootloader programs.
• This enables us to use large bootloaders or multiple OSs (ie windows/linux dual boot) NVMe - non-volitile memory express
• Supported since linux kernel 3.3
• Allows linux to use SSDs as boot drive

202.3 Alternate Bootloaders

List

• LILO
• Syslinux
• EXTLinux
• ISOLinux
• PXELinux