Grub and dracut ii
- 1. Linux boot process and management Grub and it’s pal Dracut For FredLug September 2016 By Peter Larsen
- 2. Agenda ● Boot proces overview – Bios – UEFI ● Grub fundamentals ● Initramfs fundamentals ● Diagnosis ● Demos – how to fix a broken system
- 3. Boot process overview ● Bios ● UEFI ● Kernel ● Initrd ● Systemd
- 4. Boot: BIOS Source: http://resources.infosecinstitute.com/uefi-and-tpm/ ● Traditional/Old. 16Bit, 1MB addressable space, limited boot partition size ● “Blind” - easy to inject bad code ● No real standards ● Very hardware manufactorer dependent ● Basic UI
- 5. Boot: UEFI ● Unified Extensible Firmware Interface ● Trusted Boot Source: http://www.antapex.org/diskdevices.htm
- 6. Bios/UEFI setup ● Pressing a key during boot (DEL, F1, F2 etc – depends on BIOS) ● # dmidecode Show content of bios settings and discovered hardware ● # lspci List hardware found on the PCI Bus ● # lsusb List discovered USB devices ● # lscpu List discovered CPUs and settings ● # lsscsi List SCSI devices (most drives today show up as SCSI devices) ● Hardware vendors sometimes provide executables to change BIOS settings.
- 7. Key BIOS/UEFI settings ● Boot Device / Boot Device Order ● ACPI – Advanced Configuration and Power Interface ● CPU features – important for virtualization ● USB boot enabled/disabled ● UEFI mode – in this mode, each boot device must be registered to be available. In secure mode, they must be signed to be available.
- 8. Boot: Kernel ● Kernel is loaded into memory and executed ● Bulk of kernel is compressed and will be uncompressed into specific memory locations. ● Memory and other structures are initialized ● Kernel modules are initialized – PCI and other busses are found, initialized etc. ● Mounts root device (kernel parameter) ● Executes PID 1 /init -> systemd ● First systemd process is located on initramfs.
- 9. Kernel implementation ● Most people do not compile their own kernels ● Kernels are installed via rpm or deb packages ● Located in /boot ● Fedora/CentOS/RHEL puts /boot on a primary partition of at least 512MB in size. Contains the current and 2 other kernels. ● /boot is NOT a “bootable” partition in terms of MBR (Microsoft terminology). The boot flag is ignored by linux.
- 10. Kernel options ● Too many to list here – we’ll cover some in the other sections ● Install kernel-doc for /usr/share/doc/kernel* documentation ● https://www.kernel.org/doc/Documentation/kernel-parameters.txt ● Use Grub to change/set parameters – permanent changes goes into grub.cfg ● Example (one long line): kernel /vmlinuz-3.10.0-327.36.1.el7.x86_64 root=/dev/mapper/test-root ro crashkernel=auto rd.lvm.lv=test/root rd.lvm.lv=test/swap rhgb quiet LANG=en_US.UTF-8
- 11. Boot: Initramfs ● Dynamic kernel module initializer ● Ram disk image used to “kickstart” a system ● The kernel is very large, but would be enourmous if it contained all device code statically. ● Instead the kernel loads modules based on devices identified during startup. ● Since the kernel doesn’t have all devices needed for boot, initramfs is required to initialize the right modules, so the root device can be activated/read.
- 12. GRUB: GRand Unified Bootloader ● Legacy Grub is no more ● Grub2 Staged boot ● Loads Kernel ● Mounts ram disk (initramfs)
- 13. GRUB Boot process ● POST – Stage 1/1.5 – BIOS ● Finds bootable device and transfers control to MBR ● GPT/MBR contains stage 1, very small. Fits in less than 512bytes ● GRUB stage 1.5 is located in the first 30 KB of boot device immediately following MBR. ● Stage 1.5 does better device discovery, supports larger partitions etc. and passes control to Stage 2 – UEFI ● Loads UEFI boot system, mounts EFI partition, loads GRUB stage 2 ● Stage 2 – Read Grub.cfg and displays menu and other actions. – Grub executes the commands in the boot menu
- 14. /boot and /boot/efi Partition Table: gpt Disk Flags: Number Start End Size File system Name Flags 1 1049kB 211MB 210MB fat16 EFI System Partition boot 2 211MB 525MB 315MB xfs 3 525MB 9663MB 9137MB lvm ● EFI is only used when UEFI is enabled. Mounted on /boot/efi ● /boot is partition 2 – notice NOT a boot partition ● The rest can be partitions, LVM etc. - your choice.
- 15. /boot structure /boot ├── efi │ └── EFI │ ├── BOOT │ └── redhat │ └── fonts └── grub2 └── themes └── system ● /boot/efi only exists when EFI was detected during boot ● Grub themes are rather cool. Install plymouth-theme-change to get cool graphics on your boot menu!
- 16. Grub configuration commands ● # grub2-mkconfig Create configuration/menu by scanning system for bootable images. Will detect Windows and lots of different Linux distros and boot loaders ● # grubby Command line tool for configuring grub. Very advanced. Using grub2- install is a lot easier if you are looking to fix a boot problem. Can be used to manually add menu items and set configuration options. ● # grub2-install <device> Install grub (boot) on device. Injects stage1 and 1.5 boot loaders and makes them refer to the boot device. DO NOT USE IF YOU’RE USING EFI! ● # efibootmgr Manage boot setup of EFI. List existing systems, reorder, add new system, delete systems etc.
- 17. Configuration grub.cfg ● Located in /boot/grub2/ or /boot/efi/EFI/redhat ● Created using grub2-mkconfig ● If manually editing, use /etc/grub.d to add files, then run grub2-mkconfig to generate a new configuration.
- 18. Example configuration menuentry 'Red Hat Enterprise Linux Server (3.10.0- 327.36.1.el7.x86_64) 7.2 (Maipo)' --class red --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-327.el7.x86_64- advanced-6e59c4d3-dbf6-4ba7-bd9e-7104482f66cd' { load_video set gfxpayload=keep insmod gzio insmod part_gpt insmod xfs linux /vmlinuz-3.10.0-327.36.1.el7.x86_64 root=/dev/mapper/test-root ro crashkernel=auto rd.lvm.lv=test/root rd.lvm.lv=test/swap rhgb quiet LANG=en_US.UTF-8 initrd /initramfs-3.10.0-327.36.1.el7.x86_64.img }
- 19. Kernel: Linux begins! ● Fundamentals – Processes – Memory Management – Device Management – File systems – Etc. ● Kernel starts PID 1 from root file system
- 20. Initramfs – managed by dracut ● Small root device loaded during boot right after the kernel ● Created and managed using “dracut” ● Initramfs file located in /boot with kernel ● Must match kernel version ● “lsinitrd” shows/lists content of initramfs ● Extract all using zcat | cpio -i Extract single file using “lsinitrd -f <file>”
- 21. Dracut ● Use modules to enable boot features – Examples: lvm, mdraid, luks, iscsi etc. ● List all modules: dracut –list-modules ● If transposing to new hardware, you may need to generate a new initramfs with modules fitting the hardware
- 22. September 2016 Grub and Dracut 22 Customizing initramfs on boot ● LVM – rd.lvm=0 disable LVM detection – rd.lvm.vg=<volume group name> only activate the volume groups with the given name. – rd.lvm.lv=<logical volume name> only activate the logical volumes with the given name. – rd.lvm.conf=0 remove any /etc/lvm/lvm.conf, which may exist in the initramfs
- 23. Diagnostics ● Grub Shell ● Initramfs Shell (RD) ● Emergency Shell ● Single User Mode (mode 1) ● MultiUser mode (non graphical) (mode 3) ● Graphical mode (mode 5)
- 24. Diagnostics: Grub Shell ● Configuration errors in grub.cfg is the most common cause for going to the shell ● grub> ● The cfg file is nothing but a list of commands that the grub-shell understands and executes. ● Basic commands to boot a system: set root=’hd0,msdos1’ kernel /vmlinuz-<version> root=/dev/<root partition> ro initrd /initramfs-<version>.img boot
- 25. Diagnostics: Initramfs Shell (RD) ● Initramfs is very minimal and a bad shell to do recovery in. Use for boot diagnostics, to see why a device isn’t found, responding etc. ● RD = Root Device ● Parameters are given on the kernel command line – rd.break Drop to shell Options: cmdline, pre-udev, pre-mount, mount, cleanup – rd.info Print debug information – rd.shell Drop to shell if root mounting fails – rd.udev.{info|debug} Set udev to info/debug level in output
- 26. Diagnostics: Emergency Shell/Mode ● Starts minimal shell on a system where the root device is mounted (ro). ● Full system command line options – but data disks, network and other key features are not enabled. ● Will prompt for root password! ● Kernel Parameter: systemd.unit=emergency.target
- 27. Diagnostics: Systemd debug ● Systemd startup process has problems. Used to debug services and other system management features controlled by systemd. ● Advanced system diagnostics ● Kernel Parameter: systemd.debug-shell
- 28. Diagnostics: Single User Mode (mode 1) ● Goes to shell after all core services are started. Disks mounted, devices active etc. No network. ● Will prompt for root password!! ● Kernel Parameter: systemd.target=rescue.target
- 29. Diagnostics: MultiUser mode (non graphical) (mode 3) ● Full system running – no graphical sub component. For most servers, this is the run target for a normal running system. ● Network active ● Kernel parameter: systemd.unit=multi-user.target ● Graphical subsystem does not start ● Primary services (web server, data volume) does not start
- 30. Diagnostics: Install Disk/Recovery Boot ● Alternative: Use LiveUSB/CD ● Allows full access to root device, fsck of root device and full recovery/repair ● Limited commands – Full LVM – Full File system support – Bypass root password ● Potential selinux corruption – use with CARE! – Always run “fixfiles onboot” if major changes are done to files
- 31. Demo ● Show traditional BIOS boot system ● Show EFI system ● Recover boot failure
- 32. How to reset the root password ● The old tricks are no longer valid. ● Add rd.break to kernel command line ● Command line: – # mount -o remount,rw /sysroot – # chroot /sysroot – # passwd – # touch ./autorelabel ● Exit twice and system will reboot. ● Note, system will relabel ALL files on boot – and this will take considerable time
- 33. Questions