booting bootstrap The process of starting a computer and loading the operating system is referred to as the bootstrap process, or simply booting. FreeBSD's boot process provides a great deal of flexibility in customizing what happens when you start the system, allowing you to select from different operating systems installed on the same computer, or even different versions of the same operating system or installed kernel. This chapter details the configuration options you can set and how to customize the FreeBSD boot process. This includes everything that happens until the FreeBSD kernel has started, probed for devices, and started &man.init.8;. If you are not quite sure when this happens, it occurs when the text color changes from bright white to grey. After reading this chapter, you will know: What the components of the FreeBSD bootstrap system are, and how they interact. The options you can give to the components in the FreeBSD bootstrap to control the boot process. This chapter only describes the boot process for FreeBSD running on Intel x86 systems. Turning on a computer and starting the operating system poses an interesting dilemma. By definition, the computer does not know how to do anything until the operating system is started. This includes running programs from the disk. So if the computer can not run a program from the disk without the operating system, and the operating system programs are on the disk, how is the operating system started? This problem parallels one in the book The Adventures of Baron Munchausen. A character had fallen part way down a manhole, and pulled himself out by grabbing his bootstraps, and lifting. In the early days of computing the term bootstrap was applied to the mechanism used to load the operating system, which has become shortened to booting. On x86 hardware the Basic Input/Output System (BIOS) is responsible for loading the operating system. To do this, the BIOS looks on the hard disk for the Master Boot Record (MBR), which must be located on a specific place on the disk. The BIOS has enough knowledge to load and run the MBR, and assumes that the MBR can then carry out the rest of the tasks involved in loading the operating system. BIOS Basic Input/Output System If you only have one operating system installed on your disks then the standard MBR will suffice. This MBR searches for the first bootable slice on the disk, and then runs the code on that slice to load the remainder of the operating system. If you have installed multiple operating systems on your disks then you can install a different MBR, one that can display a list of different operating systems, and allows you to choose the one to boot from. FreeBSD comes with one such MBR which can be installed, and other operating system vendors also provide alternative MBRs. The remainder of the FreeBSD bootstrap system is divided into three stages. The first stage is run by the MBR, which knows just enough to get the computer into a specific state and run the second stage. The second stage can do a little bit more, before running the third stage. The third stage finishes the task of loading the operating system. The work is split into these three stages because the PC standards put limits on the size of the programs that can be run at stages one and two. Chaining the tasks together allows FreeBSD to provide a more flexible loader. kernel init The kernel is then started and it begins to probe for devices and initialize them for use. Once the kernel boot process is finished, the kernel passes control to the user process &man.init.8;, which then makes sure the disks are in a usable state. &man.init.8; then starts the user-level resource configuration which mounts filesystems, sets up network cards to communicate on the network, and generally starts all the processes that usually are run on a FreeBSD system at startup. Master Boot Record (MBR) The FreeBSD MBR is located in /boot/boot0. This is a copy of the MBR, as the real MBR must be placed on a special part of the disk, outside the FreeBSD area. boot0 is very simple, since the program in the MBR can only be 512 bytes in size. If you have installed the FreeBSD MBR and have installed multiple operating systems on your hard disks then you will see a display similar to this one at boot time. F1 DOS F2 FreeBSD F3 Linux F4 ?? F5 Drive 1 Default: F2 Other operating systems, in particular Windows 95, have been known to overwrite an existing MBR with their own. If this happens to you, or you want to replace your existing MBR with the FreeBSD MBR then use the following command. &prompt.root; fdisk -B -b /boot/boot0 device Where device is the device that you boot from, such as ad0 for the first IDE disk, ad2 for the first IDE disk on a second IDE controller, da0 for the first SCSI disk, and so on. If you are a Linux user, however, and prefer that LILO control the boot process, you can edit the /etc/lilo.conf file for FreeBSD, or select Leave The Master Boot Record Untouched during the FreeBSD installation process. If you have installed the the FreeBSD boot manager, you can boot back into Linux and modify the LILO configuration file /etc/lilo.conf and add the following option: other=/dev/hdXY table=/dev/hdb loader=/boot/chain.b label=FreeBSD which will permit the booting of FreeBSD and Linux via LILO. In our example, we use XY to determine drive number and partition. If you are using a SCSI drive, you will want to change /dev/hdXY to read something similar to /dev/sdXY, which again uses the XY syntax. The loader=/boot/chain.b can be omitted if you have both operating systems on the same drive. You can now run /sbin/lilo -v to commit your new changes to the system, this should be verified with screen messages. Conceptually the first and second stages are part of the same program, on the same area of the disk. Because of space constraints they have been split into two, but you would always install them together. They are found on the boot sector of the boot slice, which is where boot0, or any other program on the MBR expects to find the program to run to continue the boot process. The files in the /boot directory are copies of the real files, which are stored outside of the FreeBSD filesystem. boot1 is very simple, since it too can only be 512 bytes in size, and knows just enough about the FreeBSD disklabel, which stores information about the slice, to find and execute boot2. boot2 is slightly more sophisticated, and understands the FreeBSD filesystem enough to find files on it, and can provide a simple interface to choose the kernel or loader to run. Since the loader is much more sophisticated, and provides a nice easy-to-use boot configuration, boot2 usually runs it, but previously it was tasked to run the kernel directly. >> FreeBSD/i386 BOOT Default: 0:ad(0,a)/kernel boot: If you ever need to replace the installed boot1 and boot2 use &man.disklabel.8;. &prompt.root; disklabel -B diskslice Where diskslice is the disk and slice you boot from, such as ad0s1 for the first slice on the first IDE disk. If you use just the disk name, such as ad0, in the &man.disklabel.8; command you will create a dangerously dedicated disk, without slices. This is almost certainly not what you want to do, so make sure you double check the &man.disklabel.8; command before you press Return. boot-loader The loader is the final stage of the three-stage bootstrap, and is located on the filesystem, usually as /boot/loader. The loader is intended as a user-friendly method for configuration, using an easy-to-use built-in command set, backed up by a more powerful interpreter, with a more complex command set. During initialization, the loader will probe for a console and for disks, and figure out what disk it is booting from. It will set variables accordingly, and an interpreter is started where user commands can be passed from a script or interactively. loader loader configuration The loader will then read /boot/loader.rc, which by default reads in /boot/defaults/loader.conf which sets reasonable defaults for variables and reads /boot/loader.conf for local changes to those variables. loader.rc then acts on these variables, loading whichever modules and kernel are selected. Finally, by default, the loader issues a 10 second wait for key presses, and boots the kernel if it is not interrupted. If interrupted, the user is presented with a prompt which understands the easy-to-use command set, where the user may adjust variables, unload all modules, load modules, and then finally boot or reboot. These are the most commonly used loader commands. For a complete discussion of all available commands, please see &man.loader.8; autoboot seconds Proceeds to boot the kernel if not interrupted within the time span given, in seconds. It displays a countdown, and the default time span is 10 seconds. boot -options kernelname Immediately proceeds to boot the kernel, with the given options, if any, and with the kernel name given, if it is. boot-conf Goes through the same automatic configuration of modules based on variables as what happens at boot. This only makes sense if you use unload first, and change some variables, most commonly kernel. help topic Shows help messages read from /boot/loader.help. If the topic given is index, then the list of available topics is given. include filename … Processes the file with the given filename. The file is read in, and interpreted line by line. An error immediately stops the include command. load -t type filename Loads the kernel, kernel module, or file of the type given, with the filename given. Any arguments after filename are passed to the file. ls -l path Displays a listing of files in the given path, or the root directory, if the path is not specified. If -l is specified, file sizes will be shown too. lsdev -v Lists all of the devices from which it may be possible to load modules. If -v is specified, more details are printed. lsmod -v Displays loaded modules. If -v is specified, more details are shown. more filename Displays the files specified, with a pause at each LINES displayed. reboot Immediately reboots the system. set variable set variable=value Sets the loader's environment variables. unload Removes all loaded modules. Here are some practical examples of loader usage. single-user mode To simply boot your usual kernel, but in single-user mode: boot -s To unload your usual kernel and modules, and then load just your old (or another) kernel: kernel.old unload load kernel.old You can use kernel.GENERIC to refer to the generic kernel that comes on the install disk, or kernel.old to refer to your previously installed kernel (when you have upgraded or configured your own kernel, for example). Use the following to load your usual modules with another kernel: unload set kernel="kernel.old" boot-conf To load a kernel configuration script (an automated script which does the things you would normally do in the kernel boot-time configurator): load -t userconfig_script /boot/kernel.conf kernel boot interaction Once the kernel is loaded by either loader (as usual) or boot2 (bypassing the loader), it examines its boot flags, if any, and adjusts its behavior as necessary. kernel bootflags Here are the more common boot flags: -a during kernel initialization, ask for the device to mount as the root file system. -C boot from CDROM. -c run UserConfig, the boot-time kernel configurator -s boot into single-user mode -v be more verbose during kernel startup There are other boot flags, read &man.boot.8; for more information on them. init Once the kernel has finished booting, it passes control to the user process init, which is located at /sbin/init, or the program path specified in the init_path variable in loader. The automatic reboot sequence makes sure that the filesystems available on the system are consistent. If they are not, and fsck cannot fix the inconsistencies, init drops the system into single-user mode for the system administrator to take care of the problems directly. single-user mode console This mode can be reached through the automatic reboot sequence, or by the user booting with the -s option or setting the boot_single variable in loader. It can also be reached by calling shutdown without the reboot (-r) or halt (-h) options, from multi-user mode. If the system console is set to insecure in /etc/ttys, then the system prompts for the root password before initiating single-user mode. # name getty type status comments # # If console is marked "insecure", then init will ask for the root password # when going to single-user mode. console none unknown off insecure An insecure console means that you consider your physical security to the console to be insecure, and want to make sure only someone who knows the root password may use single-user mode, and it does not mean that you want to run your console insecurely. Thus, if you want security, choose insecure, not secure. multi-user mode If init finds your filesystems to be in order, or once the user has finished in single-user mode, the system enters multi-user mode, in which it starts the resource configuration of the system. rc files The resource configuration system reads in configuration defaults from /etc/defaults/rc.conf, and system-specific details from /etc/rc.conf, and then proceeds to mount the system filesystems mentioned in /etc/fstab, start up networking services, start up miscellaneous system daemons, and finally runs the startup scripts of locally installed packages. The &man.rc.8; manual page is a good reference to the resource configuration system, as is examining the scripts themselves. shutdown Upon controlled shutdown, via shutdown, init will attempt to run the script /etc/rc.shutdown, and then proceed to send all processes the TERM signal, and subsequently the KILL signal to any that do not terminate timely.