173 lines
6 KiB
Text
173 lines
6 KiB
Text
<!-- This is a SGML version of the text on FreeBSD boot procedures
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made by Poul-Henning Kamp <phk@FreeBSD.ORG>
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This conversion has been made by Ollivier Robert.
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$Id: booting.sgml,v 1.14 1997-10-19 13:32:04 jraynard Exp $
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<!DOCTYPE linuxdoc PUBLIC "-//FreeBSD//DTD linuxdoc//EN">
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<article>
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<title>Boot overview</title>
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<author>Poul-Henning Kamp, <tt/<phk@login.dknet.dk>/</author>
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<date>v1.1, April 26th</date>
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<abstract>
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Booting FreeBSD is essentially a three step process: load the kernel,
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determine the root filesystem and initialize user-land things. This
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leads to some interesting possibilities as shown below...
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</abstract>
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<toc>
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-->
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<sect><heading>The FreeBSD Booting Process<label id="booting"></heading>
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<p><em>Contributed by &a.phk;. v1.1, April 26th.</em>
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Booting FreeBSD is essentially a three step process: load the kernel,
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determine the root filesystem and initialize user-land things. This
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leads to some interesting possibilities shown below.
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<sect1><heading>Loading a kernel</heading>
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<p>
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We presently have three basic mechanisms for loading the
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kernel as described below:
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they all pass some
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information to the kernel to help the kernel decide what to do
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next.
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<descrip>
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<tag>Biosboot</tag>
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Biosboot is our ``bootblocks''. It consists of two files which
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will be installed in the first 8Kbytes of the floppy or hard-disk
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slice to be booted from.
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Biosboot can load a kernel from a FreeBSD filesystem.
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<tag>Dosboot</tag>
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Dosboot was written by DI. Christian Gusenbauer, and is
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unfortunately at this time one of the few pieces of code that
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will not compile under FreeBSD itself because it is written for
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Microsoft compilers.
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Dosboot will boot the kernel from a MS-DOS file or from a FreeBSD
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filesystem partition on the disk. It attempts to negotiate with
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the various and strange kinds of memory manglers that lurk in
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high memory on MS/DOS systems and usually wins them for its
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case.
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<tag>Netboot</tag>
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Netboot will try to find a supported Ethernet card, and use
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BOOTP, TFTP and NFS to find a kernel file to boot.
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</descrip>
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<sect1><heading>Determine the root filesystem</heading>
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<p>
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Once the kernel is loaded and the boot-code jumps to it, the kernel
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will initialize itself, trying to determine what hardware is
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present and so on; it then needs to find a root filesystem.
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Presently we support the following types of root filesystems:
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<descrip>
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<tag>UFS</tag>
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This is the most normal type of root filesystem. It can reside on
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a floppy or on hard disk.
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<tag>MSDOS</tag>
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While this is technically possible, it is not particular useful
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because of the ``FAT'' filesystem's inability to deal with links,
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device nodes and other such ``UNIXisms''.
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<tag>MFS</tag>
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This is actually a UFS filesystem which has been compiled into
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the kernel. That means that the kernel does not really need any
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hard disks, floppies or other hardware to function.
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<tag>CD9660</tag>
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This is for using a CD-ROM as root filesystem.
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<tag>NFS</tag>
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This is for using a fileserver as root filesystem, basically
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making it a diskless machine.
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</descrip>
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<sect1><heading>Initialize user-land things</heading>
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<p>
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To get the user-land going, the kernel, when it has finished
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initialization, will create a process with ``<tt/pid == 1/'' and execute
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a program on the root filesystem; this program is normally
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``<tt>/sbin/init</tt>''.
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You can substitute any program for /sbin/init, as long as you keep
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in mind that:
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there is no stdin/out/err unless you open it yourself. If you exit,
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the machine panics. Signal handling is special for ``<tt/pid ==
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1/''.
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An example of this is the ``<tt>/stand/sysinstall</tt>''
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program on the installation floppy.
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<sect1><heading>Interesting combinations</heading>
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<p>
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Boot a kernel with a MFS in it with a special <tt>/sbin/init</tt>
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which...
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<descrip>
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<tag/A -- Using DOS/
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<itemize>
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<item>mounts your <tt/C:/ as <tt>/C:</tt>
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<item>Attaches <tt>C:/freebsd.fs</tt> on <tt>/dev/vn0</tt>
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<item>mounts <tt>/dev/vn0</tt> as <tt>/rootfs</tt>
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<item>makes symlinks<newline>
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<tt>/rootfs/bin -> /bin</tt><newline>
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<tt>/rootfs/etc -> /etc</tt><newline>
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<tt>/rootfs/sbin -> /sbin</tt><newline>
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(etc...)<newline>
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</itemize>
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Now you are running FreeBSD without repartitioning your hard disk...
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<tag/B -- Using NFS/
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NFS mounts your <tt>server:˜you/FreeBSD</tt> as
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<tt>/nfs</tt>, chroots to <tt>/nfs</tt> and executes
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<tt>/sbin/init</tt> there
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Now you are running FreeBSD diskless, even though you do not control
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the NFS server...
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<tag/C -- Start an X-server/
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Now you have an X-terminal, which is better than that dingy
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X-under-windows-so-slow-you-can-see-what-it-does thing that
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your boss insist is better than forking out money on hardware.
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<tag/D -- Using a tape/
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Takes a copy of <tt>/dev/rwd0</tt> and writes it to a remote tape
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station or fileserver.
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Now you finally get that backup you should have made a year
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ago...
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<tag>E -- Acts as a firewall/web-server/what do I know...</tag>
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This is particularly interesting since you can boot from a write-
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protected floppy, but still write to your root filesystem...
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</descrip>
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