1e30867d53
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176 lines
6.0 KiB
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176 lines
6.0 KiB
Plaintext
<!-- 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 1.3 1995/06/14 18:38:50 jfieber 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: 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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<chapt><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: 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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<sect>Loading a kernel
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<p>
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We presently have three basic mechanisms for loading the kernel:
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<enum>
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<item>biosboot
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<item>dosboot
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<item>netboot
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</enum>
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Each will be described in detail below. 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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<sect1>Biosboot
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<p>
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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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<sect1>Dosboot
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<p>
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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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isn't compilable 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 it's
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case.
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<sect1>Netboot
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<p>
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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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<sect>Determine the root filesystem
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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, and then it needs to find a root filesystem.
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Presently we support the following types of rootfilesystems:
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<itemize>
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<item>UFS
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<item>MSDOS
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<item>MFS
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<item>CD9660
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<item>NFS
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</itemize>
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<sect1>UFS
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<p>
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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 harddisk.
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<sect1>MSDOS
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<p>
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While this is technically possible, it isn't particular useful,
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because of ``FAT'' filesystems inability to make links, device
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nodes and such ``UNIXisms''.
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<sect1>MFS
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<p>
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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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disks/floppies or other HW to function.
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<sect1>CD9660
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<p>
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This is for using a CD-ROM as root filesystem.
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<sect1>NFS
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<p>
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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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<sect>Initialize user-land things
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<p>
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To get the user-land going, when the kernel has finished
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initialization, it will create a with ``<tt/pid == 1/'' and execute
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a program on the rootfilesystem, 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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<sect>Interesting combinations
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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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...<newline>
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</itemize>
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Now you run 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 run FreeBSD diskless, even though you don't 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 Xterminal, 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 our money on HW.
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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 got 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 particular 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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