105 lines
3.4 KiB
Groff
105 lines
3.4 KiB
Groff
.\" $FreeBSD$
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.Dd July 9, 1996
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.Dt NEXTBOOT 8
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.\".Os BSD 4
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.Sh NAME
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.Nm nextboot
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.Nd install a default bootstring block on the boot disk
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.Sh SYNOPSIS
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.Nm nextboot
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.Op Fl b
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.Ar filename bootstring
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.Ar
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.Nm nextboot
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.Op Fl ed
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.Ar filename
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.Sh DESCRIPTION
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The
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.Bx Free
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program
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.Nm
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controls the actions of the boot blocks at the time of the next boot.
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If compiled with the correct option,
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the boot blocks will check the nameblock for a magic number and a
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default name to use for booting. If compiled to do so they will also
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delete the name from the block, ensuring that if the boot should fail,
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then it will not be tried again. It is the job of
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.Pa /etc/rc
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to use
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.Nm
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to re-install the string if that boot is found to have succeeded.
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This allows a one-time only boot string to be used for such applications
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as remote debugging, and installation of new, untrusted kernels.
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The nameblock is defined at compile time to be the second physical block
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on the disk.
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.Pp
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The following options are available:
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.Bl -tag -width indent
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.It Fl b
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Is used for bootstrapping (initially configuring) the nameblock. Without
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this,
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.Nm
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will refuse to write to a block that does not already contain the magic
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number.
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.It Fl d
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Disable (temporarily) an existing name block by changing a bit
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in the magic number.
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.It Fl e
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Restore the good magic number on a block disabled by
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.Fl d .
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.El
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.Pp
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The
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.Fl e
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and
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.Fl d
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flags are mutually exclusive.
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.Sh DESCRIPTION
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.Nm Nextboot
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first checks that the disk has an fdisk table and checks that none of the
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partitions defined in that table include the nameblock. If the name block is
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shown to be unused, it will install the bootstrings given as arguments,
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one after the other, each preceded by a small magic number, and NULL
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terminated. The end of the list of strings is delimited by a sequence of
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0xff bytes. If the boot blocks are compiled to write back the nameblock
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after each boot, it will zero out the supplied names as it uses them,
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one per boot,
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until it reaches the 0xff, at which time it will revert to the compiled in
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boot string. At this time the nameblock will contain only zeroed out names.
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.Pp
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An example of usage might be:
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.Bd -literal
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nextboot -b /dev/rwd0 1:sd(0,a)/kernel.experimental wd(0,a)/kernel.old
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.Ed
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.Pp
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Which would instruct the boot blocks at the next boot,
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to try boot the experimental kernel off the SCSI disk.
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If for any reason this failed, the next boot attempt would
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boot the kernel
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.Pa /kernel.old
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off the IDE drive. (Assuming the write-back option were enabled) If this
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in turn failed. The compiled in default would be used.
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.Pp
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If the write-back feature is disabled, the nextboot program is a convenient way
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to change the default boot string. Note, that should the file specified in
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the nameblock be non-existent, then the name compiled into the boot blocks
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will be used for the boot rather than the next name in the nameblock. The
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nameblock is only consulted
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.Em once
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per boot.
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.Sh SEE ALSO
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.Xr boot 8 ,
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.Xr disklabel 8 ,
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.Xr fdisk 8
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.Sh BUGS
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This program works only in conjunction with the legacy boot code.
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.Pp
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The entire program should be made more user-friendly.
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The option of whether to write back or not should be stored on the
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disk and not a compile time option. I want to rethink this at some
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later stage to make it co-exist with disks that do not have
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a fdisk partitioning table (i.e. purely disklabel'd systems).
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.Pp
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Whether to write back or not should be specified at run-time in the nameblock
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so that the boot blocks need not be altered to get this feature.
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