NEXTBOOT(8) BSD System Manager's Manual NEXTBOOT(8)NAMEnextboot — install a default bootstring block on the boot disk
SYNOPSISnextboot [-b] filename bootstring file ...
nextboot [-ed] filename
DESCRIPTION
The DragonFly program nextboot controls the actions of the boot blocks at
the time of the next boot. If compiled with the correct option, the boot
blocks will check the nameblock for a magic number and a default name to
use for booting. If compiled to do so they will also delete the name
from the block, ensuring that if the boot should fail, then it will not
be tried again. It is the job of /etc/rc to use nextboot to re-install
the string if that boot is found to have succeeded. This allows a one-
time only boot string to be used for such applications as remote debug‐
ging, and installation of new, untrusted kernels. The nameblock is
defined at compile time to be the second physical block on the disk.
The following options are available:
-b Is used for bootstrapping (initially configuring) the nameblock.
Without this, nextboot will refuse to write to a block that does
not already contain the magic number.
-d Disable (temporarily) an existing name block by changing a bit in
the magic number.
-e Restore the good magic number on a block disabled by -d.
The -e and -d flags are mutually exclusive.
Nextboot first checks that the disk has an fdisk table and checks that
none of the partitions defined in that table include the nameblock. If
the name block is shown to be unused, it will install the bootstrings
given as arguments, one after the other, each preceded by a small magic
number, and NULL terminated. The end of the list of strings is delimited
by a sequence of 0xff bytes. If the boot blocks are compiled to write
back the nameblock after each boot, it will zero out the supplied names
as it uses them, one per boot, until it reaches the 0xff, at which time
it will revert to the compiled in boot string. At this time the name‐
block will contain only zeroed out names.
An example of usage might be:
nextboot-b /dev/rad0 1:da(0,a)/boot/kernel.experimental ad(0,a)/boot/kernel.old
Which would instruct the boot blocks at the next boot, to try boot the
experimental kernel off the SCSI disk. If for any reason this failed,
the next boot attempt would boot the kernel /boot/kernel.old off the IDE
drive. (Assuming the write-back option were enabled) If this in turn
failed. The compiled in default would be used.
If the write-back feature is disabled, the nextboot program is a conve‐
nient way to change the default boot string. Note, that should the file
specified in the nameblock be non-existent, then the name compiled into
the boot blocks will be used for the boot rather than the next name in
the nameblock. The nameblock is only consulted once per boot.
SEE ALSOboot(8), disklabel(8), fdisk(8)BUGS
This program works only in conjunction with the legacy boot code.
The entire program should be made more user-friendly. The option of
whether to write back or not should be stored on the disk and not a com‐
pile time option. I want to rethink this at some later stage to make it
co-exist with disks that do not have a fdisk partitioning table (i.e.
purely disklabel'd systems).
Whether to write back or not should be specified at run-time in the name‐
block so that the boot blocks need not be altered to get this feature.
BSD September 2, 2008 BSD
