GPTBOOT(8) FreeBSD System Manager's Manual GPTBOOT(8)


gptbootGPT bootcode for UFS on BIOS-based computers


gptboot is used on BIOS-based computers to boot from a UFS partition on a GPT-partitioned disk. gptboot is installed in a freebsd-boot partition with gpart(8).


The GPT standard allows a variable number of partitions, but gptboot only boots from tables with 128 partitions or less.


gptboot checks and manages several attributes of GPT UFS partitions.
Attempt to boot from this partition. If more than one partition has the bootme attribute set, gptboot will attempt to boot each one until successful.
Attempt to boot from this partition only one time. Setting this attribute with gpart(8) automatically also sets the bootme attribute. Multiple partitions may have the bootonce and bootme attributes set.
The bootfailed attribute marks partitions that had the bootonce attribute set, but failed to boot. This attribute is managed by the system. See BOOTING and POST-BOOT ACTIONS below for details.


For normal usage, the user does not have to set or manage any of the partition attributes. gptboot will boot from the first UFS partition found.

The bootonce attribute can be used for testing an upgraded operating system on an already-working computer. The existing system partition is left untouched, and the new version of the operating system to be tested is installed on another partition. The bootonce attribute is set on that new test partition. The next boot is attempted from the test partition. Success or failure will be shown in the system log files. After a successful boot of the test partition, a user script can check the logs and change the bootme attributes so the test partition becomes the new system partition. Because the bootonce attribute is cleared after an attempted boot, a failed boot will not leave the system attempting to boot from a partition that will never succeed. Instead, the system will boot from the older, known-working operating system that has not been modified. If the bootme attribute is set on any partitions, booting will be attempted from them first. If no partitions with bootme attributes are found, booting will be attempted from the first UFS partition found.


gptboot first reads the partition table. All freebsd-ufs partitions with only the bootonce attribute set, indicating a failed boot, are set to bootfailed. gptboot then scans through all of the freebsd-ufs partitions. Boot behavior depends on the combination of bootme and bootonce attributes set on those partitions.
bootonce + bootme
Highest priority: booting is attempted from each of the freebsd-ufs partitions with both of these attributes. On each partition, the bootme attribute is removed and the boot attempted.
Middle priority: booting is attempted from each of the freebsd-ufs partitions with the bootme attribute.

If neither bootonce nor bootme attributes are found on any partitions, booting is attempted from the first freebsd-ufs partition on the disk.


The startup script /etc/rc.d/gptboot checks the attributes of freebsd-ufs partitions on all GPT disks. Partitions with the bootfailed attribute generate a “boot from X failed” system log message. Partitions with only the bootonce attribute, indicating a partition that successfully booted, generate a “boot from X succeeded” system log message. The bootfailed attributes are cleared from all the partitions. The bootonce attribute is cleared from the partition that successfully booted. There is normally only one of these.


bootcode binary
parameters for the boot blocks (optional)


gptboot is installed in a freebsd-boot partition, usually the first partition on the disk. A “protective MBR” (see gpart(8)) is typically installed in combination with gptboot.

Install gptboot on the ada0 drive:

gpart bootcode -b /boot/pmbr -p /boot/gptboot -i 1 ada0

gptboot can also be installed without the PMBR:

gpart bootcode -p /boot/gptboot -i 1 ada0

Set the bootme attribute for partition 2:

gpart set -a bootme -i 2 ada0

Set the bootonce attribute for partition 2, automatically also setting the bootme attribute:

gpart set -a bootonce -i 2 ada0


gptboot appeared in FreeBSD 7.1.


This manual page written by Warren Block <wblock@FreeBSD.org>.
February 5, 2014 FreeBSD