BOOT_CONFIG(8) | System Manager's Manual | BOOT_CONFIG(8)

# NAME

`boot_config` — how to change kernel configuration at boot

# DESCRIPTION

`BOOT_CONFIG` is a kernel option that makes it possible to change the configuration at boot time. The same interface is available from userland, using the `-e` argument to config(8).

The boot time configuration is invoked by passing the `-c` option, either from the firmware prompt or, depending on the architecture, when the bootstrap loader prompts for a kernel to boot:

    >> OpenBSD/i386 BOOT 3.26
    boot> boot -c
    booting hd0a:/bsd: ...
    ...
    real mem  = 1073086464 (1023MB)
    avail mem = 1043189760 (994MB)
    User Kernel Config
    UKC>

Changes made can be saved for the next reboot, by using config(8). However, those changes would not persist across system upgrades and would prevent kernel relinking. To ensure these changes are carried over to upgraded kernels, they can be saved to the bsd.re-config(5) configuration file.

# COMMANDS

`add` dev
  Add a device through copying another.
`base` `8` | `10` | `16`
  Change the base of numbers displayed and entered.
`change` devno | dev
  Modify one or more devices.
`disable` devno | dev
  Disable one or more devices.
`enable` devno | dev
  Enable one or more devices.
`exit`
  Continue boot.
`find` devno | dev
  Find one or more devices.
`help`
  Give a short summary of all commands and their arguments.
`list`
  Show all known devices, a screen at a time.
`lines` [count]
  Set the number of rows per page.
`quit`
  Continue boot.
`show` [attr [val]]
  Show all devices for which attribute attr has the value val.
`verbose`
  Toggle the autoconfig verbose variable.

# EXAMPLES

The Ethernet card is not detected at boot because the kernel configuration does not match the physical hardware configuration, e.g. wrong IRQ in OpenBSD/i386. The Ethernet card is supposed to use the ne(4) driver.

    UKC> find ne
    24 ne0 at isa0 port 0x240 size 0 iomem 0xd8000 iosiz 0 irq 9 drq -1 drq2 -1 flags 0x0
    25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0
    26 ne* at isapnp0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 flags 0x0
    27 ne* at pci* dev -1 function -1 flags 0x0
    28 ne* at pcmcia* function -1 irq -1 flags 0x0
    UKC>

ne1 seems to match the configuration except it uses IRQ 5 instead of IRQ 10. So the irq on ne1 should be changed via the change command. The device can be specified by either name or number.

    UKC> change ne1
    25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0
    change (y/n) ? y
    port [0x300] ?
    size [0] ?
    iomem [-1] ?
    iosiz [0] ?
    irq [10] ? 5
    drq [-1] ?
    drq2 [-1] ?
    flags [0] ?
    25 ne1 changed
    25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 5 drq -1 drq2 -1 flags 0x0
    UKC>

The show command is useful for finding which devices have a certain attribute. It can also be used to find those devices with a particular value for an attribute.

    UKC> show slot
      2 ahc* at eisa0 slot -1 flags 0x0
     10 uha* at eisa0 slot -1 flags 0x0
     12 ep0 at eisa0 slot -1 flags 0x0
     17 ep* at eisa0 slot -1 flags 0x0
    102 ahb* at eisa0 slot -1 flags 0x0
    103 fea* at eisa0 slot -1 flags 0x0
    UKC> show port 0x300
     25 ne1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0
     72 we1 at isa0 port 0x300 size 0 iomem 0xcc000 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0
     75 el0 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 9 drq -1 drq2 -1 flags 0x0
     77 ie1 at isa0 port 0x300 size 0 iomem -1 iosiz 0 irq 10 drq -1 drq2 -1 flags 0x0
    UKC>

It is possible to add new devices, but only devices that were linked into the kernel. If a new device is added, following devices will be renumbered.

    UKC> find ep
     11 ep0 at isa0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 drq2 -1 flags 0x0
     12 ep0 at eisa0 slot -1 flags 0x0
     13 ep0 at pci* dev -1 function -1 flags 0x0
     14 ep* at isapnp0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 flags 0x0
     15 ep* at isa0 port -1 size 0 iomem -1 iosiz 0 irq -1 drq -1 drq2 -1 flags 0x0
     16 ep* at eisa0 slot -1 flags 0x0
     17 ep* at pci* dev -1 function -1 flags 0x0
     18 ep* at pcmcia* dev -1 irq -1 flags 0x0
    UKC> add ep1
    Clone Device (DevNo, 'q' or '?') ? 13
    Insert before Device (DevNo, 'q' or '?') 14
     14 ep1 at pci* dev -1 function -1
    UKC> change 14
     14 ep1 at pci* dev -1 function -1
    change (y/n) ? y
    dev [-1] ? 14
    function [-1] ?
    flags [0] ? 18
     14 ep1 changed
     14 ep1 at pci* dev 14 function -1 flags 0x12
    UKC>

When configuration is completed, booting can proceed by issuing the `quit` or `exit` commands.

    UKC> quit
    Continuing...
    mainbus0 (root)

# SEE ALSO

bsd.re-config(5), config(8)

# AUTHORS

Mats O Jansson <moj@stacken.kth.se>

May 5, 2025 | Debian