BOOT_CONFIG(8)

📦 in nixpkgs unstable

💅 BOOT_CONFIG(8) — like, how to change your kernel config at boot, bestie

okay so this is the boot_config manual page — System Manager's Manual energy, and it's literally your guide to poking at the kernel config before the kernel even finishes waking up. main character energy fr fr 🎀

✨ NAME ✨

boot_config — the whole "how to change kernel configuration at boot" situation 🦋

🌸 DESCRIPTION 🌸

BOOT_CONFIG is a kernel option that makes it possible to change the configuration at boot time. no cap. and the same interface is available from userland too, using the -e argument to config(8). iykyk 💅

So the boot-time config gets invoked by passing the -c option — either from the firmware prompt, or, depending on the architecture, when the bootstrap loader prompts you 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). BUT — and this is the important part, bestie — those changes would not persist across system upgrades and would prevent kernel relinking 😭. To make sure your changes get carried over to upgraded kernels, they can be saved to the bsd.re-config(5) configuration file. slay 💖

👑 COMMANDS 👑

  • add dev — add a device through copying another. iconic cloning behaviour 🧚
  • 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. bye bestie 🫶
  • 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 saga

So 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. and the Ethernet card is supposed to use the ne(4) driver. classic 🙈

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, either way works 💅

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

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. double duty queen 👑

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>

💀 adding a whole new device

it is possible to add new devices, but only devices that were linked into the kernel. that's the catch, bestie. and 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>

⭐ finishing up

when configuration is completed, booting can proceed by issuing the quit or exit commands. go forth, bestie 🌸

UKC> quit
Continuing...
mainbus0 (root)

💜 SEE ALSO 💜

🎀 AUTHORS 🎀

Mats O Jansson <moj@stacken.kth.se> — the icon who made this 🫶

✨ that's the whole vibe ✨ — this beauty came from the Debian edition of the System Manager's Manual, dated May 5, 2025, and it's BOOT_CONFIG(8) all the way down. mwah 💋

📦 where this page comes from