Having previously setup serial console access for Linux systems, here’s a recipe to do this for FreeBSD as well.

In my case, I needed it for a OOB (out of band) BMC (Baseboard Management Controller) from Dell (iDRAC), but it should apply to any system.

Create or add to (in a SINGLE LINE) /boot.config:

-Dh

That will setup a “dual console” for the boot loader, mirroring i/o for the boot loader to both standard video console and the serial console. See also boot(8) and boot.config(5) man pages.

Add to /boot/loader.conf (or /boot/loader.conf.local):

boot_multicons="YES"
boot_serial="YES"
comconsole_speed="57600"
comconsole_port="0x2f8"
console="comconsole,vidconsole"

That completes the setup, providing sufficient hints to the loader to provide to the kernel to again mirror i/o to both serial and video consoles. See also loader(8) and loader.conf(5) man pages.

Note two potentially unique options that may not map directly to your configuration:

  • comconsole_speed="57600" - for a slower, older machine.
  • comconsole_port="0x2f8" - ttyu1 (com2), rather than default ttyu0.

In my case (Dell iDRAC), I needed to set the console interface to a different UART, hence the comconsole_port= option, otherwise the default UART would be used. You should be able to find the address of the serial console via dmesg, as in:

# dmesg | grep uart
uart0: <16550 or compatible> port 0x3f8-0x3ff irq 4 flags 0x10 on acpi0
uart1: <16550 or compatible> port 0x2f8-0x2ff irq 3 on acpi0
uart1: console (57600,n,8,1)

For your case, best to confirm both com port and speed via the documentation for your BMC/DRAC/CMC/whatever.

Useful references: