FreeBSD serial console on another port
On many of my FreeBSD servers I use a serial console. It's pretty straightforward to set up as the Handbook illustrates.
Combined with a serial port server with SSH access (thanks XS4all!) or, say, HP's iLO which can also provides SSH access to the serial port it's a very nice way to be able to access your server even if you need to boot to single-user or if you locked yourself out. Even better, combine this with a firmware that knows something about console redirection and you also get at the BIOS setup over the same port.
I recently found myself in a situation where I couldn't use COM1 as the serial console port on a new server. The Handbook says:
If there is no COM1 (sio0), get one. At this time, there is no way to select a port other than COM1 for the boot blocks without recompiling the boot blocks.
[…]
25.6.4.2. Using a Serial Port Other Than sio0 for the Console
Using a port other than sio0 as the console requires the boot blocks, the boot loader, and the kernel to be recompiled as follows.
which sounds a bit scary. I'm not foreign to the notion of running a
customized kernel. However, on a stock x86 server in co-lo which I hope
to be able to update with freebsd-update(8), I don't really want to
run a custom kernel. Thinking it through a little and looking at the
uart(4) manual page I got an idea.
There is an alternative to compile your own kernel, and I admit it might fall under the tip "get one" above: Change your COM3 (or whatever) to COM1! Here's how I did it:
Normal /boot/device.hints lists the first serial port as:
hint.uart.0.at="isa" hint.uart.0.port="0x3F8" hint.uart.0.flags="0x10" hint.uart.0.irq="4"
Note that sio(4) as mentioned in the Handbook is not listed in
device.hints anymore.
The serial port I want to use as console is on the PCI bus, so I changed
uart.0 to:
hint.uart.0.at="pci" hint.uart.0.port="0xEC00" hint.uart.0.flags="0x10" hint.uart.0.irq="B"
and it just works! The details were taken from what the BIOS setup told me about what it called "COM3". The AMI BIOS of a Supermicro C7X58 motherboard can see at least a 16C550 compatible PCI serial cards just fine.
If you later want to enter the BIOS setup over the serial port, use ESC 4 when it tells you to press F4. Move around by pressing arrow keys and TAB after each arrow key.
Written by MC on Setting Orange, the 43 day of The Aftermath in the YOLD 3179 ().