Using a Raspberry Pi as a serial console server for FreeBSD

My home server runs FreeBSD 11.0, and, of course, has full disk encryption. This requires a passphrase to be entered while booting, so I need a keyboard and monitor attached, and have to be in the same room for restarts and after power outages. This is not acceptable at all. There is no need for a display and keyboard otherwise, and I want to be able to enter the passphrase remotely, but without compromising security. Luckily, the server has a serial port and I have a spare Raspberry Pi. So I can connect the serial port to the Raspberry Pi, SSH into it and use the serial console of the server just like physical access. Hooray for stuff from the 1960s still being useful!

So first things first, I need to connect the Raspberry Pi to the serial console. Since it only has USB (aside from GPIO), a USB to serial port adapter is necessary. The Raspberry Pi runs FreeBSD 11.0, which has good out-of-the-box support for the Prolific PL2303 chipset, so I just ordered a cheap adapter based on it. It probably works with Linux as well, if you’re using Raspbian. You can use dmesg to check if it was detected correctly:

# dmesg | grep Prolific
ugen0.4: <Prolific Technology Inc.> at usbus0
uplcom0: <Prolific Technology Inc. USB-Serial Controller, class 0/0,
rev 2.00/3.00, addr 4> on usbus0

Don’t forget (like I did) that the USB to serial port adapter only provides you with a serial port. To connect it to the server’s serial port, you still need a null modem cable or adapter.

Now that we have the hardware, it’s time to set up the server to use the serial console. This is pretty straight-forward, just add the following to your /boot/loader.conf and reboot, as described in the FreeBSD handbook:

boot_multicons="YES"
boot_serial="YES"
console="comconsole,efi"

If you don’t use EFI yet, replace efi with vidconsole. This outputs all kernel messages also to the serial console and lets you type in the passphrase during the boot process. You can also only use comconsole, but then there won’t be much output on a real monitor in case you need one.

Additionally, you may like to see a login prompt after the boot process is done, in case the server’s network is down and SSH doesn’t work. A terminal on the serial port can be activated by adding the following line in /etc/ttys:

ttyu0   "/usr/libexec/getty std.9600" dialup  on secure

Now you should already be able to use the serial console via the Raspberry Pi, with a tool of your choice:

# tip ucom1
# screen /dev/cuaU0 9600
# cu -l /dev/cuaU0 -s 9600

If you don’t want to be root to access the console (you don’t), you can add your user to the group dialer:

# pw groupmod dialer -M solence

Now that all is up and running, be aware that the serial console works like a monitor. If you log in via serial console and only close serial connection, it’s just like turning off your monitor, you’re still logged in! So be aware to always log out before closing the serial connection.

 

Increasing the port speed

This setup is usable, but painfully slow, because the default port speed is still 9,600 baud. In case you don’t remember the ancient unit baud, it’s symbols per second. This equals bits/sec, so 9,600 baud convert to 1,200 bytes/sec. Considering that the standard terminal line has 80 characters, that’s only 15 lines/sec.

So increasing the serial port speed to the maximum of 115,200 baud would be a good idea. This equals 14,000 bytes/sec or 180 terminal lines/sec, which sounds much better. Some tweaking on the server’s side is required, however.

First, add the desired speed to /boot/loader.conf:

comconsole_speed="115200"

Then change the speed in the corresponding entry in /etc/ttys:

ttyu0   "/usr/libexec/getty std.115200" dialup  on secure

In theory, this is sufficient and only requires a reboot to work, but in my case I also had to configure the serial port device. Check with stty if the port has the correct speed on the server:

# stty -f /dev/ttyu0

If the speed is set to 115,200 baud, everything is fine. If it’s still 9,600 baud, it has to be adjusted like described in this great tutorial.

After that’s done, the serial console can be used with the increased speed on the Raspberry Pi:

$ screen /dev/cuaU0 115200
$ cu -l /dev/cuaU0 -s 115200

You can also add an alias in /etc/remote for easy use with tip on the Raspberry Pi:

server:dv=/dev/cuaU0:br#115200:pa=none:

Now you can just use:

$ tip server

If your Raspberry Pi is accessible remotely, directly or via VPN, power outages or remote maintenance are no problem anymore. But make sure you can still reach the Raspberry Pi when the server you connected it to is down, otherwise all this would be pointless. Also, now you don’t even need keyboard and monitor at all, because it’s way more convenient to use SSH and the serial console than getting up and moving over to the box 🙂

One thought to “Using a Raspberry Pi as a serial console server for FreeBSD”

Leave a Reply

Your email address will not be published. Required fields are marked *