This short guide by user solence will show you how to set your Raspberry Pi as a serial console server for FreeBSD. Follow the link below for the full set of instructions.

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 apdater 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:

Using a Raspberry Pi as a serial console server for FreeBSD