I have long been a huge fan of having serial console on my serversâ€“it can really save the day when a mistake is made. Yesterday, one of my coworkers botched the sshd_config in an upgrade of a server, so the server came up fine, but without sshd. As a result, the system was not accessible for remote login via the network.
Over the years, I have done serial console in many ways. I began with a single null modem cable between the back of two servers. Next, I utilized a RocketPort multi-port serial card with 8 serial ports on it. These days, I have moved on to employing big serial console servers such as those made by OpenGear, providing up to 48 ports. They also have ancillary features such as providing a Nagios platform and Environmental monitoring.
No matter your physical connectivity, I recommend using Conserver. This helps by logging what is happening on the console, which can be very handy if you need to see what happened in the past whether it be a function of the system, or to see who did what. It also provides multi-user access, so you can watch while someone else is working and both of you can collaborate on fixing a problem.
In order for the previous technologies to be useful, the servers require configuration as well. The first step is to configure the BIOS for serial console redirection. Once this has been performed, the OS will need to be configured to present a console login via the serial port. The FreeBSD Handbook explains how to do this Here.