44.14. Decapitating Your Machine -- Serial Consoles
Often server machines are placed in a rack in a colocation facility, in some back closet, or in some other out of the way place. This can make it really inconvenient to access the server's console should something go wrong or need diagnosing; hauling a monitor and keyboard into your server storage area is a real pain. If you've got your server mounted in a rack, there are devices that are essentially a flat screen monitor, keyboard, and mouse mounted in a sliding rack shelf, which work well, but they're expensive.
A simple and cheap solution is to change the console from the normal monitor/keyboard/mouse to one of the serial ports. The serial port can be hooked via null modem to a terminal server or another machine, allowing controlled access, or you can just plug your laptop into it with a null modem when you need to diagnose problems or reboot.
Linux has a howto describing details of dealing with serial consoles at http://www.linuxdoc.org/HOWTO/Remote-Serial-Console-HOWTO/. Essentially, you provide options to the boot loader and kernel to tell them to use your serial port as a console, and then configure getty to accept logins on that serial port. The HOWTO shows various potential configurations and demonstrates proper setup on each.
FreeBSD's handbook has a chapter on setting up serial consoles. Again, you have to tell the boot loader and the kernel to use the serial port, and then edit /etc/ttys to enable getty on that serial port. FreeBSD can also be configured to decide whether to use a normal console or serial console based on whether or not a keyboard is plugged in. NetBSD and OpenBSD are configured similarly.
Solaris is even easier: just unplug the keyboard before you boot the machine. Solaris uses a serial console by default if no keyboard is plugged in at boot time. If you want to set it explicitly to use a serial console even if the keyboard is plugged in, just set input-device and output-device to ttya (or ttyb if you want it on the second serial port) in the boot eeprom.
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