A well known feature of Sun's Sparc machines can be configured in Linux: the serial console. Here are the steps you have to do to be able to use the serial console:

Setting up a serial tty

What's that? A serial tty can be used to login via nullmodem cable. So you don't need to connect monitor and keyboard if you did something wrong with your firewall config A notebook and a nullmodem cable are sufficient.

It's very easy: edit the file /etc/inittab, search for some lines like these:

1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2

and add a line

S0:12345:respawn:/sbin/agetty -L 19200 ttyS0 vt102

Explanation: The S0 is a symbolical name of the console, the 2345 says, start in runlevels 2,3,4 and 5, respawn starts the console if shutdown, mingetty is the getty (the program that represents the console) and -L 19200 ttyS0 setup the serial line 1 with a baud rate of 19200.

Setting up grub for use with the serial line

In /boot/grub/menu.lst add the lines
serial unit=0 speed=19200
terminal --timeout 10 serial console

Telling the linux kernel to write boot messages to the serial line

Add the options to the kernel in /boot/grub/menu.lst:
console=ttyS0,19200 console=tty0
Example kernel line:
kernel /vmlinuz-2.6.27.23-0.1-default root=/dev/vg_system/root showopts ide=nodma apm=off noresume edd=off powersaved=off nohz=off highres=off processor.max_cstate=1 x11failsafe vga=0x314 console=ttyS0,19200 console=tty0

With Xen

... tbc