Monday, March 8, 2010

Setup Your Linux Server to Use a Serial Console

Do you have a linux server without a keyboard or monitor? Need to administer the server on-site but don't want to lug over a monitor and keyboard (or kvm)? Then setup the server to output the console to a serial port and use screen/minicom (Hyperterminal or putty in Windows) to console into the server over a serial cable.

To set this up, you need to edit /etc/inittab to tell it to start a terminal on the serial port for the console. If you want to see the kernel load and see all the services start then you also need to configure grub to use the serial port as well.

Edit /etc/inittab and add the line starting with "co" to the file (substitute the device name of your serial port for ttyS0 below):

co:2345:respawn:/sbin/agetty ttyS0 9600 vt100-nav # ADD THIS LINE
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6

To watch the kernel load (and all the services) you must configure grub to enable the console option in the kernel on boot. Edit /boot/grub.conf and add the "serial" and "terminal" lines below, and modify the "kernel" line to include the console specification:

serial --unit=0 --speed=9600
terminal --timeout=5 serial console
title CentOS (2.6.9-55.0.2.EL)
root (hd1,0)
kernel /vmlinuz-2.6.9-55.0.2.EL ro root=/dev/VolGroup00/LogVol00 console=ttyS0,9600n8
initrd /initrd-2.6.9-55.0.2.EL.img

To connect to the server just hook a serial cable from your laptop to the server. Set the speed to 9600, no parity and 8bits. Boot up the server, you should see kernel output, services load, and finally a login prompt. After you disconnect you can easily re-connect and log in: just fire up your terminal emulator, connect your serial cable, hit enter a few times and you should get a prompt to log in.

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