Debian Linux: Set a Serial Console

How do I set up a serial console on Debian Linux HP server for troubleshooting and login purpose?

To setup a serial console you need to edit the following files under Debian Linux:

1. /boot/grub/menu.lst or /etc/default/grub (recommended for grub2)
2. /etc/inittab
3. /etc/securetty

Our Setup

You can list your working serial port under Linux as follows:
# setserial -g /dev/ttyS[0123]
Sample outputs:

/dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4
/dev/ttyS1, UART: unknown, Port: 0x02f8, IRQ: 3
/dev/ttyS2, UART: unknown, Port: 0x03e8, IRQ: 4
/dev/ttyS3, UART: unknown, Port: 0x02e8, IRQ: 3

/dev/ttyS0 (COM1) is detected and working serial console under Linux. For testing purpose, I'm going to set baud rate to 19200 and terminal type to vt100.

Grub Configuration (Grub2)

Edit /etc/default/grub, enter:
# vi /etc/default/grub
Append / modify as follows:

GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,19200n8'
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --speed=19200 --unit=0 --word=8 --parity=no --stop=1"

Save and close the file. Next run the following command to generate a grub2 config file /boot/grub/grub.cfg :
# update-grub

A Note About Grub Legacy (older version)

If you are not using GRUB2 update /boot/grub/menu.lst (grub legacy) with kernel line as follows with console port, and baud rate:

title           Debian GNU/Linux, kernel 2.6.32-5-486
root            (hd0,0)
kernel          /boot/vmlinuz-2.6.32-5-486 root=UUID=b598f856-2a2a-46d5-b60a-95826cfc7bf6 ro console=ttyS0,19200 earlyprint=serial,ttyS0,19200
initrd          /boot/initrd.img-2.6.32-5-486

Save and close the file.

/etc/inittab Configuration

Edit /etc/inittab, enter:
# vi /etc/inittab
You need put a getty on a serial line for a terminal:

T0:23:respawn:/sbin/getty -L ttyS0 19200 vt100

Save and close the file.

/etc/securetty Configuration

Edit /etc/securetty, enter:
# vi /etc/securetty
Make sure UART serial ports /dev/ttyS0 is listed:

ttyS0

Save and close the file. Now, you can reboot the server for testing purpose.

How Do I Connect Using a Serial Console?

You need to use the minicom command, which is a communication program that runs under most unices. You can install minicom as follows on your Debian / Ubuntu desktop system:
# apt-get install minicom
For rpm based distro such as RHEL / Fedora / CentOS Linux, enter:
# yum install minicom
Run minicom as follows to create a configuration file:
# minicom -s -c on
Press [down] key and select Serial port setup:

Fig.01: Setting up minicom serial port

1. Press A to setup serial device name such as /dev/ttyS0
2. Press E to setup Bps/Par/Bits (baud rate)
3. Press [ESC] to exit
4. From menu select "Save setup as DFL"
5. Exit

Alternatively, you can create minicom config DFL file as follows:
# cat /etc/minicom/minirc.dfl
pu port /dev/ttyS0
pu baudrate 19200
pu bits 8
pu parity N
pu stopbits 1
pu rtscts No

Save and close the file. To connect to a serial console simply type the minicom command as follows:
# minicom
Sample outputs:

Fig.02: Minicom in action