Linux: Configure / Enable Serial Console By Editing GRUB Boot Loader

How do I configure and enable console redirection in the BIOS? I want see my remote dedicate server boot information and any diagnostic information that is normally printed to the screen. I've already enabled serial ports in my server BIOS and wired them up. Now how do I configure and access Linux over a serial console?

The serial console is helpful in troubleshooting difficult problems. This is useful if you are remotely administrate the Linux server even if the network has failed or crashed. In order to use serial console you need to configure GRUB boot loader and make changes to /etc/inittab file. Following instructions are tested on :

Tutorial details
Difficulty	Intermediate (rss)
Root privileges	Yes
Requirements	None

[1] Cent OS version 4/5

[2] Redhat Enterprise Linux (RHEL) version 3/4/5

[3] Debian version 3/4/5/6

Step # 1: Serial redirection in the GRUB

Open the grub configuration file /boot/grub/menu.lst (Debian / Ubuntu Linux) or /boot/grub/grub.conf (Redhat/CentOS/Fedora Core Linux), enter:
# vi /boot/grub/grub.conf
Append the following lines below "hiddenmenu" option:

serial --unit=1 --speed=19200
terminal --timeout=8 console serial

The first line tells GRUB to use the first serial port at a baud rate of 19200
The second line gives the user 9 seconds to decide where GRUB should output it's information.
Please adjust port number and speed as per your setup.

Next make sure splashimage options is disabled as graphics can't be displayed across the serial port. Remove splashimage line or just comment it out by prefixing # symbol:
#splashimage=(hd0,0)/grub/splash.xpm.gz

Step # 2: Enabling serial output from the Linux kernel

Find the kernel line (grub config file) which corresponds to your currently running kernel. Add the following at the end of that line - console=tty0 console=ttyS0,9600n8:

title Red Hat Enterprise Linux ES (2.6.9-42.0.10.ELsmp)
        root (hd0,0)
        kernel /vmlinuz-2.6.9-42.0.10.ELsmp ro root=LABEL=/ console=tty0 console=ttyS1,19200n8
        initrd /initrd-2.6.9-42.0.10.ELsmp.img

Save and close the file. When multiple console= options are listed, the Linux kernel will output (tty0, ttyS1) to both when possible. Now the kernel errors get logged with on both the normal VGA console and on the serial console.

Step #3: Logging in via the serial console

Finally open the /etc/inittab file, enter:
# vi /etc/inittab
Append the following line:
1:23:respawn:/sbin/agetty -h -L ttyS1 19200 vt100
Save and close the file. The above line allows you to login via the serial device. You are attaching agetty process to serial consol. Your Linux distribution may have the above line.

Step # 4: Test your setup

Next, reboot the server. Make sure you connect to the serial console using IPMI Java client or IPMIView Linux client. Finally, you should see the following message on screen:
Press any key to continue.
Now hit any key to launch GRUB on the serial console. Please note that if you do nothing, it will open server's VGA console.

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{ 4 comments… read them below or add one }

1 John Doe July 6, 2011 at 7:32 pm

I started banging my head on the wall when I got to “now reboot your server”. Contradiction in terms.

2 Stefan Lasiewski January 12, 2012 at 11:49 pm

Append the following line:
1:23:respawn:/sbin/agetty -h -L ttyS1 19200 vt100

The first column ’1′ may cause problems on most RHEL5 installations. In my case, this cause the error ‘INIT: /etc/inittab[69]: duplicate ID field “1″‘ upon boot.

This is because the first column is a unique identifier, and is commonly used to identify tty1:

# Run gettys in standard runlevels
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

A better name would be something like S0:

S0:2345:respawn:/sbin/agetty -h -L ttyS0 9600 vt100

3 Chris Quinn February 21, 2012 at 6:19 pm

This is awesome! Thank you very much this is what I was looking for. I got IPMI working on my Supermicro server!

4 Jorge Medina November 25, 2012 at 6:33 pm

Why people use console=tty0 in grub, afaik that tty/file doesn’t exist by default in any linux distribution, I realized that when people set tty0 in grub the system boots but without a valid tty and without login, you have to type alt+f1 to get a valid tty.

To fix it you have to set grub to a valid tty/console, like this:

GRUB_CMDLINE_LINUX=”console=tty1 console=ttyS0,115200n8″

Best regards.