Linux Serial Consoles for Servers and Clusters

As packaged by most distributions, the Linux kernel and bootloader select the directly attached video controller and keyboard as console, but this is easily changed. When a PC-based system boots, the bootloader is the first program to be loaded off the disk. The three major bootloaders in popular use on Linux systems are GRUB, LILO and SYSLINUX (used on boot floppies); all of them support serial consoles. Next, the Linux kernel needs to be told to use a serial port for its console, which can be handled at compile time or by passing kernel command-line options from the bootloader configuration. Finally, if you want to be able to log in on the console, you need to configure a getty process to run after the system is up.

We discuss the kernel configuration next because it is a prerequisite to understanding the bootloader config later on. The most flexible way to configure the kernel console is with the options passed on the kernel command line. You can append arguments to the command line from the bootloader. Here is an example of the kernel command-line syntax:

console=ttyS0,9600n8

This tells the kernel to use ttyS0 (the first serial port discovered by the kernel), running at 9,600 baud, no parity and 8 bits. The kernel defaults to one stop bit. This is the most common speed and configuration for a serial console, which is why most serial ports and terminals default to 9600n8. It is possible to append more than one console= argument to the command line; kernel messages then are output to all of them, but only the last one is used for input.

GRUB is a flexible bootloader with excellent support for serial consoles. When properly configured, GRUB allows multiple devices to be used as a console. Listing 1 shows an example grub.conf file (usually /boot/grub/grub.conf and symlinked to /etc/grub.conf) as configured by the Red Hat/Fedora Core installer. Yours may be slightly different.

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub
# after making changes to this file
# NOTICE:You have a /boot partition.
#        This means that all kernel
#        and initrd paths are relative
#        to /boot/, eg.
#        root (hd0,1)
#        kernel /vmlinuz-version ro root=/dev/hda6
#        initrd /initrd-version.img
#boot=/dev/hda
default=0
timeout=10
splashimage=(hd0,1)/grub/splash.xpm.gz
title Red Hat Linux (2.4.20-8)
        root (hd0,1)
        kernel /vmlinuz-2.4.20-8 ro root=LABEL=/
        initrd /initrd-2.4.20-8.img

The first thing to do is remove all splashimage directives. In some early versions, these directives confuse GRUB and make it default to the video console. Then add a serial and terminal line. The serial line initializes the serial port to the proper baud and settings. In the terminal line, we configure GRUB to send prompts to both the serial port and to the keyboard and monitor. You can press any key on either, and it becomes the default console. The --timeout=10 argument tells GRUB to default to the first device listed in the terminal line after ten seconds. We also modified the kernel command line to include the option that tells the Linux kernel to use the serial port as console. Listing 2 shows the complete modified grub.conf file.

The LILO bootloader, although much more mature than GRUB, is less feature-rich. We must configure LILO and pass options to the kernel to use a serial port. To do this, we add:

serial=<port>,<bps><parity><bits>

where port 0 is the first serial port detected by LILO. Also, the append= line is modified to include the kernel options. After modifying the /etc/lilo.conf file, be sure to run LILO to update the bootloader. The completed lilo.conf file is shown in Listing 3.