CentOS 8 And RHEL 8 Serial Console Setup Guide For Debugging

July 11, 2025 by StackCamp Team 61 views

How to Configure Serial Console on CentOS 8 / Red Hat 8 for Debugging

In the realm of system administration and debugging, having a reliable serial console setup is paramount. A serial console acts as a direct communication channel with your server, especially crucial when network access is unavailable or the system is in a state where typical console access is impossible. This comprehensive guide will walk you through the intricacies of setting up a serial console on CentOS 8 and Red Hat 8 (RHEL 8) systems. If you've previously configured a serial console on CentOS 7, you'll find some familiar steps, but also key differences that are vital to understand for a successful setup on newer systems.

Before diving into the configuration, let's emphasize why a serial console is an indispensable tool for system administrators. Imagine a scenario where your server fails to boot due to kernel panics or file system corruption. In such situations, network connectivity might be down, rendering SSH access useless. A serial console provides a direct, low-level interface to the system, allowing you to monitor the boot process, diagnose issues, and perform recovery tasks. It's akin to having a physical keyboard and monitor attached directly to the server, regardless of the network state. For those who rely on serial consoles for debugging, this guide provides a detailed walkthrough tailored for CentOS 8 and RHEL 8.

Before you begin, ensure you have the following:

A CentOS 8 or Red Hat 8 system.
Root access or sudo privileges.
A serial cable (usually a null modem cable) to connect your server to another machine (or a terminal server).
A machine with a serial terminal program (such as PuTTY, Minicom, or screen) to connect to the serial console.

Step 1: Identify the Serial Port

The first step is to identify the serial port you'll be using. Common serial ports are /dev/ttyS0 (COM1), /dev/ttyS1 (COM2), and so on. You can usually determine the correct port by consulting your server's hardware documentation or by trial and error. Connect your serial cable and attempt to connect using different port names until you get a response.

Step 2: Edit GRUB Configuration

The GRUB (Grand Unified Bootloader) configuration needs to be modified to enable the serial console. This involves editing the /etc/default/grub file. Open this file with your favorite text editor as root:

sudo vi /etc/default/grub

Locate the line that starts with GRUB_CMDLINE_LINUX. This line contains kernel parameters. Add the following parameters to enable the serial console:

console=tty0 console=ttyS0,115200n8

console=tty0: This directs kernel messages to the primary console.
console=ttyS0,115200n8: This enables the serial console on /dev/ttyS0 with a baud rate of 115200, no parity, and 8 data bits. You can adjust the baud rate if necessary, but 115200 is a common and recommended setting.

If you have a GRUB_CMDLINE_LINUX line that looks like this:

GRUB_CMDLINE_LINUX="crashkernel=auto rhgb quiet"

Modify it to include the serial console parameters:

GRUB_CMDLINE_LINUX="crashkernel=auto rhgb quiet console=tty0 console=ttyS0,115200n8"

Additionally, you may need to add or modify the GRUB_TERMINAL setting to ensure GRUB itself uses the serial console. Add or modify the following line:

GRUB_TERMINAL="serial console"

If a GRUB_TERMINAL line already exists, ensure it includes both serial and console. If it only contains console, append serial to it.

Finally, for GRUB to be aware of the serial port, you might need to add the following line as well:

GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"

This command sets the serial port parameters for GRUB. Ensure the speed matches the baud rate you specified in the GRUB_CMDLINE_LINUX line.

Step 3: Apply GRUB Changes

After modifying /etc/default/grub, you need to apply the changes by regenerating the GRUB configuration file. This is done using the grub2-mkconfig command. Run the following command as root:

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

This command reads the settings in /etc/default/grub and creates a new GRUB configuration file. If you are using UEFI, the configuration file might be located at /boot/efi/EFI/centos/grub.cfg or /boot/efi/EFI/redhat/grub.cfg, depending on your system. Adjust the output path accordingly.

Step 4: Configure the System Console

Next, you need to configure the system to use the serial console after the kernel has booted. This involves editing the /etc/securetty file. This file lists the terminals on which root login is allowed. Open the file as root:

sudo vi /etc/securetty

Add the following line to the file:

ttyS0

This allows root login on the serial console. Without this, you might be able to see the boot messages, but you won't be able to log in.

Step 5: Configure Systemd for Serial Console

CentOS 8 and RHEL 8 use systemd as the system and service manager. You need to ensure that systemd knows about the serial console. Create a new systemd service file for the serial console. Create the file /etc/systemd/system/serial-getty@.service with the following content:

[Unit]
Description=Serial Getty on %I
Documentation=man:systemd-getty-generator(8) man:agetty(8)
After=network.target
After=systemd-user-sessions.service
Wants=systemd-user-sessions.service

[Service]
ExecStart=-/sbin/agetty -o '-p -- \u' --keep-baud 115200,38400,9600 %I $TERM
Type=idle
Restart=always
UtmpIdentifier=%I
TTYPath=/dev/%I
TTYReset=yes
TTYVHangup=yes
KillMode=mixed
TimeoutStopSec=0

[Install]
WantedBy=getty.target

This service file defines how the serial console getty (the program that manages login prompts on terminals) should be started. It specifies the baud rates to use, the terminal type, and other settings.

Now, enable and start the serial console service. Run the following commands as root:

sudo systemctl enable serial-getty@ttyS0.service
sudo systemctl start serial-getty@ttyS0.service

The first command enables the service to start on boot, and the second command starts it immediately.

Step 6: Reboot and Test

After completing the configuration, reboot your system to apply the changes:

sudo reboot

Connect to your server using your serial terminal program. Use the same settings you specified in the GRUB configuration (typically 115200 baud, 8 data bits, no parity, 1 stop bit). You should see the GRUB menu and the kernel boot messages on the serial console. After the system boots, you should see a login prompt. Log in with your username and password to verify that the serial console is working correctly.

If you encounter issues, here are some common problems and solutions:

No output on the serial console:
- Double-check your serial cable connection.
- Verify that you are using the correct serial port and baud rate.
- Ensure that the console parameters are correctly set in /etc/default/grub.
- Make sure the GRUB_TERMINAL setting includes both serial and console.
- Verify that the GRUB_SERIAL_COMMAND is correctly configured.
- Check if the ttyS0 entry exists in /etc/securetty.
- Ensure the serial-getty@ttyS0.service is enabled and running.
Garbled output:
- This usually indicates a baud rate mismatch. Ensure that the baud rate in your terminal program matches the baud rate specified in the GRUB configuration.
Login prompt not appearing:
- Check if the ttyS0 entry is present in /etc/securetty.
- Verify that the serial-getty@ttyS0.service is enabled and running.
Kernel panic or other boot issues:
- The serial console should provide valuable debugging information. Examine the output closely to identify the cause of the problem.

Setting up a serial console on CentOS 8 and RHEL 8 is a crucial task for system administrators who need reliable access to their servers, especially during emergencies or when network connectivity is compromised. This guide has provided a detailed, step-by-step process to configure a serial console, from identifying the serial port to configuring GRUB and systemd. By following these instructions, you can ensure that you have a robust and dependable serial console setup for your systems. Remember, a properly configured serial console can be a lifesaver when troubleshooting critical issues, making it an invaluable tool in your system administration arsenal. The ability to debug and recover systems via a serial console is a fundamental skill for any serious system administrator, and this guide equips you with the knowledge to confidently configure this essential tool on modern Linux systems.