🧱 Building better initramfs: A deep dive into dracut on Fedora & RHEL

Understanding how to use dracut is critical for kernel upgrades, troubleshooting boot issues, disk migration, encryption, and even kernel debugging.

🚀 Introduction: What is dracut?

dracut is a powerful tool used in Fedora, RHEL, and other distributions to create and manage initramfs images—the initial RAM filesystem used during system boot. Unlike older tools like mkinitrd, dracut uses a modular approach, allowing you to build minimal or specialized initramfs tailored to your system.

📦 Installing dracut (if not already available)

dracut comes pre-installed in Fedora and RHEL. If it is missing, install it with:

$ sudo dnf install dracut

Verify the version:

$ dracut --version

📂 Basic usage

📌 Regenerate the current initramfs

$ sudo dracut --force

This regenerates the initramfs for the currently running kernel.

📌 Generate initramfs for a specific kernel

$ sudo dracut --force /boot/initramfs-$(uname -r).img $(uname -r)

Or Manually!

$ sudo dracut --force
$ sudo dracut --force /boot/initramfs-5.14.0-327.el9.x86_64.img 5.14.0-327.el9.x86_64

🧠 Understanding key dracut options (with examples)

–force

Force regeneration even if the file already exists:

$ sudo dracut --force

–kver <kernel-version>

Generate initramfs for a specific kernel:

$ sudo dracut --force --kver 5.14.0-327.el9.x86_64

–add <module> / –omit <module>

Include or exclude specific modules (e.g., lvm, crypt, network).

Include LVM module only:

$ sudo dracut --force --add lvm

Omit network module:

$ sudo dracut --force --omit network

–no-hostonly

Build a generic initramfs that boots on any compatible machine:

$ sudo dracut --force --no-hostonly

–hostonly

Create a host-specific image for minimal size:

$ sudo dracut --force --hostonly

–print-cmdline

Show the kernel command line:

$ dracut --print-cmdline

–list-modules

List all available dracut modules:

$ dracut --list-modules

–add-drivers “driver1 driver2”

Include specific drivers:

$ sudo dracut --add-drivers "nvme ahci" --force

🧪 Test cases and real-world scenarios

1. LVM root disk fails to boot after migration

$ sudo dracut --force --add lvm --hostonly

2. Initramfs too large

Shrink it by omitting unused modules:

$ sudo dracut --force --omit network --omit plymouth

3. Generic initramfs for provisioning

$ sudo dracut --force --no-hostonly --add network --add nfs

4. Rebuild initramfs for rollback kernel

$ sudo dracut --force /boot/initramfs-5.14.0-362.el9.x86_64.img 5.14.0-362.el9.x86_64

🪛 Advanced use: Debugging and analysis

Enable verbose output:

$ sudo dracut -v --force

Enter the dracut shell if boot fails:

Use rd.break in the GRUB kernel line.

📖 Where is dracut configuration stored?

There are two locations where configuration setting may occur.

The global settings location is at:

/etc/dracut.conf

and the drop-in location is at:

/etc/dracut.conf.d/*.conf

Example using the drop-in location:

$ cat /etc/dracut.conf.d/custom.conf

The contents might appear as follows for omitting and adding modules:

omit_dracutmodules+=" plymouth network "
add_dracutmodules+=" crypt lvm "

⚠️ Note: Always include a space at the beginning and end of the value when using += in these configuration files. These files are sourced as Bash scripts, so

add_dracutmodules+=" crypt lvm "

ensures proper spacing when multiple config files are concatenated. Without the spaces, the resulting string could concatenate improperly (e.g., mod2mod3) and cause module loading failures.

🧠 Deep dive: /usr/lib/dracut/modules.d/ – the heart of dracut

The directory /usr/lib/dracut/modules.d includes all module definitions. Each contains:

• A module-setup.sh script
• Supporting scripts and binaries
• Udev rules, hooks, and configs

List the modules using the following command:

$ ls /usr/lib/dracut/modules.d/

Example output:

01fips/ 30crypt/ 45ifcfg/ 90lvm/ 95resume/
02systemd/ 40network/ 50drm/ 91crypt-gpg/ 98selinux/

Inspect specific module content (module-setup.sh, in this example) using this:

$ cat /usr/lib/dracut/modules.d/90lvm/module-setup.sh

You can also create custom modules at this location for specialized logic.

🏁 Final thoughts

dracut is more than a utility—it’s your boot-time engineer. From creating lightweight images to resolving boot failures, it offers unparalleled flexibility.

Explore man dracut, read through /usr/lib/dracut/modules.d/, and start customizing.

💡 This article is dedicated to my wife, Rupali Suraj Patil, for her continuous support and encouragement.