🧱 Building Better Initramfs: A Deep Dive into `dracut` on Fedora & RHEL

# 🧱 Building better initramfs: A deep dive into *dracut* on Fedora & RHEL **By Suraj Rajendra Patil (Fedora ID: suraj522)** --- ## 🚀 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. Understanding how to use *dracut* is critical for kernel upgrades, troubleshooting boot issues, disk migration, encryption, and even kernel debugging. --- ## 📦 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 /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 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 dracut shell if boot fails: Use `rd.break` in the GRUB kernel line. --- ## 📖 Where is dracut config stored? * `/etc/dracut.conf` – global config * `/etc/dracut.conf.d/*.conf` – drop-in configs Example: ``` $ cat /etc/dracut.conf.d/custom.conf 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 This directory includes all module definitions. Each contains: * A `module-setup.sh` script * Supporting scripts and binaries * Udev rules, hooks, and configs List modules: ``` $ ls /usr/lib/dracut/modules.d/ ``` Example output: ``` 01fips/ 30crypt/ 45ifcfg/ 90lvm/ 95resume/ 02systemd/ 40network/ 50drm/ 91crypt-gpg/ 98selinux/ ``` Inspect a module: ``` $ cat /usr/lib/dracut/modules.d/90lvm/module-setup.sh ``` You can also create custom modules here 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.*