dnf history undo versus GRUB kernel fallback for config preservation after a failed upgrade

Recovery Decision: Package Rollback vs. Kernel Fallback

To recover a Rocky Linux 9.x system while maintaining configuration consistency, you must choose between reverting the software state or reverting the boot environment. Neither method provides a fully automated "snapshot" restoration of configuration files.

Comparison of Recovery Mechanisms

Feature	dnf history undo	GRUB Kernel Fallback
Package Versions	Reverts to pre-transaction versions	Keeps upgraded package versions
Config Files (/etc)	Not restored; remains in upgraded state	Preserved in upgraded state
Systemd Units	May leave orphaned units/services	Keeps upgraded units; may mismatch kernel
Boot Kernel	Remains current (unless kernel was rolled back)	Boots previous known-good kernel

Analysis of Configuration and State

dnf history undo: This command reverts the package installation state but does not track or revert changes made to configuration files in /etc. If the upgrade process modified a config file or deleted one, dnf history undo will not restore the original file. This creates configuration drift, where old package binaries are paired with new configuration syntax, often leading to service failure.

GRUB Kernel Fallback: This method preserves the upgraded package set and all configuration changes. The system boots using an older kernel, but the services running in userspace are the upgraded versions. The primary risk here is a kernel-userspace mismatch, where an upgraded service requires a kernel module or feature not present in the fallback kernel.

Recommended Recovery Steps

1. Identify the failure: If the system cannot boot, use GRUB fallback first to regain access.
2. Evaluate drift: If you must use dnf history undo, manually verify critical configurations. Use the following to check for modified files:

   rpm -V package_name

3. Clean orphaned units: After a dnf undo, check for failed services that no longer have supporting binaries:

   systemctl list-units --failed

4. Reconcile: Manually compare current /etc files against your last known-good backup to resolve drift.

Missing Diagnostic Detail: Did the failed upgrade include a kernel update or a major version jump (e.g., 9.x to 9.y)? If the kernel was the primary cause of failure, GRUB fallback is the priority; if application-level dependencies broke, dnf undo is required.