Migrating from RHEL to AlmaLinux: Using the Build System for Package Control

The Migration Challenge: Maintaining Package Control Without RHEL Subscriptions

When Red Hat ended free RHEL subscriptions, many enterprises faced a critical decision: pay for support or migrate to a downstream rebuild. AlmaLinux emerged as a popular choice because it maintains full binary compatibility with RHEL, but organizations often need more than replacement packages—they need control over their own package builds.

How AlmaLinux Build System (ABS) Enables Independent Package Management

The AlmaLinux Build System (ABS) is an automated infrastructure that rebuilds packages from upstream source RPMs (SRPMs). Unlike a simple binary copy, ABS provides a mechanism for organizations to rebuild packages with custom modifications while maintaining compatibility with the broader AlmaLinux ecosystem. This is particularly valuable for organizations with compliance requirements or custom patches.

ABS uses mock build environments—standardized chroots that isolate the build process. These environments ensure that packages built on ABS match the exact toolchain and dependency versions used by AlmaLinux, producing RPMs with identical signatures and file placements to official releases. The system supports parallel builds across AlmaLinux 8, 9, and 10, using consistent toolchain versions and dependency resolution.

Practical Example: Rebuilding a Custom Package with ABS

Let's walk through rebuilding a modified nginx package for AlmaLinux 9. This example assumes you have access to the AlmaLinux build infrastructure and basic familiarity with RPM packaging.

1. Obtain the SRPM: Download the nginx source RPM from the AlmaLinux repository.

   wget https://repo.almalinux.org/almalinux/9/BaseOS/x86_64/os/Packages/nginx-1.20.1-10.el9.src.rpm

2. Install build dependencies: On your build machine, install the necessary tools:

   sudo dnf install rpm-build rpmdevtools mock

3. Set up the build environment: Create a mock configuration for AlmaLinux 9:

   sudo mock -r almalinux-9-x86_64 --init

   This creates an isolated build root matching AlmaLinux 9's environment.
4. Modify and rebuild: Extract the SRPM, make your changes, then rebuild:

   rpm -i nginx-*.src.rpm
   rpmbuild -ba ~/rpmbuild/SPECS/nginx.spec

After building, verify the package matches AlmaLinux expectations:

rpm -q --provides nginx-custom
rpm -V nginx-custom

Trade-offs and Limitations

While ABS provides powerful rebuild capabilities, there are important considerations:

• Binary compatibility dependency: The system assumes accurate SRPM reconstruction. Any divergence from upstream build processes could introduce subtle incompatibilities that break dependent packages.
• Build timing: During large-scale updates, the build infrastructure may experience queue delays, potentially making security patches available later than RHEL's direct release cycle.
• Custom package validation: Organizations with existing custom RHEL packages must thoroughly test compatibility with ABS-rebuilt packages before production deployment.

Verification and Next Steps

Before deploying rebuilt packages, validate them against known RHEL systems:

1. Check the AlmaLinux Build System status page for current build queue and package availability.
2. Use rpm -V verification against a known RHEL system to confirm file integrity.
3. Compare package checksums and file manifests between AlmaLinux and RHEL releases.
4. Verify build timestamps and changelogs in the AlmaLinux package repository metadata.

Start with non-critical packages to build confidence in the process, then gradually expand to production workloads. The key is balancing the independence that custom rebuilds provide with the stability that binary compatibility ensures.