Microsoft has posted a preview ISO of Azure Linux 4.0, the first public look at the next major version of the open-source distribution that runs much of the company’s cloud infrastructure. The release, which Microsoft says is strictly for evaluation and not for production, brings a significant change: a migration from the legacy DNF package manager to DNF5. The ISO, available from the project’s GitHub repository, gives IT teams, developers, and cloud architects an early chance to test compatibility before it becomes the default host for Azure services.

What’s Actually Changed in Azure Linux 4.0

The headlining shift is the adoption of DNF5. For years, Azure Linux—originally known as CBL-Mariner—relied on DNF, the standard package manager for RPM-based systems. DNF5 is a ground-up rewrite that merges DNF and the obsolete YUM, promising faster performance, a cleaner codebase, and a more consistent command-line experience. Red Hat led the DNF5 effort, and it’s been rolling out across Fedora and other RPM distros. Microsoft’s move aligns Azure Linux with that broader ecosystem direction.

Beyond the package manager, details are sparse. The preview ISO doesn’t come with a detailed changelog, and Microsoft’s advisory frames this as a work-in-progress build for early validation. Based on the artifacts, the kernel version appears to be a recent 6.x LTS release—likely 6.6 or newer—and the core userland has been refreshed. However, the company isn’t publishing comprehensive documentation for version 4.0 yet, leaving testers to discover specific component versions on their own.

What hasn’t changed: Azure Linux 4.0 remains a minimal, container-optimized distribution. It has no desktop environment, no user-facing GUI tools, and is designed solely as a lightweight host for containers and cloud workloads. That design philosophy continues unchanged.

What This Means for You

If you run Azure Kubernetes Service (AKS) clusters or other Azure infrastructure, Azure Linux is the default OS for many node pools. Currently, version 3.0 is the stable channel. When 4.0 reaches general availability, it will replace 3.0 as the new default. The DNF5 migration could break existing tooling, scripts, or container builds that depend on DNF commands or plugin behavior. That’s why Microsoft is urging evaluation now—so you’re not caught off guard when production upgrades arrive.

For teams that build custom images or use Azure Linux as a base for containers, the shift to DNF5 means you’ll need to update Dockerfiles and package installation logic. DNF5 aims for backward compatibility with most DNF commands, but it removes deprecated functions and enforces stricter transaction handling. Testing your build pipelines with this preview will surface any gaps early.

Developers who use Azure Linux locally can run the ISO in Hyper‑V, VirtualBox, or any hypervisor to verify application compatibility on the newer kernel and libraries. Just remember: this isn’t for production. Microsoft is explicit—stick with Azure Linux 3.0 for live workloads and test 4.0 only in isolated environments.

How We Got Here: A Quick History of Azure Linux

Azure Linux started in 2020 as CBL-Mariner, an internal Microsoft distribution built to have a consistent, secure, and streamlined OS for Azure services. It was described as a “cloud native infrastructure Linux distribution” focused on containers and the edge. The project went open source under the MIT license, with source code on GitHub.

In 2023, with version 2.0, Microsoft rebranded it to Azure Linux and began positioning it as a supported host for AKS and other Azure workloads. Version 3.0 followed, bringing newer kernels and package versions. Now, version 4.0’s public preview—especially the package manager swap—shows Microsoft’s commitment to transparency and early feedback. By adopting DNF5, Azure Linux tracks a significant upstream development, reinforcing that it’s not a fork that goes its own way.

What You Should Do Now

  1. Download the ISO from the official GitHub releases page (link below). Look for a file named azurelinux-4.0-preview-*.iso. Verify the checksum to ensure integrity.
  2. Spin up a VM. Azure Linux needs at least 1 GB of RAM and a 4 GB virtual disk; the minimal installer walks you through partitioning. For container-based testing, pull the container image from Microsoft Container Registry: mcr.microsoft.com/azurelinux/base/core:4.0-preview and run it with Podman or Docker.
  3. Focus your testing on:
    - Package management: Try installing, removing, and updating packages using dnf5 (the dnf command may still work as a symlink). Check that infrastructure‑as‑code scripts handle the transition.
    - Kernel compatibility: Load any custom kernel modules you rely on.
    - Application performance: Run typical workloads and compare behavior against Azure Linux 3.0.

Microsoft hasn’t published a GA timeline. Historically, releases have taken several months from preview to stable, but feedback could accelerate or slow that. Watch the Azure Linux GitHub repository for updates and a release candidate announcement.

Outlook

This preview offers a concrete look at Azure Linux’s future. The DNF5 migration signals that Microsoft wants the distro to stay current with the RPM ecosystem, and it gives teams a head start on compatibility testing. While production use is forbidden today, evaluating now can smooth the transition later. Block time for testing, monitor GitHub for the release candidate, and provide feedback—it may well shape the final OS.