Microsoft on May 18, 2026 opened the public preview of Azure Linux 4.0 for Azure Virtual Machines, while simultaneously marking the general availability of Azure Container Linux, an immutable operating system purpose-built for container workloads. The dual rollout underscores Microsoft's deepening investment in Linux as a first-class citizen across its cloud platform.

Azure Linux, formerly codenamed CBL-Mariner, is Microsoft's own Linux distribution, originally built for internal use in services like Xbox, Azure Kubernetes Service, and Azure Arc. It emerged publicly in 2021 and has since become the recommended guest OS for many Azure workloads. Version 4.0 represents a significant refresh, transitioning to a more recent Long-Term Support (LTS) kernel and bundling updated system packages.

What is Azure Linux?

Azure Linux is a lightweight, security-hardened Linux distribution developed and maintained by Microsoft. It uses the RPM package manager and borrows packaging conventions from Fedora, but it is not a fork. The system is built from source with a minimal footprint, designed to boot quickly and run efficiently on Azure's hypervisor. It ships with a curated set of packages, omitting desktop environments and other utilities unnecessary in cloud infrastructure.

This "by Microsoft, for Azure" philosophy means Azure Linux integrates tightly with Azure services, offering optimized performance for networking, storage, and management agents. It supports both x86_64 and ARM64 architectures, allowing consistent experiences across VM types. Since its public debut, Azure Linux has been offered as a free guest OS option on Azure Virtual Machines, with automatic updates, security patches, and a predictable release cadence.

Azure Linux 4.0 Public Preview: What's New

The 4.0 release enters public preview on May 18, 2026, with new VM images available across all Azure regions. Microsoft has not yet published detailed release notes, but early indicators point to these enhancements:

  • Kernel version bump: Azure Linux 4.0 likely ships with Linux kernel 6.6 LTS or later, bringing improved hardware support, better I/O performance, and the latest security mitigations.
  • Updated system libraries: Core packages such as glibc, OpenSSL, and systemd receive major version upgrades, ensuring compatibility with modern applications and compliance with the latest cryptographic standards.
  • Enhanced Hyper-V integration: Refined drivers for Azure's virtualized network and storage stacks, potentially lowering latency and increasing throughput for premium VM SKUs.
  • Image size reduction: Continuing the trend of lean images, 4.0 may strip further unnecessary components, leading to faster boot times and smaller attack surfaces.
  • Security hardening: New compiler-based defenses, stricter kernel lockdown configurations, and automated update mechanisms via Azure's rolling update service.

Azure Linux releases follow an LTS model, with each major version supported for approximately two years of security updates. Version 4.0 succeeds the 3.0 branch and will eventually become the default recommended image for new deployments once it reaches general availability.

Azure Container Linux Goes GA

In parallel with the 4.0 preview, Microsoft announced that Azure Container Linux has reached general availability. This immutable OS variant is stripped to the essentials needed to run containers, with a read-only root filesystem and atomic image-based updates. It is the operating system that powers Azure Kubernetes Service (AKS) nodes, Azure Container Instances, and other container-centric services.

An immutable OS cannot be altered after boot; updates occur by downloading a new signed system image and rebooting into it. This model eliminates configuration drift, simplifies rollbacks, and drastically reduces patching complexity. Azure Container Linux supports both standard Linux containers and Windows containers via Hyper-V isolation, making it a versatile foundation for mixed-workload clusters.

The GA release brings the same kernel and package baseline as Azure Linux 4.0, ensuring consistency across VM and container environments. Key features include:

  • Declarative configuration: System settings are defined in YAML at provisioning time, not modified ad-hoc.
  • Atomic A/B updates: Two system partitions allow seamless failover if an update fails.
  • Integration with Azure Monitor: Built-in agents send telemetry and logs directly to Azure Monitor and Log Analytics.
  • Support for confidential containers: TEE-based attestation and encrypted memory for sensitive workloads.

By pairing Azure Linux 4.0 for general-purpose VMs with Azure Container Linux for containers, Microsoft offers a cohesive Linux stack that spans traditional IaaS and modern containerized applications.

Fedora Roots?

Azure Linux is often described as "Fedora-based," but this label requires context. The distribution uses RPM packaging, systemd, and many utilities that originated in the Fedora ecosystem. Microsoft's engineers follow Fedora's packaging guidelines where possible, and they pull source code from the Fedora project for numerous core packages. However, Azure Linux is not a derivative or fork of Fedora; it is an independent distribution built from the ground up by Microsoft's Linux Systems Group.

The Fedora resemblance is a deliberate choice: RPM and DNF are familiar to enterprise Linux administrators, and aligning with Fedora's package layout eases the porting of third-party software. Most RPMs built for Fedora or RHEL can be installed on Azure Linux without modification. This compatibility lowers the barrier for organizations migrating on-premises workloads to Azure.

How to Get Started

Azure Linux 4.0 Public Preview

To deploy a preview VM, navigate to the Azure portal, create a new virtual machine, and select "Azure Linux" from the image gallery. For programmatic deployments, use the Azure CLI:

az vm create \\
  --resource-group myResourceGroup \\
  --name myAzureLinuxVM \\
  --image Microsoft.AzureLinux:azure-linux:x86_64:4.0 \\
  --admin-username azureuser \\
  --generate-ssh-keys

Replace the image URN with arm64:4.0 to test on ARM-based instances. Microsoft recommends using this preview for development and testing; production workloads should remain on the 3.0 branch until 4.0 reaches GA.

Azure Container Linux

For AKS users, simply create a new node pool specifying --os-sku AzureLinux:

az aks nodepool add \\
  --resource-group myResourceGroup \\
  --cluster-name myAKSCluster \\
  --name containerpool \\
  --os-sku AzureLinux \\
  --node-vm-size Standard_D4s_v5

All new AKS clusters created with the Azure Portal will default to Azure Container Linux for agent nodes. Existing clusters can migrate node pools by creating new pools and draining old ones.

Impact on the Azure Ecosystem

The preview of Azure Linux 4.0 and the GA of Azure Container Linux solidify Microsoft's strategy of offering a vertically integrated Linux stack. This brings several benefits:

  • Reduced support complexity: With a single vendor for both the hypervisor and guest OS, Azure support engineers can troubleshoot issues faster without the usual finger-pointing between OS and cloud providers.
  • Cost optimization: Azure Linux images are free; there is no license cost. Combined with the efficient resource usage of a minimal OS, this can lower overall VM spend.
  • Security agility: Microsoft's direct control over the kernel and packages means zero-day patches can be tested and deployed within hours, not days.
  • Unified management: Tools like Azure Policy and Azure Automanage can natively configure and update Azure Linux instances, simplifying governance at scale.

For Windows enthusiasts, the growing prominence of Azure Linux might seem like a departure. In reality, it reflects Microsoft's long-term vision of Azure as a multi-platform cloud where Linux runs as many workloads as Windows. The same Hyper-V virtualization stack that powers Windows Server underpins Azure Linux VMs, and technologies like WSL2 have already brought the Linux kernel into Windows 10 and 11. Azure Linux is a natural extension of that convergence.

Looking Ahead

Microsoft has not announced a GA date for Azure Linux 4.0, but based on past releases, the preview phase may last three to six months. During this period, the team will collect telemetry, address bugs, and finalize the package set. Enterprise customers can influence the direction by filing issues on the Azure Linux GitHub repository and participating in the Azure feedback hub.

As immutable Linux distributions like Flatcar and Bottlerocket gain traction, Azure Container Linux positions Microsoft to compete directly in the container-optimized OS market. Its tight Azure integration and unified support model may prove decisive for shops already invested in the Microsoft ecosystem.

In the meantime, the public preview of Azure Linux 4.0 is an invitation to test, benchmark, and plan. For anyone running Linux on Azure, the availability of a Microsoft-maintained, Fedora-compatible, and free OS is a compelling proposition that only grows with each release.