Microsoft has introduced a public preview of Azure Linux 4.0, a significant update to its cloud-focused Linux distribution. This new version builds on a modernized software base, featuring Kernel 6.18 LTS, dnf5, glibc 2.42, OpenSSL 3.5, and Python 3.14. Initially, it is available for Azure Virtual Machines and VM Scale Sets, along with container images, with further support for Azure Kubernetes Service (AKS) and the Windows Subsystem for Linux (WSL) anticipated.
Azure Linux, which is based on Fedora, is specifically designed for cloud and server workloads. The aim is to provide a unified Linux base that standardizes operations across virtual machines, Kubernetes nodes, and containers, thus reducing the variety that companies need to manage and secure. This latest version has been employed internally at Microsoft since 2023 and includes applications such as AKS, Azure SQL, and Azure Cosmos DB.
The core upgrade of Azure Linux 4.0 involves a comprehensive update to its system base. The new Linux Kernel 6.18 LTS aims to enhance Hyper-V integration, providing better support for current GPUs and AI accelerators, in addition to updated drivers.
The updated packaging system, dnf5, has undergone a complete re-implementation in C++. This transition is expected to lead to quicker package resolution, reduced dependencies, and lowered memory usage. Added improvements include enhanced memory management and threading optimizations in glibc 2.42, as well as systemd 258, which is geared toward rapid system boot and effective service management. Furthermore, OpenSSL 3.5 provides initial support for post-quantum cryptography methods.
Alongside the release of Azure Linux 4.0, Microsoft is launching Azure Container Linux (ACL), a more secure and compliant variant for Kubernetes environments. Unlike the general Azure Linux version, ACL will not receive package updates; rather, it will be replaced with complete images, minimizing the attack surface. This operating model shares similarities with specialized container operating systems, such as Flatcar Linux or Fedora CoreOS, where SELinux is active in enforcement mode by default.
For conventional workloads, the standard Azure Linux remains available with full package management utilizing dnf5. Both versions will use the same kernel, accompanying security updates, and the same Azure integration.
Microsoft has emphasized the security features of this distribution, asserting that Azure Linux will support Secure Boot, Trusted Launch, SELinux, and a cryptographically secure supply chain featuring signed packages and Software Bill of Materials (SBOM). Certification for FIPS 140-3 is in progress and expected to align with the public release.
The maintenance cycle is designed to be predictable, with security updates scheduled monthly and new hardware support introduced annually via updated Hardware Enablement Kernels (HWE). Critical and high-priority security vulnerabilities will receive backported patches between regular updates.
Azure Linux 4.0 is currently available for Azure Virtual Machines, VM Scale Sets, and container base images in public preview. Microsoft plans to introduce AKS support, WSL integration, and language-specific runtime containers for Python, Node.js, Java, and .NET in the future. Importantly, there are no additional licensing costs for Azure Linux itself; only the Azure resources used will incur charges.
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