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New release on November 11, 2025

Fluent Bit v4.2.0 is out!

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Release Notes v4.2.0

Fluent Bit is a Fast and Lightweight Telemetry Agent for Linux, BSD, MacOS and Windows. We are proud to announce the availability of Fluent Bit v4.2.0.

Fluent Bit v4.2.0
* Copyright (C) 2015-2025 The Fluent Bit Authors
* Fluent Bit is a CNCF graduated project under the Fluent organization
* https://fluentbit.io



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_|   |_|__,_|___|_| |_|__| ____/|_|__|   _/     |_(_)_____/


         Fluent Bit v4.2 – Direct Routes Ahead
     Celebrating 10 Years of Open, Fluent Innovation!



[2025/11/12 02:05:19.470120000] [ info] [fluent bit] version=4.2.0, commit=6bc014390c, pid=24361
[2025/11/12 02:05:19.470475000] [ info] [storage] ver=1.5.4, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/11/12 02:05:19.470483000] [ info] [simd    ] disabled
[2025/11/12 02:05:19.470487000] [ info] [cmetrics] version=1.0.5
[2025/11/12 02:05:19.470493000] [ info] [ctraces ] version=0.6.6
[2025/11/12 02:05:19.470863000] [ info] [output:stdout:route_default] worker #0 started
[2025/11/12 02:05:19.470880000] [ info] [sp] stream processor started
[2025/11/12 02:05:19.470902000] [ info] [engine] Shutdown Grace Period=5, Shutdown Input Grace Period=2

For people upgrading from previous versions you must read the Upgrading Notes section of our documentation:

https://docs.fluentbit.io/manual/4.2/installation/upgrade_notes

Introduction

Fluent Bit, a CNCF graduated project under the umbrella of Fluentd, announces the availability of v4.2.0.

In every release, there are many improvements and fixes, on this notes we will refer to the major changes that will make your infrastructure happier ;)


🔥 Highlights


New Features


Direct Routing

The v4.2 release introduces a powerful new Direct Routing capability that allows inputs to specify routes directly to outputs, bypassing the traditional routing mechanism. This enables more efficient and predictable data flow patterns, with support for:

  • Conditional Routing: Route logs based on conditions and context variables
  • Label-based Matching: Match routes using labels and plugin names stored in chunk metadata
  • Route Persistence: Direct routes are preserved across chunk restores and reloads
  • YAML Configuration Support: Configure input routes and outputs directly in YAML format

This feature provides greater flexibility and control over how data flows through your Fluent Bit pipeline, enabling more sophisticated routing scenarios.

Dead Letter Queue (DLQ)

A new Dead Letter Queue feature has been added to preserve invalid chunks for later verification and troubleshooting. This helps with:

  • Data Preservation: Invalid chunks are preserved instead of being lost
  • Troubleshooting: Investigate and debug problematic data without losing it
  • Filesystem Storage: DLQ support for filesystem chunks with safe deletion checks
Routing Metrics & Observability

Enhanced observability with comprehensive routing metrics:

  • Routing Metrics: Track routing performance, matched/unmatched logs, and dropped events
  • Metrics Export: Routing metrics are now integrated into the /metrics endpoint
  • Context-based Routing: Support for context variables in conditional routing for more dynamic routing decisions
OpenTelemetry Enhancements

Improved OpenTelemetry support with:

  • Unified Trace JSON Parser: New unified interface for OpenTelemetry trace handling
  • Error Status Propagation: Better error handling and status propagation for traces
  • Strict ID Decoding: Enhanced validation for log and trace IDs

Plugin Improvements


  • Tail Input: Long line truncation support for handling extremely long log lines
  • Syslog Input: Octet counting support for improved RFC compliance
  • Forward Input: Fixed username parsing and authentication issues
  • Node Exporter Metrics: Added netstat Linux collector
  • Systemd Input: Explicit restarts for journal data cursor
  • Kubernetes Filter: Improved EKS environment detection using service account issuer
  • HTTP Output: Added PUT method support
  • Vivo Exporter: Updated API endpoints to /api/v1/ with groups support
  • S3 Output: Fixed segmentation fault on dry-run
  • OpenSearch Output: Improved AWS signature handling on retry/error

⚙️ General Improvements


  • Storage: Fixed heap corruption in chunk sort parser, improved chunk state management
  • Reload: Fixed race condition between watchdog start and async cancellation
  • WASM: Fixed heap leakages in WASM plugin execution
  • TLS/OpenSSL: Implemented certificate thumbprint handling for better certificate management
  • Thread Storage: Improved thread-local storage initialization using pthread_once
  • Windows: Enhanced Windows container support with improved DLL handling and vcpkg upgrades
  • Packaging: Added missing SASL dependencies for Rocky/AlmaLinux, fixed GPG key import with redirect support
  • Build System: Added strict pointer types option for enhanced code safety

Libraries

  • chunkio: upgrade to v1.5.4
  • ctraces: upgrade to v1.0.5
  • monkey: upgrade to v1.8.5
  • wamr: upgrade to v2.4.1

Contributors

On every release, there are many people involved doing contributions on different areas like bug reporting, troubleshooting, documentation and coding, without these contributions from the community, the project won’t be the same and won’t be in the good shape that it is now. So THANK YOU! to everyone who takes part of this journey!

Join us

We want to hear about you, our community is growing and you can be part of it!, you can contact us at:

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