Nginx 502 Bad Gateway with PHP-FPM

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

Web requests return HTTP 502, disrupting PHP-based applications.

Environment & Reproduction

Ubuntu 18.04 running Nginx with PHP-FPM over socket or TCP upstream.

Root Cause Analysis

PHP-FPM worker pool is down, overloaded, or misconfigured versus Nginx upstream settings.

Quick Triage

Check php-fpm service state, socket path, and recent error log entries.

Step-by-Step Diagnosis

Validate pool config, process limits, and Nginx fastcgi pass directives.

Solution – Primary Fix

Restart and tune PHP-FPM, align socket permissions, and reload Nginx config.

Solution – Alternative Approaches

Use TCP upstream with health checks or scale workers for traffic spikes.

Verification & Acceptance Criteria

Application endpoints return 200 responses and no new upstream connection errors appear.

Rollback Plan

Restore prior php-fpm pool settings if new tuning introduces instability.

Prevention & Hardening

Monitor worker saturation and configure alerting for fastcgi upstream failures.

Related Errors & Cross-Refs

connect() to unix socket failed, upstream prematurely closed connection, and 504 timeouts.

Related tutorial: View the step-by-step tutorial for Ubuntu 18.04 LTS.

View all Ubuntu 18.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Nginx and PHP-FPM tuning documentation for production stability on Ubuntu.