Ubuntu 18.04 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

Users receive 502 responses and dynamic pages fail to render.

Environment & Reproduction

Nginx with PHP-FPM backend on Ubuntu 18.04 web servers.

Root Cause Analysis

PHP-FPM not running, wrong socket path, or backend resource exhaustion.

Quick Triage

Check php-fpm service status and active fastcgi socket path.

Step-by-Step Diagnosis

Review nginx and php-fpm logs, verify pool config, and test socket permissions.

Solution – Primary Fix

Align fastcgi_pass with active socket, restart php-fpm, and reload nginx.

Solution – Alternative Approaches

Use TCP upstream for php-fpm where socket permission issues persist.

Verification & Acceptance Criteria

Application endpoints return 200 and upstream errors drop to zero.

Rollback Plan

Revert to previous pool and site config if performance degrades.

Prevention & Hardening

Add health checks and process limits tuned for expected traffic.

Related Errors & Cross-Refs

connect to unix failed, upstream prematurely closed connection, 502 bad gateway.

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 integration documentation and tuning references.