Ubuntu 14.04 LTS PHP-FPM 502 Bad Gateway with Nginx

📖 ~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

Dynamic PHP pages fail with 502, breaking customer-facing web functions.

Environment & Reproduction

Typical in Nginx plus PHP-FPM deployments after config edits or package updates.

Root Cause Analysis

Nginx upstream points to missing socket/process or PHP-FPM pool crashes under load.

Quick Triage

Check php5-fpm process status, socket path, and Nginx error log entries.

Step-by-Step Diagnosis

Validate upstream endpoint consistency and inspect PHP-FPM pool and permission settings.

Solution – Primary Fix

Align socket configuration, restart services, and tune pool limits for workload stability.

Solution – Alternative Approaches

Switch to TCP upstream or isolate heavy PHP workloads into dedicated pools.

Verification & Acceptance Criteria

HTTP probes return 200 for PHP routes and error rate normalizes.

Rollback Plan

Reinstate previous Nginx and PHP-FPM configs from versioned backups.

Prevention & Hardening

Add health checks, config tests in CI, and resource alerts for FPM saturation.

Related Errors & Cross-Refs

Related to Apache proxy failures, MySQL outages, and disk full log issues.

Related tutorial: View the step-by-step tutorial for Ubuntu 14.04 LTS.

View all Ubuntu 14.04 LTS tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Nginx upstream docs, PHP-FPM tuning guides, and Ubuntu service debugging references.