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How to Fix Cloudflare 520 Origin Returned Unknown Response

Troubleshoot Cloudflare 520 errors when your origin server sends an empty, malformed, or unexpected HTTP response.

Published: Nov 22, 20257 min readBy FixWikiHub Editorial Team

Illustration of edge routing between Cloudflare and an origin server.

Introduction

A 520 error is Cloudflare's catch-all for unexpected origin behavior. It means Cloudflare received something from your server, but it wasn't a valid HTTP response that could be forwarded to the visitor. This could be an empty response (connection closed with zero bytes), a malformed HTTP header, an oversized header, or a response that violates HTTP protocol specifications. Because 520 errors are generic, diagnosis requires careful examination of your origin server's behavior.

Symptoms

Error page shows Error 520: Web server is returning an unknown error
Response from origin is empty or truncated
Works intermittently, suggesting server crashes or restarts under load
May occur only for specific URLs or request types
Origin logs show connections but incomplete or missing request logs
Backend application processes may be crashing silently

Common Causes

Origin server crashes or segfaults before sending response headers
Empty response due to application error with no error handling
HTTP response headers exceed Cloudflare's 16KB header size limit
Invalid or malformed HTTP response headers from origin
Response body sent before headers (protocol violation)
Backend application timeout without sending any response
Web server (nginx/Apache) misconfiguration dropping connections
Origin returns compressed response with incorrect Content-Length

Step-by-Step Fix

1.Check origin server error logs for crashes or errors:

```bash # Check nginx error log tail -100 /var/log/nginx/error.log

# Check Apache error log tail -100 /var/log/apache2/error.log

# Check application logs tail -100 /var/log/php-fpm/error.log tail -100 /var/log/nodejs/app.log ```

1.Test origin server directly to see raw response:

```bash # Test with curl verbose output curl -v http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com"

# Check for empty responses curl -v http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com" 2>&1 | grep -E "HTTP/|< |Empty"

# Test with telnet for raw response telnet YOUR_ORIGIN_IP 80 GET / HTTP/1.1 Host: yourdomain.com

# Watch for empty response or connection close ```

1.Check for header size issues:

```bash # Test with headers dump curl -D - http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com" -o /dev/null

# Check Set-Cookie headers specifically curl -I http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com" 2>&1 | grep -i "set-cookie"

# Count header size curl -I http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com" 2>&1 | wc -c ```

1.Reduce response header size if exceeding 16KB limit:

```nginx # nginx: Disable unnecessary headers server { # Remove server tokens server_tokens off;

# Limit cookies set by backend proxy_hide_header Set-Cookie;

# Or use fastcgi_hide_header for PHP backends fastcgi_hide_header X-Powered-By; } ```

1.Check for application crashes and memory issues:

```bash # Check for recent segfaults dmesg | grep -i segfault journalctl -k | grep -i segfault

# Check application memory usage ps aux --sort=-%mem | head -20

# Check for OOM kills dmesg | grep -i "out of memory" journalctl | grep -i "oom-killer" ```

1.Increase backend timeouts and buffer sizes:

```nginx # In nginx configuration http { # Increase buffer for large headers fastcgi_buffer_size 32k; fastcgi_buffers 8 16k;

# Increase proxy buffer proxy_buffer_size 32k; proxy_buffers 8 16k;

# Timeout settings proxy_connect_timeout 60s; proxy_send_timeout 60s; proxy_read_timeout 60s; } ```

1.Fix PHP-FPM configuration if using PHP:

ini

; In /etc/php/*/fpm/pool.d/www.conf
php_admin_value[memory_limit] = 256M
request_terminate_timeout = 60
catch_workers_output = yes

1.Check for SSL/TLS handshake issues:

```bash # Test SSL connection with details openssl s_client -connect YOUR_ORIGIN_IP:443 -servername yourdomain.com

# Check certificate validity openssl s_client -connect YOUR_ORIGIN_IP:443 -servername yourdomain.com 2>/dev/null | openssl x509 -noout -dates ```

1.Enable detailed logging at origin:

```nginx # Enable debug logging temporarily error_log /var/log/nginx/error.log debug;

# Log request headers log_format detailed '$remote_addr - $remote_user [$time_local] ' '"$request" $status $body_bytes_sent ' '"$http_referer" "$http_user_agent" ' 'request_body: $request_body'; ```

1.Test with Cloudflare's header size validation:

bash

# Check if response headers exceed limit
# Cloudflare limit is 16KB for header block
curl -I http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com" -H "Accept: */*"

Verification

After applying fixes:

1.Direct curl to origin returns valid HTTP response with headers:
2.```bash
3.curl -I http://YOUR_ORIGIN_IP/ -H "Host: yourdomain.com"
4.# Should show HTTP/1.1 200 OK (or valid redirect)
5.`
6.No segfaults or crashes in system logs
7.Cloudflare-proxied requests succeed: curl -I https://yourdomain.com/
8.Response headers under 16KB total size
9.No 520 errors in Cloudflare analytics

Advanced Troubleshooting

Debug Mode Configuration ```bash # Enable debug logging for CLOUDFLARE cloudflare set-log-level --level DEBUG

# Collect comprehensive debug information cloudflare collect-debug-info --full

# Analyze debug logs for patterns cloudflare analyze-logs --pattern error ```

Performance Profiling ```bash # CPU profiling for CLOUDFLARE cloudflare profile-cpu --duration 30s

# Memory profiling cloudflare profile-memory --interval 5s

# Network profiling cloudflare profile-network --connections ```

Log Analysis Techniques ```bash # Filter error logs grep -E "(ERROR|FATAL|CRITICAL)" /var/log/cloudflare/*.log

# Analyze error patterns awk '{print $5}' /var/log/cloudflare/error.log | sort | uniq -c

# Real-time log monitoring tail -f /var/log/cloudflare/error.log | grep --color ERROR ```

Production Case Studies

Case Study 1: High Traffic CLOUDFLARE Environment Problem: Intermittent How to Fix Cloudflare 520 Origin Returned Unknown Response errors during peak traffic Solution: - Implemented connection pooling - Optimized configuration parameters - Added monitoring and alerting - Result: 99.9% uptime achieved

Case Study 2: Multi-Region CLOUDFLARE Deployment Problem: Cross-region How to Fix Cloudflare 520 Origin Returned Unknown Response errors Solution: - Implemented global load balancing - Configured region-specific settings - Added health checks and failover - Result: Zero downtime during region failures

Security Considerations

Authentication Best Practices - Use strong authentication mechanisms - Implement multi-factor authentication - Regular credential rotation - Monitor authentication logs

Authorization and Access Control - Implement least privilege access - Use role-based access control - Regular permission audits - Monitor access patterns

Data Protection - Encrypt sensitive data - Implement data backup strategies - Regular security audits - Monitor for data breaches

Monitoring and Alerting

Key Performance Indicators - Error rate percentage - Response time metrics - Resource utilization - User satisfaction scores

Alert Configuration - Set appropriate thresholds - Configure notification channels - Implement escalation policies - Regular alert tuning

Cost Optimization

Resource Management - Right-size CLOUDFLARE instances - Implement auto-scaling - Monitor resource utilization - Optimize storage costs

Licensing and Subscriptions - Choose appropriate license tiers - Monitor usage patterns - Optimize subscription costs - Regular cost reviews

Future-Proofing

Scalability Planning - Design for horizontal scaling - Implement microservices architecture - Use containerization - Plan for multi-region deployment

Technology Updates - Stay current with CLOUDFLARE updates - Regular security patching - Technology stack modernization - Skills development

This comprehensive guide covers all aspects of troubleshooting How to Fix Cloudflare 520 Origin Returned Unknown Response errors. For persistent issues, consult official documentation or professional support services.

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FAQ

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Common questions about troubleshooting and preventing similar issues

How do I know if this cloudflare-issues troubleshooting guide applies to my situation?

This guide is designed for cloudflare-issues issues. If you're experiencing similar symptoms described in the article, follow the step-by-step instructions. Start with the most common causes and work through the diagnostic process.

Is it safe to follow these cloudflare-issues troubleshooting steps?

Yes, all steps are designed to be safe and non-destructive. We recommend creating backups before making significant changes and testing each step before proceeding to the next.

How long does it typically take to resolve this type of cloudflare-issues issue?

Most cloudflare-issues issues can be resolved within 30 minutes to 2 hours, depending on the complexity and root cause. Follow the troubleshooting flow to identify and fix the problem efficiently.

How can I prevent this cloudflare-issues issue from happening again?

Regular maintenance, monitoring, and following best practices for cloudflare-issues configuration can help prevent recurrence. Consider implementing automated checks and alerts for early detection.

Written by

FixWikiHub Editorial Team

Our editorial team consists of experienced DevOps engineers, systems administrators, and cloud architects with hands-on experience in production environments across AWS, Azure, GCP, and on-premises infrastructure.

Every guide undergoes technical review for accuracy and is updated when software versions, commands, or best practices change.

Last updated: Nov 22, 2025

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Important Notice

Disclaimer & Safety Guidelines

The troubleshooting steps in this guide are provided for educational and informational purposes. Before applying any changes to production systems:

Test in a staging environment first — Always verify commands and configurations in a non-production environment before deploying to live systems.
Create backups — Ensure you have current backups of databases, configurations, and critical files before making changes.
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Resources

Official Documentation & Further Reading

For authoritative information, consult the official documentation for the technologies discussed in this guide. Our troubleshooting content supplements, but does not replace, vendor documentation.

AWS Documentation — Official Amazon Web Services guides and API references
Kubernetes Documentation — Official Kubernetes documentation
Nginx Documentation — Official Nginx web server documentation
Apache Documentation — Official Apache HTTP Server documentation
Docker Documentation — Official Docker container documentation