Among all the error codes returned by the Cloudflare edge proxy, Error 520 (Web Server Returned an Unknown Error) is notoriously the most frustrating to diagnose.
Unlike Error 521 (which cleanly tells you the web server daemon is down) or Error 522 (which indicates a connection timeout), Error 520 is a Catch-All protocol mismatch.
It means that Cloudflare successfully established a TCP connection with your origin server and sent an HTTP request, but your origin server returned an empty, malformed, or corrupt response that violated the HTTP specification.
In this technical troubleshooting guide, we walk you through the diagnostic process to isolate the root cause on cPanel, Apache, and Nginx servers, and implement permanent production fixes.
🔍 The 4 Primary Root Causes of Cloudflare Error 520
When Cloudflare marks an origin response as “Unknown,” it almost always stems from one of four technical triggers:
| Root Cause | Technical Trigger | Typical Fix Location |
|---|---|---|
| Oversized HTTP Response Headers | Origin response headers exceed Cloudflare’s strict 16KB limit. | Tune cookie bloat, debug headers, or Nginx buffer directives. |
| PHP-FPM Process Segfault or Crash | PHP-FPM worker crashes mid-execution (SIGSEGV) without output. | Inspect /var/log/php-fpm.log and allocate more memory. |
| Premature TCP Connection Reset (RST) | Origin server or firewall drops connection before sending body. | Check web server keepalive_timeout and CSF firewall flood limits. |
| Completely Empty Response (0-Byte Reply) | Script dies silently without returning an HTTP status line. | Check WordPress PHP error logs and fatal syntax errors. |
🛠️ Step 1: Check for Oversized HTTP Response Headers (>16KB)
The #1 cause of Error 520 on WordPress and WooCommerce sites is oversized HTTP response headers.
Cloudflare enforces a strict architectural limit: the total combined size of all HTTP response headers cannot exceed 16KB (16,384 bytes).
If a bloated plugin (such as an unoptimized analytics tracker, social login tool, or session manager) writes dozens of massive Set-Cookie headers or excessive debugging headers, the header block breaches 16KB. Cloudflare immediately aborts the connection and throws Error 520.
How to Inspect Origin Headers via Terminal:
Bypass Cloudflare and curl your origin server directly via its real IP address:
curl -svo /dev/null -H "Host: yourdomain.pk" https://YOUR_ORIGIN_IP/Look at the response headers. If you see dozens of repeated cookies like this:
< Set-Cookie: woocommerce_items_in_cart=...
< Set-Cookie: wp_session_token=...
< Set-Cookie: ... [hundreds of lines]You have cookie/header bloat!
The Fix in Nginx (/etc/nginx/nginx.conf):
If your origin uses Nginx as a reverse proxy, increase internal FastCGI header buffers to prevent truncation, and strip unnecessary debug headers:
fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;
proxy_buffer_size 32k;
proxy_buffers 4 64k;
proxy_busy_buffers_size 64k;Reload Nginx:
nginx -t && systemctl reload nginx🛠️ Step 2: Diagnose Crashed PHP-FPM Worker Processes
If a PHP script encounters a segmentation fault (segfault), an infinite recursion loop, or exhausts system RAM:
- The PHP-FPM child worker process is killed instantly by the Linux kernel (
OOM KillerorSIGSEGV). - Because the worker died abruptly, it cannot send an HTTP 500 error code back to Apache/Nginx.
- Apache/Nginx sends an empty, truncated response to Cloudflare, triggering Error 520.
How to Check PHP-FPM Logs via SSH:
Inspect your PHP-FPM error log in real time:
# On Ubuntu/Debian:
tail -n 100 -f /var/log/php8.3-fpm.log
# On cPanel / AlmaLinux:
tail -n 100 -f /opt/cpanel/ea-php83/root/usr/var/log/php-fpm/error.logWhat to Look For:
If you see entries like:
WARNING: [pool www] child 14829 exited on signal 11 (SIGSEGV)
WARNING: [pool www] server reached pm.max_children settingYour PHP workers are either running out of memory or crashing on a broken PHP extension.
The Fix:
Increase worker pool capacity and memory limits in your pool configuration (/etc/php/8.3/fpm/pool.d/www.conf):
pm = dynamic
pm.max_children = 50
pm.start_servers = 10
pm.min_spare_servers = 5
pm.max_spare_servers = 20
pm.max_requests = 1000Restart PHP-FPM:
systemctl restart php8.3-fpm🛠️ Step 3: Check Keep-Alive Timeouts Between Cloudflare & Origin
Cloudflare maintains persistent TCP connections (Keep-Alive) with your origin server to accelerate subsequent visitor requests.
If your origin web server’s keepalive_timeout is configured too aggressively (e.g., 5 seconds), the server may send a TCP FIN or RST packet to close the socket at the exact instant Cloudflare transmits a new incoming request.
Cloudflare receives a TCP reset on an open socket and displays Error 520.
The Fix in Apache (httpd.conf / /etc/apache2/apache2.conf):
Ensure KeepAlive is enabled and timeout is set to at least 60 to 75 seconds:
KeepAlive On
MaxKeepAliveRequests 100
KeepAliveTimeout 75The Fix in Nginx (/etc/nginx/nginx.conf):
keepalive_timeout 75s;
keepalive_requests 1000;🛠️ Step 4: Verify Firewall Rate Limiting (CSF & iptables)
Many webmasters in Pakistan run ConfigServer Security & Firewall (CSF) on their cPanel servers.
If CSF’s Port Flood Protection (PORTFLOOD) or Connection Tracking (CT_LIMIT) modules are set to strict defaults:
- Because all visitor traffic arrives through a handful of Cloudflare proxy IP addresses, CSF flags Cloudflare as an active TCP flood attack.
- CSF silently drops active connections mid-transmission, causing Cloudflare to receive 0-byte responses and show Error 520.
The Fix:
Ensure all official Cloudflare IP ranges are added to /etc/csf/csf.ignore and /etc/csf/csf.allow, then reload:
csf -ra⚡ Eliminate Origin Proxy Errors with Nextgen Cloud VPS
High-traffic websites demand robust origin server hardware that never drops connections under load:
- Deploy on Nextgen Cloud VPS in Pakistan with dedicated KVM virtualization, tuned PHP-FPM pools, and pure NVMe storage.
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📚 Related SSL, Cloudflare & Troubleshooting Guides
- How to Fix Error 525 (SSL Handshake Failed) on Cloudflare & cPanel – Resolve origin TLS negotiation failures.
- Fix 502 Bad Gateway: Nginx & PHP-FPM Troubleshooting Guide – Stop PHP-FPM socket drops and upstream gateway errors.
- Fix 500 Internal Server Error in WordPress: Complete cPanel & Nginx Troubleshooting Guide – Step-by-step diagnostic guide for web server crashes.
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