HTTP troubleshooting
521 Web Server Is Down
Your uptime monitor detected that the request reached the Cloudflare edge network, but Cloudflare was blocked or unable to establish a basic TCP connection to your origin backend web server.
What 521 Status Code Means
- 1.
Origin server crash
The primary hosting server or web software has completely stopped running or dropped offline.
- 2.
Firewall connection block
A local server firewall or security plugin is blocking Cloudflare IP addresses, misidentifying them as an attack.
- 3.
Uptime monitor behavior
Our monitor isolates the crash directly to your backend host, as the proxy network itself is responsive.
Troubleshooting checklist
- 1.
Check origin host status
Log into your hosting dashboard directly to see if the main server machine is powered on and running.
- 2.
Inspect security access logs
Review fail2ban, iptables, or local security logs to check if Cloudflare proxy IPs were banned.
- 3.
Verify web service daemons
Confirm that NGINX, Apache, or your Node app instance is active and listening on the expected ports.
- 4.
Check server port openings
Ensure ports 80 and 443 are fully open and accepting external incoming connections on the host machine.
How to Fix It
- 1.
Restart the web server
Reboot your hosting software stack to clear stuck connection processes and open up listeners.
- 2.
Whitelist proxy network IPs
Add all official Cloudflare IP ranges to your local firewall and hosting provider safelists immediately.
- 3.
Adjust host security levels
Tune down aggressive local rate limiters that might trigger blocks during heavy monitoring pings.
- 4.
Fix internal server routing
Correct any local loopback network bugs that prevent the web daemon from binding to public network slots.
After the Service is Fixed
- 1.
Verify with Uptime Monitor
Confirm that your uptime monitoring charts return to green once the origin host opens back up.
- 2.
Monitor network traffic levels
Watch inbound connection volume to verify that local systems are not dropping connections under heavy loads.
- 3.
Automate service checks
Set up internal server watchdog scripts to automatically revive dead web daemons before edge blocks trigger.
- 4.
Maintain security safelists
Update your local network rules automatically whenever your reverse proxy provider adds new IP blocks.