Dealing with a web server that is down can be incredibly frustrating. Especially when all you get is a vague error message like error code 521. But don’t worry, this guide will explain what error 521 means, why you might be getting it, and most importantly – how to fix it.
What is Error Code 521?
Error 521 is a generic HTTP status code that indicates your web server is refusing connections. It’s also sometimes referred to as the “Web Server Is Down” error.
Specifically, error 521 means that the origin web server has rejected the connection from Cloudflare. This could happen for a variety of reasons, but most commonly it’s because:
- The web server is overloaded and cannot handle any more traffic
- There is a misconfiguration on the web server blocking requests
- The web server is temporarily down for maintenance
So in summary, 521 errors mean your website’s web server is up, but it is actively refusing connections.
Common Causes of Error 521
There are a few common issues that can trigger a 521 error on your website:
1. Web Server Overloaded
One of the most common triggers for a 521 error is simply having too much traffic for your webserver to handle. Most websites can only support a certain number of concurrent connections and requests.
If you get an unexpected spike in visitors, your web server could become overloaded and start rejecting connections. This leads to 521 errors for users trying to access your site.
2. Web Server Misconfigurations
Another common source of 521 errors is having the wrong firewall, server, or middleware configurations. For example:
- Blocking the Cloudflare IP ranges from accessing your server
- Having incorrect reverse proxy settings
- Failing to configure CORS (Cross-Origin Resource Sharing) properly
Misconfigured security and access policies can block legitimate connections from Cloudflare servers and return error 521.
3. Server Outages
If your web server itself crashes or needs to be rebooted, this can obviously prevent any connections and lead to error 521. Most hosting providers have at least some degree of downtime for server maintenance and upgrades.
Software bugs, hardware failures, power outages, and various other issues can all cause your web server to be temporarily unavailable and unable to respond to requests.
4. Domain Name Resolution Failures
For your web server to be reachable, your domain name needs to properly resolve to the server’s IP address. Any DNS issues, like incorrect A records, misconfigured reverse DNS, or domain registration problems can break this resolution.
When the domain fails to point to the correct web server IP, it will trigger 521 errors.
5. Network Connectivity Issues
Problems with the network connection between Cloudflare and your web server can result in error 521. This could be due to:
- Bad networking hardware like faulty routers, switches, or cabling
- Network congestion or saturation
- Routing issues causing packet loss
- Firewalls, ACLs, or Cloud WAFs blocking traffic
If the packets can’t make the trip from Cloudflare to your web server and back, all requests will fail with a 521 error.
How to Fix Error Code 521
Fixing error 521 on your website will require investigating which of these potential causes is the source of your problem. Here are the general steps you can take:
1. Check Web Server Health
First, check whether your origin web server itself is up and running properly. Log into the server and try accessing the application locally to verify it is responding.
If the server is down entirely, work with your hosting provider to get it back online. Rebooting hardware, troubleshooting software issues, and restarting services may help get the server running again.
2. Check for Resource Constraints
If the web server is up, the next step is checking for resource constraints. Monitor the application performance and server workloads to see if the server is overloaded or lacks compute resources.
Adding more RAM, and CPU cores, or optimizing database queries can help improve performance. Alternatively, you may need to scale up to a larger server instance size if the workload is too great.
3. Review Security Policies and Configs
Double-check that security groups, firewall policies, web server configs, and middleware settings allow traffic between Cloudflare and your origin server. Scan server logs to confirm requests are being received but rejected.
Tweaking iptables firewall rules, modifying application configs, and updating WAF settings may be required to permit access.
4. Verify Network Connectivity
Ping and traceroute from Cloudflare servers to your origin web server to test basic network connectivity. Check interface statistics for signs of network saturation or packet loss.
If needed, work with your data center or hosting provider to troubleshoot and repair any network infrastructure issues.
5. Validate DNS Resolution
Confirm that your domain name properly resolves to your web server’s public IP address. Check DNS records and registration details for any incorrect or outdated entries that could be misdirecting traffic.
Update any A records, CNAMEs, or name server (NS) records as needed to ensure proper name-to-IP mapping.
6. Try Testing From Different Locations
Another troubleshooting technique is to try accessing your website from different geographic regions. Use a VPN or proxy service to test from various locations.
If the 521 error only occurs in certain areas, it’s likely a routing-related issue between Cloudflare and your web server. Work with your hosting provider’s support team to diagnose.
7. Check Cloudflare Settings
Finally, go through your Cloudflare account settings and confirm that:
- Cloudflare is enabled for your domain
- Proxy status is set to “Proxied”, not “DNS Only”
- The original IP address is specified correctly
Double-check check these settings match the architecture and configs on your origin server as well.
Preventing Web Server Errors
Some best practices that can help avoid 521 errors and other connectivity problems include:
- Monitor server workloads and scale capacity proactively before overload
- Implement redundant servers and failover clustering for high-availability
- Enable health checks to route traffic away from unhealthy origin servers
- Setup network fault tolerance with multiple providers
- Validate DNS, firewalls, and proxy settings during config changes
- Use Cloudflare features like Load Balancing and Waiting Rooms to absorb traffic spikes
Catching server issues before they occur will provide the best user experience and limit downtime.
Conclusion
Error 521 indicates your web server is refusing connections and not available to respond to requests. Causes range from server overloads to DNS problems and network outages. By following a structured troubleshooting approach, you can identify the root cause and get services restored.
The key is using tools to monitor server health, double-check configurations, and test connectivity from end to end. With the right solutions in place, you can avoid or quickly resolve error 521, keeping your website stable and users happy.
