Troubleshooting Server Response Test Method for Cloudflare Error 521

If your site uses Cloudflare CDN, sometimes you may encounter a headache problem: an "Error 521 Web Server Is Down" pop-up when you visit a page. But this doesn't necessarily mean that the server is really down, it's likely that the server just isn't responding correctly to Cloudflare's requests.

Step 1: Check server firewall settings

The most common cause of Error 521 One of the reasons for this is that the server firewall is blocking Cloudflare's IP address. First make sure that the server allows Cloudflare requests.

Cloudflare will use certain IP addresses to connect to your site, these IP addresses are officially provided and you can go to the official documentation (Google "Cloudflare IP ranges" to find them) to see the full list.

Common Practice:

sudo ufw status

If you find that Cloudflare's IP is blocked, you can release it with the following command:

sudo ufw allow from  to any port 80,443

To save time, it is recommended to add all IP addresses provided by Cloudflare directly to thefirewallswhitelisting to avoid similar problems in the future.

Step 2: Check if the web service is running properly

Ensure that the web service(e.g. Apache or Nginx) is running properly and listening on the correct port.

Check the way:

Apache:sudo systemctl status apache2

Nginx:sudo systemctl status nginx

Make sure that the service state is active (running) and not stopped or reporting errors. If it is stopped, try restarting it:sudo systemctl restart apache2

maybe sudo systemctl restart nginx

Step 3: Manually Test Server Port Connectivity

You can simulate Cloudflare access directly with the command line tool curl or telnet to see how the server responds.

Test command (replace below with your server IP):

curl -I http://
curl -I https://

or use telnet to check the port:

telnet  80
telnet  443

If it connects normally, you will see a message back from the server indicating that the port is open and responding to requests normally.

Step 4: Bypass Cloudflare Authentication Server Response (Modify Local Hosts)

Another more intuitive way to test is to modify the hosts file of your local computer and directly access the server IP to see if the website can be opened normally:

The hosts file for Windows systems is at:C:\Windows\System32\drivers\etc\hosts

The hosts file for a Mac or Linux system is at:/etc/hosts

Add it at the end of the hosts file:Your server IP Your domain name

Example:123.123.123.123 example.com

Then save and flush the DNS cache:

Executed under Windows (Administrator CMD):ipconfig /flushdns

Performed under Mac:dscacheutil -flushcache

Once you've made the changes, use your browser to access your domain name (at this point it will directly access your server IP, bypassing Cloudflare) and see if the page displays properly:

If you can access it normally, the server side is fine and the problem is between Cloudflare and the server.
If you cannot access it, you need to check the server itself (ports, web service status, etc.).

Step 5: Check if the SSL certificate is OK (if using HTTPS)

If your site has enabled HTTPSThe problem may also occur with Error 521, which usually means that the server's SSL ConfigurationThere are questions.

You can run it server-side:

openssl s_client -connect Your server IP:443

Check to see if the server is correctly providing the SSL CertificateIf the SSL handshake fails, you may need to regenerate or repair the certificate. If the SSL handshake fails, you may need to regenerate or repair the certificate.

Step 6: Check Server Load

Sometimes, the server's CPU or memory usage is too high, which can also cause Cloudflare connections to time out and Error 521 to appear.

Use the command to view the server load:

top

If you find that the CPU or memory usage is close to 100%, it is possible that the server is underperforming or there is a process that is taking up too many resources, at which point you need to optimize the configuration or upgrade the server.

Final step: check that Cloudflare is configured correctly

In the DNS area of the Cloudflare panel, confirm:

Are the DNS records set up correctly and in the "orange cloud" state (Proxied).

Verify that the server IP address is correct and has not been entered incorrectly.

In the "SSL/TLS" settings, make sure:

SSL mode is set correctly (usually Flexible or Full)

Origin certificates and server-side certificates are configured consistently.

If all of the above are checked and the server itself responds normally, and Cloudflare is configured correctly, you've basically ruled out a server-side problem.

summarize

To troubleshoot Error 521, the point is to confirm that the server is rejecting the Cloudflare Check that the firewall is not blocking Cloudflare. Check to see if Cloudflare is blocked by your firewall, and make sure that your web service is running properly, that your ports are open, and that your SSL configuration is working correctly. If the server is responding normally, the problem probably lies in the Cloudflare settings. Troubleshooting item by item, basically can find the reason.

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