How to Fix Error Code 521: WordPress Cloudflare Error Troubleshoot

Key highlights

• Understand what triggers Error Code 521 and how Cloudflare-WordPress connection failures disrupt your site
• Learn step-by-step troubleshooting methods to diagnose and resolve server connectivity issues quickly
• Discover proven Cloudflare configuration adjustments that prevent future 521 errors from occurring
• Explore immediate fixes that restore your website’s connection and get your site back online
• Uncover advanced solutions for persistent server communication problems between Cloudflare and your WordPress host

Has your WordPress site suddenly gone dark with an Error Code 521 message? You’re not alone. This frustrating Cloudflare error occurs when your CDN can’t establish a connection to your origin server, leaving your visitors staring at a blank screen. Understanding how to fix Error Code 521 is crucial for maintaining your site’s uptime and credibility.

The good news? Cloudflare error troubleshooting doesn’t have to be complicated. While Error 521 seems intimidating at first, it’s actually one of the more straightforward connection issues to resolve once you understand what’s causing it.

In this comprehensive guide, we’ll walk you through proven methods on how to fix Cloudflare Error 521 on your WordPress site. You’ll learn how to identify the root cause, whether it’s a server configuration issue, firewall problem, or hosting service interruption.

What is Error 521 and which category does it belong to?

Error code 521 is a Cloudflare-specific status code that belongs to the 5xx server error category. Unlike client-side 4xx errors, this category indicates that the problem stems from the server side rather than the visitor’s browser or internet connection. Specifically, an Error 521 means that while your origin server is online, it is actively refusing the connection request from Cloudflare’s edge servers. This distinction is crucial for troubleshooting, as it confirms the server is operational but blocking the specific attempt to communicate.

While standard HTTP errors often provide generic messages like “Service Unavailable,” this custom Cloudflare error offers precise diagnostics about the breakdown in the TCP connection. In a typical setup, Cloudflare acts as a reverse proxy, forwarding visitor requests to your WordPress site. When the origin server—often due to strict firewall rules or a closed port 443—rejects this handshake, Cloudflare returns Error 521. This differs from a timeout (Error 522), where the server simply fails to respond, highlighting that the issue is a specific rejection of the connection attempt.

Understanding this technical context is essential when learning how to fix error code 521 effectively. Because the error confirms that your server is reachable but restrictive, you can bypass general connectivity checks and focus immediately on security configurations. Recognizing that this is a “refused connection” event rather than a server crash saves time and directs your efforts toward the correct solution: ensuring your hosting provider or firewall is configured to accept legitimate traffic from Cloudflare’s network.

Why should you care about Cloudflare error codes? 

Understanding and addressing Cloudflare error codes, such as Error 521, is crucial for several reasons: 

1. Website availability  

Cloudflare error codes serve as indicators of potential issues that can impact the availability of your website.  

Error 521 explicitly indicates a communication breakdown between Cloudflare and the origin server, resulting in downtime. 

Identifying and resolving these errors promptly is essential to ensure that your website remains accessible to visitors and minimize disruptions to your online presence. 

2. User experience  

Encountering error messages such as error 521 when trying to access your site can significantly impact visitors’ experience. 

These errors can frustrate users, reduce their trust in your website and potentially drive them to seek information or services elsewhere. 

Proactively addressing Cloudflare error codes can provide your audience with a seamless and reliable browsing experience, fostering positive interactions and engagement. 

3. SEO impact  

Website downtime or persistent errors can have detrimental effects on your search engine optimization (SEO) efforts. Search engines prioritize websites that offer consistent performance and accessibility to users. Therefore, unresolved Cloudflare error codes may result in lowered search engine rankings, reduced organic traffic and diminished visibility in search results. 

Promptly resolving these errors can help maintain a healthy website performance that supports your SEO strategies and attracts organic traffic. 

4. Business reputation  

The reliability and availability of your website reflect your brand’s reputation and credibility.  

When your website has frequent errors and downtime, it can harm your business’s image and make it seem unreliable to your audience. 

Taking care of Cloudflare error codes promptly shows that you are committed to having a stable and trustworthy online presence, which can boost customer confidence in your brand and services. 

5. Security concerns  

Sometimes, Cloudflare error codes might indicate security issues or mistakes that could put your website at risk. Ignoring these errors could make your site vulnerable to cyber threats, data breaches or other security risks. 

By actively monitoring and fixing Cloudflare error codes, you can make your website more secure and better protected against potential attacks, keeping sensitive information safe. 

What causes error 521 with WordPress and Cloudflare? 

Several factors can cause the occurrence of Cloudflare Error 521, including: 

• Origin server downtime: If the origin server hosting your website is down or unreachable, Cloudflare’s edge servers will be unable to establish a connection, resulting in Error 521.
• Firewall restrictions: Strict firewall rules or security settings on the origin server can sometimes block incoming connections from Cloudflare’s IP addresses, leading to Error 521. 
• Server configuration issues: Misconfigurations on the origin server, such as incorrect server settings, IP address restrictions or network configurations, may prevent Cloudflare from establishing a connection and trigger Error 521. 
• Traffic overload: Sudden spikes in traffic or high server load can overwhelm the origin server, preventing it from responding to requests from Cloudflare and resulting in Error 521. 
• DNS resolution problems: Issues with DNS resolution, such as incorrect DNS settings or delays in DNS propagation, can hinder Cloudflare from reaching the origin server, resulting in Error 521. 
• SSL/TLS certificate mismatch: If the SSL/TLS certificate installed on the origin server does not match the settings configured in Cloudflare, the handshake will fail, triggering Error 521. 
• Routing or network connectivity issues: Problems with routing or network connectivity between Cloudflare’s edge servers and the origin server can disrupt communication and cause Error 521. 

How Error 521 differs from other Cloudflare 5xx Errors?

While the 5xx series broadly indicates server-side problems, each Cloudflare error code points to a specific breakdown in the communication chain. Error code 521 is unique because it specifically signals that your origin server is online but actively refusing the connection request from Cloudflare, much like a security guard blocking an entrance. This stands in contrast to other errors where the server might be completely unreachable, actively crashing, or simply struggling to process requests in time.

Understanding these nuances is vital for effective troubleshooting. For example, treating a 521 refusal like a timeout issue could lead you to waste time optimizing site speed when the real culprit is a firewall rule. The table below compares Error 521 against other common status codes to help you identify exactly where the connection is failing.

Error code	Meaning	Key difference from 521
521	Web Server Is Down	Origin server refuses the connection (e.g., firewall block).
520	Web Server Returns an Unknown Error	A catch-all for invalid or empty responses, not a refusal.
522	Connection Timed Out	The server is too slow to establish a connection (handshake).
523	Origin Is Unreachable	Network routing issues prevent finding the server.
524	A Timeout Occurred	Server connects but takes too long to send data back.
525	SSL Handshake Failed	Failure occurs specifically during SSL/TLS encryption setup.
526	Invalid SSL Certificate	The server’s certificate is invalid, expired, or untrusted.

How to fix the Cloudflare error 521 in five steps? 

To fix Cloudflare Error 521, ensure your server is online, whitelist Cloudflare IPs, enable port 443, install a Cloudflare origin certificate and contact your hosting provider for support. Follow these five key steps:

1. Check whether your server is offline
2. Whitelist all of Cloudflare’s IP addresses
3. Ask your hosting provider to enable port 443
4. Create and upload a Cloudflare origin certificate
5. Contact your hosting provider for further support

Let’s explore all steps one by one in more detail:

Step 1: Check whether your server is offline 

Cloudflare Error 521 often happens when your origin server is down or unreachable. Here’s how to check your server status:

1. Server availability check

Access your server directly by entering the server’s IP address or domain in a web browser. A successful response indicates that the server is functioning correctly. If you can reach your server directly, the issue may lie in the communication between Cloudflare and the origin server. 

2. Check server logs

Review the server logs to identify any recent errors or issues impacting its availability. Look for error messages indicating server downtime or connectivity problems. Analyze the logs for indications of high server load, network issues, firewall blocks or other factors that could prevent Cloudflare from reaching the server. 

3. Verify firewall settings

Ensure that your server’s firewall settings allow incoming connections from Cloudflare’s IP addresses. Check for any rules blocking Cloudflare’s requests and adjust the firewall configuration accordingly. Whitelist Cloudflare’s IP ranges to allow traffic from their network to reach your server without being blocked by security measures. 

4. Test server response

Run diagnostic tools or commands like ping or traceroute to test the response time and connectivity between Cloudflare’s edge servers and your origin server. Verify that the server responds to Cloudflare requests promptly and that there are no network issues causing delays or interruptions. 

5. Update DNS settings

Ensure that your DNS settings point to the correct IP address of your origin server. Make any necessary adjustments to the DNS records to ensure that Cloudflare can successfully resolve the server’s address. Check for any DNS propagation issues that may be delaying the resolution of your server’s IP address and update the settings as needed. 

Resolving DNS_PROBE_FINISHED_NXDOMAIN errors

The DNS_PROBE_FINISHED_NXDOMAIN error indicates that the Domain Name System cannot resolve your domain to an IP address. Unlike Error 521, where the server refuses the connection, this error means the browser cannot locate the server at all. This is often caused by outdated local cache or DNS propagation delays. Follow these steps to resolve the issue:

1. Clear browser cache: In Chrome, enter chrome://net-internals/#dns in the address bar and click “Clear host cache.”
2. Flush system DNS: On Windows, run ipconfig /flushdns in the Command Prompt. For Mac, use sudo dscacheutil -flushcache in the Terminal.
3. Check configurations: Verify your DNS records in the Cloudflare dashboard and confirm nameserver settings with your registrar.
4. Wait for propagation: Global DNS updates can take up to 48 hours to fully propagate.

Most DNS errors are temporary and easily fixable. Following these steps usually clears the cache and restores access to your site.

Step 2: Whitelist all of Cloudflare’s IP addresses  

Whitelisting Cloudflare’s IP addresses on your server is important in resolving Cloudflare Error 521 and ensuring smooth communication between Cloudflare’s edge servers and your origin server. 

Here’s how to effectively whitelist Cloudflare’s IP addresses: 

1. Obtain Cloudflare’s IP ranges

Cloudflare provides a list of IP address ranges used by their edge servers. This list is in the Cloudflare dashboard under the Firewall or Network settings. Make sure to have the most up-to-date list of Cloudflare’s IP ranges to whitelist all the relevant addresses. 

2. Access server firewall settings

To access the firewall settings, log in to your server’s control panel or terminal. You may use tools like iptables (Linux), Windows Firewall (Windows Server) or a third-party firewall management interface. 

3. Whitelist Cloudflare IP addresses

Add rules to allow incoming connections from Cloudflare’s IP ranges. Depending on your server setup, you can create specific rules to permit HTTP (port 80) and HTTPS (port 443) traffic from Cloudflare’s IPs.

Ensure that the rules are configured to accept incoming connections from Cloudflare’s IP addresses and that they do not conflict with existing firewall rules. 

4. Test connectivity

After whitelisting Cloudflare’s IP addresses: 

• Test the connectivity between Cloudflare’s edge servers and your origin server. 
• Use tools like cURL or online services to send requests from Cloudflare’s network to your server and verify that the connections are successful. 
• Monitor the server logs for any blocked requests or errors that could indicate issues with the whitelisting configuration. 

5. Regularly update IP whitelists

Cloudflare periodically updates its IP address ranges as it expands its network. It is essential to stay informed about these updates and adjust your whitelist accordingly to ensure continued communication with Cloudflare’s servers.

Set up a process to regularly review and update the whitelisted IP addresses to maintain secure and reliable connectivity between Cloudflare and your server. 

Step 3: Ask your hosting provider to enable port 443  

When encountering Cloudflare error 521, reaching out to your hosting provider to enable port 443 can be a crucial step in resolving the issue and ensuring secure communication between Cloudflare’s edge servers and your origin server.

Here’s an overview of how you can effectively address this point: 

1. Identify the issue

Before contacting your hosting provider, make sure that Error 521 is caused by port 443 being disabled on your server. Port 443 is crucial for HTTPS connections, and enabling it will help establish secure communication channels. 

2. Contact support

Contact your hosting provider’s support team through live chat, email or phone support. Clearly explain the issue with Error 521 and request help enabling port 443 on your server. 

3. Provide server details

When contacting your hosting provider, share relevant server information such as the server type (e.g., Apache, Nginx), server configuration and operating system. This information will help the support team accurately enable port 443 on your server. 

4. Request port activation

Emphasize enabling port 443 on your server to facilitate secure HTTPS connections. Stress that enabling port 443 is crucial for resolving Error 521 and ensuring secure data transmission between Cloudflare and your server. 

5. Verify activation

After your hosting provider activates port 443, confirm that the port is open for HTTPS traffic. Use network diagnostic tools or commands to check the status of port 443 and ensure that it is accessible for secure connections. 

6. Test connectivity

Once port 443 is enabled, test the connectivity between Cloudflare and your origin server by initiating HTTPS requests. Monitor network logs or use online tools to confirm that HTTPS traffic is successfully reaching your server, indicating that port 443 activation has resolved Error 521. 

Step 4: Create and upload a Cloudflare origin certificate  

To fix SSL-related causes of Error 521, generate a Cloudflare origin certificate and install it on your server to enable secure HTTPS communication with Cloudflare. Here’s how:

1. Generate a Cloudflare origin certificate 

a. Start the certificate creation process: Log in to your Cloudflare account, select the affected domain, then navigate to SSL/TLS → Origin Server. Click ‘Create Certificate’ to begin generating a new Cloudflare origin certificate.

b. Configure certificate settings: Choose Cloudflare to generate both the private key and CSR for you. Confirm your domain appears under Hostnames, select the certificate validity period and ensure the key format is set to PEM. Once verified, click ‘Create’.

c. Copy and install the certificate on your server: After the certificate is generated, Cloudflare will display the origin certificate and private key. Copy both and install them on your web server. This completes the secure SSL connection between Cloudflare and your origin.

2. Download the certificate files

After creating the Cloudflare Origin Certificate, download the certificate files to your local system. These files typically include the certificate itself, the private key and any intermediate certificates necessary for the chain of trust. 

3. Upload the certificate to your server

Access your server’s control panel or SSH terminal to upload the Cloudflare Origin Certificate files. Depending on your server configuration, you may need to place the certificate files in specific directories or upload them through the server interface. 

Also, ensure that you securely transfer the certificate files to your server and that the private key remains confidential and protected. 

4. Configure SSL/TLS settings

Once the certificate files are uploaded to your server, configure your SSL/TLS settings to associate the Cloudflare Origin Certificate with your domain. Update the server configuration files (e.g., Apache config, Nginx config) to use the certificate for HTTPS connections. 

5. Restart the server

After updating the SSL/TLS settings, restart your web server to apply the changes and activate the Cloudflare Origin Certificate. This step ensures the server recognizes the new certificate and establishes secure connections using HTTPS on port 443. 

6. Test connectivity and encryption

Verify that the Cloudflare Origin Certificate is correctly installed by testing the HTTPS connectivity between Cloudflare and your origin server. Use online tools or browsers to confirm that secure HTTPS connections are established without errors. 

Next, check the browser address bar for encryption details to ensure that the connection is secure and that the Cloudflare Origin Certificate is being utilized. 

Step 5: Contact your hosting provider 

If previous steps don’t resolve the issue, your hosting provider may need to step in. They can check server-side configurations, firewall rules or blocked ports causing Error 521.

1. Identify the issue

Before contacting your hosting provider, make sure that Error 521 is caused by server-related issues or misconfigurations. Understanding the root cause will help you explain the problem effectively to the support team. 

2. Choose the communication channel

Contact your hosting provider through their support channels, which may include live chat, email, ticketing systems or phone support. Select the communication method that best suits your urgency and preference for resolving the issue. 

3. Describe the error

Clearly describe the Cloudflare Error 521 you are experiencing and provide specific details, such as when the error started occurring, the impact on your website’s availability and any troubleshooting steps you have already taken. 

4. Request assistance

Ask your hosting provider to assist in investigating the connectivity issues between Cloudflare and your server. Request them to check server logs, firewall settings and server configurations to identify any issues causing Error 521. 

5. Provide server details

Share relevant server information with the support team, including your server type (e.g., Apache, Nginx), hosting plan and any specific configurations that may affect communication with Cloudflare’s edge servers. 

6. Collaborate on solutions

Work collaboratively with your hosting provider to troubleshoot and resolve the connectivity issues causing Error 521. Follow their instructions, provide any additional information they request and implement suggested solutions to address the problem effectively. 

7. Follow up and verify fixes

After your hosting provider takes action to resolve the issue, follow up to ensure that the connectivity problems are resolved. Test the connection between Cloudflare and your server to verify that Error 521 has been successfully addressed. 

8. Seek additional guidance

If your hosting provider’s initial solutions do not resolve Error 521, seek additional guidance or escalate the issue if necessary. Your hosting provider’s support team should assist you in finding a resolution that ensures seamless communication with Cloudflare. 

When dealing with Error 521 issues, Bluehost’s 24/7 technical support team can quickly diagnose and resolve connectivity problems between your server and Cloudflare. Our specialists examine server configurations, firewall settings and port restrictions to identify the root cause. We provide temporary workarounds and maintain detailed documentation throughout the process to keep your WordPress site running smoothly.

Contact Bluehost 24/7 customer support now for expert assistance with your Error 521 issue and get your site back online fast.

Common Error 521 scenarios and quick solutions

Before diving into complex diagnostics, review these frequent real-world scenarios where error code 521 often appears. In many cases, a recent change triggers the connection refusal, and a simple adjustment can restore connectivity immediately. Compare your situation to this checklist; if these quick fixes do not resolve the Cloudflare error, proceed to the comprehensive five-step diagnosis guide below.

Symptom	Likely cause	Quick solution
Error after plugin update	Plugin conflict crashing service	Deactivate recent plugins via FTP or File Manager.
Error after site migration	DNS points to old IP	Update A record to the new server IP address.
Error after firewall changes	Blocked Cloudflare IPs	Whitelist all Cloudflare IP ranges in your firewall.
Error during traffic spike	Server resource overload	Check CPU/RAM; upgrade plan or optimize caching.
Error after HTTPS setup	Port 443 closed	Contact host to open port 443 for secure connections.
Server maintenance	Web server service stopped	Restart Apache/Nginx service or contact support.

Final thoughts

Successfully resolving Error Code 521 and other Cloudflare errors requires understanding the connection between Cloudflare’s edge servers and your origin server. By following the systematic troubleshooting steps outlined in this guide from checking server status and firewall settings to verifying SSL certificates, you can effectively fix Cloudflare error issues and restore functionality.

Remember, how to fix Error Code 521 isn’t just about quick fixes; it’s about implementing long-term solutions for stable website performance.

Need additional assistance with Cloudflare error troubleshooting? Connect with Bluehost 24/7 customer support team is ready to help you resolve connectivity issues.

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