Diagram explaining Error 521 on cloud hosting, showing connection issues between Cloudflare and origin server

Error 521 is one of the most common issues with website owners using cloud hosting and Cloudflare. In this article we will look at everything you need to know about Error 521, what caused the issue, and some actual solutions to get your site back live as fast as possible.

What Is Error 521?

Error 521 (aka “Web Server Is Down”) is most frequently due to the inability for Cloudflare to connect to your origin server. Error 521 is an HTTP error, like other errors before it, but it specifically means there is a failure in communication between the edge servers on Cloudflare’s network and the infrastructure with your hosting provider.

When users experience Error 521, they see a screen that tells them “The web server is not returning a connection, and as a result, the web page is not displaying”. If this problem isn’t fixed quickly, Error 521 can severely impact your website’s accessibility and user experience, and also your search engine ranking.

Common Causes of Error 521

It’s important to understand the reasons for Error 521 so that you can apply sensible fixes. Below are the most common causes of the Error 521:

List of common causes for Cloudflare Error 521, including server downtime, firewall blocking Cloudflare IPs, and SSL misconfiguration

1. Origin Server Down

The most simple possibility, is if your web server down, outright, or unresponsive, for a variety of reasons:

Server Maintenance, or Liver Updates

Failures of Hardware

Data Center Power Failures

Expired Resources: (CPU, memory, disk space)

Server Crash/System Instability

2. Firewall Configurations

Misconfigured firewalls will block Cloudflare’s IP’s which can lead to communication issues:

Server Firewall Blocking Cloudflare IP’s

Network-level Firewall blocking

Security Plugins that block normal traffic

Rate Limiting Rules impose Cloudflare Requests

3. DNS Configurations

If DNS settings are incorrect doesn’t send Cloudflare to the correct server, or send it to a non-existing endpoint:

Stale (A instructions) point to old server IP’s

Expired or Missing DNS Entries

TTL (Time to Live) are preventing propagation delay

Mis-defined CNAME records

4. SSL/TLS Certificate Problems

Certificate problems can also prohibit secured connections:

The SSL certificate has expired.

A self-signed certificate from Cloudflare was not trusted.

There were problems with the certificate chain.

There were compatibility issues with your SSL/TLS settings.

5. Connection Problems

There could be potential problems with the infrastructure Cloudflare has to make a connection with your host.

Local Internet service problems.

Routing issues.

DDoS attacks on your server.

Rate congestion or dropped packets.

6. Server Resource Problems

If all the limits on server resources have been reached it may not be operable:

Traffic spikes have created very high CPU utilization.

Limits have been reached for memory.

Limits have been reached for disk space.

Limits have been reached to the database connection limits of the database system.

Step By Step Solutions

Error 521 Solutions

Infographic showing solutions to resolve Error 521, such as server restarts, firewall rules update, and SSL mismatch fixes

Solution 1: Check the Status of your Server

To check if your origin server is online, you can do one of these steps:

Directly access your server. To see if you can directly access your site using the server address.

Access your hosting control panel. To simply log on to the control panel from your hosting provider and access the server status.

Use server monitoring services. Services like Pingdom, UptimeRobot, etc to see if the origin server is accessible.

Contact the host provider. If the server cannot be accessed, contact the hosting provider immediately.

Solution 2: Check Cloudflare Settings

Check the values you entered into the Cloudflare setting

1. DNS Records: Make sure your A records are pointing to the correct IP address of your server
2. SSL Mode: Verify that your site’s SSL/TLS encryption level is consistent with your server level
3. Proxy Status: Ensure key records are proxied through Cloudflare (using the orange cloud icon)
4. Origin Rules: Go through any origin rules that may block connectivity

Solution 3: Whitelist Cloudflare IP Ranges

Whitelisting Cloudflare IP addresses in server firewall to prevent blocking and ensure proper website connection

Whitelist Cloudflare’s IP ranges with server firewall:

1. What are the Current IP Ranges: Refer to Cloudflare IP Ranges page for the most updated IP4 and IP6 addresses
2. Server Firewall: Whitelist Cloudflare IPs in server firewall
3. Security Plug-ins: Whitelist Cloudflare IPs in WordPress related security plug-ins
4. Network Firewalls: Ensure network-level firewalls are not blocking Cloudflare traffic

Solution 4: Examine SSL Certificate Settings.

When troubleshooting certificate problems you may want to think about looking at:

1. Expired or Valid: Look to see if your SSL cert has expired and is valid
2. Certificate Chain: Look to see if you have a full chain of certs installed in full
3. SSL Mode: Make sure Cloudflare’s SSL Mode is the same as your server ssl
4. SSL Certificate Authority: Make sure you are using certs from a trusted Certificate Authority that Cloudflare recognize

Solution 5: Look at Server Resource Use.

When troubleshooting resource use you may want to think about looking at:

1. Resource Use: Check CPU, Memory and Disk Use
2. Database Optimisation: Clean up and optimised database and be more efficient with resource use
3. Caching: Use Caching on your server to lower resource use
4. Upgrade Hosting: If you see resource usage exceed limits or near the max host it may be worthwhile to upgrade hosting if appropriate

Solution 6: Tune the Configuration of Your Servers

Modify server configurations for improved compatibility with Cloudflare:

1. Keepalive: Increase the number of keepalive timeout values
2. Connection Limits: Adjust the limit on max number of connections
3. Timeout Values: Configure settings for the appropriate timeout value values
4. Load balancer: If you are on a high traffic website, utilize load balancing.

Advanced Troubleshoot Diagnostics

Utilize the Cloudflare analytics

Use Cloudflare analytics to find trends:

Error Analytics: Analyze error values, and times of peaks of errors.

Traffic: Detect patterns of traffic spikes, that may correlate to errors.

Performance: Check response time metrics, and average success rates of your connections.

Log File Review 

Review your server logs to see what you might find:

Access Logs: Look for trends in your failed requests. 

Error Logs: Identify if there were server-side errors from logs concurrent with the time of each error521 incident.

Cloudflare Logs: You can use Cloudflare Logs (an enterprise only feature) to conduct a more in-depth analysis. (Cloudflare Logs are very help and combination of Cloudflare logs and server access logs have solved problems for my customers). 

Network Diagnostics

Network diagnostics interface showing connection status and server response times

Conduct network-level diagnostics:

Traceroute: Trace the path between Cloudflare, and your server. 

Ping: Do Ping tests to confirm basic connectivity to your server

Port Scans: Verify that the ports needing to be open are indeed open, and that your server can be reached.

Avoidance Techniques

Looking and Alerting

Use proper monitoring tools:

Uptime Monitoring: Notify me about downtime of my server.

Resource Monitoring: Track CPU, RAM, and Disk Usage.

SSL Certificate Monitoring: Notify me when the SSL certificate is about to expire.

Performance Monitoring: Notify me when response time breach 1 seconds and reports error rates.

Routine Maintenance

Establish routine base procedures:

Server Update: Update the server software is up to date.

Security Patches: Apply security patches.

Certificates Re-issue: Issue certificates automatically.

Back-up Procedures: Have back-ups in place and regularly tested for validity.

Redundancy and Failover

Build some redundancy in your infrastructure:

Load Balancing: Distribute traffic to multiple server.

Failover Reservations: Automatic fail on backup servers.

Certificate Management: Configure Cloudflare to do all above maximizing benefit.

Geographic Distribution: Consider multi-region deployment of critical application.

When to Contact Support

Infographic listing scenarios for contacting support, such as persistent server errors, DNS issues, or firewall blocks

If you have decided to take the plunge and contact support, then you should be able to figure out when to reach out for support:

1. Repeat Problems: If problems have persisted even after you have worked to attempt to solve a problem.
2. Complex Architecture: If you are working with architecture at the enterprise level.
3. Urgent Incidents: If you are working with some mission-critical application that has hit a road block that needs an immediate response.
4. Did you change anything? If you are migrating a server, or making a handicap overhaul of your configuration.

When contacting the support team at your hosting provider, you will want to have all the details that you can tell them about the following:

1. When you started to see the error?
2. Have you made any recent changes on your server, or in your Cloudflare account?
3. What have you already done to try to mitigate the issue?
4. To point out any error patterns, or timing observations.

Conclusion

The Error 521 can be a complex error to diagnose and troubleshoot, but applying a systematic approach and taking reasonable preventative steps can minimize the impact to your website. Error 521 generally reflects a communication issue between Cloudflare and your origin server, so the majority of your subsequent investigation will take place on the server side — mostly around accessibility, any firewall rules, or DNS.

Monitoring regularly, performing maintenance on time, and knowing the hosting infrastructure inside out, will minimize the chances of Error 521 happening and will reduce your response time to recover. The solutions and preventative approach outlined should help you keep running a dependable website that your visitors can access reliably.

If you continue to experience Error 521 or run into it in more complex hosting environments, don’t hesitate to work with your hosting provider’s technical support or consider working with an outside consultant to help chart a path forward with a web infrastructure specialist who can work in context of your configuration.

Frequently Asked Questions (FAQ)

Frequently Asked Questions

What does Error 521 mean?

Error 521 means Cloudflare is unable to connect to your origin server. It’s a connection timeout error and not really a server error.

How long will it take to fix Error 521?

Once you identify the root cause, most Error 521 issues can be resolved within 15 – 30 minutes. Server downtime might take longer.

Can Error 521 impact my SEO rankings?

Yes, as you have prolonged Error 521 your SEO will be negatively impacted meaning search engines cannot access your content.

Is Error 521 the same as Error 502?

No. Error 521 is a specific Cloudflare connection error while the Error 502 is just a general bad gateway error.

Why do I get Error 521 randomly?

Random occurrences are typically indicative of server resource issues, network issues, and intermittent connectivity issues.

Can I prevent Error 521 entirely?

While you cannot prevent all occurrences, the monitoring, proper server maintenance, and configuration is on your part will significantly reduce instances of Error 521.

Does Error 521 mean my website is hacked?

Not necessarily. Error 521 is usually a connectivity issue, however a security plugin may block legitimate traffic from Cloud flare.

Should I disable Cloudflare to fix Error 521?

Disabling Cloudflare is a wet bandage, it won’t fix your connection issue. You should seek to rectify your underlying connectivity issue.

How do I identify whether my server is the cause of Error 521?

You can try checking your website without Cloudflare by using the IP address from your hosting provider. If you can visit your website using the IP address, then there is a problem with Cloudflare.

Can Error 521 happen from too much traffic?

Yes, a traffic surge can overwhelm you server resources, possibly preventing it from responding to Cloudflare’s connection request.