Cloudflare Error 521 This is a common HTTP error that indicates that the server was unable to establish a connection to Cloudflare's network, usually expressed as "Web server is down". This error can cause websites to become inaccessible, which can be very inconvenient for users. Understanding the causes of Error 521 and taking the appropriate precautions will allow you to keep your website running properly under Cloudflare protection and avoid similar problems.

1. Causes of Cloudflare Error 521

Error 521 is usually caused by several reasons:

• Server refuses to connect
  Cloudflare accesses your website through a reverse proxy. If a web server (e.g. Apache, Nginx, etc.) rejects a request from Cloudflare, a 521 error occurs.
• Firewall setting error
  hostingfirewallsor security settings may be blocking Cloudflare's IP address, preventing a connection from being established. A firewall that is not properly configured to allow Cloudflare's IP address is a common cause of error 521.
• Web server down or not responding
  If the web server crashes or is in a maintenance state, it does not respond to Cloudflare requests, resulting in a 521 error.
• DNS configuration error
  DNS SettingsWhen there is a problem, Cloudflare will not be able to resolve the domain name correctly and thus will not be able to establish a connection with your server.

2. How to Resolve and Avoid Cloudflare Errors 521

There are several steps you can take to avoid or fix Cloudflare error 521:

2.1 Checking Web Server Status

First, your Web server (computer)It needs to be in working order. If the server is down for failure or maintenance, Cloudflare cannot connect to the site. You can check the web server status in the following two ways:

• Server Health Check: Check the server's operating logs for crashes or unusual downtime. If the server needs to be restarted, be sure to complete the operation.

• Monitoring Tools: Use monitoring tools (e.g. UptimeRobot, Pingdom) to check if the server is online and to ensure that the server is always up and running.

2.2 Ensure that the server allows Cloudflare connections

To avoid 521 errors, your server should allow access from Cloudflare's IP address.Cloudflare uses a series of IP addresses to proxy theweb trafficIf the server blocks these IP addresses, Cloudflare will not be able to access the site.

• Configuring the Firewall: Log in to the server's firewall configuration screen and add Cloudflare's IP address range to the whitelist. You can find a list of all IP addresses on the official Cloudflare website.

• Check IP Blacklist: Make sure your firewall or security plugin has not mistakenly blacklisted Cloudflare's IP address. This will avoid blocking normal traffic.

2.3 Verifying DNS Settings

Make sure that the domain name resolution is set up correctly and is pointing to the correct IP address of the web server. If the DNS is misconfigured, Cloudflare cannot resolve the web address correctly, resulting in a 521 error.

• Checking DNS Configuration: Log in to the domain name registrar control panel and check if the DNS settings are correct. the A record or CNAME record should point to the Web server IP address correctly.

• Cloudflare DNS Settings: In the Cloudflare control panel, check that the DNS settings are correct and verify that all records (e.g., A records, CNAME records, etc.) point to the correct servers.

2.4 Troubleshooting Problems with Web Server Logs

If you have verified that the server is running and the DNS configuration is correct and you are still getting error 521, you can check your server's DNS configuration by viewing the Web Server logsfile to diagnose the problem.

• Viewing the Error Log: By Apache or Nginx's error logs to check if any connection requests from Cloudflare were denied or lost.

• Network Connection Log: Check the network connection logs to see if any requests from Cloudflare were accidentally blocked or lost.

2.5 Upgrade Cloudflare and Server Plans

In some cases, Error 521 may be caused by insufficient hardware resources of the Cloudflare program or web server you are using. For example, if the server is overloaded, it may not be able to handle requests from Cloudflare.

• Upgrade the Cloudflare program: Upgrade your Cloudflare plan as needed. The higher-end plans offer stronger protection and more resources for sites with higher traffic or higher requirements.
• Optimize server performance: Check the performance of your Web server to make sure it is powerful enough to handle large amounts of traffic. You can increase the server's processing power by increasing memory, optimizing database queries, and using content caching.

summarize

To avoid these problems, webmasters should ensure that their servers are running properly, firewalls are configured correctly, DNS settings are correct, and web server logs are checked regularly. Additionally, using Cloudflare Advanced Plan and optimizing server performance can also help to prevent 521 ErrorThe

Timely troubleshooting and taking appropriate measures can greatly reduce the occurrence of Error 521 and keep your website running safely and stably under Cloudflare protection.

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