If the "Error 521" error occurs occasionally on your site and is not fixed to a particular page, then the problem may be with the server and the CDN The communication between them. This is a problem that comes and goes and is difficult to reproduce, but for the user it means that "the site won't open".

What is Error 521?

Error 521 is a common Cloudflare error message that means:

The web server rejected Cloudflare's connection request.

The user request has gone through the CDN, but on return to the source, thesource stationThe server didn't respond or simply closed the connection, causing the access to fail.

There are many possible reasons for this, such as:

• Source station down or being restarted
• Firewall blocked CDN request
• Too many connections to the server causing new connections to be rejected
• Nginx or Apache crashes or is misconfigured

Often the problem is not the CDN's fault, but rather the source site "dropping the ball".

Recommendations for monitoring tools: first identify when the problem occurs

Error 521 is highly episodic, so you need aMonitoring tools that can record and provide timely warningsThe first thing you need to do is to know when and how often the faults occur.

The following tools are recommended:

1. UptimeRobot (free + cost-effective)

• Support HTTP(s), Ping and other monitoring methods
• You can set up a 5-minute monitoring (free version) to check if the site is working.

• Logging of "downtime period" when 521 error occurs, support email or Telegram notification

Suitable for individual webmasters, small and medium-sized enterprises, with Cloudflare It's very convenient.

2. Better Uptime (with screenshots + event logging)

• Each failure will take a screenshot of the current page status, making it easy to confirm whether the error is 521
• Notifications can be integrated with Slack, Discord, phone calls, etc.
• Provide historical event records, suitable for troubleshooting reference

Suitable for team use and more friendly to troubleshooting details.

3. Cloudflare's own analytical functions

If you're using Cloudflare, you can see it in Dashboard → Website → Analytics:

• Return code statistics (including frequency of occurrence of 521)
• Number of failures to return to source
• Geographic distribution of requests

It's not particularly detailed, but combined with other tools it's enough to determine if it's a source station issue.

Log Analysis Methodology: Locating Server-Side Problems

Monitoring tools can only tell you that "errors were reported", but why they were reported, or rely on the server logs.

Below are the locations of several key log files and how to analyze them:

1. Viewing Nginx or Apache Logs

Common log paths:

• Nginx::/var/log/nginx/error.log
• Apache:/var/log/apache2/error.log

Open the log file and search for keywords:

grep "connection refused" /var/log/nginx/error.log

Or:

tail -n 100 error.log

If the logs show something like connection reset by peer maybe upstream timed out, indicating that there is a response problem with the server itself.

2. Check that the firewall is not blocking the Cloudflare IP

The server may have enabled thefirewalls(such as iptables or firewalld), sometimes recognizes Cloudflare's IP segment as the source of an attack.

You can check if the port is closed with the following command:

sudo iptables -L -n

Or confirm that Cloudflare's IP segment is released:

sudo ufw allow from 173.245.48.0/20

You can also visit the official Cloudflare IP list page to make sure they are all added to the whitelist.

3. Analysis of system resource utilization

Error 521 Sometimes the server is "stuck", not refusing to connect, but "busy".

After logging in to the server, you can view the current load with the following command:top

Or use:htop

Focused Look:

• Is the CPU usage too high?
• Is the memory full?
• Are there a lot of PHP-FPM or MySQL process jamming

Finding a problem and restarting the service or expanding the configuration in a timely manner is the key to solving it.

Recommendations for real-world troubleshooting process

1. Confirmation of point in time: View the timeframe in which the error occurred via UptimeRobot or Cloudflare
2. Checking the error log: Check Nginx / Apache logs for error messages
3. probefirewalls: Confirm that Cloudflare's back-origin requests are intercepted.
4. Checking system resources: Whether the server is overloaded and the network is unstable
5. Combined analysis of multiple sources: Summarize monitoring screenshots + log messages to cross-reference them

If the problem is recurring, it is advisable to consider:

• Replacement of servers with more stable servers
• Implementing load balancing
• Conducting a websiteCache OptimizationReduced pressure on source stations

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