What is the difference between Error 521 and Error 520, 522? In a single article

in using Cloudflare or other reverse proxies, many webmasters will encounter the error page, which displays Error 521,Error 520 maybe Error 522. These errors look similar, but the causes and solutions are different. Without understanding the differences, it can be difficult to quickly pinpoint the problem. In this article, we figure out the difference between these three errors in simple terms.

1. What is Error 521?

Error 521: Web server is down
This is one of the most common errors. It indicates that Cloudflare has tried to connect to the source server, but the server rejected the connection request.

1.1 Common causes

• Source server down or not up
• Firewall blocking Cloudflare's IP
• Web Services (Nginx(Apache) process hangs
• Server port is closed

1.2 Characteristics

• The page will just say "Web server is down".
• The server port is usually 80 or 443
• You can see the request rejection in the server logs

2. What is Error 520?

Error 520: Web server returns an unknown error
The 520 error is relatively ambiguous; it means that Cloudflare received an unrecognizable response after making a request to the source.

2.1 Common causes

• The web server returned an abnormal HTTP response header
• The source station suddenly closed the connection
• Empty responses or incorrectly formatted content
• Server-side code reports an error but does not handle it correctly

2.2 Characteristics

• The cause of the error is not intuitive and usually needs to be looked atserver (computer)log (computing)
• In many cases it's a problem with PHP or application layer code
• It's not necessarily a connection failure at the network level

3. What is Error 522?

Error 522: Connection timed out
A 522 error means that Cloudflare successfully contacted the source station, but the connection was not established within the specified time.

3.1 Common causes

• Server response is too slow
• Process is full and cannot handle new connections
• comprehensive databaseQueries take too long
• Insufficient bandwidth at the source, resulting in handshake timeout

3.2 Characteristics

• The error message is "Connection timed out".
• Slow website access and final trigger timeout
• Commonly used in high traffic or underperforming servers

4. Core differences between the three types of errors

error code	descriptive	trigger scene	Focused Direction of Examination
521	Web server refuses to connect	Source site hangs, firewall blocks	Checking Web service operation, firewall settings
520	unknown error	Abnormal response, program crash	View application logs, check code and output
522	Connection timeout	Excessive server pressure, insufficient bandwidth	Optimize performance, check network and database

One sentence summary:

• 521 It's "the server is ignoring me."
• 520 It's "The server said something I don't understand."
• 522 It's "the server is too slow to wait for a response".

5. Ideas for solutions

5.1 Responding to Error 521

• Verify that the web service (Nginx/Apache) is running

• Check if the firewall is blocking Cloudflare IP
• Check whether server ports 80 and 443 are open to the public.

5.2 For Error 520

• Open the server error log and troubleshoot for abnormal outputs

• Verify that PHP and the application are not crashing
• Repair of irregularities HTTP Header or empty response

5.3 Responding to Error 522

• View Server CPU, Memory Usage

• Optimize database queries and caching strategies
• Use load balancing or higher performance hosting

6. Why it is important to understand these differences

If you only see the error page and don't know the difference, you may keep thinking it's the same problem. But in reality:

• 521 More biased toward server denial of connection
• 520 Often an application layer problem
• 522 is a performance and network bottleneck

When correctly differentiated, the problem can be quickly narrowed down, thus saving troubleshooting time.

summarize

When the site appears Error 521, 520 or 522 When you do, don't panic. While similar in appearance, they essentially represent three different kinds of failures: connection denied, abnormal response, and timeout waiting. Understanding the difference will allow you to target your server, code, or performance bottlenecks and get your site back to normal faster.

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