Fixing “Upstream Prematurely Closed Connection” Error in Nginx

I. Introduction

Nginx is a popular and widely used open-source web server software. It acts as a reverse proxy server for HTTP, HTTPS, SMTP, and other protocols. However, like any software, Nginx is not immune to errors. One of the most common errors that Nginx users encounter is the “upstream prematurely closed connection” error. In this article, we will discuss what causes this error and how to fix it.

II. What Causes the Error?

The “upstream prematurely closed connection” error occurs when Nginx receives a premature end-of-file (EOF) from the upstream server it is communicating with. This can be caused by several factors, including:

• Timeout: The most common cause of this error is a timeout. When an upstream server takes too long to respond, Nginx will close the connection and return this error.
• Memory Issues: Another common cause of this error is low memory on the upstream server. If the server runs out of memory, it may close the connection with Nginx.
• Network Issues: Network problems such as high latency or high packet loss can also cause this error.

III. How to Fix the Error

To fix the “upstream prematurely closed connection” error, there are several steps that can be taken, including:

• Increase Timeout: To fix the timeout issue, you can increase the timeout value in the Nginx configuration file. The timeout value can be set in the “http” block in the Nginx configuration file. The default value is 60 seconds, but you can increase it to a higher value to avoid the error. For example:

http {
    ...
    proxy_connect_timeout 120s;
    proxy_send_timeout 120s;
    proxy_read_timeout 120s;
    ...
}

• Monitor Memory Usage: Monitoring the memory usage on the upstream server is also important. If you see that the server is running out of memory, you may need to increase the amount of memory available or optimize the code running on the server to reduce memory usage.
• Check Network: If you suspect that the error is caused by network issues, you can check the network for any problems. You can also use network monitoring tools to monitor network performance and identify any potential issues.

IV. Conclusion

The “upstream prematurely closed connection” error is a common issue in Nginx. The error is caused by a number of factors, including timeouts, memory issues, and network problems. To fix the error, you can increase the timeout value, monitor memory usage, and check the network for any issues. By understanding what causes this error and how to fix it, you can keep your Nginx server running smoothly.