Why Am I Seeing NGINX 502 Bad Gateway: PHP-FPM?

Understanding the NGINX Error 502 Bad Gateway

The NGINX Error 502 Bad Gateway is a widespread issue encountered by website users. This article delves into the various potential causes behind this error and offers solutions for both end-users and web developers. We’ll explore the key takeaways, what NGINX 502 Bad Gateway signifies, the role of PHP-FPM, possible reasons for encountering the error, and how to resolve it. Additionally, we’ll discuss using MetricFire’s platform for monitoring NGINX and conclude with a summary of the article’s key points.

1. Key Takeaways

The NGINX Error 502 Bad Gateway is an indication that the server is receiving an error from another server, typically stemming from a failure to connect to PHP-FPM or PHP-FPM not responding. Utilizing PHP-FPM in conjunction with NGINX can enhance website performance and reduce resource consumption. Possible reasons for this error include inactive NGINX or PHP-FPM, firewall restrictions, domain name issues, or server unavailability. End-users can resolve the error by reloading the page, clearing browser cache and cookies, trying a different browser, or restarting their router. MetricFire’s platform offers NGINX status monitoring, and its Telegraf plugin enables users to visualize NGINX server status and 502 errors.

2. Understanding NGINX 502 Bad Gateway

The NGINX 502 Bad Gateway error occurs when a server acts as a proxy, receiving information from another server and encountering an error while connecting. This error arises when NGINX fails to connect to PHP-FPM or PHP-FPM becomes unresponsive. It is known by various names such as HTTP Error 502, 502 Service Temporarily Overloaded, and more. The appearance of the error page can vary from site to site.

3. The Role of PHP-FPM

PHP-FPM, or PHP-FastCGI Process Manager, is a web request processing tool for PHP applications. When used with NGINX, it significantly boosts website performance while reducing resource consumption. PHP-FPM operates as a separate service, processing web requests over a TCP/IP socket, with NGINX handling HTTP requests and PHP-FPM interpreting PHP code.

4. Possible Reasons for 502 Bad Gateway

There are several potential causes for the 502 Bad Gateway error:

a. NGINX is not running

• Verify NGINX’s status with the command: systemctl status nginx
• If it shows as inactive (dead), start NGINX to resolve the issue.

b. PHP-FPM is not running

• On Linux, check PHP-FPM’s status using: ps aux | grep php-fpm
• If no PHP-FPM processes are displayed, start PHP-FPM to resolve error 502.

c. PHP-FPM timeout expires

• This can lead to a 502 error if the server takes too long to respond.
• Adjust the PHP-FPM timeout or investigate why your application is unresponsive.

d. Firewall blocks requests

• Firewalls, DDoS protection systems, or CMS security plugins can block requests between servers.

e. Domain name is not allowed

• Ensure the domain name resolves to the correct IP address by changing the DNS server if needed.

f. Server is unavailable

• Check if the original server is down or if there is no connection to it.

5. How to Solve the 502 Error

As a website visitor, you can take the following steps to resolve the 502 error:

a. Reload the page: Often, the error is temporary, and a page refresh can rectify it.

b. Clear browser cache and cookies: Deleting old or corrupted files and data may resolve the issue.

c. Try using a different browser: The problem could be specific to your current browser.

d. Restart your computer and network equipment: Rebooting may resolve connectivity problems.

e. Check plugins and extensions: Disable any plugins or extensions in your browser that may be causing the issue.

f. Change your DNS server: Switching to Open DNS or Google DNS servers might help.

As a web developer, consider the following steps to address the 502 error:

a. Check server availability: Ensure your server is up and running.

b. Examine firewall logs: Investigate any unusual crashes in the firewall logs.

c. Disable Cloudflare: If using Cloudflare, disabling it might resolve the 502 Bad Gateway error if a limit is reached.

6. Using MetricFire to Monitor NGINX

MetricFire offers a platform for monitoring NGINX statuses and provides a Telegraf plugin for visualizing NGINX server status and 502 errors. This tool is valuable for identifying bottlenecks and troubleshooting errors in real-time. Hosted Graphite and Grafana enable users to monitor system metrics effectively.

7. Conclusion

In conclusion, we have explored the meaning of error 502, its potential causes, and ways to resolve it. Monitoring tools like hosted Graphite and Grafana offered by MetricFire are essential for error elimination and efficient system performance monitoring. To integrate Graphite and Grafana with your system, consider booking a demo with MetricFire’s team or signing up for their free trial.