502 Bad Gateway Error is a common issue that can occur when using NGINX as a reverse proxy. This error occurs when NGINX is unable to connect to the upstream server, which can be caused by a number of different factors. In this blog post, we’ll go through the steps to troubleshoot and fix this error.

Step 1: Check the Upstream Server The first step in troubleshooting a 502 Bad Gateway Error is to check the status of the upstream server. This is the server that NGINX is trying to connect to in order to retrieve the requested content. The upstream server can be a web server, application server, or a load balancer.

You can use the command “telnet” to check if the upstream server is responding to the connection. For example:

Copy codetelnet upstream-server-ip 80

If the upstream server is responding, you should see a message like:

Copy codeTrying upstream-server-ip...
Connected to upstream-server-ip.
Escape character is '^]'.

If the upstream server is not responding, you may need to check the network settings or the configuration of the upstream server.

Step 2: Check the NGINX Configuration If the upstream server is responding, the next step is to check the NGINX configuration. The NGINX configuration file is usually located in the /etc/nginx/nginx.conf file or in the /etc/nginx/conf.d/ directory.

In the configuration file, you need to check the server block that is handling the request. Make sure the server block contains the correct IP address and port number of the upstream server.

You can also check the error log file to see if there are any issues with the configuration. The error log file is usually located in the /var/log/nginx/error.log file.

Step 3: Check the Firewall If the upstream server and NGINX configuration are correct, the next step is to check the firewall. Make sure that the firewall is not blocking the connection between NGINX and the upstream server.

You can use the iptables command to check the firewall rules. For example:

Copy codeiptables -L

This command will display all the firewall rules that are currently in place. If you find any rules that are blocking the connection, you can delete them using the iptables command.

Step 4: Check the Resource Usage Another possible cause of the 502 Bad Gateway Error is that the upstream server or NGINX is running out of resources. This can happen when the server is handling a large number of requests or when there are other processes that are consuming too many resources.

You can check the resource usage by using the command “top” or “htop” in the terminal. This will show you the processes that are currently running on the server and the resources they are using.

If you find that the upstream server or NGINX is running out of resources, you may need to upgrade the server or optimize the configuration.

In conclusion, a 502 Bad Gateway Error can be caused by a number of different factors. By following these steps, you should be able to troubleshoot and fix the issue. Remember to always check the upstream server, NGINX configuration, firewall, and resource usage when encountering this error.

Also read: Block IP Address Using .htaccess method

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