(13: Permission Denied) While Connecting to Upstream

The (13: Permission Denied) while connecting to upstream error in Nginx typically indicates a permission issue when Nginx tries to communicate with an upstream server (e.g., FastCGI, a proxy server, or a backend application). This error can occur due to various reasons, including file system permissions, incorrect ownership of Unix sockets, or network restrictions.

Here’s how you can troubleshoot and resolve this issue:

1. Check Permissions of the Unix Socket

If you are using a Unix socket to communicate with an upstream server (like FastCGI), ensure that the socket file has the correct permissions and ownership.

Steps:

1. Locate the Socket File: Find the location of the Unix socket file used in your Nginx configuration. This is often specified in the fastcgi_pass or proxy_pass directive.

    

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   location /fcgi-bin/ {
       fastcgi_pass unix:/var/run/yourapp/yourapp.sock;
       # Other directives...
   }
   

2. Check Permissions: Verify the permissions and ownership of the socket file. For example:

    

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   ls -l /var/run/yourapp/yourapp.sock
   

   Ensure the socket file is readable and writable by the Nginx user (often www-data or nginx).

3. Update Permissions or Ownership: Adjust permissions or ownership as needed:

    

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   sudo chown www-data:www-data /var/run/yourapp/yourapp.sock
   sudo chmod 660 /var/run/yourapp/yourapp.sock
   

   Replace www-data with the user Nginx runs as, and adjust the socket path as necessary.

2. Check Nginx User Permissions

Ensure that the user Nginx is running as has the necessary permissions to access the Unix socket or the upstream service.

Steps:

1. Identify Nginx User: Check which user Nginx is running as in the Nginx configuration file (/etc/nginx/nginx.conf or similar).

    

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   user www-data;
   

2. Verify User Access: Ensure that this user has access to the necessary files and directories, including the Unix socket.

3. Check Upstream Service Configuration

Ensure that the upstream service (e.g., FastCGI, application server) is configured correctly and is running. Check if it’s binding to the correct Unix socket or network port.

Steps:

1. Verify Service Status: Check the status of your upstream service to ensure it is running and bound to the correct socket.

    

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   sudo systemctl status yourapp
   

2. Check Service Configuration: Verify the configuration of the upstream service to ensure it matches the configuration specified in Nginx.

4. Review SELinux or AppArmor Policies

If you are using SELinux or AppArmor, these security modules might restrict access to files and sockets. Check if SELinux or AppArmor policies are causing the permission issues.

Steps:

1. Check SELinux Status:

    

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   getenforce
   

   If SELinux is in enforcing mode, you may need to adjust policies or switch to permissive mode for testing.

2. Set SELinux to Permissive Mode (for testing):

    

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   sudo setenforce 0
   

   Remember to revert to enforcing mode and properly configure policies for production environments.

3. AppArmor: If using AppArmor, check the AppArmor profiles to ensure they are not blocking access.

5. Check Nginx Error Logs

Look at the Nginx error logs for more details about the permission denied error. The logs may provide additional context about what is being blocked.

sudo tail -f /var/log/nginx/error.log

6. Restart Services

After making changes, restart Nginx and the upstream service to apply the new configuration.

sudo systemctl restart nginx
sudo systemctl restart yourapp

Summary

• Check Unix Socket Permissions: Ensure that the socket file has the correct permissions and ownership.
• Verify Nginx User Permissions: Ensure the Nginx user has access to the socket or network port.
• Review Upstream Service: Ensure the upstream service is running and correctly configured.
• Check Security Modules: Ensure SELinux or AppArmor are not blocking access.
• Review Logs: Check Nginx error logs for additional context.

By following these steps, you can diagnose and fix the (13: Permission Denied) error in Nginx related to upstream connections.

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