How to Fix '(13: Permission denied) while connecting to upstream' Nginx Error

1. Check Nginx User Permissions

To fix the Nginx permission denied error, check the Nginx user permissions. Look at the Nginx.conf file to see the user and group settings. Make sure the Nginx user can access the files and directories. You may need to change file and directory permissions with the chmod command.

2. Examine SELinux Settings

SELinux can block Nginx from accessing files or making network connections. Check the SELinux status and change the boolean values if needed. Use the 'setsebool' command to change these settings. You can toggle the SELinux boolean value for httpd network connect with:

setsebool httpd_can_network_connect on -p

3. Review Nginx Configuration Files

Check your Nginx.conf file for errors. Look at the upstream server settings. Make sure all paths and server addresses are right and that Nginx can access these resources.

4. Analyze Nginx Error Logs

Nginx error logs can help you find the permission denied problem. Look at the error logs, usually in /var/log/nginx/error.log, for permission denied messages. Use the grep command to find these errors:

grep denied /var/log/nginx/error.log

5. Investigate Firewall Rules

Your firewall might be blocking Nginx connections. Check your firewall rules and make sure they let Nginx connect to the needed ports and IP addresses. You may need to add new rules or change existing ones to give Nginx the right access.

6. Verify Upstream Server Availability

Check that your upstream servers are running and reachable. Try to connect to these servers from your Nginx server to rule out network or configuration issues. You can use telnet or curl to test these connections and make sure the upstream servers are responding.

If you're still having problems, you might see a 502 Bad Gateway error when connecting to upstream. In this case, check your application server and make sure it's running and configured correctly.

Remember, if you change SELinux settings, you might need to run:

audit2allow -m mynginx
sudo semodule -i mynginx.pp

This creates and installs a custom policy module based on the audit logs.

Using audit2allow to Generate SELinux Policies

The audit2allow tool helps create SELinux policies when standard policies don't work. To use it:

1. Check SELinux logs for denied actions:

   grep "denied" /var/log/audit/audit.log

2. Use audit2allow to generate a policy:

   audit2allow -m mynginx sudo semodule

3. Apply the new policy:

   mynginx sudo semodule -i mynginx.pp

This process creates and installs a SELinux module based on recent denied actions.

Adjusting File Ownership and Permissions

Correct file ownership and permissions help solve Nginx permission issues:

1. Set Nginx files and directories ownership:

   chown -R nginx:nginx /var/www/html

2. Set permissions:

   chmod 755 /var/www/html
   chmod 644 /var/www/html/*

These commands give the Nginx user and group ownership of web files and set read, write, and execute permissions. Change paths as needed for your setup.