The error indicates that SELinux is preventing Apache from connecting to the database. The command setsebool -F httpd_can_network_connect_db on allows the Apache web server to make network connections to the database, resolving the permission denied error without disabling SELinux entirely.

The user mask (umask) feature defines the default permissions Linux assigns to the file or directory. The user mask is an octal value that represents the bits to be removed from the octal mode 666 permissions for files or the octal mode 777 permissions for directories.

Some good security practices when hardening a Linux server are:
Removing unnecessary packages (B) to reduce the attack surface and eliminate potential vulnerabilities Disabling the SSH root login possibility (E) to prevent unauthorized access and brute-force attacks on the root account

The reason the firewall rules are not active is that the firewalld service is not enabled. This means that the service will not start automatically at boot time or after a system reload. To enable the firewalld service, the systems administrator needs to use the command sudo systemct1 enable firewalld. This will create a symbolic link from the firewalld service file to the appropriate systemd target, such as multi-user.target. Enabling the service does not start it immediately, so the systems administrator also needs to use the command sudo systemct1 start firewalld or sudo systemct1 reload firewalld to activate the firewall rules.