The administrator should only allow secure traffic and DNS inquiries on the server, which means that only ports 22, 53, and 443 should be open. Port 22 is used for SSH (Secure Shell), which is a protocol that allows secure remote login and command execution over a network connection using a command-line interface (CLI). Port 53 is used for DNS (Domain Name System), which is a service that translates domain names into IP addresses and vice versa. Port 443 is used for HTTPS (Hypertext Transfer Protocol Secure), which is a secure version of HTTP that encrypts the data exchanged between a web browser and a web server.
Reference: https://tools.cisco.com/security/center/resources/dns_best_practices