How can clients verify the legitimacy of a DNS server?

Clients can verify the legitimacy of a DNS server by using DNSSEC protocols. DNSSEC, or Domain Name System Security Extensions, is designed to add a layer of security to the DNS infrastructure. It ensures that responses to DNS queries are authentic and have not been tampered with during transmission. This is achieved through cryptographic signatures that validate the integrity and authenticity of the data provided by the DNS server.

When a client queries a DNS server that supports DNSSEC, it receives not only the requested records but also digital signatures that can be verified against the public key of the domain. If the signatures match, the client can be confident that the response is legitimate and has not been altered, which is crucial for preventing various attacks such as DNS spoofing or cache poisoning.

The other options, while important for general security practices, do not specifically address the verification of a DNS server's legitimacy. Checking firewall settings, using secure connections, and updating software regularly contribute to overall security but do not specifically confirm that the DNS server is authentic or has not been compromised.