Drive encryption is a form of data-at-rest encryption that encrypts the entire hard drive or solid state drive. This means that all the data on the drive, including the operating system, applications, and files, are protected from unauthorized access. Drive encryption is usually implemented at the hardware or firmware level, and requires a password, PIN, or biometric authentication to unlock the drive. Drive encryption is the most comprehensive and secure way to achieve data-at-rest encryption, as it prevents anyone from accessing the data without the proper credentials, even if they physically remove the drive from the server.
References: CompTIA Server+ Study Guide, Chapter 9: Security, page 367.

Explanation
The administrator should perform an antivirus scan on all servers within the cluster and reboot each server, and block access to 208.206.12.9 from all servers on the network. These actions will help to remove any malware that may have infected the application server and prevent any further unauthorized traffic to the known-bad public IP address. An antivirus scan can detect and remove malicious software that may be sending data to an external source, and a reboot can clear any temporary files or processes that may be related to the malware. Blocking access to 208.206.12.9 from all servers on the network can prevent any future attempts to communicate with the malicious IP address.
References: CompTIA Server+ SK0-005 Certification Study Guide, Chapter 3, Lesson 3.4, Objective 3.4; Chapter 6, Lesson 6.2, Objective 6.2

Explanation
The best way to complete the request to harden the server is to disable port 389 on the server. Port 389 is the default port used by LDAP (Lightweight Directory Access Protocol), which is a protocol that allows access and modification of directory services over a network. LDAP can be used for authentication, authorization, or information retrieval purposes. However, LDAP does not encrypt its data by default, which can expose sensitive information or credentials to attackers who can intercept or modify the network traffic. Therefore, port 389 should be disabled on a web server that only hosts websites and does not need LDAP functionality.
Alternatively, port 636 can be used instead of port 389 to enable LDAPS (LDAP over SSL/TLS), which encrypts the data using SSL/TLS certificates.

Explanation
The domain search variable is used to specify a list of domains that are appended to a hostname when resolving it. If the servername is not fully qualified, the resolver will try each domain in the list until it finds a match or fails. By configuring the domain search variable, the technician can avoid typing the full domain name every time they want to ping a server. Verified References: [How to configure DNS suffixes on Linux systems]

A DNS entry is a record that maps a domain name to an IP address. If the DNS entry for the new application is incorrect, users will not be able to resolve the domain name to the correct IP address and port number. This will prevent them from accessing the application, even if the firewall rules allow port 8443. To fix this issue, the server administrator should verify and update the DNS entry for the new application.
References: CompTIA Server+ Study Guide, Chapter 6: Networking, page 230.