What type of authentication server is NOT supported on FortiGate:

FortiGate supports several types of authentication servers that allow for centralized user management and authentication for network access. Among the options provided, SSH is not an authentication server; rather, it is a protocol used for secure remote login and command execution on network devices.

TACACS+, LDAP, and RADIUS are all authentication methods that FortiGate supports. TACACS+ (Terminal Access Controller Access-Control System Plus) is used for network device access control and can manage user credentials centrally. LDAP (Lightweight Directory Access Protocol) is utilized for accessing and maintaining distributed directory information services and is commonly used for directory-based authentication. RADIUS (Remote Authentication Dial-In User Service) is widely used for authenticating remote users and devices, providing centralized authentication, authorization, and accounting.

Since SSH does not function as an authentication server but rather serves as a secure channel for accessing devices, it is the correct choice in this context.