zas_db: zas_db authenticates users against an LDAP database, supporting the following authentication methods: username/password, S/Key, CryptoCard RB1, LDAP binding, GSSAPI/Kerberos5, and x.509.

file: Authentication through the traditional htpasswd file. Supports the username/password authentication method.

PAM: Authentication using Pluggable Authentication Modules. Any authentication method supported in PAM can be used.

RADIUS: Authentication using a RADIUS server. Supports the username/password and challange/response authentication methods.