AS_db: AS_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: It is an authentication through the traditional htpasswd file. it supports the username/password authentication method.

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

RADIUS: It is an authentication using a RADIUS server. It supports the username/password and challange/response authentication methods.