As Internet communications are carried out on the basis of IP addresses, appropriate DNS server settings are required for mapping domain names (host names) to corresponding IP addresses, or the PXE may fail to connect to the given host.
Therefore, DNS server settings are important for external authentication. With appropriate DNS settings, the PXE can resolve the external authentication server's name to an IP address for establishing a connection. If the SSL encryption is enabled, the DNS server settings become critical since only fully qualified domain name can be used for specifying the LDAP server.
For information on external authentication, see Setting Up External Authentication.