Q: Do I need a Windows Client Access License (CAL) for machines that just use Windows Server for DNS or DHCP?
A: The only time you don't require a Windows CAL is when accessing Windows Server hosted services from the Internet in an unauthenticated manner. Machines using Windows Server for DNS or DHCP require Windows CALs. This is confirmed in Microsoft's licensing documentation.
"Any direct or indirect access of Windows Server requires a CAL, except for anonymous access through the Internet. For example, the use of DNS—a service that helps route network traffic—requires the purchase of a Windows Server license and CALs to use and access this particular role in managing your organization’s domain names. Even with infrequent or occasional use, access of Windows Server DNS capabilities requires a CAL."