The dns keyword requires that the naming service used on your machine is DNS. If the name service is not DNS, you should use the ip keyword instead.
The dns keyword is used to specify that a bind operation must originate from a specific domain or host machine. The dns keyword uses this syntax:
dns = "DNS_Hostname" or dns != "DNS_Hostname"
The dns keyword requires a fully qualified DNS domain name. Granting access to a host without specifying the domain creates a potential security threat. For example, the following expression is allowed but not recommended:
dns = "legend.eng";
You should use a fully qualified name such as:
dns = "legend.eng.example.com";
The dns keyword allows wild cards.
dns = "*.example.com";
The bind rule is evaluated to be true if the client accessing the directory is located in the named domain. This can be useful for allowing access only from a specific domain. Note that wild cards do not work if your system uses a naming service other than DNS.