The domain name of DNS name server.
The value of this object reflects the value of
cmgwConfigDomainName from the entry with a value of
'dnsServer(2)' for object cmgwConfigDomainNameEntity of
cMediaGwDomainNameConfigTable.
If there is no entry in cMediaGwDomainNameConfigTable with
'dnsServer(2)' of cmgwConfigDomainNameEntity, then
the value of this object will be empty string.