Can we assume that the OP's Safari will clear local storage?
I think you will find there are considerably more Flash files than just cookies. There is a web based Flash utility to inhibit local storage although it does not remove history items.
Your remark about a greyed out DNS server perplexed me for a while. I suspect you are referring to the situation when a first local DNS server is inserted where only a DHCP server existed before. If so this is nothing to do with Open, ISP or any other specific types of DNS server. We do not know if the OP already had non-DHCP DNS servers listed. It is important to give general advice which will work in a wide range of cases although even I have assumed the OP is not behind a firewall with a proxy server!