ad content. Hosts file and DNS manipulation Most operating systems, even those which are aware of the Domain Name System (DNS), still offer backward compatibility with a locally administered list of foreign hosts. This configuration, for historical reasons, is stored in a flat text file that by default contains very few hostnames and their associated IP addresses. Editing this hosts file is simple and effective because most DNS clients will read the local hosts file before querying a remote DNS server. Storing black-hole entries in the hosts file prevents the browser from accessing an ad server by manipulating the name resolution of the ad server to a local or nonexistent IP address (127.0.0.1 or 0.0.0.0 are typically used for IPv4 addresses). While simple to implement, these methods can be circumvented by advertisers, either by hard-coding, the IP address of the server that hosts the ads (this, in its turn, can be worked around by changing the local routing table by using for example iptables or other blocking firewalls), or by loading the advertisements from the same server that serves the main content; blocking name resolution of this server would also block the useful content of the site. Using a DNS sinkhole by manipulating the hosts file exploits the fact that most operating systems store a file with IP address, domain name pairs which is consulted by most browsers before using a DNS server to look up a domain name. By assigning the loopback address to each known ad server, the user directs traffic intended to reach each ad server to the local machine or to a virtual black hole of /dev/null or bit bucket. DNS filtering Advertising can be blocked by using a DNS server which is configured to block access to domains or hostnames which are known to serve ads by spoofing the address. Users can choose to use an already modified DNS server or set up a dedicated device running adequate software such as a Raspberry Pi running Pi-hole themselves. Manipulating DNS is a widely employed method to manipulate what the end-user sees from the Internet but can also be deployed locally for personal purposes. China runs its own root DNS and the EU has considered the same. Google has required their Google Public DNS be used for some applications on its Android devices. Accordingly, DNS addresses/domains used for advertising may be extremely vulnerable to a broad form