Editing Ad blocking (section)

===Hosts file and DNS manipulation===
{{Further|Hosts (file)#Extended applications}}
{{Unreferenced section|date=April 2023}}
Most [[operating system]]s, 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 address]]es. 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 (networking)|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 (<code>[[127.0.0.1]]</code> or <code>[[0.0.0.0]]</code> 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]].