DNS hijacking is a process in which an individual redirects queries to a domain name server (DNS). It may be accomplished through the use of malicious software or unauthorized modification of a server. Once the individual has control of the DNS, they can direct others who access it to a web page that looks the same, but contains extra content such as advertisements. They may also direct users to pages containing malware or a third-party search engine.
DNS hijacking is also done by some Internet service providers, such as Comcast, so that they can link users to their own search pages when they visit a web page that no longer exists. Many claim this is to improve the users experience; however, this can also be another great source of extra revenue since they control the site and get paid off any advertisement clicks. Currently, there are no laws against an ISP doing this to its users.
How DNS Hijacking Works?
As mentioned before, DNS is the one that is responsible for mapping the user friendly domain names to their corresponding IP addresses. This DNS server is owned and maintained by your Internet service provider (ISP) and many other private business organizations. By default, your computer is configured to use the DNS server from the ISP. In some cases, your computer may even be using the DNS services of other reputed organizations such as Google. In this case, you are said to be safe and everything seems to work normally.
But, imagine a situation where a hacker or a malware program gains unauthorized access to your computer and changes the DNS settings, so that your computer now uses one of the rogue DNS servers that is owned and maintained by the hacker. When this happens, the rogue DNS server may translate domain names of desirable websites (such as banks, search engines, social networking sites etc.) to IP addresses of malicious websites. As a result, when you type the URL of a website in the address bar, you may be taken to a fake website instead of the one you are intending for. Sometimes, this can put you in deep trouble!
Src: gohacking, Computer Hope