DNS spoofing

DNS spoofing, also referred to as DNS cache poisoning, is a form of computer security hacking in which corrupt Domain Name System data is introduced into the DNS resolver's cache, causing the name server to return an incorrect IP address. This results in traffic being diverted to the attacker's computer (or any other computer).

A domain name system server translates a human-readable domain name (such as example.com) into a numerical IP address that is used to route communications between nodes. Normally if the server doesn't know a requested translation it will ask another server, and the process continues recursively. To increase performance, a server will typically remember (cache) these translations for a certain amount of time. This means if it receives another request for the same translation, it can reply without needing to ask any other servers, until that cache expires.

When a DNS server has received a false translation and caches it for performance optimization, it is considered poisoned, and it supplies the false data to clients. If a DNS server is poisoned, it may return an incorrect IP address, diverting traffic to another computer (often an attacker's).

Normally, a networked computer uses a DNS server provided by an Internet service provider (ISP) or the computer user's organization. DNS servers are used in an organization's network to improve resolution response performance by caching previously obtained query results. Poisoning attacks on a single DNS server can affect the users serviced directly by the compromised server or those serviced indirectly by its downstream server(s) if applicable.

To perform a cache poisoning attack, the attacker exploits flaws in the DNS software. A server should correctly validate DNS responses to ensure that they are from an authoritative source (for example by using DNSSEC); otherwise the server might end up caching the incorrect entries locally and serve them to other users that make the same request.

...
Wikipedia