What is DNS Cache Poisoning?
Whenever your computer contacts a domain name like “google.com,” it must first contact its DNS server. The DNS server responds with one or more IP addresses where your computer can reach google.com. Your computer then connects directly to that numerical IP address. DNS converts human-readable addresses like “google.com” to computer-readable IP addresses like “126.96.36.199”.
The Internet doesn’t just have a single DNS server, as that would be extremely inefficient. Your Internet service provider runs its own DNS servers, which cache information from other DNS servers. Your home router functions as a DNS server, which caches information from your ISP’s DNS servers. Your computer has a local DNS cache, so it can quickly refer to DNS lookups it’s already performed rather than performing a DNS lookup over and over again.
A DNS cache can become poisoned if it contains an incorrect entry. For example, if an attacker gets control of a DNS server and changes some of the information on it — for example, they could say that google.com actually points to an IP address the attacker owns — that DNS server would tell its users to look for Google.com at the wrong address. The attacker’s address could contain some sort of malicious phishing website
DNS poisoning like this can also spread. For example, if various Internet service providers are getting their DNS information from the compromised server, the poisoned DNS entry will spread to the Internet service providers and be cached there. It will then spread to home routers and the DNS caches on computers as they look up the DNS entry, receive the incorrect response, and store it.