DNS Cache Poisoning
DNS servers are constantly sending out questions (“What’s the IP address of www.hot-girls.com ?”) and receiving answers (“www.hot-girls.com is at 210.247.219.18″).
They don’t actually authenticate the source of the answers — there’s no way for your DNS server to be sure that the answer actually came from the REAL hot-girls.com. Some DNS servers don’t even check that they asked a question that corresponds to an answer they received, and just believe any answer someone sends them.
The simplest form of cache poisoning is simply sending fake answers to someone’s DNS server; for each safeguard a DNS server might apply to prevent cache poisoning there’s usually a workaround that goes one step further. This is why SSH has all that stuff with strict checking of host keys.
