DNS poisoning
DNS poisoning is one of my favorite subjects, just in the aspect of how effective it can be. You have nothing to fear… but an attacker with your DNS cache. DNS is simply there because humans are ill-advised and, in some cases, ill-equipped. It's hard for humans to remember a number. Can you imagine if you had to remember the IP address of a website instead of simply typing in a name? We associate and remember names better than we do numbers.
We're all familiar with the Uniform Resource Locator (URL) box where we type in the name of a website we'd like to go to. DNS oversees taking the name and converting it down to an IP address using tables. These tables can be distributed across multiple systems. Some of them are internal, while others are external names.
A DNS server typically hosts these databases. If the DNS server oversees looking at internal names and names of servers in your environment, that would be your internal DNS name server...