There are several obstacles an attacker like Eve must overcome to issue a fake DNS response that will be accepted. First, an attacker must issue a response to her own DNS query before the authoritative name server is given a chance to respond. This obstacle is easily overcome, however, because if the attacker forces the target name server to query external authoritative name servers, she can expect that her immediate, direct response will be received before these external name servers have a chance to perform a lookup and issue a reply.
Source: Introduction to computer security(Michael_Goodrich,_Roberto_Tamassia under Network Security chapter 6 (DNS Cache Poisoning))
What is an external authoritative name server? How does it help the attacker to poison the