resolver 和 name server 的不同

resolver 一般是指系统提供解析服务的库文件，是它去向 name server 进行查询。简单的解释如下：
The resolver is the client half of the Domain Name System. It's responsible for translating a program's request for host information into a query to a name server and for translating the response into an answer for the program.


Name servers' query messages aren't that much different from resolvers' query messages in the first place. The primary difference in the query messages is that resolvers request recursive resolution and name servers seldom do. Requesting recursion is the default with nslookup, so you have to explicitly turn it off. The difference in operation between a resolver and a name server is that the resolver applies the search list, and the name server doesn't. By default, nslookup applies the search list, so that must be explicitly turned off as well. Of course, judicious use of the trailing dot will have the same effect.