前面文章提到域名的主从 nameservers，里面用到的例子是使用了三家服务商，一共设置了 7 个 nameservers。
那么这里有一个问题就提出来了，最好给域名设置多少个DNS解析服务器（nameserver）？
根据规范，RFC1537 的说明。在第六节里有例子。
For each top level domain there should be 2 secondary servers in Europe and 2 in the USA, but there may of course be more on either side. An excessive number of nameservers is not a good idea though; a recommended maximum is 7
nameservers. In Europe, EUnet has offered to run secondary server for each European top level domain.
意思是使用过多的 nameserver 不是一个好想法，建议的最大数目是 7。而例子里是一个主 nameserver，然后两个 secondary 服务器放在美国，另外两个放在欧洲，一共是 5个而已。
而使用 BuddyNS 的免费工具检查域名的DNS Delegation的情况是给出的警告信息是这样的：
g2list.win delegates its DNS to 7 servers. There are very few advantages in running more than 5 servers, and some disadvantages:
- Bigger response packets. Some DNS implementations of these servers may cause TrunCation (TC) of DNS responses, based on how they build the Additional section. This causes major slowdowns, and in some cases entire resolution errors.
- Harder maintenance. Each system is as weak as its weakest node.
意思就是你用了 7 个服务器，使用超过 5 个服务器只能带来极少的好处，而坏处多多。比如更大的响应包，延迟更久，甚至解析错误，维护更加困难。
所以这里DavidYin给出自己的建议。