8.8.8.8

据http://www.infoq.com/news/2009/12/Public-DNS-Google所言：
Google is offering two DNS servers for public use, namely 8.8.8.8 and 8.8.4.4, in an attempt to further speed up browsing.
google提供了两个DNS服务器： 8.8.8.8 和8.8.4.4

主要是为了解决三个问题：速度、安全、效率

Google finds this DNS latency unacceptable and has opened up two globally distributed DNS servers to public use hoping to improve browsing speed. They are trying to address three main issues:

* Speed: Resolver-side cache misses are one of the primary contributors to sluggish DNS responses. Clever caching techniques can help increase the speed of these responses. Google Public DNS implements pre-fetching: before the TTL on a record expires, we refresh the record continuously, asynchronously and independently of user requests for a large number of popular domains. This allows Google Public DNS to serve many DNS requests in the round trip time it takes a packet to travel to our servers and back.
* Security: DNS is vulnerable to spoofing attacks that can poison the cache of a name server and can route all its users to a malicious website. Until new protocols like DNSSEC get widely adopted, resolvers need to take additional measures to keep their caches secure. Google Public DNS makes it more difficult for attackers to spoof valid responses by randomizing the case of query names and including additional data in its DNS messages.
* Validity: Google Public DNS complies with the DNS standards and gives the user the exact response his or her computer expects without performing any blocking, filtering, or redirection that may hamper a user’s browsing experience.

目前我试用中，巧合的是发现换了这个DNS后google.cn无法访问了，google.com可以访问。