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Google Public DNS is a Domain Name System (DNS) service offered by Google. It functions as a recursive name server providing domain name resolution for any host on the Internet. The service was announced on 3 December 2009, in an effort described as making the web faster and more secure. According to Google, as of 2013[update], Google Public DNS is the largest public DNS service in the world, handling more than 130 billion requests per day.
Google Public DNS operates recursive name servers for public use at the following IP addresses: 18.104.22.168 and 22.214.171.124 for IPv4 service, as well as 2001:4860:4860::8888 and 2001:4860:4860::8844, for IPv6 access. The addresses are mapped to the nearest operational server by anycast routing.
The service does not use conventional DNS name server, such as BIND, instead relying on a custom-built implementation, with limited IPv6 support, conforming to the DNS standards set forth by the IETF. It fully supports the DNSSEC protocol since 19 March 2013. Previously Google Public DNS accepted and forwarded DNSSEC-formatted messages but did not perform validation.
There have been instances of DNS providers practicing DNS hijacking while processing queries, that is, redirecting web browsers to an advertisement site operated by the provider when a nonexistent domain name is entered. This is considered an intentional breaking of the DNS specification. The Google service correctly replies with a non-existent domain (NXDOMAIN) response. The correct implementation of the DNS specification is a reason to justify using the service.
The Google service also addresses DNS security. A common attack vector is to interfere with a DNS service to achieve redirection of web pages from legitimate to malicious servers. Google documents efforts to be resistant to DNS cache poisoning, including “Kaminsky Flaw” attacks as well as denial-of-service attacks.
Google claims various efficiency and speed benefits, such as using anycast routing to send user requests to the closest data center, over-provisioning servers to handle denial-of-service attacks, and load-balancing servers using two cache levels, with a small per-host cache containing the most popular names and another pool of servers partitioned by the name to be looked up. This second level cache reduces the fragmentation and cache miss rate that can result from increasing the number of servers.
It is stated that for the purposes of performance and security, only the querying IP address, which is deleted after 24-48 hours, ISP, and location information (kept permanently) are stored on the servers.
At the launch of Google Public DNS, it did not directly support DNSSEC. Although RRSIG records of course could be queried, the AD flag (Authenticated Data, meaning the server was able to validate signatures for all of the data) was never set in the launch version. This was upgraded on 28 January 2013, when Google's DNS servers silently started providing DNSSEC validation information, but only if the client explicitly set the DNSSEC OK (DO) flag on its query. This service requiring a client-side flag was replaced on 6 May 2013 with full DNSSEC validation by default, meaning all queries will be validated unless clients explicitly opt out.
Since June 2014, Google Public DNS automatically detects nameservers that support edns-client-subnet (ECS) options as defined in the IETF draft (by probing nameservers at a low rate with ECS queries and caching the ECS capability), and will send queries with ECS options to such nameservers automatically.
In March 2014, use of Google Public DNS was blocked in Turkey after it was used by users to circumvent the blocking of Twitter, which took effect on 20 March 2014 under court order. The block was the result of earlier remarks by Prime Minister Tayyip Erdogan who vowed to "wipe out Twitter" following damaging allegations of corruption in his inner circle. The method became popular after it was determined that a simple domain name block was used to enforce the ban, which would easily be bypassed by using an alternate DNS system. Activists distributed information on how to use the service, and spray-painted the IP addresses used by the service as graffiti on buildings. Following the discovery of this method, the block was changed to directly block Twitter's IP address, and Google Public DNS was blocked entirely.