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What is Root Hints?

Friday, October 28, 2011

Root hints are DNS data stored in a DNS server. The root hints provides a list of preliminary resource records that can be used by the DNS service to locate other DNS servers that are authoritative for the root of the DNS domain namespace tree. Root hints are used to prepare servers authoritative for non-root zones so that they can learn and discover authoritative servers that manage domains located at a higher level or in other subtrees of the DNS domain namespace. These hints are essential for servers authoritative at lower levels of the namespace when locating and finding servers under these conditions.

For example, If a DNS server (ServerA.corp.com) is answering a query for a domain, such as the anITKB.com domain, ServerA needs some assistance to locate an authoritative server (ServerB.anITKB.com) for this domain. In order for ServerA to find ServerB, or any other servers that are authoritative for the anITKB.com domain, it needs to be able to query the root servers for the DNS namespace. The root servers can then refer ServerA to the authoritative servers for the [com] domain. The DNS servers for the [com] domain can, in turn, offer ServerA a referral to ServerB or other servers that are authoritative for the anITKB.com domain.

By default, the DNS Server service implements root hints using a file, named Cache.dns, stored in the %systemroot%\System32\Dns folder on the server computer. This file normally contains the NS and A resource records for the Internet root servers. If, however, you are using the DNS Server service on a private network, you can edit or replace this file with similar records that point to your own internal root DNS servers.

To get more information about the current 13 Root DNS servers (there are actually more, but distributed via anycast), visit this link: http://icannwiki.org/Root_servers.

Note:
If you are operating internal root servers, do not use root hints. Instead, delete the Cache.dns file entirely for any of your root servers. Your internal root server(s) need(s) to host a forward lookup zone called “.”

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