Exam 70-291 – DNS Pt. 3

DNS is required by Active Directory, so you can definitely expect this to be a popular topic on exam 70-291 (as well as many of the other MCSA/MCSE exams). There is no getting around it – you must learn as much as possible about DNS.

A DNS server can perform recursive or iterative queries. With a recursive query, the DNS client requires the DNS server to respond with the IP address of the request or an error message that the requested name does not exist. The DNS server can not refer the client to another DNS server if it is unable to map the request to an IP address. When a DNS server receives a recursive request, it queries other DNS servers until it finds the information or until the query fails. In other words, the DNS server can not refer the DNS client to another DNS server, as it can with an iterative query.

With an iterative query, the DNS server uses zone information and its cache to return the best possible answer to the client. If the DNS server does not have the requested information, it can refer the client to another DNS server.

For example, when a DNS client enters wp3.lockergnome.com into a browser, the following process occurs:

  1. A DNS request is sent to the local DNS server. This may be a DNS server on the client’s local network or a DNS server at the client’s Internet service provider (ISP).

  2. Before forwarding the request to a root server, the DNS server checks its local cache to determine whether the name has recently been resolved. If there is an entry in the local cache, the IP address is returned to the client.
  3. If no entry exists in the cache for the hostname, the request is sent to a root name server.
  4. The root name server refers the request to a name server responsible for the first-level domain within the hostname. For example, the root name server would refer the request to the lockergnome.com DNS server.
  5. The request is referred to second-level DNS servers, and then third-level DNS servers, until one of them can resolve the request to an IP address and return the results back to the client.

Now that you have a general idea what happens when a DNS client attempts to resolve a host name to an IP address, you are ready to begin looking at several other aspects of DNS such as the different types of DNS servers, zone types, and how to install DNS on a server running Windows Server 2003.

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