You have been recently promoted to the position of a Network Administrator of a

Question

You have been recently promoted to the position of a Network Administrator of a network which has 15 Windows Servers 2012 EE & 200 Windows 7 workstations.   Recently some of the users are having difficulty resolving some domain names. You are now responsible to provide some solutions. In addition, you have been asked to train a new network coordinator who asked you question on DNS caching. Describe how DNS servers cache data to prevent redundant inquiries. She also asked you to explain the differences between recursive and iterative queries, and the function of a DNS forwarder. Finally she wants you to explain the need for reverse name resolution and how it works.

Explanation / Answer

Hi

Here i will be covering DNS Caching, Forwarder, Recursive, Iterative resolution & Inverse DNS.

DNS caching:

If a DNS Lookup sounds like it might take a few days, it doesn't. It may however take a few seconds. And it happens many times a day. It may occur several times before a full page can be displayed, if the page is composed of elements on different servers (ads, for example). There are times when the Internet is so congested that the wait is annoying. And if the DNS request requires a dial-up connection to be established, the browser may sometimes time out, requiring the user to renew the request. Every time the browser seeks a page on a website, the DNS lookup is repeated. If the user visits twenty pages on one site, the lookup is repeated twenty times. All for the same identical IP address. If the lookup takes three seconds, that is a total of sixty seconds spent waiting.

One solution to these delays is to store a copy of the domain names and their corresponding addresses in a computer on the LAN. This is called DNS caching.

When a caching DNS server tracks down the answer to a client's query, it returns the answer to the client. But it also stores the answer in its cache for the period of time allowed by the records' TTL value. The cache can then be used as a source for subsequent requests in order to speed up the total round-trip time.

Forwarding DNS Server

A forwarding DNS server will look almost identical to a caching server from a client's perspective, but the mechanisms and work load are quite different.

A forwarding DNS server offers the same advantage of maintaining a cache to improve DNS resolution times for clients. However, it actually does none of the recursive querying itself. Instead, it forwards all requests to an outside resolving server and then caches the results to use for later queries.

Recursive Resolution
The client (resolver) can ask for a recursive answer from a name server. This means that the resolver expects the server to supply the final answer. If the server is the authority for the domain name, it checks its database and responds. If the server is not the authority, it sends the request to another server (the parent usually) and waits for the response. If the parent is the authority, it responds; otherwise, it sends the query to yet another server. When the query is finally resolved, the response travels back until it finally reaches the requesting client. This is called recursive resolution.

Iterative Resolution

If the client does not ask for a recursive answer, the mapping can be done iteratively. If the server is an authority for the name, it sends the answer. If it is not, it returns (to the client) the IP address of the server that it thinks can resolve the query. The client is responsible for repeating the query to this second server. If the newly addressed server can resolve the problem, it answers the query with the IP address; otherwise, it returns the IP address of a new server to the client. Now the client must repeat the query to the third server. This process is called iterative resolution because the client repeats the same query to multiple server

Reverse DNS & How It works:

DNS originally included a feature called inverse querying that would allow this type of “opposite” resolution. A resolver could send a query which, instead of having a name filled in and a space for the server to fill in the IP address, had the IP address and a space for the name. The server would check its resource records and return the name to the resolver.

The inverse domain is used to map an address to a name. This may happen, for example, when a server has received a request from a client to do a task. Although the server has a file that contains a list of authorized clients, only the IP address of the client (extracted from the received IP packet) is listed. The server asks its resolver to send a query to the DNS server to map an address to a name to determine if the client is on the authorized list. This type of query is called an inverse or pointer (PTR) query.

Thanks

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