With respect to Domain Name System (DNS): (I) Why DNS is required? (Describe wit
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With respect to Domain Name System (DNS): (I) Why DNS is required? (Describe with respect to the Internet protocol stack and Web servers) (II) Why DNS is operated in a distributed and hierarchical manner? (III) Run the following command in linux terminal: host -v uiowa.edu. Include the result and discuss about that. (IV) How can you modify your operating system configurations so that when a DNS query is made, that is sent to a specific DNS sever (e.g., Google's)? (V) Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT of RTT_1 + RTT_2 +... + RTT_n. Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let RTT_0 denote the RTT between the local host and the server containing the object. Assuming zero transmission time of the object, how much time el apses from when the client clicks on the link until the client receives the object?Explanation / Answer
1) The Domain Name System (aka DNS) is used to resolve human-readable hostnames like www.Dyn.com into machine-readable IP addresses like 204.13.248.115. DNS also provides other information about domain names, such as mail services.It is like a phone book for the Internet. If you know a person’s name but don’t know their telephone number, you can simply look it up in a phone book.
2) The DNS is operated in distributed and hierarchical manner becuase of the following reason:-
The first step in this process begins when you ask your computer to resolve a hostname, such as visiting http://google.com. The first place your computer looks is its local DNS cache, which stores information that your computer has recently retrieved.
If your computer doesn’t already know the answer, it needs to perform a DNS query to find out.
If the information is not stored locally, your computer queries (contacts) your ISP’s recursive DNS servers. These specialized computers perform the legwork of a DNS query on your behalf. Recursive servers have their own caches, so the process usually ends here and the information is returned to the user.
If the recursive servers don’t have the answer, they query the root nameservers. A nameserver is a computer that answers questions about domain names, such as IP addresses. The thirteen root nameservers act as a kind of telephone switchboard for DNS. They don’t know the answer, but they can direct our query to someone that knows where to find it.
The root nameservers will look at the first part of our request, reading from right to left — www.google.com — and direct our query to the Top-Level Domain (TLD) nameservers for .com. Each TLD, such as .com, .org, and .us, have their own set of nameservers, which act like a receptionist for each TLD. These servers don’t have the information we need, but they can refer us directly to the servers that do have the information.
The TLD nameservers review the next part of our request — www.google.com — and direct our query to the nameservers responsible for this specific domain. These authoritative nameservers are responsible for knowing all the information about a specific domain, which are stored in DNS records. There are many types of records, which each contain a different kind of information. In this example, we want to know the IP address for www.google.com, so we ask the authoritative nameserver for the Address Record (A).
The recursive server retrieves the A record for dyn.com from the authoritative nameservers and stores the record in its local cache. If anyone else requests the host record for dyn.com, the recursive servers will already have the answer and will not need to go through the lookup process again. All records have a time-to-live value, which is like an expiration date. After a while, the recursive server will need to ask for a new copy of the record to make sure the information doesn’t become out-of-date.
Armed with the answer, recursive server returns the A record back to your computer. Your computer stores the record in its cache, reads the IP address from the record, then passes this information to your browser. The browser then opens a connection to the webserver and receives the website.
This entire process, from start to finish, takes only milliseconds to complete.
3)The host command is used here is for DNS lookup and the option -v gives us the verbose output of the command ie TTL time of each DNS servers alog with their ip addresses.
4) The first thing you have to do for sending a request to a particular DNS is to run nslookup or host -v command to look for DNS servers available.Write down the DNS you want to use.
The second thing you have to do is to Go to your connection properties => Internet Procol (TCP/IP). (the place where you can set a static IP)
Here you can manually define which DNS you want to use for each connection.
5)2RTT_0 + RTT_1 + ... + RTT_n
as RTT_0 is spent on connection request, and the other RTT_0 on requesting the URL webpage.
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