please give me clear picture

Section 1: DNS nslookup

The Domain Name System (DNS) translates hostnames to IP addresses, fulfilling a critical role in the Internet infrastructure. In this lab, well take a closer look at the client side of DNS. Recall that the clients role in the DNS is relatively simple a client sends a query to its local DNS server, and receives a response back. Much can go on under the covers, invisible to the DNS clients, as the hierarchical DNS servers communicate with each other to either recursively or iteratively resolve the clients DNS query. From the DNS clients standpoint, however, the protocol is quite simple a query is formulated to the local DNS server and a response is received from that server.

In this lab, well make extensive use of the nslookup tool, which is available in most Linux/Unix, Mac, and Microsoft platforms today.

1. Start in Linux/Unix/Mac by opening a terminal.

2. Or in Windows, open the Command Prompt (Press x and the windows logo key at the same time, then click Run, type cmd).

In it is most basic operation, nslookup tool allows the host running the tool to query any specified DNS server for a DNS record. The queried DNS server can be a root DNS server, a top-level-domain DNS server, an authoritative DNS server, or an intermediate DNS server (see the textbook for definitions of these terms). To accomplish this task, nslookup sends a DNS query to the specified DNS server, receives a DNS reply from that same DNS server, and displays the result.

Now consider the second command:

nslookup type=NS mit.edu

In this example, we have provided the option -type=NS and the domain mit.edu. This causes nslookup to send a query for a type-NS record to the default local DNS server. In words, the query is saying, please send me the host names of the authoritative DNS for mit.edu. (When the type option is not used, nslookup uses the default, which is to query for type A records.) The answer, displayed in the above screenshot, first indicates the DNS server that is providing the answer (which is the default local DNS server) along with three MIT nameservers. Each of these servers is indeed an authoritative DNS server for the hosts on the MIT campus. However, nslookup also indicates that the answer is non-authoritative, meaning that this answer came from the cache of some server rather than from an authoritative MIT DNS server. Finally, the answer also includes the IP addresses of the authoritative DNS servers at MIT. (Even though the type-NS query generated by nslookup did not explicitly ask for the IP addresses, the local DNS server returned these for free and nslookup displays the result.)

Now finally consider the third command:

nslookup www.aiit.or.kr bitsy.mit.edu

In this example, we indicate that we want to the query sent to the DNS server bitsy.mit.edu rather than to the default DNS server (dns-prime.poly.edu). Thus, the query and reply transaction takes place directly between our querying host and bitsy.mit.edu. In this example, the DNS server bitsy.mit.edu provides the IP address of the host www.aiit.or.kr, which is a web server at the Advanced Institute of Information Technology (in Korea).

Now that we have gone through a few illustrative examples, you are perhaps wondering about the general syntax of nslookup commands. The syntax is:

nslookup option1 option2 host-to-find dns-server

In general, nslookup can be run with zero, one, two or more options. And as we have seen in the above examples, the dns-server is optional as well; if it is not supplied, the query is sent to the default local DNS server. Now that we have provided an overview of nslookup, it is time for you to test drive it yourself. Answer the following questions in your lab report:

3. Run nslookup to obtain the IP address of a Web server in Asia. Submit your screenshot of nslookup command. What is the IP address of that server?

4. Run nslookup to determine the authoritative DNS servers for a university in Europe, e.g, University of Cambridge. Submit your screenshot of nslookup command.

5. Run nslookup so that one of the DNS servers obtained in Question 4 is queried for the mail servers for Yahoo! Mail (mail.yahoo.com). Submit your screenshot of nslookup command. What is its IP address?