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server iteratively contacted several other DNS servers to get the answer, as described in Section 2.5 of the textbook. 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”. Thiscauses 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
4answer 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 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 , 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. Do the following (and write down the results): 1. Run nslookup to obtain the IP address of a Web server in Asia. IP address = 184.108.40.206
52. Run nslookup to determine the authoritative DNS servers for a university in Europe. 3. Run nslookup so that one of the DNS servers obtained in Question 2 is queried for the mail servers for Yahoo! mail. [ipconfig ]ipconfig (for Windows) and ifconfig (for Linux/Unix) are among the most useful little utilities in your host, especially for debugging network issues. Here we’ll only describe ipconfig, although the Linux/Unix ifconfig is very similar. ipconfig can be used to show your current TCP/IP information, including your address, DNS server addresses, adapter type and so on. For example, if you all this information about your host simply by entering ipconfig \all
6into the Command Prompt, as shown in the following screenshot.