COMP 3533 Network Infrastructure

COMP 3533 Lab 3 - DNS Questions + Answers Network Infrastructure and Security (Mount Royal University) COMP 3533 Lab 3 - DNS Questions + Answers Network Infrastructure and Security (Mount Royal University) lOMoARcPSD|lOMoARcPSD| Mount Royal University Department of Mathematics and Computing COMP 3533 Network Infrastructure Lab 3: DNS October 02, 2017 Due: October 10, 2017 Submission: Submit an electronic copy on Blackboard As described in Section 2.5 of the textbook, the Domain Name System (DNS) translates hostnames to IP addresses, fulfilling a critical role in the Internet infrastructure. In this lab, we’ll take a closer look at the client side of DNS. Recall that the client’s role in the DNS is relatively simple – a client sends a query to its local DNS server, and receives a response back. As shown in Figures 2.21 and 2.22 in the textbook, 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 client’s DNS query. From the DNS client’s standpoint, however, the protocol is quite simple – a query is formulated to the local DNS server and a response is received from that server. Before beginning this lab, you’ll probably want to review DNS by reading Section 2.5 of the text. In particular, you may want to review the material on local DNS servers, DNS caching, DNS records and messages, and the TYPE field in the DNS record. 1. nslookup In this lab, we’ll make extensive use of the nslookup tool, which is available in most Linux/Unix and Microsoft platforms today. To run nslookup in Linux/Unix, you just type the nslookup command on the command line. To run it in Windows, open the Command Prompt and run nslookup on the command line. 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. Downloaded by Austine Wanjala ()lOMoARcPSD| Figure 1 The above screenshot shows the results of four independent nslookup commands (displayed in the Windows Command Prompt). In this example, the client host is located at MRU, where the default local DNS server is . When running nslookup, if no DNS server is specified, then nslookup sends the query to the default DNS server, which in this case is . Consider the first command: nslookup In words, this command is saying “please send me the IP address for the host ”. As shown in the screenshot, the response from this command provides two pieces of information: (1) the name and IP address of the DNS server that provides the answer; and (2) the answer itself, which is the host name and IP address of . Although the response came from the local Downloaded by Austine Wanjala ()lOMoARcPSD| DNS server at Mount Royal University, it is quite possible that this local DNS 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 In this example, we have provided the option “-type=NS” and the domain “”. 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 ”. (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 mtroyal nameservers. Each of these servers is indeed an authoritative DNS server for the hosts on the MRU 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 MRU DNS server. Finally, the answer also includes the IP addresses of the authoritative DNS servers at MRU. (Even though the type NS querygenerated by nslookup did not explicitly ask for the IP addresses, the local DNS server returned these “for free” and nslookup displays the result.) Now consider the third command: nslookup