Download Ethereal Lab: DNS and more Summaries Computer Networks in PDF only on Docsity!
Ethereal Lab: DNS
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 2.26 and 2.18 in the textbook, much can go on within the network, 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 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 nslookup on the command line. To run it in Windows, open the Command Prompt and run nslookup on the command line. Read the man page for nslookup to learn about all of its functionalities.
In its most basic operation, the 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.
The above screenshot shows the results of three independent nslookup commands (displayed in the Windows Command Prompt). In this example, the client host is located on the campus of Polytechnic University in Brooklyn, where the default local DNS server is dns-prime.poly.edu. When running nslookup, if no DNS server is specified, then nslookup sends the query to the default DNS server, which in this case is dns- prime.poly.edu. Consider the first command:
nslookup www.mit.edu
In words, this command is saying “please send me the IP address for the host www.mit.edu”. 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 www.mit.edu. Although the response came from the local DNS server at Polytechnic 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 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
Open Ethereal and load the file dns-ethereal-trace-1. Enter “ip.addr
==128.238.38.160” into the filter. This is the IP address on which the trace was captured. This filter removes all packets that neither originate nor are destined to the capturing host. The packet capture was generated by opening a web browser, and then visiting the web page: http://www.ietf.org. Once the page was loaded, packet capture was stopped.
Answer the following questions:
- Locate the DNS query and response messages. Are then sent over UDP or TCP?
- What is the destination port for the DNS query message? What is the source port of DNS response message?
- To what IP address is the DNS query message sent? This is the IP address of a local DNS server.
- Examine the DNS query message. What “Type” of DNS query is it? Does the query message contain any “answers”?
- Examine the DNS response message. How many “answers” are provided? What do each of these answers contain?
- Consider the subsequent TCP SYN packet sent by the host. Does the destination IP address of the SYN packet correspond to any of the IP addresses provided in the DNS response message?
- This web page contains images. Before retrieving each image, does the host issue new DNS queries?
Part 2.2: Now let’s examine with nslookup.
Load dns-ethereal-trace-2. This trace was created by performing an nslookup on
www.mit.edu
The trace should look like the following:
We see from the above screenshot that nslookup actually sent three DNS queries and received three DNS responses. For the purpose of this assignment, in answering the following questions, ignore the first two sets of queries/responses, as they are specific to nslookup and are not normally generated by standard Internet applications. You should instead focus on the last query and response messages.
- What is the destination port for the DNS query message? What is the source port of DNS response message?
- To what IP address is the DNS query message sent?
- Examine the DNS query message. What “Type” of DNS query is it? Does the query message contain any “answers”?
- Examine the DNS response message. How many “answers” are provided? What do each of these answers contain?
Part 2.3: Now load the following file: dns-ethereal-trace-3. This trace was generated by issuing the following command:
nslookup –type=NS mit.edu
