Wireshark Lab DNS v6 01 Supplement to Computer Networking A Top Down th Approach 6 ed J F Kurose and K W Ross Tell me and I forget Show me and I remember Involve me and I understand Chinese proverb 2005 21012 J F Kurose and K W Ross All Rights Reserved 1 As described in Section 2 5 of the text 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 References to figures and sections are for the 6 edition of our text Computer Networks A Top down Approach 6 ed J F Kurose and K W Ross Addison Wesley Pearson 2012 1 th th 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 dnsprime 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 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 WARNING the example below uses the server bitsy mit edu When you try this command at home please replace bitsy mit edu with Google s public DNS server 8 8 8 8 since bitsy mit edu is no longer in use and it will not respond 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 Do the following and write down the results 1 Run nslookup to obtain the IP address of a Web server in Asia What is the IP address of that server 2 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 What is its IP address 2 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 into the Command Prompt as shown in the following screenshot ipconfig is also very useful …