P19. In this problem, we use the useful tool dig tool available on Unix and Linu
ID: 3623627 • Letter: P
Question
P19. In this problem, we use the useful tool dig tool available on Unix and Linux hosts to explore the hierarchy of DNS servers. Recall that in Figure 2.21, a DNS server higher in the DNS hierarchy delegates a DNS query to a DNS server lower in the hierarchy, by sending back to the DNS client the name of that lower-level DNS server. First read the man page for dig, and then answer the following questions.a. Starting with a root DNS server (from one of the root servers [a-m].root-servers.net), initiate a sequence of queries for the IP address for your department's Web server by using dig. Show the list of the names of DNS servers in the delegation chain in answering your query.
b. Repeat part a) for several popular Web sites, such as google.com, yahoo.com, or amazon.com.
Explanation / Answer
The format to invoke dig is
dig @<name.of.dns.server> <record-type><domain-name>
where,
Let us consider the root server A.ROOT-SERVERS.NET
As we need to know the IP address, we use the type ‘A’.
For example, if we perform the dig on the website google.com, the command to be used is:
dig A.ROOT-SERVERS.NET A google.com
It provides the required output.
The list of the names of DNS servers in the delegation chain for this query are
ns1.google.com
ns4.google.com
ns2.google.com
ns.3google.com
Similarly, if the command is used for yahoo.com, the format is
dig A.ROOT-SERVERS.NET A yahoo.com
and the name servers are
ns3.yahoo.com
ns2.yahoo.com
ns4.yahoo.com
ns8.yahoo.com
ns1.yahoo.com
ns5.yahoo.com
ns6.yahoo.com
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