Question

2 DNS: Domain Name System (a) Root DNS server (b)TLD DNS server d Local DNS server dns.poly.edu (c) Authoritative DNS server dns.umass.edu Requesting host cis.poly.edu gaia.cs.umass.edu A) Explain the four classes of DNS servers (a, b, c and d) using the above topology B) Explain the procedure of a host to resolve a DNS query using an Iterative query C) Explain the procedure of a host to resolve DNS query using a Recursive Query D) Compare both queries (Iterative and recursive queries) in terms of pros and cons

Answer 1

Ques A.

The Administrator of DNS is structured in a hierarchy using different zones.

In this topology the hierarchy of classes starts from the root DNS nameservers:

ClASS 1 Root DNS nameserver - It operate in the root zone i.e these servers answer the queries that and recorded within the root zone.All the lookups are stored in the Root and once a lookup hits root server it travels down to the hierarchy till authoritative servers.

Root servers refer the queries to Top level domain servers just next to root servers.

CLASS 2 TLD servers - TLD nameservers store the information of all domain names and that of which share a common extension such as .com .org etc

If google.com is searched then after the root nameservers response the TLD server for .com domain will be reached by the resolver. Which would point towards the authoritative server for that extension of domain.

CLASS 3 Authoritative servers - After TLD server the authoritative servers do the final step towards the IP address journey.

Authoritative servers contain the information about the specific domain it serves.The authoritative servers provide resolver with the IP address of that server found in DNS A record or if it has a CNAME record it will point resolver to an alias server where a whole new procedure of lookup will be started by recursive resolver to procure the record by authoritative nameserver.

CLASS 4 Local servers- A local DNS server is which performs the lookup and is actually located at the network in whoch the system of user is connected,suppose if we use an internet provider server then local DNS is working at the IPS.

QUESTION C

In the recursive the DNS server receiving the query does the work of fetching the answers and give it back to user.

The procedure goes like :

1) Suppose the query domain is www.example.com operating systems resolver will send query to DNS server for the A record.

2)any server which gives the request will search for the IP address in the tables and if it does not have the result it will send the request to a DNS root servers which are themselves iterative servers.

3)the DNS root servers will give the response as the list of servers which are responsible for handling.com.

4) DNS server will choose a server from the list of server given by the root DNS server to look up for the query www.example.com.

5) Similar to the rootserver the GTDL's servers are also iterative in nature so they provide list of IP addresses of the day DNS server responsible for the domain we queried for to our DNS server.

6) Our DNS server will select an IP address from the list of IP dresses of the authoritative server and query for the A record of www.example.com. authoritative servers reply with a record of the specific domain queried for.

7) our DNS server will reply back with the IP domain pair and other resources if available. Our browser will send request to the IP given for the webpage www.example.com.

Question B

Iterative domain name servers work in the flow do not give the answer for the query but they reply with list of DNS servers which might have the result

Step 1)for example if we query for www.example.com or operating system will query for record A of the the domain to the DNS server.

Step 2) DNS server lookup for the the IP for the query domain and will reply with the list of addresses of root servers state of querying the root servers.

Step3) Now our operating system resolver will query to the root servers.

Step4) This procedure goes until the IP address for the query is fetched.

Question D

Iterative queries :

Pro's -

1) less risk of DOS attacks.

2) authorised use of resources.

Con's-

1)It can not provide definitive answers.

2) returns an answer without referring to other DNS servers.

Recursive queries :

Pro's-

1) provides definite answers.

2) Gives service from querying to returning the answers to the client.

Con's-

1) DOS attacks.

2) DNS cache poisoning.

3) unauthorised use of resources.

4) when DNS servers are not configured properly when the problem of root name server performance degradation is faced.

Thankyou.