DNS

Question 1

What is a iterative query?

Answer 1

• A client asks the DNS server for an answer, and the server returns the best answer.
• Usually stored in server’s cache.
• If the server doesn’t know the answer, it may direct the client to another server through a referral.

Question 2

What is a recursive query?

Answer 2

-The client sends a query to a name server, asking it to respond the requested answer or with an error message.

Error messages:

The server can’t come up with the right answer.
The domain name doesn’t exist.

Here’s what happens to resolve the request:

1. The resolver sends a recursive DNS query to its local DNS server asking for the IP address of www.whitehouse.gov. The local name server is responsible for resolving the name, and it cannot refer the resolver to another name server.
2. The local name server checks its zones, and it finds no zones corresponding to the requested domain name.
3. The root name server has authority for the root domain and will reply with the IP address of a name server for the .gov top-level domain.
4. The local name server sends an iterative query for www.whitehouse.gov to the Gov name server.
5. The Gov name server replies with the IP address of the name server servicing the domain www.whitehouse.gov
6. The local name server sends an iterative query forwww.whitehouse.gov to the whitehouse.gov name server.
7. The whitehouse.gov name server replies with the IP address corresponding to www.whitehouse.gov.
8. The local name server sends the IP address of www.whitehouse.gov back to the original resolver.

Question 3

What I is an inverse query?

Answer 3

-Inverse queries use pointer (PTR) records.

Instead of supplying a name and then asking for an IP address, the client first provides the IP address and then asks for the name. Because there’s no direct correlation in the DNS namespace between a domain name and its associated IP address, this search would be fruitless without the use of the in-addr.arpa domain. Nodes in the in-addr.arpa domain are named after the numbers in the dotted-octet representation of IP addresses. However, because IP addresses get more specific from left to right and domain names get less specific from left to right, the order of IP address octets must be reversed when building the in-addr.arpa tree. With this arrangement, administration of the lower limbs of the DNS in-addr.arpa tree can be given to companies as they are assigned their Class A, B, or C subnet address or delegated even further down thanks to Variable Length Subnet Masking (VLSM). Once the domain tree is built into the DNS database, a special PTR record is added to associate the IP addresses with the corresponding hostnames. In other words, to find a hostname for the IP address 206.131.234.1, the resolver would query the DNS server for a PTR record for 1.234.131.206.in-addr.arpa. If this IP address is outside the local domain, the DNS server will start at the root and sequentially resolve the domain nodes until arriving at 234.131.206.in-addr.arpa, which would contain the PTR record for the desired host.