How DNS server works

The DNS works in a manner pretty similar to that of a telephone directory inquiry service. You dial up the inquiry service and ask for a person's telephone number, based on the name. If the person is local, the directory service immediately comes up with the answer. However, if the person happens to be staying in another state, the directory service either directs your call to that state's telephone directory inquiry service, or asks you to call them. Furthermore, if the person is in another country, the directory service takes help of their international counterparts. This is very similar to the way a DNS server works. In the case of the telephone directory service, you tell a person's name and ask for the telephone number. In the case of DNS, you specify the domain name and ask for its corresponding IP address.

Basically, the DNS servers do two things tirelessly:

• Accepting requests from programs for mapping domain names into IP addresses.

• Accepting requests from other DNS servers to map domain names into IP addresses. When such a request comes in, a DNS server has the following options:

• It can supply the IP address because it already knows the IP address for the domain.

• It can contact another DNS server and try to locate the IP address for the name requested. It may have to do this more than once. Every DNS server has an entry called alternate DNS server, which is the DNS server it should get in touch with for unresolved domains. The DNS hierarchy specifies how the chains between the various DNS servers should be established for this purpose.

• It can simply say, 'I do not know the IP address for the domain name you have requested, but here is the IP address for a name server that knows more than I do.' In other words, it suggests the name of another DNS server.

• It can return an error message because the requested domain name is invalid or does not exist. Suppose one host is interested in knowing the IP address of the server at IBM.com. For this purpose, it contacts its nearest DNS server. The DNS server looks at the list of domain names and their IP addresses. It finds an entry for the domain and sends it back to the client computer. However, when the DNS server receives another request from another computer for jklm.com, it replies saying that such a domain name does not exist. As we know, for this it might need to consult other DNS servers to see if they have any idea about this domain name, or it might need to suggest the name of the DNS server that the host should contact itself.

For using DNS, an application program performs the following operations:
I . The application program interested in obtaining the IP address of another host on the Internet calls a library procedure called resolver, sending it the domain name for which the corresponding IP address is to be located. The resolver is an application program running on the host.

2. The resolver sends a UDP packet to the nearest DNS server (called the local DNS server).
3. The local DNS server looks up the domain name and returns the IP address to the resolver.
4. The resolver returns the IP address to the calling application program.

Using the IP address thus obtained, the calling application establishes a transport layer (TCP) connection with the destination, or sends UDP packets, as appropriate. All this happens without the end user being aware of it. When you key in the domain name such as honda.auto.com, to see its Web site, internally, the DNS is used to get the IP address and then the connection is established.

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