DNS (Domain Name System)
We human beings can be identified in many ways. For example, we can be identified by the names that appear on our birth certificates. We can be identified by our driver’s license numbers. Just as humans can be identified in many ways ,so too can Internet hosts.Hostnames (such as cnn.com yahoo.com) are mnemonic and are therefore appreciated by humans. Hostnames can consist of variable-length alphanumeric characters, they would be difficult to process by routers.For these reasons, hosts are also identified by so-called IP addresses.
An IP address consists of four bytes and has a rigid hierarchical structure. An IP address looks like 126.96.36.199, where each period separates one of the bytes expressed in decimal notation from 0 to 255.
People prefer the more mnemonic hostname identifier, while routers prefer fixed-length, hierarchically structured IP addresses. In order to reconcile these preferences, we need a directory service that translates hostnames to IP addresses. This is the main task of the Internet’s domain name system (DNS).
The DNS is a distributed database implemented in a hierarchy of DNS servers, and an application-layer protocol that allows hosts to query the distributed database.DNS consists of name servers and resolvers.
We will focus on the hostname-to-IP-address translation service. DNS in the user’s host then takes over, sending a query message into the network. All DNS query and reply messages are sent within UDP datagrams to port 53. DNS in the user’s host receives a DNS reply message that provides the desired mapping. This mapping is then passed to the invoking application. Thus, from the perspective of the invoking application in the user’s host, DNS is a black box providing a simple, straightforward translation service.
There are three classes of DNS servers (root DNS servers, top-level domain (TLD) DNS servers, and authoritative DNS servers) organized in a hierarchy.
Suppose a DNS client wants to determine the IP address for the hostname www.amazon.com. To a first approximation, the following events will take place. The client first contacts one of the root servers, which returns IP addresses for TLD servers for the top-level domain com. The client then contacts one of these TLD servers, which returns the IP address of an authoritative server for amazon.com. Finally, the client contacts one of the authoritative servers for amazon.com, which returns the IP address for the hostname www.amazon.com.
1- Root DNS servers : In the Internet there are 13 root DNS servers (labeled A through M).Although we have referred to each of the 13 root DNS servers as if it were a single server,each “server” is actually a network of replicated servers,for both security and reliability purposes.
2- Top-level domain (TLD) servers : These servers are responsible for top-level domains such as com, org, net, edu, and gov, and all of the country top-level domains such as uk, fr, ca, and jp.
3- Authoritative DNS servers. Every organization with publicly accessible hosts (such as Web servers and mail servers) on the Internet must provide publicly accessible DNS records that map the names of those hosts to IP addresses. An organization’s authoritative DNS server houses these DNS records. An organization can choose to implement its own authoritative DNS server to hold these records; alternatively, the organization can pay to have these records stored in an authoritative DNS server of some service provider. Most universities and large companies implement and maintain their own primary and secondary (backup) authoritative DNS server.