What is DNS ?

WHAT IS DNS?

• Domain Name System (DNS)
• Domain Name System (DNS) is a system for naming computers and network services that maps those names to network addresses and organizes them into a hierarchy of domains.
• DNS naming is used on TCP/IP networks, such as the Internet and most corporate networks, to locate computers and services by using user-friendly names.
• When a user enters the DNS name of a computer in an application, DNS can look up the name and provide other information that is associated with the computer, such as its IP address or services that it provides for the network. This process is called name resolution.
• Name systems, such as DNS, make it easier to use network resources by providing users with a way to refer to a computer or service by a name that is easy to remember.
• DNS looks up that name and provides the numeric address that operating systems and applications require to identify the computer on a network.
• For example, users enter www.DigeshTuts.com instead of the numeric IP address of the server to identify a Facebook Web server on the Internet.

Basic definition of DNS server:

• A TCP/IP application protocol that enables a DNS server to resolve (translate):

1.  Domain and computer names to IP addresses
2. IP addresses to domain and computer names

• DNS servers provide the DNS namespace for an enterprise
• One of the requirements for using Active Directory on a Windows Server 2008 network is to have a DNS server on the network.

Two types of DNS server:

1.  Preferred DNS server:
   • The client makes a query or update.
   • The preferred server responds to the query or update first.
2.  Alternate DNS server:
   •  If the preferred DNS server does not respond, the query or update is redirected to the alternate server.
   • If the alternate DNS server does not respond, the query of update is redirected to the next configured alternate DNS server.
   • If none of the DNS servers respond to the query or update, then it fails.
   •  Any server which does not respond is removed from the clients list temporarily.

What is DNS Zones:

•  When we work with DNS, we have many files to manage.Each DNS server may be responsible for many different DNS zones.
• Each zone contains its own files and folders that will require some degree of effort in order to create, maintain, update, manage, and secure.
• As we work with our DNS environment, we will likely break down our forest into smaller segments that in DNS are referred to as zones.

Basic concept of Forward look up ,Reverse look up Zones:

• If the records in our DNS zone database are designed to allow the resolution of a name to an IP address, we would say the zone is working forward, and the zone database type we would create is called a forward lookup zone.
•  If our object is to provide an option for the finding of names based on a provided IP address, then we would say our zone is working in reverse, or we would create a reverse lookup zone.
• What if we wanted a DNS server that only resolved the names of other authoritative DNS servers in our environment? You would create a stub zone to serve our purposes.

Types of zones:( In Forward look up )

•  As we learned earlier, when we install DNS as part of Active Directory, the appropriate forward lookup zones for the domain is created automatically. If we choose to add zones or if we are not using DNS as an integrated part of Active Directory, we will use DNS Manager to create and manage forward lookup zones.
• Not all forward lookup zones are created equally; there are actually three different types, called primary zones, secondary zones, and stub zones.

1.  Primary zones are zones that are created and stored on the local server. They can be updated and maintained directly on the server and can also receive replicated updates from other servers.
2. Secondaryzones are zones that are stored on the local server; however, all of their information comes from updates received from another designated primary server. Secondary servers are a good way to help share the workload that might otherwise be forced onto a standard primary server.
3.  Stub zones create a copy of only the name server records for a given zone. This zone type is useful in helping clients find and query the appropriate internal DNS name server.

Advantages of DNS:

• The most visible one is that it achieves transparency without losing scalability.
• It is transparent because the clients are obliged to use the addresses provided by the authoritative DNS server, and cannot establish whether these addresses belong to the home machine of the service or to any of its replicas.
• DNS as a distributed name resolution service proved to be very efficient, even though the amount of people using it has increased tremendously with the growth of the Internet.
• Another vital advantage of using DNS to redirect clients is that it is a natural way of informing the clients about the service addresses. It is used by many existing network services, and is very likely to be used by those to come as well.
• Moreover, DNS is supported by a huge infrastructure of millions of DNS servers, capable of caching the answers our redirector generates. Once we make this infrastructure work for us, both efficiency and availability of our redirector considerably increase.
•  One more important advantage of DNS is that it allows multiple replicate addresses to be returned, enabling the client to choose one from them.
• The last advantage of DNS-based redirection is its good maintainability. Deployment of the complete redirection mechanism boils down to launching a single modified DNS server, and subsequently delegating a service domain to this server.
• From this moment on this server is responsible for answering requests for the service address. No other modification of the DNS infrastructure is necessary.

Disadvantages of DNS:

• On the other hand, using DNS-based redirection leads to a few difficulties. The first of them is caused by the fact that DNS queries carry no information about the client that triggered the name resolution.
• All that the service-side DNS server knows is the network address of the DNS server that asks about the service location.
• Therefore, we have to assume that clients always use a DNS server that is close to them, and approximate a client's location to that of its DNS server. Whether we consider it to be a drawback or not depends on the accuracy we want to achieve.
• Studies show that 64% of clients are located in the same network as their DNS servers. Thus, as long as we do not need strict per-client redirection, the location of the client DNS server approximates the client well enough.