Domain Name System
- 1. DOMAIN NAME SYSTEM M.Owais Asghar Jara
- 2. DOMAIN NAME SYSTEM (DNS) People prefer to use names instead of the addresses so the Internet needs to have a directory system that can map a name to an address. As internet is so huge today so a central directory can not hold that much mapping and disadvantage is if central computer fails whole communication will collapse. So information is distributed among many computers in this world. In this method the host that needs mapping can contact to its closest computer holding the needed information. This method is used by DOMAIN NAME SYSTEM (DNS).
- 3. CONTINUED…. The following six steps map the host to an IP address. 1. The user passes the host name to the file transfer client. 2. The file transfer client passes the host name to the DNS client. 3. Each computer after booting Knows the address of one DNS server. The DNS client sends a message to a DNS server with a query that gives the file transfer server user name using the known IP address of the DNS server. 4. The DNS server responds with the IP address of the desired file transfer server. 5. The DNS server passes the IP address of the file transfer client. 6. The file transfer client now uses the received IP address to access the file transfer server.
- 5. NAME SPACE A ‘Name Space’ that maps each address to a unique name can be organized in two ways: 1. Flat Name Space: A name in this space is a sequence of characters without structure. The names may or may not have a common section if they do, it has no meaning. Disadvantage can not be used in large systems because it must be centrally controlled to avoid ambiguity and duplication. 2. Hierarchical Name Space: In this space each name is made of several parts. The first can define the name of the organization, second can define its nature, 3rd can define the departments and so on. In this case the authority to assign and control can be decentralized. the first organization can choose first.com and now the hosts can choose name of
- 7. DOMAIN NAMES AND LABELS LABEL: Each node in a tree has a label which is a string of maximum 63 characters. The root label is null string (Empty). DNS requires children of a node have different label which guarantees the uniqueness of the domain name. DOMAIN NAME: Each node in a tree has a domain name. A full domain name is a sequence of labels separated by dots (.) Always read from node up to the root. Fully Qualified Domain Name (FQDN) is that if a name ends with a dot that means null label. Partially Qualified Domain Name(PQDN) is a name if it does not end with a null string i.e. it starts from node but does not end at root. (Can be made FQDN by supplying the missing part called suffix).
- 9. DOMAIN A domain is a subtree of the domain name space. The name of the domain is the name of the node at the top of the subtree. (Reliability)
- 10. HIERARCHY OF NAME SERVERS The whole information can not be stored in the single computer. Solution is divide the whole space into many domains based on the first level. DNS allows domains to be further divided into the sub-domains where each server is responsible of for its domain that can be large or small. The domain server that has sub-domain servers will must have some way to contact to its sub-domains.
- 12. ZONE What a server is responsible for or has authority over is called ZONE. It is contiguous part of entire tree. If a server accepts responsibility of a domain and does not divide it into sub domains then domain and zone are same thing. The server will keep record of nodes in a database called a zone file. If a server divides into subdomains then server and domains are two different things. Information about the noes in the subdomains is stored in the servers at lower levels, with the original server keeping some sort of reference to the lower levels.
- 14. ROOT SERVERS A server whose Zone consists of the whole tree. Keeps reference to other servers in its zone and delegates it authority to other servers. 1. Primary Server: A server that stores a file about the zone for which it is in authority. It is responsible for creating, maintaining and updating the zone file. And stores that file on local disk. 2. Secondary Server: It only copies the information in the primary server and stores it on its own disk. It is not responsible for updating etc. If that is required it must be done through primary server. A sort of backup if primary server fails. A server can be primary server for a specific zone and secondary server for another zone therefore we must be careful about which zone we refer to.
- 15. DNS IN THE INTERNET Generic Domain: Country Domain Inverse Domain Inverse domain are difficult to track, which could be used to find the name of a host when given IP address.
- 19. RESOLUTION