10 - Domain Name System.ppt

10 - Domain Name System.ppt

• 1.
  Domain Name System
• 2.
  Uses • translates domainnames (computer hostnames) to IP addresses. • lists mail exchange servers accepting e-mail for each domain. • an essential component of contemporary Internet use. • Originally, each computer on the network retrieved a file called HOSTS which mapped an address (such as 192.0.34.166) to a name (such as www.example.net.) • The Hosts file still exists on most modern operating systems, and allows users to specify an IP address to use for a hostname without checking the DNS. • Modifying the hosts file is done on each and every PC (tiresome!!)
• 3.
  How the DNSworks • The domain name space consists of a tree of domain names. Each node or leaf in the tree has one or more resource records, which hold information associated with the domain name. • The tree sub-divides into zones. A zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. • A resolver (DNS server) looks up the information associated with nodes. A resolver knows how to communicate with name servers by sending DNS requests, and heeding DNS responses.
• 4.
  • A domainname usually consists of two or more parts (technically labels), separated by dots. For example wikipedia.org. • The rightmost label conveys the top-level domain (for example, the address en.wikipedia.org has the top-level domain org). • Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it.
• 5.
  The address resolution mechanism •DNS resolution process for ahost.inadomain.com 1. the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time. 2. query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the com top level domain). 3. querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.com in our example). 4. repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought.
• 7.
  • DNS says,“Go ask the people who manage it, and they will tell you.” Improvement • results of a query are cached for a period of time after a successful answer (the server providing the answer initially dictates the period of validity, which may vary from just seconds to days or even weeks).
• 8.
  Types of NameServers • Name servers that hold all information on hosts within a zone are called authoritative for this zone, and sometimes are referred to as master name servers. Any query for a host within this zone will end up at one of these master name servers. • To distribute work loads, master name servers have secondary servers, which transfer the zone data from the primary (master) server at regular intervals. – provides backup • A name server that is not authoritative for any domain which is able to conduct DNS queries for the applications running on the local network and cache the information, is called a caching-only server.
• 9.
  Reverse Lookups • Findingthe IP address belonging to a host is certainly the most common use for the Domain Name System, but sometimes you'll want to find the canonical hostname corresponding to an address. • Finding this hostname is called reverse mapping, and is used by several network services to verify a client's identity. • When using a single hosts file, reverse lookups simply involve searching the file for a host that owns the IP address in question.
• 10.
  • Detailed tutorialon the net at http://www.learntcpip.com/DNS/License_DNS.htm