Chapter 29 Domain Name System.ppt


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Chapter 29 Domain Name System.ppt

  1. 1. Chapter 29 Domain Name System (DNS) <ul><li>Allows users to reference computer names via symbolic names </li></ul><ul><li>translates symbolic host names into associated IP addresses </li></ul><ul><li>A global directory service </li></ul>
  2. 2. DNS Design <ul><li>naming scheme used in the Internet where each computer name consists of a sequence of alpha-numeric segments separated by periods forming unique fully qualified domain name (FQDN) </li></ul><ul><li>Domain names are hierarchical (fig 29.2) . </li></ul><ul><li>Top level domain names (fig 29.1) : com, edu, gov, mil, net, org, arpa, country code. </li></ul><ul><li>To obtain a domain, an organization must register with one of the ICANN accredited registrars (eg. A unique domain suffix is assigned to each organization. </li></ul>
  3. 3. DNS Design (cont) <ul><li>Each organization has authority over the hostnames under its domain name. </li></ul><ul><li>DNS namespace divided into zones </li></ul><ul><li>Each authoritative DNS server is responsible for the names in its zone. </li></ul><ul><li>DNS naming system comprised of a large distributed database. </li></ul><ul><li>DNS servers may be maintained by separate organizations </li></ul><ul><li>DNS servers are linked to one another (fig 29.3) </li></ul>
  4. 4. DNS Server Hierarchy <ul><li>DNS servers are arranged in a hierarchy that matches the naming hierarchy, with each being the authority for part of the naming hierarchy. </li></ul><ul><li>A root server (eg. occupies the top of the hierarchy, and is an authority for the top-level domains (eg. .com) </li></ul><ul><li>A single server must be responsible for all computers that have a given suffix. </li></ul><ul><li>All domain name servers are linked together to form a unified system. </li></ul><ul><li>Each server knows how to reach a root server and how to reach servers that are authorities for names further down the hierarchy. </li></ul>
  5. 5. DNS Client-Server Model <ul><li>The client (eg. nslookup) places the name to be translated in a DNS request message to a name server (named), which finds the corresponding address and sends a reply message. </li></ul><ul><li>If name server cannot answer a request, it temporarily becomes the client of another name server, until a server is found that can answer the request. </li></ul>
  6. 6. Name Resolution <ul><li>translation of a domain name by software (name resolver) into an equivalent IP address </li></ul><ul><li>Host name is said to resolved to an address. </li></ul><ul><li>In Unix, name resolver uses gethostbyname routine. </li></ul><ul><li>Each resolver is configured with the address(es) of a local domain name server(s). </li></ul><ul><li>When a DNS request contains a name for which a server is an authority, the server answers the request directly to the resolver(client). </li></ul><ul><li>Iterative query resolution </li></ul><ul><ul><li>when a request arrives for a name server outside the set for which the server is an authority, the server becomes a client of a root server and of other servers down the hierarchy until a server which has authority for the domain is found . </li></ul></ul>
  7. 7. Optimization of DNS Performance <ul><li>root server replication </li></ul><ul><li>DNS caching used by local server to minimize frequency of contact to authoritative server </li></ul>
  8. 8. Types of DNS Entries <ul><li>Each entry in a DNS database consists of a domain name, a record type and a value. </li></ul><ul><li>DNS query requests specify both a domain name and a type; the server only returns a binding that matches the type of the query. </li></ul><ul><li>Example record types: </li></ul><ul><ul><li>A (address) </li></ul></ul><ul><ul><li>MX(mail exchanger type used by email software) </li></ul></ul><ul><ul><li>CNAME (aliasing) </li></ul></ul><ul><ul><li>NS (Name server) </li></ul></ul>
  9. 9. Abbreviations and the DNS <ul><li>domain name server does not understand abbreviation and only responds to a full name. </li></ul><ul><li>Resolvers are programmed to try a set of suffixes, allowing user to use abbreviations (no suffix) for local names </li></ul>