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Chapter 29 Domain Name System.ppt
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Chapter 29 Domain Name System.ppt

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Transcript

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