Chapter 29 Domain Name System DNS

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