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

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).(fig 29.2) Top level domain names (fig 29.1): com, edu, gov, mil, net, org, arpa, country code.(fig 29.1) 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.

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)(fig 29.3)

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.

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.

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.

Optimization of DNS Performance root server replication DNS caching used by local server to minimize frequency of contact to authoritative server

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 software) – CNAME (aliasing) – NS (Name server)

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