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