2.4.1 Services Provided by DNS

Example of a DNS lookup

1. The same user machine runs the client side of DNS application. 2. The browser extracts the hostname, www.someschool.edu , from the URL and passes the hostname to the client side of the DNS application. 3. The DNS client sends a query containing the hostname to a DNS server. 4. The DNS client eventually receives a reply, which includes the IP address for the hostname. 5. Once the browser receives the IP address from DNS, it can initiate a TCP connection to the HTTP server located at port 80 at that IP address.

Host Aliasing

A host with a complicated hostname can have one or more alias names. DNS can be invoked by an application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host.

Load Distribution

DNS is also used to perform load distribution among replicated servers, such as replicated Web servers. Busy websites, such as cnn.com, are replicated over multiple servers, with each server running on different end systems and each having a different IP address. for replicated Web servers, a set of IP address is thus associated with a canonical hostname

Mail Server Aliasing

For obvious reasons, it is highly desirable that e-mail address be mnemonic (easy to remember). DNS can be invoked by a mail application to obtain the canonical hostname for a supplied alias hostname as well as the IP address of the host. In fact, the MX record permits a company's mail server and Web server to have identical (aliased) hostnames.

DNS

The DNS is (1) a distributed database implemented in a hierarchy of DNS servers, and (2) an application layer protocol that allows hosts to query the distributed database. The DNS protocol runs over UDP and uses port 53. DNS is commonly employed by other application-layer protocols - including HTTP and SMTP to translate user supplied hostnames to IP addresses.