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Understanding DNS
A domain name system (DNS) is a distributed hierarchical system that converts hostnames to IP addresses. DNS is divided into sections called zones. Each zone has name servers that respond to the queries belonging to their zones.
It is easier for most people to remember names rather than numbers especially if those numbers are IPv4 or IPv6 addresses. Because of this, DNS servers are used to map device hostnames to IP addresses. DNS allows you to use names to designate key external systems such as file and log servers that your device might need to contact. The DNS server maintains a centralized repository for device hostnames on the network, ensuring that each device hostname is unique. This centralized repository makes it easier to query and to administer translations between the network IP addresses and hostnames. You can configure your device to query one or more DNS servers by specifying the DNS server IP addresses in your Junos OS configuration.
DNS Components
DNS includes three main components:
- DNS resolver—Resides on the client side of the DNS. When a user sends a hostname request, the resolver sends a DNS query request to the name server to request the hostname's IP address.
- Name server—Processes the DNS query requests received from the DNS resolver and returns the IP address to the resolver.
- Resource records—Data elements that define the basic structure and content of DNS.
DNS Server Caching
A DNS name server is responsible for providing the hostname IP address to users. The TTL field in the resource record defines the period for which DNS query results are cached. When the TTL value expires, the name server sends a fresh DNS query and updates the cache.
