DNS Resolver

Domain Name System (DNS) servers store and manage information about domains and respond to resolution requests for clients (in some cases millions of times each day). The DNS database is a distributed name database stored on many DNS servers. DNS uses a hierarchical tree structure for its name space and a hierarchical tree for name authorities and registration.

Since information in DNS is stored in a distributed form, there is no single server that has information about every domain in the system. The process of resolution instead relies on the hierarchy of name servers as described above.

At the top of the DNS hierarchy is the root domain and the root name servers. These are the most important servers because they maintain information about the top-level domains within the root. They also know the servers that can be used to resolve domains one level below them. Those servers can reference servers that are responsible for second-level domains. Thus, a DNS resolution requests might be sent to more than one server.

An Embedded DNS Resolver is capable of resolving the IP address of a host from the host's name. It does this by sending DNS requests to a DNS Server. The IP address of a DNS Server is specified in the configuration or can be obtained from the DHCP Server for the Local Area Network.

The Embedded DNS Resolver caches the resolved IP addresses. The length of time the resolved host IP address is kept in the local cache depends on the Time to Live (TTL) timeout. This is returned in an answering packet from the DNS Server. The next time a DNS is requested, the cache table is checked first. If a valid host is found, the IP address is resolved from the cache and no actual DNS request is sent to the DNS Server.

You must use the DNS Resolver when a remote host uses a Dynamic IP, which changes each time the remote host logs on to the internet.