A DNS resolver is a dedicated computer for the name resolution service on the network. It performs only one task; resolve names with IP addresses. Since it resolves names with IP addresses, we call it a resolver computer. In simple words, a resolver computer is a computer that resolves names with IP addresses on the network at the fastest possible speed. It can be a DNS server or a system that is configured to communicate with DNS servers to resolve IP addresses.
Based on the size and requirement, a network can have single or multiple DNS servers. For example, the Internet the largest computer network uses thousands of DNS servers. If a network has only one DNS server and does not use external DNS servers, the role of the resolver system and the DNS server is the same. In this situation, network computers can directly communicate with the DNS server for name resolution. Since the DNS server directly resolves queries, it also works as a resolver system.
If a network has multiple DNS servers or uses external DNS servers, the role of the resolver system and the DNS server is different. In this situation, network computers do not directly communicate with DNS servers for name resolution. They send queries to the resolver system and the resolver system communicates with DNS servers to solve queries.
Characteristics/functions of a resolver system
It removes the burden of managing records in the hosts file from other computers. Once a resolver system is configured, other computers of the network do not need to maintain records in their hosts files.
It reduces the number of DNS servers' IP addresses you need to configure on each network system. Suppose, your network uses multiple DNS servers, without a resolver system, you need to configure the IP address of all DNS servers on all computers. But with the resolver system, you only need to configure the resolver system's IP address on each computer.
It improves the network's performance and efficiency. If the resolver system is configured, only the resolver computer communicates with DNS servers. Other computers don't need to waste their hardware resources to communicate with DNS servers to resolve queries.
It maintains a list of all available DNS servers and sends queries to them in sequence until it gets an authoritative answer for each query.
How does a resolver system work?
Let's take an example to understand how this process works.
Suppose, a system wants to know the IP address of Google.com. For this, it creates a query to know the IP address associated with the name Google.com and sends that query to the resolver system. If the resolver system is a DNS server, it checks its database, finds the IP address associated with the name Google.com, creates a reply message that includes the IP address associated with the name Google.com, and sends that message back to the system which sent the query message.
If the resolver system is not a DNS server, then it works as a mediator between the local system and DNS servers. It receives queries from the local system and forwards that query to all configured DNS servers in a sequence until it gets the IP address of the name it is looking for.
Once it gets the IP address associated with name, it does not send the query to the next DNS server for that name. Usually, resolver systems are also configured to cache the result. If caching is enabled, the resolver system saves a copy of all replies it receives from DNS servers. Next time, if it receives a query for the same name, instead of forwarding the query to DNS servers, it uses locally saved copies to answer the query.
The sender system learns the IP address of Google.com from the reply message it receives from the resolver system and uses that IP address to access Google.com.