How do DNS clients and servers communicate during queries?

1 DNS Client

A DNS client, also known as a resolver, is a software component that initiates a DNS query when a user requests a domain name. The resolver can be part of the operating system, the browser, or a third-party application. The resolver's main function is to send a DNS query message to a DNS server and receive a DNS response message with the requested information or an error code.

2 DNS Server

A DNS server, also known as a name server, is a hardware or software device that stores and provides DNS records for domain names. There are different types of DNS servers, such as authoritative, recursive, caching, and forwarding servers, that perform different roles in the DNS hierarchy. A DNS server's main function is to process DNS query messages from resolvers and return DNS response messages with the relevant records or referrals.

3 DNS Query Message

A DNS query message is a packet of data that contains the domain name that the resolver wants to resolve and other parameters, such as the query type, the class, and the recursion desired flag. The query type specifies the kind of record that the resolver is looking for, such as A (address), MX (mail exchange), or NS (name server). The class indicates the protocol family, such as IN (internet) or CH (chaos). The recursion desired flag tells the server whether to perform recursion or not.

4 DNS Response Message

A DNS response message is a packet of data that contains the answer to the DNS query message and other information, such as the response code, the authority section, and the additional section. The response code indicates the status of the query, such as NOERROR (success), NXDOMAIN (non-existent domain), or SERVFAIL (server failure). The authority section lists the authoritative servers for the domain name. The additional section provides additional records that may be useful for the resolver.

5 DNS Protocol

The DNS protocol is a set of rules and formats that define how DNS clients and servers communicate during queries. The DNS protocol uses UDP (User Datagram Protocol) as the default transport layer protocol for sending and receiving DNS messages. UDP is a connectionless and unreliable protocol that does not guarantee delivery, order, or integrity of packets. However, UDP is fast and efficient for small and simple transactions, such as DNS queries. The DNS protocol also supports TCP (Transmission Control Protocol) as an alternative transport layer protocol for larger or complex transactions, such as zone transfers or DNSSEC (Domain Name System Security Extensions) validation.

6 DNS Recursion

DNS recursion is a process in which a DNS server queries other DNS servers on behalf of a resolver until it finds the answer or reaches an error. DNS recursion can be performed by recursive servers, which are designed to handle multiple queries from resolvers and cache the results, or by forwarding servers, which are configured to forward queries to another server without caching. DNS recursion can speed up the resolution process and reduce the network traffic, but it can also introduce security and performance issues, such as cache poisoning or denial-of-service attacks.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?