Glossary
What is DNS?
The system that translates domain names into IP addresses and other record information.
DNS lets a client discover the records associated with a name before connecting to a service. Different record types serve different purposes, such as A and AAAA for addresses or MX for mail servers. When checking a change, compare the same name and record type across resolvers, then check the web connection separately.
Example
An A query for example.com asks for an IPv4 address; an MX query asks for mail server names. These queries answer different questions.
Common issue
A successful DNS answer does not prove the web server is working. Incorrect records, cached answers and HTTP connection failures need separate checks.