Answer:
ICMP echo requests.
Step-by-step explanation:
A Domain Name System (DNS) can be defined as a naming database in which internet domain names (website URLs) are stored and translated into their respective internet protocol (IP) address. This simply means that, DNS is used to connect uniform resource locator (URL) or web address with their internet protocol (IP) address.
When troubleshooting Domain Name System (DNS) problems, one helpful feature the public servers usually support is an ICMP echo requests.
ICMP is an acronym for Internet Control Message Protocol and it is a standard protocol for communicating network errors in the form of messages such as Time exceeded and Destination unreachable.
In Computer Networking, an echo reply and echo request are implemented with a command utility referred to as ping. When a user sends an echo request message with the ping program, a successful attempt will return an echo reply message.
Hence, when troubleshooting Domain Name System (DNS) problems, a user should ping a public server such as a Google DNS server with the IP address (8.8.8.8 or 8.8.4.4), Cloudfare with 1.1.1.1 or the common 4.2.2.2 DNS server. If successful, it means the name-server was resolved properly into an IP address and therefore, there's an active connection on the network.