Reachability testing lets you know how accessible your server is to the Internet.
Unless your server has a public IP address and is connected to the Internet without a router or gateway, not all the active services might be available to clients on the Internet. Reachability testing shows which services are available to such clients.
How does reachability testing work?
The Server app securely connects to Apple and asks automated servers at Apple to try connecting to your server for each service that’s enabled. Because Apple servers are outside your local network, they can simulate external clients trying to connect to your server. The results are shown in the Server app.
The Apple servers check for:
An externally accessible IP address.
A public host name.
A handshake to each service enabled on the server. No information besides the connection availability is exchanged.
Properly configured port forwarding, if necessary.
Apple doesn’t retain any of your personal information and transmits only enough information about your configuration to let Apple’s servers know what kinds of connection to check for.
Why should I use reachability testing?
This service provides a reliable way to have an external source check your network connectivity. It’s frustrating when you believe everything is configured correctly, but you have no way to test the connections. This service gives you instant feedback about whether your configuration actually provides external accessibility.
How do I disable this?
To disable reachability testing, see Disable reachability tests for macOS Server.