Devices shown with only an old DNS domain name

Does WiFi Guard save device DNS domain names or obtain them each time a device is detected?

The reason I ask, is because lots of my devices are shown with an old DNS Domain name, and none with the current DNS domain name.

WiFi Guard does not cache or store device names. It keeps a record of each device's MAC address, IP assignments over time, and any category or description you set, but not its name.

The old DNS domain names you are seeing are most likely coming from cached data elsewhere, such as in your operating system or at the router level. A quick way to check is to run the following command in a Command Prompt window (replace x.x.x.x with the device IP):

tracert x.x.x.x

Look at the line after Tracing route to… If it shows the old domain name there as well, the cache issue is outside WiFi Guard.

Please give that a try and let me know what result you get.

That returns host names with the new DNS domain name.

Where does WIFi Guard actually get the names from (those in the Name column of the scan result screen)?

Here is exactly how WiFi Guard decides the "Name" shown for a device. It simply tries a few discovery methods in order and uses the first one that succeeds. There is no internal caching of names:

1. NetBIOS: If it returns a name, it is used.
2. mDNS (Bonjour/Zero-conf).
3. SNMP: Typically reads sysName.0
4. Reverse DNS: If there is a PTR record (or other reverse-lookup path) that yields a hostname, it is used.
5. HTTP page title (only if the host responded to ping)

In short: the first successful source in that list "wins". If you are seeing an old domain or unexpected label, it is coming from whichever of these answered first, often your router/DNS, a stale NBNS responder, or an mDNS announcement on the LAN.

With Wireshark, you can capture traffic during a scan and check which of these methods actually returned the name. Filtering for NBNS, mDNS, DNS, or SNMP packets around the device's IP will quickly reveal where the information is coming from.

It is only Linux machines returning the old DNS domain name, via mDNS.

I had to update /etc/avahi/avahi-daemon.conf on each Raspberry PI, to use a DNS domain name other than local.

Unfortunately, I have a few devices with Linux embedded that will return .local forever.