Browser autostarts when ethernet is connected. How?

Question

I plugged my Windows 10 laptop into a router from our new isp. We only got the fibre, no services set up yet (The fibre can carry phones, tv and internet)

Immediately when I plugged the cable in, the browser opened and showed "Welcome to your fiber connection from X-corp. Call us at 555-555-5555 for to get internet" (Name changed ;-)

How does that work? Note, I wasn't trying to go to a website and got redirected, no, the browser started "magically".

Can windows get a command over dhcp?

I can see some use for this "autostart" on our corporate network, yet I fear black-hat use of this.

Answer 1

Captive Portal detection is an OS built-in and although something you mostly see/experience on WiFi connections also works for wired connections. There is no DHCP magic going on there.

See for instance https://msdn.microsoft.com/en-us/library/windows/hardware/dn408681.aspx

Windows 8 and Windows 8.1 supports captive portal networks by immediately opening the web browser if a captive portal is detected. The user sees your branded web page in the foreground of their device, which helps them to understand what actions they should take to authenticate by using the captive portal.