I have set up a Management Interface and assigned it to the "Listen Interfaces" in System: Settings: Administration. For testing purposes, I created a firewall rule for the Management Interface that passes to all destinations. I created a ISC DHCPv4 services and am able to get an i.p. of 192.168.100.50. I then try to open the web interface at 192.168.100.1, but instead of getting the form-based authentication page, I get a basic authentication prompt.
This has worked for me before in an older version of OPNsense, but I'm starting fresh with 26.1. Is there something else I need to set up to get this to work?
I think what you see is probably an overlay of several misconfigurations. OpnSense does not have a basic authentication for starters.
Maybe what you see is the basic login of your ONT or modem, which often has a management IP of 192.168.100.1. For this very reason, the use of that specific subnet is discouraged here (https://forum.opnsense.org/index.php?topic=47099).
That being said, you should not have the ONT on your management network, because it much likely also is your WAN.
Thanks, I set it to 10.10.60.0/24 and now it's working.