I feel like I'm missing some important point about certificates.

[dwasifar]

I think I'm failing to understand how certs are supposed to work in OPNsense.

I have valid certs for my webservers, so I tried loading one of them into OPNsense.  It offers the cert to the web browser when I connect over LAN, but the browser complains because the cert is owned by [mydomain].com instead of 192.168.1.1.  I created a host override to allow connecting to OPNsense with [hostname].[mydomain].com instead of by local subnet IP address, but then OPNsense thinks it's a rebind attack and won't let me in. 

What am I missing here?

[bartjsmit]

You're almost there :)

System, Settings, Administration, untick 'Disable DNS Rebinding Checks'

Bart...