Is it possible to use OpnSense as a ddns server? I have a business location with a static IP and my home and a second business location with dynamic IPs. I would like to be able to use the one at the static location as a ddns server for the other two. I don't plan to host any websites but do need to access all locations to monitor security cameras, access remote computers, and possibly run an ftp server. I would like to be able to enter the static ip:port and be forwarded to the corresponding remote machine.
You would not need to if you use a VPN, like wireguard. The connection could always be build towards the location with the static IP. After that, all traffic is routed over the VPN and all your IPs are RFC1918. You can even use your local DNS and do not have to exposed any forwarded ports to the internet.
That was not an answer to the OP's question. (Posting I was referring to has vanished...)
AFAIK, OpnSense can at this time only be used as a DDNS client. I know of no OpnSense package to enable DDNS server capabilities, but of course one could use any FreeBSD package that does this.
Probably doable with BIND on server side and RFC-2136 on the client side (there is a plugin for this). Not sure about BIND having this configurable via the GUI currently, though.