> By instant I didn't mean "in the EXACT same time". A few seconds delay over
> the Internet is acceptable, and the application has been prepared for this.

And you told me that replication is "no-go"...

>>> Like any other ipfw.
>
> Please explain this. Firewalls and routers are not in my area of expertise,
> but since I'm forced to do everything myself, I could use some help /
> suggestions.

I am also not an expert in firewalls. You should go to specialized
forum and ask if it is possible:
1) Forward connection to any port from host in white list.
2) Add source host to white list on connect to port 3050.
3) Remove the host from white list after disconnect.

Perhaps, some cheap routers don't allow ssh connections and direct
modification of ipfw rules. In this case you will have to modify it's
firmware.

> I've read a paper on Firebird and events which said that Classic + Events +
> Firewall over the Internet is not doable, but again, I might have misread it
> or the paper might have been wrong...

May be the author of the paper didn't have enough fantasy to imagine
the way...

SY, SD.