Lightmorphic Flow 0.6.0
Pre-release
Beta.
The computer being controlled now asks
Discovery used to work the wrong way round: the computer with the keyboard shouted its presence at the network, and anything with a firewall dropped it without a word. That is why two machines could sit there saying nothing found with no explanation.
It is now a question and an answer. The computer being controlled asks, from a port of its own, and the one with the keyboard replies straight back to it. A firewall lets that reply through as a matter of course, because it is an answer to a question that machine already asked.
What this means: the computers being controlled need no firewall change and no password. Only the machine with the mouse and keyboard has to accept anything coming in, and only that one is offered the button.
This is the same reason Deskflow was never affected — its client dials out and nothing listens on it.
Installing
sudo apt install ./lightmorphic-flow_0.6.0_all.deb
More at flow.lightmorphic.com.