The rockspec code has a built-in, tacit requirement to use git protocol. From behind a firewall, one must use HTTPS. luarocks has no proxy environment variable to configure this from the command line, so the installation fails.
Fortunately, there is a way to reconfigure git:
sudo git config --global url."https://".insteadOf git://
This forces all git protocol references to use HTTPS instead. The installation succeeds, and my model code runs to the point of first coding failure.
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solved Can’t install distlearn through a firewall