Comment 6 for bug 1781775

first you need to check the HTTP port that vlc is using (for me it was 8010)...

open vlc > tools > preferences > select "all" (at the bottom) > stream output > stout stream > chromecast > HTTP Port = ????.

open gufw,
click "+" to add a new rule,
"Advanced" tab,
"Name" = VLC Chromecast
"Policy" = Allow,
"Direction" = Both
"Protocol" = TCP
"To / Port" = <add your port number here>.
clicked on "Add" then "Close".

Alternatively you could assign a static IP to your chromecast's MAC address via your router's set up page...

then open gufw,
click "+" to add a new rule,
"Advanced" tab,
"Name" = Chromecast,
"Policy" = Allow,
"Direction" = Both,
"Protocol" = TCP
in the "From" field, enter EG: 192.168.1.6,
clicked on "Add" then "Close".