Could not connect websocket using Action Cable - ruby-on-rails

Question

Could not connect websocket using Action Cable in Rails 5.1. HTTP server is Unicorn on nginx and adapter is Redis.
Rails configuration is the following.
# config/environments/production.rb
config.action_cable.disable_request_forgery_protection = true
nginx configuration is the following.
upstream unicorn {
server unix:/rails/current/tmp/sockets/unicorn.sock;
}
server {
listen 80;
charset utf-8;
server_name sub.example.com;
root /rails/current/public;
location / {
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $http_host;
proxy_redirect off;
if (!-f $request_filename) {
proxy_pass http://unicorn;
break;
}
}
location /cable {
proxy_pass http://unicorn;
proxy_http_version 1.1;
proxy_set_header Upgrade "websocket";
proxy_set_header Connection "Upgrade";
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto https;
proxy_redirect off;
}
...
}
Errors in console of web browser are the following.
WebSocket connection to 'wss://sub.example.com/cable' failed: Error during WebSocket handshake: Unexpected response code: 404
Errors in Rails are the following.
[ERROR] Failed to upgrade to WebSocket (REQUEST_METHOD: GET, HTTP_CONNECTION: close, HTTP_UPGRADE: )
[INFO] Finished "/cable/"[non-WebSocket] for xxx.xxx.xxx.xxx at 2017-xx-xx
A strangest thing is a error in Rails does not have HTTP_UPGRADE value. But HTTP request headers of web browser include Upgrade key and websocket value. Also setting "websocket" for proxy header in nginx configurations.
What should I do?

Answer 1

On the back of that writeup, I just realized the universal solution is
a simple change to the config/secrets.yml file to reference the
ENV["PORT"] setting
...
# Be sure to restart your server when you modify this file.
development:
secret_key_base: 231bf79489c63f8c8facd7...
action_cable_url : http://localhost:<%= ENV["PORT"] %>
test:
secret_key_base: 1ab8adbcf8410aebb...
action_cable_url : http://localhost:<%= ENV["PORT"] %>
# Do not keep production secrets in the repository,
# instead read values from the environment.
production:
secret_key_base: <%= ENV["SECRET_KEY_BASE"] %>
action_cable_url : <%= ENV["SERVER_PORT"] %>
.
You can also have this in your dot file:
export PORT=3000
source