I might be totally off track, but I think yours is the key question. Because I was fooled as well I’d like to elaborate:
From doc:
`:check_origin` - if the transport should check the origin of requests
when the `origin` header is present
Now the “origin” header from first random doc:
indicates the origin (scheme, hostname, and port) that **caused** the request
Caused… which is the website itself you connected to in the first place (ah-ah moment to me.. this explains why I find domain names in the “check_origin”!)
BUT assuming you have a config such this
config :mygoodservice, MyGoodService.Endpoint,
https: [
port: 8443,
...
],
url: [host: "mygoodservice.com"],
What is the proper origin? You are conveniently told that by the console error (which I got with the default check_origin: true config):
[error] Could not check origin for Phoenix.Socket transport.
Origin of the request: https://mygoodservice.com:8443
I suspect that just letting the default value true is not actually defaulting to the origin you are prompted about in the console 
.. (maybe it’s forgetting the port? or I’m doing bad somwhere else… dunno).
Anyway setting the whole exact string reported by the console solved for me!
Also: if you are troubleshooting start/stop/restarting your service, I’d close the tab and open a new one.. for what is worth I’ve seen strange things happening on firefox (which might well be consequence of me doing this late night).
Cheers
