Cross-domain WebSocket connection failed during WebSocket handshake: Unexpected response code: 400 - node.js

Question

I am trying to use socket.io in a Node js application. I have the application sitting on a subdomain and the front-end running on the www version of the domain.
Running the front-end and Node js service on the same domain is not an option.
Sending data back and forth from the client to the server seems to be working. I have sent data both ways and it has worked fine.
However, In the console of the browser I get the following error.
WebSocket connection to 'wss://subdomain.domain.com/socket.io/?EIO=3&transport=websocket&sid=6bNHWyXcCdlMI0HHAAAB' failed: Error during WebSocket handshake: Unexpected response code: 400
My Nginx configuration looks like this:
# HTTP - redirect all requests to HTTPS:
server {
listen 80;
listen [::]:80 default_server ipv6only=on;
return 301 https://$host$request_uri;
}
# HTTPS - proxy requests on to local Node.js app:
server {
listen 443;
server_name subdomain.domain.com;
ssl on;
# Use certificate and key provided by Let's Encrypt:
ssl_certificate /etc/letsencrypt/live/subdomain.domain.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/subdomain.domain.com/privkey.pem;
ssl_session_timeout 5m;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_prefer_server_ciphers on;
ssl_ciphers '*******';
# Pass requests for / to localhost:3000:
location / {
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-NginX-Proxy true;
proxy_pass http://localhost:3000/;
proxy_ssl_session_reuse off;
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
proxy_redirect off;
proxy_http_version 1.1;
proxy_set_header Connection "upgrade";
}
}
Both the client and the Node js are using https.
Does anyone know what is causing this issue and how it could be fixed?
Thank you

Answer 1

It looks like you forgot about Upgrade header. It's required if you want to use Nginx as a reverse proxy for WebSockets.
As said here, just try to add one more header:
proxy_set_header Upgrade $http_upgrade;