I ran into this issue as well and found this post. Ultimately none of these answers solved my problem, instead I had to put in a rewrite rule to strip out the location /rt as the backend my developers made was not expecting any additional paths:
+-(william@wkstn18)--(Thu, 05 Nov 20)-+
+-(~)--(16:13)->wscat -c ws://WebsocketServerHostname/rt
error: Unexpected server response: 502
Testing with wscat repeatedly gave a 502 response. Nginx error logs provided the same upstream error as above, but notice the upstream string shows the GET Request is attempting to access localhost:12775/rt and not localhost:12775:
2020/11/05 22:13:32 [error] 10175#10175: *7 upstream prematurely closed
connection while reading response header from upstream, client: WANIP,
server: WebsocketServerHostname, request: "GET /rt/socket.io/?transport=websocket
HTTP/1.1", upstream: "http://127.0.0.1:12775/rt/socket.io/?transport=websocket",
host: "WebsocketServerHostname"
Since the devs had not coded their websocket (listening on 12775) to expect /rt/socket.io but instead just /socket.io/ (NOTE: /socket.io/ appears to just be a way to specify websocket transport discussed here). Because of this, rather than ask them to rewrite their socket code I just put in a rewrite rule to translate WebsocketServerHostname/rt to WebsocketServerHostname:12775 as below:
upstream websocket-rt {
ip_hash;
server 127.0.0.1:12775;
}
server {
listen 80;
server_name WebsocketServerHostname;
location /rt {
proxy_http_version 1.1;
#rewrite /rt/ out of all requests and proxy_pass to 12775
rewrite /rt/(.*) /$1 break;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $host;
proxy_pass http://websocket-rt;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection $connection_upgrade;
}
}