How to dynamically assign port to proxy_pass

Hi everybody, I used two streaming’s servers which host many webradios streams (over 25). Each stream use a port between 8000 and 9000.

I have 3 servers : -> nginx load-balancer and reverse-proxy -> 2 streaming’s server (webradios) on ports between 3000 and 4000 .

I use Nginx to :

• allow HTTPS for all webradios

• change server on failure (ex.: server down). (high-availability / load-balance)

I’m trying to do something like this :

1. When user listen a stream on https://hosting.mydomain.com:8xxx , I want to transparently (proxify) send the request on one of the two Icecast’s server.

2. If HTTPS and Icecast_1 is alive then send the request to Icecast_1.

3. If HTTPS and Icecast_1 is down then send the request to Icecast_2.

This is the current configuration of Nginx:

#Streaming's cluster :
upstream backend {
    ip_hash;
    server 10.0.0.1 ;
    server 10.0.0.2 ;
}


#SSL for all
server {

        listen 8000-9000 ssl ;
        server_name hosting.mydomain.com;

        access_log /var/log/nginx/reverse-ssl-access.log;
        error_log /var/log/nginx/reverse-ssl-error.log;

        # ssl on;
        ssl_protocols TLSv1.2 TLSv1.1 TLSv1 ;
        ssl_certificate /etc/letsencrypt/live/live.mydomain.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/live.mydomain.com/privkey.pem;

        location / {
               resolver 8.8.8.8;
               proxy_pass http://backend:$server_port;
        }

}

code

I try to do something like this : “http://backend:$server_port”.

In the web browser I’ve the error : 502 Bad gateway

In the errors logs on the server : *1 no resolver defined to resolve backend .

Can you help me ?