Append upstream hostname into Host header

Hi,

We are using nginx for load balancing dynamically discovered upstream endpoints. We are in need to attach upstream hostname as header Host in upstream requests. For example below is my existing conf extract.

upstream products {
server hostname1:port1;
server hostname2:port2;
}
server {
location /products {
proxy_pass http://products
}
}

All requests are load balanced and working fine. Now we are in need to add http header "Host: hostnameX" (ex: hostname1 or hostname2, not its IP) in all requests to upstream. How can we get this done in nginx?

Regards