Nginx changes url to backend url - nginx

Question

i have 2 backend servers listening on 80 and 443 ports:
a.example.com
b.example.com
when i get http://a.example.com at 80 port, backend internal nginx redirect it to 443 port: https://a.example.com, but when i get https port it is not redirect it, opening normally. I have not permissions to change backend nginx configurations.
So, and i have 1 load balancer nginx server, witch route requests to upstreams (backends):
upstream backends {
server 172.20.1.2:443; #a.example.com
server 172.20.2.2:443; #b.example.com
}
server {
listen 80;
server_name a.example.com;
location / {
proxy_pass https://backends;
proxy_set_header HOST a.example.com;
}
}
when i get response from second backend (b.example.com) my URL in browser is changes to https://b.example.com, but i requested https://a.example.com
Q: How i can get always https://a.example.com url from both backends? For example, when a.example.com backend not responding, lb request to second backend (b.example.com), but response url must be a.example.com
Thx!

Answer 1

for routing of https(443) url you need to listen on 443 in nginx also. Eg.
server {
listen 443 ssl;
server_name a.example.com;
ssl_certificate a.example.com.crt;
ssl_certificate_key a.example.com.key;
ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_ciphers HIGH:!aNULL:!MD5;
location / {
proxy_pass https://backends;
proxy_set_header HOST a.example.com;
}
}
and for dynamic routing to any upstreams try this :
upstream backends {
least_conn;
server 172.20.1.2:443; #a.example.com
server 172.20.2.2:443; #b.example.com
}
Hope this will works for you.
Helpful Links :-
https://docs.nginx.com/nginx/admin-guide/load-balancer/http-load-balancer/