nginx-reverse-proxy-keep-domain.conf

server {
  ...
  server_name exposed.domain.com;
  ...
  location / {
    proxy_pass "http://internal.domain.com:1234";
    proxy_pass_request_headers on;
    proxy_set_header Host $host;
    proxy_redirect "http://exposed.domain.com:1234" "https://exposed.domain.com";
  }
}