reverse proxy with dns control

[arx]

like something like that

server {
	listen port;
	location / {
if ($http_host ~ "dnsinput:port")  {
		
		proxy_buffering off;
        proxy_set_header X-Real-IP  $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header Host $host;
        proxy_pass   dnsoutput:port;
	}
  }
}

it gives me syntax error

I get the input from the file
GET /favicon.ico HTTP/1.1" 404 548 "http://dnsinput:port/" 
in access.log

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,293951,293956#msg-293956