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