reverse proxy with dns control

[arx]

the important thing for me is the dns, the port is static and no server do
the check,
the problem that I don't know how it is calls the variable with what name
the client makes the request (the dns that is invoked in the GET method by
the client) present in access.log

[14/Apr/2022:12:22:57 +0200] "GET /favicon.ico HTTP/1.1" 404 548
"http://DNS:PORT/" "Mozilla/5.0 (........

I have to check on dns, and when I go to put the condition (as in the
previous example it gives me a syntax error)

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,293951,293958#msg-293958