nginx.conf (rewrite subdomain)

server {
    listen       80;
    server_name  .domainname.com;

    if ($http_host = 'admin.domainname.com') {
        rewrite ^(?!/wp-admin)(.*)$ https://www.domainname.com$1 permanent;
    }

    location / {
        echo "URL is $scheme://$http_host$uri\n";
    }

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   html;
    }
}