nginx proxy to different local backend sites, depending on a variable

gistfile1.txt

map $http_x_forwarded_for $backend_site {
  ~^[6] www1.zahna.com;
  default www2.zahna.com;
}
server {
  listen 80;
  listen 443 ssl;
  server_name dfas78dsf9sa.cloudfront.net;
  access_log /dev/null;
  location / {
    proxy_set_header Host $backend_site;
    proxy_set_header X-Forwarded-Host $host;
    proxy_pass http://127.0.0.1;
  }
}