http {

  # Split clients (approximately) equally based on
  # client ip address
  split_clients $remote_addr $cdn_host {
    33% cdn1;
    33% cdn2;
    - cdn3;
  }

  server {
    server_name example.com;

    # Use the variable defined by the split_clients block to determine
    # the rewritten hostname for requests beginning with /images/
    location /images/ {
      rewrite ^ http://$cdn_host.example.com$request_uri? permanent;
    }
  }
}