redirect to main domain (e.g. from domain.com.host.com to domain.com)

_config.php

if (Director::isLive()) {
	$hostname = gethostname();
	$http_host = $_SERVER['HTTP_HOST'];

	if (strpos($http_host, $hostname) !== false) {
		//redirect to original domain

		$destURL = str_replace('.' . $hostname, '', $http_host);

		$response = new SS_HTTPResponse();
		$response->redirect('http://' . $destURL, 301);

		HTTP::add_cache_headers($response);

		// TODO: Use an exception - ATM we can be called from _config.php, before Director#handleRequest's try block
		$response->output();
		die;
	}

	Director::forceSSL();
	Director::forceWWW();
}

This redirects e.g. on a hetzner managed server, where every account has a subdomain of the hostname, e.g. example.com.dedi1234.your-server.de which should redirect to example.com in live.

Used together with some SilverStripe methods to force ssl and www subdomain.