add forward-confirmed reverse DNS

inc/config.php

@@ -66,7 +66,10 @@
 	// Use `host` via shell_exec() to lookup hostnames, avoiding query timeouts. May not work on your system.
 	// Requires safe_mode to be disabled.
 	$config['dns_system'] = false;
-	
+
+	// Check validity of the reverse DNS of IP addresses. Highly recommended.
+	$config['fcrdns'] = true;
+
 	// When executing most command-line tools (such as `convert` for ImageMagick image processing), add this
 	// to the environment path (seperated by :).
 	$config['shell_path'] = '/usr/local/bin';

inc/functions.php

@@ -2265,6 +2265,7 @@ function rDNS($ip_addr) {
 
 	if (!$config['dns_system']) {
 		$host = gethostbyaddr($ip_addr);
+		$isip = filter_var($host, FILTER_VALIDATE_IP);
 	} else {
 		$resp = shell_exec_error('host -W 1 ' . $ip_addr);
 		if (preg_match('/domain name pointer ([^\s]+)$/', $resp, $m))
@@ -2273,6 +2274,10 @@ function rDNS($ip_addr) {
 			$host = $ip_addr;
 	}
 
+	if ($config['fcrdns'] && !$isip && gethostbyname($host) != $ip_addr) {
+		$host = $ip_addr;
+	}
+
 	if ($config['cache']['enabled'])
 		cache::set('rdns_' . $ip_addr, $host);