Gazelle/sections/tools/services/get_host.php

<?php
#declare(strict_types=1);

if (isset($_SERVER['http_if_modified_since'])) {
    header('Status: 304 Not Modified');
    error();
}

header('Expires: '.date('D, d-M-Y H:i:s \U\T\C', time() + 3600 * 24 * 120)); //120 days
header('Last-Modified: '.date('D, d-M-Y H:i:s \U\T\C', time()));

if (!check_perms('users_view_ips')) {
    error('Access denied.');
}
if (empty($_GET['ip'])) {
    error('No IP given.');
}

$IP = $_GET['ip'];
$Delimiter = $IP[strcspn($IP, ':.')];
$OctOrHextets = explode($Delimiter, $IP);


if ($Delimiter === '.' && sizeof($OctOrHextets) === 4) { // IPv4
    if (($OctOrHextets[0] === 127 || $OctOrHextets[0] === 10)
    || ($OctOrHextets[0] === 192 && $OctOrHextets[1] === 168)
    || ($OctOrHextets[0] === 172 && ($OctOrHextets[1] >= 16 && $OctOrHextets[1] <= 32))
  ) {
        error('Invalid IPv4 address.');
    }
    foreach ($OctOrHextets as $Octet) {
        if ($Octet > 255 || $Octet < 0) {
            error('Invalid IPv4 address.');
        }
    }
} elseif (sizeof($OctOrHextets) <= 8) { // IPv6
    foreach ($OctOrHextets as $Hextet) {
        if (strlen($Hextet) > 4) {
            error('Invalid IPv6 address.');
        }
    }
} else {
    error('Invalid IP address.');
}

$Host = Tools::lookup_ip($IP);

if ($Host === '') {
    trigger_error('Tools::get_host_by_ajax() command failed with no output, ensure that the host command exists on your system and accepts the argument -W');
} elseif ($Host === false) {
    echo 'Could not retrieve host.';
} else {
    echo $Host;
}