pixelfed/app/Services/DomainService.php

<?php

namespace App\Services;

use Illuminate\Support\Facades\Cache;
use Illuminate\Support\Facades\Redis;

class DomainService
{
	const CACHE_KEY = 'pf:services:domains:';

    public static function hasValidDns($domain)
    {
        if(!$domain || !strlen($domain) || strpos($domain, '.') == -1) {
            return false;
        }

        if(config('security.url.trusted_domains')) {
            if(in_array($domain, explode(',', config('security.url.trusted_domains')))) {
                return true;
            }
        }

        return Cache::remember(self::CACHE_KEY . 'valid-dns:' . $domain, 14400, function() use($domain) {
            return count(dns_get_record($domain, DNS_A | DNS_AAAA)) > 0;
        });
    }
}