fix(domains): resolve server hostname to IP for sslip.io domains

Closes #12009

bootstrap/helpers/shared.php

@@ -1497,7 +1497,25 @@ function sslip(Server $server)
     }
     $ip = $server->ip === 'host.docker.internal' ? base_ip() : $server->ip;
 
-    return 'http://'.sslipHostLabel($ip).'.sslip.io';
+    return 'http://'.sslipHostLabel(resolveHostToIp($ip)).'.sslip.io';
+}
+
+/**
+ * Resolve a hostname to its IPv4 (or else IPv6) address. sslip.io only answers for names that
+ * contain an IP address. Returns the input unchanged if it is already an IP or does not resolve.
+ */
+function resolveHostToIp(string $host): string
+{
+    if (filter_var(normalizeIpAddress($host), FILTER_VALIDATE_IP) !== false) {
+        return $host;
+    }
+    $ipv4 = gethostbyname($host);
+    if (filter_var($ipv4, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4) !== false) {
+        return $ipv4;
+    }
+    $ipv6 = data_get(@dns_get_record($host, DNS_AAAA), '0.ipv6');
+
+    return is_string($ipv6) && $ipv6 !== '' ? $ipv6 : $host;
 }
 
 function service_templates_cache_key(): string

tests/Feature/SslipServerHostnameTest.php

@@ -0,0 +1,15 @@
+<?php
+
+use App\Models\Server;
+
+it('builds the sslip.io domain from the IP address of the server', function (string $address, string $expected) {
+    $server = new Server(['ip' => $address]);
+    $server->id = 1;
+
+    expect(sslip($server))->toBe($expected);
+})->with([
+    'IPv4' => ['188.245.12.182', 'http://188.245.12.182.sslip.io'],
+    'IPv6' => ['2a01:4f8:c016:bd23::1', 'http://2a01-4f8-c016-bd23--1.sslip.io'],
+    'resolvable hostname' => ['localhost', 'http://127.0.0.1.sslip.io'],
+    'unresolvable hostname' => ['coolify-test.invalid', 'http://coolify-test.invalid.sslip.io'],
+]);