Add remote host validation API

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

11 files changed, 318 insertions, 133 deletions

diff --git a/lib/composer/composer/autoload_classmap.php b/lib/composer/composer/autoload_classmap.php
index 2b5475f6efb..397f36e660b 100644
--- a/lib/composer/composer/autoload_classmap.php
+++ b/lib/composer/composer/autoload_classmap.php
@@ -535,6 +535,7 @@ return array(
     'OCP\\Security\\ICredentialsManager' => $baseDir . '/lib/public/Security/ICredentialsManager.php',
     'OCP\\Security\\ICrypto' => $baseDir . '/lib/public/Security/ICrypto.php',
     'OCP\\Security\\IHasher' => $baseDir . '/lib/public/Security/IHasher.php',
+    'OCP\\Security\\IRemoteHostValidator' => $baseDir . '/lib/public/Security/IRemoteHostValidator.php',
     'OCP\\Security\\ISecureRandom' => $baseDir . '/lib/public/Security/ISecureRandom.php',
     'OCP\\Security\\ITrustedDomainHelper' => $baseDir . '/lib/public/Security/ITrustedDomainHelper.php',
     'OCP\\Security\\VerificationToken\\IVerificationToken' => $baseDir . '/lib/public/Security/VerificationToken/IVerificationToken.php',
@@ -1288,7 +1289,6 @@ return array(
     'OC\\Http\\Client\\Client' => $baseDir . '/lib/private/Http/Client/Client.php',
     'OC\\Http\\Client\\ClientService' => $baseDir . '/lib/private/Http/Client/ClientService.php',
     'OC\\Http\\Client\\DnsPinMiddleware' => $baseDir . '/lib/private/Http/Client/DnsPinMiddleware.php',
-    'OC\\Http\\Client\\LocalAddressChecker' => $baseDir . '/lib/private/Http/Client/LocalAddressChecker.php',
     'OC\\Http\\Client\\NegativeDnsCache' => $baseDir . '/lib/private/Http/Client/NegativeDnsCache.php',
     'OC\\Http\\Client\\Response' => $baseDir . '/lib/private/Http/Client/Response.php',
     'OC\\Http\\CookieHelper' => $baseDir . '/lib/private/Http/CookieHelper.php',
@@ -1362,6 +1362,8 @@ return array(
     'OC\\NaturalSort_DefaultCollator' => $baseDir . '/lib/private/NaturalSort_DefaultCollator.php',
     'OC\\NavigationManager' => $baseDir . '/lib/private/NavigationManager.php',
     'OC\\NeedsUpdateException' => $baseDir . '/lib/private/NeedsUpdateException.php',
+    'OC\\Net\\HostnameClassifier' => $baseDir . '/lib/private/Net/HostnameClassifier.php',
+    'OC\\Net\\IpAddressClassifier' => $baseDir . '/lib/private/Net/IpAddressClassifier.php',
     'OC\\NotSquareException' => $baseDir . '/lib/private/NotSquareException.php',
     'OC\\Notification\\Action' => $baseDir . '/lib/private/Notification/Action.php',
     'OC\\Notification\\Manager' => $baseDir . '/lib/private/Notification/Manager.php',
@@ -1517,6 +1519,7 @@ return array(
     'OC\\Security\\RateLimiting\\Backend\\MemoryCacheBackend' => $baseDir . '/lib/private/Security/RateLimiting/Backend/MemoryCacheBackend.php',
     'OC\\Security\\RateLimiting\\Exception\\RateLimitExceededException' => $baseDir . '/lib/private/Security/RateLimiting/Exception/RateLimitExceededException.php',
     'OC\\Security\\RateLimiting\\Limiter' => $baseDir . '/lib/private/Security/RateLimiting/Limiter.php',
+    'OC\\Security\\RemoteHostValidator' => $baseDir . '/lib/private/Security/RemoteHostValidator.php',
     'OC\\Security\\SecureRandom' => $baseDir . '/lib/private/Security/SecureRandom.php',
     'OC\\Security\\TrustedDomainHelper' => $baseDir . '/lib/private/Security/TrustedDomainHelper.php',
     'OC\\Security\\VerificationToken\\CleanUpJob' => $baseDir . '/lib/private/Security/VerificationToken/CleanUpJob.php',
diff --git a/lib/composer/composer/autoload_static.php b/lib/composer/composer/autoload_static.php
index ad62c3585e6..4c430720ef1 100644
--- a/lib/composer/composer/autoload_static.php
+++ b/lib/composer/composer/autoload_static.php
@@ -568,6 +568,7 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OCP\\Security\\ICredentialsManager' => __DIR__ . '/../../..' . '/lib/public/Security/ICredentialsManager.php',
         'OCP\\Security\\ICrypto' => __DIR__ . '/../../..' . '/lib/public/Security/ICrypto.php',
         'OCP\\Security\\IHasher' => __DIR__ . '/../../..' . '/lib/public/Security/IHasher.php',
+        'OCP\\Security\\IRemoteHostValidator' => __DIR__ . '/../../..' . '/lib/public/Security/IRemoteHostValidator.php',
         'OCP\\Security\\ISecureRandom' => __DIR__ . '/../../..' . '/lib/public/Security/ISecureRandom.php',
         'OCP\\Security\\ITrustedDomainHelper' => __DIR__ . '/../../..' . '/lib/public/Security/ITrustedDomainHelper.php',
         'OCP\\Security\\VerificationToken\\IVerificationToken' => __DIR__ . '/../../..' . '/lib/public/Security/VerificationToken/IVerificationToken.php',
@@ -1321,7 +1322,6 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OC\\Http\\Client\\Client' => __DIR__ . '/../../..' . '/lib/private/Http/Client/Client.php',
         'OC\\Http\\Client\\ClientService' => __DIR__ . '/../../..' . '/lib/private/Http/Client/ClientService.php',
         'OC\\Http\\Client\\DnsPinMiddleware' => __DIR__ . '/../../..' . '/lib/private/Http/Client/DnsPinMiddleware.php',
-        'OC\\Http\\Client\\LocalAddressChecker' => __DIR__ . '/../../..' . '/lib/private/Http/Client/LocalAddressChecker.php',
         'OC\\Http\\Client\\NegativeDnsCache' => __DIR__ . '/../../..' . '/lib/private/Http/Client/NegativeDnsCache.php',
         'OC\\Http\\Client\\Response' => __DIR__ . '/../../..' . '/lib/private/Http/Client/Response.php',
         'OC\\Http\\CookieHelper' => __DIR__ . '/../../..' . '/lib/private/Http/CookieHelper.php',
@@ -1395,6 +1395,8 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OC\\NaturalSort_DefaultCollator' => __DIR__ . '/../../..' . '/lib/private/NaturalSort_DefaultCollator.php',
         'OC\\NavigationManager' => __DIR__ . '/../../..' . '/lib/private/NavigationManager.php',
         'OC\\NeedsUpdateException' => __DIR__ . '/../../..' . '/lib/private/NeedsUpdateException.php',
+        'OC\\Net\\HostnameClassifier' => __DIR__ . '/../../..' . '/lib/private/Net/HostnameClassifier.php',
+        'OC\\Net\\IpAddressClassifier' => __DIR__ . '/../../..' . '/lib/private/Net/IpAddressClassifier.php',
         'OC\\NotSquareException' => __DIR__ . '/../../..' . '/lib/private/NotSquareException.php',
         'OC\\Notification\\Action' => __DIR__ . '/../../..' . '/lib/private/Notification/Action.php',
         'OC\\Notification\\Manager' => __DIR__ . '/../../..' . '/lib/private/Notification/Manager.php',
@@ -1550,6 +1552,7 @@ class ComposerStaticInit749170dad3f5e7f9ca158f5a9f04f6a2
         'OC\\Security\\RateLimiting\\Backend\\MemoryCacheBackend' => __DIR__ . '/../../..' . '/lib/private/Security/RateLimiting/Backend/MemoryCacheBackend.php',
         'OC\\Security\\RateLimiting\\Exception\\RateLimitExceededException' => __DIR__ . '/../../..' . '/lib/private/Security/RateLimiting/Exception/RateLimitExceededException.php',
         'OC\\Security\\RateLimiting\\Limiter' => __DIR__ . '/../../..' . '/lib/private/Security/RateLimiting/Limiter.php',
+        'OC\\Security\\RemoteHostValidator' => __DIR__ . '/../../..' . '/lib/private/Security/RemoteHostValidator.php',
         'OC\\Security\\SecureRandom' => __DIR__ . '/../../..' . '/lib/private/Security/SecureRandom.php',
         'OC\\Security\\TrustedDomainHelper' => __DIR__ . '/../../..' . '/lib/private/Security/TrustedDomainHelper.php',
         'OC\\Security\\VerificationToken\\CleanUpJob' => __DIR__ . '/../../..' . '/lib/private/Security/VerificationToken/CleanUpJob.php',
diff --git a/lib/private/Http/Client/Client.php b/lib/private/Http/Client/Client.php
index d4dba3e5a44..2e370395132 100644
--- a/lib/private/Http/Client/Client.php
+++ b/lib/private/Http/Client/Client.php
@@ -37,8 +37,11 @@ use GuzzleHttp\Client as GuzzleClient;
 use GuzzleHttp\RequestOptions;
 use OCP\Http\Client\IClient;
 use OCP\Http\Client\IResponse;
+use OCP\Http\Client\LocalServerException;
 use OCP\ICertificateManager;
 use OCP\IConfig;
+use OCP\Security\IRemoteHostValidator;
+use function parse_url;
 
 /**
  * Class Client
@@ -52,19 +55,18 @@ class Client implements IClient {
 	private $config;
 	/** @var ICertificateManager */
 	private $certificateManager;
-	/** @var LocalAddressChecker */
-	private $localAddressChecker;
+	private IRemoteHostValidator $remoteHostValidator;
 
 	public function __construct(
 		IConfig $config,
 		ICertificateManager $certificateManager,
 		GuzzleClient $client,
-		LocalAddressChecker $localAddressChecker
+		IRemoteHostValidator $remoteHostValidator
 	) {
 		$this->config = $config;
 		$this->client = $client;
 		$this->certificateManager = $certificateManager;
-		$this->localAddressChecker = $localAddressChecker;
+		$this->remoteHostValidator = $remoteHostValidator;
 	}
 
 	private function buildRequestOptions(array $options): array {
@@ -181,7 +183,13 @@ class Client implements IClient {
 			return;
 		}
 
-		$this->localAddressChecker->throwIfLocalAddress($uri);
+		$host = parse_url($uri, PHP_URL_HOST);
+		if ($host === false || $host === null) {
+			throw new LocalServerException('Could not detect any host');
+		}
+		if (!$this->remoteHostValidator->isValid($host)) {
+			throw new LocalServerException('Host violates local access rules');
+		}
 	}
 
 	/**
diff --git a/lib/private/Http/Client/ClientService.php b/lib/private/Http/Client/ClientService.php
index e868d4af7a5..bbc2330176f 100644
--- a/lib/private/Http/Client/ClientService.php
+++ b/lib/private/Http/Client/ClientService.php
@@ -33,6 +33,7 @@ use OCP\Http\Client\IClient;
 use OCP\Http\Client\IClientService;
 use OCP\ICertificateManager;
 use OCP\IConfig;
+use OCP\Security\IRemoteHostValidator;
 
 /**
  * Class ClientService
@@ -46,17 +47,16 @@ class ClientService implements IClientService {
 	private $certificateManager;
 	/** @var DnsPinMiddleware */
 	private $dnsPinMiddleware;
-	/** @var LocalAddressChecker */
-	private $localAddressChecker;
+	private IRemoteHostValidator $remoteHostValidator;
 
 	public function __construct(IConfig $config,
 								ICertificateManager $certificateManager,
 								DnsPinMiddleware $dnsPinMiddleware,
-								LocalAddressChecker $localAddressChecker) {
+								IRemoteHostValidator $remoteHostValidator) {
 		$this->config = $config;
 		$this->certificateManager = $certificateManager;
 		$this->dnsPinMiddleware = $dnsPinMiddleware;
-		$this->localAddressChecker = $localAddressChecker;
+		$this->remoteHostValidator = $remoteHostValidator;
 	}
 
 	/**
@@ -73,7 +73,7 @@ class ClientService implements IClientService {
 			$this->config,
 			$this->certificateManager,
 			$client,
-			$this->localAddressChecker
+			$this->remoteHostValidator,
 		);
 	}
 }
diff --git a/lib/private/Http/Client/DnsPinMiddleware.php b/lib/private/Http/Client/DnsPinMiddleware.php
index 00bc209d7b1..294a23f9de1 100644
--- a/lib/private/Http/Client/DnsPinMiddleware.php
+++ b/lib/private/Http/Client/DnsPinMiddleware.php
@@ -25,20 +25,21 @@ declare(strict_types=1);
  */
 namespace OC\Http\Client;
 
+use OC\Net\IpAddressClassifier;
+use OCP\Http\Client\LocalServerException;
 use Psr\Http\Message\RequestInterface;
 
 class DnsPinMiddleware {
 	/** @var NegativeDnsCache */
 	private $negativeDnsCache;
-	/** @var LocalAddressChecker */
-	private $localAddressChecker;
+	private IpAddressClassifier $ipAddressClassifier;
 
 	public function __construct(
 		NegativeDnsCache $negativeDnsCache,
-		LocalAddressChecker $localAddressChecker
+		IpAddressClassifier $ipAddressClassifier
 	) {
 		$this->negativeDnsCache = $negativeDnsCache;
-		$this->localAddressChecker = $localAddressChecker;
+		$this->ipAddressClassifier = $ipAddressClassifier;
 	}
 
 	/**
@@ -133,7 +134,10 @@ class DnsPinMiddleware {
 					$curlResolves["$hostName:$port"] = [];
 
 					foreach ($targetIps as $ip) {
-						$this->localAddressChecker->throwIfLocalIp($ip);
+						if (!$this->ipAddressClassifier->isLocalAddress($ip)) {
+							// TODO: continue with all non-local IPs?
+							throw new LocalServerException('Host violates local access rules');
+						}
 						$curlResolves["$hostName:$port"][] = $ip;
 					}
 				}
diff --git a/lib/private/Http/Client/LocalAddressChecker.php b/lib/private/Http/Client/LocalAddressChecker.php
deleted file mode 100644
index eb24f002d7d..00000000000
--- a/lib/private/Http/Client/LocalAddressChecker.php
+++ /dev/null
@@ -1,102 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-/**
- * @copyright Copyright (c) 2021, Lukas Reschke <lukas@statuscode.ch>
- *
- * @author Lukas Reschke <lukas@statuscode.ch>
- *
- * @license GNU AGPL version 3 or any later version
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as
- * published by the Free Software Foundation, either version 3 of the
- * License, or (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program. If not, see <http://www.gnu.org/licenses/>.
- *
- */
-namespace OC\Http\Client;
-
-use IPLib\Address\IPv6;
-use IPLib\Factory;
-use IPLib\ParseStringFlag;
-use OCP\Http\Client\LocalServerException;
-use Psr\Log\LoggerInterface;
-use Symfony\Component\HttpFoundation\IpUtils;
-
-class LocalAddressChecker {
-	private LoggerInterface $logger;
-
-	public function __construct(LoggerInterface $logger) {
-		$this->logger = $logger;
-	}
-
-	public function throwIfLocalIp(string $ip) : void {
-		$parsedIp = Factory::parseAddressString(
-			$ip,
-			ParseStringFlag::IPV4_MAYBE_NON_DECIMAL | ParseStringFlag::IPV4ADDRESS_MAYBE_NON_QUAD_DOTTED
-		);
-		if ($parsedIp === null) {
-			/* Not an IP */
-			return;
-		}
-		/* Replace by normalized form */
-		if ($parsedIp instanceof IPv6) {
-			$ip = (string)($parsedIp->toIPv4() ?? $parsedIp);
-		} else {
-			$ip = (string)$parsedIp;
-		}
-
-		$localRanges = [
-			'100.64.0.0/10', // See RFC 6598
-			'192.0.0.0/24', // See RFC 6890
-		];
-		if (
-			(bool)filter_var($ip, FILTER_VALIDATE_IP) &&
-			(
-				!filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE) ||
-				IpUtils::checkIp($ip, $localRanges)
-			)) {
-			$this->logger->warning("Host $ip was not connected to because it violates local access rules");
-			throw new LocalServerException('Host violates local access rules');
-		}
-	}
-
-	public function throwIfLocalAddress(string $uri) : void {
-		$host = parse_url($uri, PHP_URL_HOST);
-		if ($host === false || $host === null) {
-			$this->logger->warning("Could not detect any host in $uri");
-			throw new LocalServerException('Could not detect any host');
-		}
-
-		$host = idn_to_utf8(strtolower(urldecode($host)));
-		// Remove brackets from IPv6 addresses
-		if (strpos($host, '[') === 0 && substr($host, -1) === ']') {
-			$host = substr($host, 1, -1);
-		}
-
-		// Disallow local network top-level domains from RFC 6762
-		$localTopLevelDomains = ['local','localhost','intranet','internal','private','corp','home','lan'];
-		$topLevelDomain = substr((strrchr($host, '.') ?: ''), 1);
-		if (in_array($topLevelDomain, $localTopLevelDomains)) {
-			$this->logger->warning("Host $host was not connected to because it violates local access rules");
-			throw new LocalServerException('Host violates local access rules');
-		}
-
-		// Disallow hostname only
-		if (substr_count($host, '.') === 0 && !(bool)filter_var($host, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6)) {
-			$this->logger->warning("Host $host was not connected to because it violates local access rules");
-			throw new LocalServerException('Host violates local access rules');
-		}
-
-		$this->throwIfLocalIp($host);
-	}
-}
diff --git a/lib/private/Net/HostnameClassifier.php b/lib/private/Net/HostnameClassifier.php
new file mode 100644
index 00000000000..626aa47083e
--- /dev/null
+++ b/lib/private/Net/HostnameClassifier.php
@@ -0,0 +1,74 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * @copyright 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @author 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace OC\Net;
+
+use function filter_var;
+use function in_array;
+use function strrchr;
+use function substr;
+use function substr_count;
+
+/**
+ * Classifier for network hostnames
+ *
+ * @internal
+ */
+class HostnameClassifier {
+	private const LOCAL_TOPLEVEL_DOMAINS = [
+		'local',
+		'localhost',
+		'intranet',
+		'internal',
+		'private',
+		'corp',
+		'home',
+		'lan',
+	];
+
+	/**
+	 * Check host identifier for local hostname
+	 *
+	 * IP addresses are not considered local. Use the IpAddressClassifier for those.
+	 *
+	 * @param string $hostname
+	 *
+	 * @return bool
+	 */
+	public function isLocalHostname(string $hostname): bool {
+		// Disallow local network top-level domains from RFC 6762
+		$topLevelDomain = substr((strrchr($hostname, '.') ?: ''), 1);
+		if (in_array($topLevelDomain, self::LOCAL_TOPLEVEL_DOMAINS)) {
+			return true;
+		}
+
+		// Disallow hostname only
+		if (substr_count($hostname, '.') === 0 && !filter_var($hostname, FILTER_VALIDATE_IP, FILTER_FLAG_IPV6)) {
+			return true;
+		}
+
+		return false;
+	}
+}
diff --git a/lib/private/Net/IpAddressClassifier.php b/lib/private/Net/IpAddressClassifier.php
new file mode 100644
index 00000000000..d4698864ec8
--- /dev/null
+++ b/lib/private/Net/IpAddressClassifier.php
@@ -0,0 +1,81 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * @copyright 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @author 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace OC\Net;
+
+use IPLib\Address\IPv6;
+use IPLib\Factory;
+use IPLib\ParseStringFlag;
+use Symfony\Component\HttpFoundation\IpUtils;
+use function filter_var;
+
+/**
+ * Classifier for IP addresses
+ *
+ * @internal
+ */
+class IpAddressClassifier {
+	private const LOCAL_ADDRESS_RANGES = [
+		'100.64.0.0/10', // See RFC 6598
+		'192.0.0.0/24', // See RFC 6890
+	];
+
+	/**
+	 * Check host identifier for local IPv4 and IPv6 address ranges
+	 *
+	 * Hostnames are not considered local. Use the HostnameClassifier for those.
+	 *
+	 * @param string $ip
+	 *
+	 * @return bool
+	 */
+	public function isLocalAddress(string $ip): bool {
+		$parsedIp = Factory::parseAddressString(
+			$ip,
+			ParseStringFlag::IPV4_MAYBE_NON_DECIMAL | ParseStringFlag::IPV4ADDRESS_MAYBE_NON_QUAD_DOTTED
+		);
+		if ($parsedIp === null) {
+			/* Not an IP */
+			return false;
+		}
+		/* Replace by normalized form */
+		if ($parsedIp instanceof IPv6) {
+			$ip = (string)($parsedIp->toIPv4() ?? $parsedIp);
+		} else {
+			$ip = (string)$parsedIp;
+		}
+
+		if (!filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE)) {
+			/* Range address */
+			return true;
+		}
+		if (IpUtils::checkIp($ip, self::LOCAL_ADDRESS_RANGES)) {
+			/* Within local range */
+			return true;
+		}
+
+		return false;
+	}
+}
diff --git a/lib/private/Security/RemoteHostValidator.php b/lib/private/Security/RemoteHostValidator.php
new file mode 100644
index 00000000000..e48bd862472
--- /dev/null
+++ b/lib/private/Security/RemoteHostValidator.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * @copyright 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @author 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace OC\Security;
+
+use OC\Net\HostnameClassifier;
+use OC\Net\IpAddressClassifier;
+use OCP\IConfig;
+use OCP\Security\IRemoteHostValidator;
+use Psr\Log\LoggerInterface;
+use function strpos;
+use function strtolower;
+use function substr;
+use function urldecode;
+
+/**
+ * @internal
+ */
+final class RemoteHostValidator implements IRemoteHostValidator {
+	private IConfig $config;
+	private HostnameClassifier $hostnameClassifier;
+	private IpAddressClassifier $ipAddressClassifier;
+	private LoggerInterface $logger;
+
+	public function __construct(IConfig $config,
+								HostnameClassifier $hostnameClassifier,
+								IpAddressClassifier $ipAddressClassifier,
+								LoggerInterface $logger) {
+		$this->config = $config;
+		$this->hostnameClassifier = $hostnameClassifier;
+		$this->ipAddressClassifier = $ipAddressClassifier;
+		$this->logger = $logger;
+	}
+
+	public function isValid(string $host): bool {
+		if ($this->config->getSystemValueBool('allow_local_remote_servers', false)) {
+			return true;
+		}
+
+		$host = idn_to_utf8(strtolower(urldecode($host)));
+		// Remove brackets from IPv6 addresses
+		if (strpos($host, '[') === 0 && substr($host, -1) === ']') {
+			$host = substr($host, 1, -1);
+		}
+
+		if ($this->hostnameClassifier->isLocalHostname($host)
+			|| $this->ipAddressClassifier->isLocalAddress($host)) {
+			$this->logger->warning("Host $host was not connected to because it violates local access rules");
+			return false;
+		}
+
+		return true;
+	}
+}
diff --git a/lib/private/Server.php b/lib/private/Server.php
index 1da1b614b5b..03aa75060df 100644
--- a/lib/private/Server.php
+++ b/lib/private/Server.php
@@ -105,8 +105,6 @@ use OC\Files\Type\Loader;
 use OC\Files\View;
 use OC\FullTextSearch\FullTextSearchManager;
 use OC\Http\Client\ClientService;
-use OC\Http\Client\DnsPinMiddleware;
-use OC\Http\Client\LocalAddressChecker;
 use OC\Http\Client\NegativeDnsCache;
 use OC\IntegrityCheck\Checker;
 use OC\IntegrityCheck\Helpers\AppLocator;
@@ -858,7 +856,7 @@ class Server extends ServerContainer implements IServerContainer {
 		$this->registerAlias(\OCP\Security\ISecureRandom::class, SecureRandom::class);
 		/** @deprecated 19.0.0 */
 		$this->registerDeprecatedAlias('SecureRandom', \OCP\Security\ISecureRandom::class);
-
+		$this->registerAlias(\OCP\Security\IRemoteHostValidator::class, \OC\Security\RemoteHostValidator::class);
 		$this->registerAlias(IVerificationToken::class, VerificationToken::class);
 
 		$this->registerAlias(ICrypto::class, Crypto::class);
@@ -890,22 +888,11 @@ class Server extends ServerContainer implements IServerContainer {
 
 		$this->registerAlias(ICertificateManager::class, CertificateManager::class);
 		$this->registerAlias(IClientService::class, ClientService::class);
-		$this->registerService(LocalAddressChecker::class, function (ContainerInterface $c) {
-			return new LocalAddressChecker(
-				$c->get(LoggerInterface::class),
-			);
-		});
 		$this->registerService(NegativeDnsCache::class, function (ContainerInterface $c) {
 			return new NegativeDnsCache(
 				$c->get(ICacheFactory::class),
 			);
 		});
-		$this->registerService(DnsPinMiddleware::class, function (ContainerInterface $c) {
-			return new DnsPinMiddleware(
-				$c->get(NegativeDnsCache::class),
-				$c->get(LocalAddressChecker::class)
-			);
-		});
 		$this->registerDeprecatedAlias('HttpClientService', IClientService::class);
 		$this->registerService(IEventLogger::class, function (ContainerInterface $c) {
 			return new EventLogger($c->get(SystemConfig::class), $c->get(LoggerInterface::class), $c->get(Log::class));
diff --git a/lib/public/Security/IRemoteHostValidator.php b/lib/public/Security/IRemoteHostValidator.php
new file mode 100644
index 00000000000..99f149aee04
--- /dev/null
+++ b/lib/public/Security/IRemoteHostValidator.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * @copyright 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @author 2022 Christoph Wurst <christoph@winzerhof-wurst.at>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+namespace OCP\Security;
+
+/**
+ * Validator for remote hosts
+ *
+ * @since 26.0.0
+ */
+interface IRemoteHostValidator {
+
+	/**
+	 * Validate if a host may be connected to
+	 *
+	 * By default, Nextcloud does not connect to any local servers. That is neither
+	 * localhost nor any host in the local network.
+	 *
+	 * Admins can overwrite this behavior with the global `allow_local_remote_servers`
+	 * settings flag. If the flag is set to `true`, local hosts will be considered
+	 * valid.
+	 *
+	 * @param string $host hostname of the remote server, IPv4 or IPv6 address
+	 *
+	 * @return bool
+	 * @since 26.0.0
+	 */
+	public function isValid(string $host): bool;
+}