From: Vladimír Čunát <vladimir.cunat@nic.cz>
Date: Tue, 2 Apr 2024 15:39:05 +0000 (+0200)
Subject: daemon/tls: respect crypto policy overrides in OS
X-Git-Tag: v6.0.8~29^2
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fenvironments%2Fdocs-develop-tls-hs7nlh%2Fdeployments%2F3707;p=thirdparty%2Fknot-resolver.git

daemon/tls: respect crypto policy overrides in OS
---

diff --git a/NEWS b/NEWS
index b70d259d0..e56187f17 100644
--- a/NEWS
+++ b/NEWS
@@ -1,3 +1,11 @@
+Knot Resolver 6.0.8 (2024-0m-dd)
+================================
+
+Improvements
+------------
+- TLS (DoT, DoH): respect crypto policy overrides in OS (!1526)
+
+
 Knot Resolver 6.0.7 (2024-03-27)
 ================================
 
diff --git a/daemon/tls.c b/daemon/tls.c
index 1512f0451..e8dff76c5 100644
--- a/daemon/tls.c
+++ b/daemon/tls.c
@@ -71,22 +71,24 @@ static int client_verify_certificate(gnutls_session_t tls_session);
 static struct tls_credentials *tls_credentials_reserve(struct tls_credentials *tls_credentials);
 
 /**
- * Set mandatory security settings from
- * https://tools.ietf.org/html/draft-ietf-dprive-dtls-and-tls-profiles-11#section-9
+ * Set restrictions on TLS features, in particular ciphers.
+ *
+ * We explicitly disable features according to:
+ * https://datatracker.ietf.org/doc/html/rfc8310#section-9
+ * in case the gnutls+OS defaults allow them.
  * Performance optimizations are not implemented at the moment.
+ *
+ * OS defaults are taken into account, e.g. on Red Hat family there's
+ * /etc/crypto-policies/back-ends/gnutls.config and update-crypto-policies tool.
  */
 static int kres_gnutls_set_priority(gnutls_session_t session) {
-	static const char * const priorities =
-		"NORMAL:" /* GnuTLS defaults */
-		"-VERS-TLS1.0:-VERS-TLS1.1:" /* TLS 1.2 and higher */
-		 /* Some distros by default allow features that are considered
-		  * too insecure nowadays, so let's disable them explicitly. */
-		"-VERS-SSL3.0:-ARCFOUR-128:-COMP-ALL:+COMP-NULL";
+	static const char * const extra_prio =
+		"-VERS-TLS1.0:-VERS-TLS1.1:-VERS-SSL3.0:-ARCFOUR-128:-COMP-ALL:+COMP-NULL";
 	const char *errpos = NULL;
-	int err = gnutls_priority_set_direct(session, priorities, &errpos);
+	int err = gnutls_set_default_priority_append(session, extra_prio, &errpos, 0);
 	if (err != GNUTLS_E_SUCCESS) {
 		kr_log_error(TLS, "setting priority '%s' failed at character %zd (...'%s') with %s (%d)\n",
-			     priorities, errpos - priorities, errpos, gnutls_strerror_name(err), err);
+			     extra_prio, errpos - extra_prio, errpos, gnutls_strerror_name(err), err);
 	}
 	return err;
 }
diff --git a/doc/user/config-network-server-tls.rst b/doc/user/config-network-server-tls.rst
index b74617540..420de59ca 100644
--- a/doc/user/config-network-server-tls.rst
+++ b/doc/user/config-network-server-tls.rst
@@ -105,6 +105,16 @@ Configuration options for DoT and DoH
 A self-signed certificate is generated by default.
 For serious deployments it is strongly recommended to configure your own TLS certificates signed by a trusted CA.
 
+Knot Resolver respects system-wide cryptographic policies. If you are using a
+distro that ships such a package, you may use `crypto-policies
+<https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9/html/security_hardening/using-the-system-wide-cryptographic-policies_security-hardening>`_
+to additionally harden DoT/DoH encryption by disabling certain cipher suites,
+outdated TLS protocols etc.
+
+Please note that Knot Resolver already disallows the usage of TLS 1.1 and lower
+as per section 9 of :rfc:`8310`. This **cannot** be overridden by system-wide
+policies.
+
 .. option:: network/tls:
 
    .. option:: cert-file: <path>