add support for certificate transparency requirement

Resolving DoT and DoH can now require CT logged certificates. At least
two trusted log entries are required for certificates with a lifetime of
180 days or less and at least three trusted log entries are required
otherwise.

This policy is a sane choice for PKI deployments certificate logs have
value (WebPKI) and thus the limits are not tweakable.

.clang-format

@@ -79,5 +79,6 @@ PenaltyExcessCharacter: 100
 Standard: Cpp11
 ContinuationIndentWidth: 8
 ForEachMacros: [ 'cds_lfs_for_each', 'cds_lfs_for_each_safe', 'cds_list_for_each_entry_safe', 'ISC_LIST_FOREACH', 'ISC_LIST_FOREACH_SAFE', 'ISC_LIST_FOREACH_REV', 'ISC_LIST_FOREACH_REV_SAFE' ]
+TypenameMacros: [ 'STACK_OF' ]
 RemoveParentheses: ReturnStatement
 RemoveSemicolon: true

bin/named/transportconf.c

@@ -128,6 +128,9 @@ add_doh_transports(const cfg_obj_t *transportlist, dns_transport_list_t *list) {
 				       dns_transport_set_cafile);
 		parse_transport_option(doh, transport, "remote-hostname",
 				       dns_transport_set_remote_hostname);
+		parse_transport_option(
+			doh, transport, "certificate-transparency",
+			dns_transport_set_certificate_transparency);
 	}
 
 	return ISC_R_SUCCESS;
@@ -180,6 +183,9 @@ add_tls_transports(const cfg_obj_t *transportlist, dns_transport_list_t *list) {
 				       dns_transport_set_cafile);
 		parse_transport_option(tls, transport, "remote-hostname",
 				       dns_transport_set_remote_hostname);
+		parse_transport_option(
+			tls, transport, "certificate-transparency",
+			dns_transport_set_certificate_transparency);
 	}
 
 	return ISC_R_SUCCESS;

doc/misc/options

@@ -360,6 +360,7 @@ statistics-channels {
 tls <string> {
 	ca-file <quoted_string>;
 	cert-file <quoted_string>;
+	certificate-transparency <boolean>;
 	cipher-suites <string>;
 	ciphers <string>;
 	dhparam-file <quoted_string>;

lib/dns/include/dns/transport.h

@@ -66,6 +66,8 @@ dns_transport_get_prefer_server_ciphers(const dns_transport_t *transport,
 					bool		      *preferp);
 bool
 dns_transport_get_always_verify_remote(dns_transport_t *transport);
+bool
+dns_transport_get_certificate_transparency(dns_transport_t *transport);
 /*%<
  * Getter functions: return the type, cert file, key file, CA file,
  * hostname, HTTP endpoint, HTTP mode (GET or POST), ciphers, cipher suites,
@@ -132,6 +134,10 @@ dns_transport_set_prefer_server_ciphers(dns_transport_t *transport,
 void
 dns_transport_set_always_verify_remote(dns_transport_t *transport,
 				       const bool	always_verify_remote);
+
+void
+dns_transport_set_certificate_transparency(dns_transport_t *transport,
+					   const bool certificate_transparency);
 /*%<
  * Setter functions: set the type, cert file, key file, CA file,
  * hostname, HTTP endpoint, HTTP mode (GET or POST), ciphers, cipher suites, TLS

lib/dns/transport.c

@@ -61,6 +61,7 @@ struct dns_transport {
 		uint32_t protocol_versions;
 		ternary_t prefer_server_ciphers;
 		bool always_verify_remote;
+		bool certificate_transparency;
 	} tls;
 	struct {
 		char *endpoint;
@@ -380,6 +381,25 @@ dns_transport_get_always_verify_remote(dns_transport_t *transport) {
 	return transport->tls.always_verify_remote;
 }
 
+void
+dns_transport_set_certificate_transparency(
+	dns_transport_t *transport, const bool certificate_transparency) {
+	REQUIRE(VALID_TRANSPORT(transport));
+	REQUIRE(transport->type == DNS_TRANSPORT_TLS ||
+		transport->type == DNS_TRANSPORT_HTTP);
+
+	transport->tls.certificate_transparency = certificate_transparency;
+}
+
+bool
+dns_transport_get_certificate_transparency(dns_transport_t *transport) {
+	REQUIRE(VALID_TRANSPORT(transport));
+	REQUIRE(transport->type == DNS_TRANSPORT_TLS ||
+		transport->type == DNS_TRANSPORT_HTTP);
+
+	return transport->tls.certificate_transparency;
+}
+
 isc_result_t
 dns_transport_get_tlsctx(dns_transport_t *transport, const isc_sockaddr_t *peer,
 			 isc_tlsctx_cache_t *tlsctx_cache, isc_mem_t *mctx,
@@ -427,6 +447,8 @@ dns_transport_get_tlsctx(dns_transport_t *transport, const isc_sockaddr_t *peer,
 		const char *key_file = dns_transport_get_keyfile(transport);
 		const bool always_verify_remote =
 			dns_transport_get_always_verify_remote(transport);
+		const bool certificate_transparency =
+			dns_transport_get_certificate_transparency(transport);
 		char peer_addr_str[INET6_ADDRSTRLEN] = { 0 };
 		isc_netaddr_t peer_netaddr = { 0 };
 		bool hostname_ignore_subject;
@@ -529,6 +551,14 @@ dns_transport_get_tlsctx(dns_transport_t *transport, const isc_sockaddr_t *peer,
 
 		isc_tlsctx_enable_dot_client_alpn(tlsctx);
 
+		if (certificate_transparency) {
+			result = isc_tlsctx_require_certificate_transparency(
+				tlsctx);
+			if (result != ISC_R_SUCCESS) {
+				goto failure;
+			}
+		}
+
 		isc_tlsctx_client_session_cache_create(
 			mctx, tlsctx,
 			ISC_TLSCTX_CLIENT_SESSION_CACHE_DEFAULT_SIZE,

lib/isc/include/isc/tls.h

@@ -608,6 +608,9 @@ isc_tlsctx_set_random_session_id_context(isc_tlsctx_t *ctx);
  *\li   'ctx' - a valid non-NULL pointer;
  */
 
+isc_result_t
+isc_tlsctx_require_certificate_transparency(isc_tlsctx_t *ctx);
+
 #define isc_tlserr2result(category, module, funcname, fallback)            \
 	isc__tlserr2result(category, module, funcname, fallback, __FILE__, \
 			   __LINE__)

lib/isc/tls.c

@@ -885,7 +885,7 @@ isc_tlsctx_enable_peer_verification(isc_tlsctx_t *tlsctx, const bool is_server,
 
 isc_result_t
 isc_tlsctx_load_client_ca_names(isc_tlsctx_t *ctx, const char *ca_bundle_file) {
-	STACK_OF(X509_NAME) * cert_names;
+	STACK_OF(X509_NAME) *cert_names;
 	REQUIRE(ctx != NULL);
 	REQUIRE(ca_bundle_file != NULL);
 
@@ -1530,6 +1530,68 @@ isc_tlsctx_set_random_session_id_context(isc_tlsctx_t *ctx) {
 		SSL_CTX_set_session_id_context(ctx, session_id_ctx, len) == 1);
 }
 
+static int
+transparency_cb(const CT_POLICY_EVAL_CTX *ctx, const STACK_OF(SCT) *scts,
+		void *arg) {
+	size_t trusted, needed;
+	int daydiff;
+	X509 *x509;
+	SCT *sct;
+
+	REQUIRE(arg == NULL);
+
+	if (scts == NULL) {
+		goto failure;
+	}
+
+	x509 = CT_POLICY_EVAL_CTX_get0_cert(ctx);
+	INSIST(x509 != NULL);
+
+	if (ASN1_TIME_diff(&daydiff, NULL, X509_get0_notBefore(x509),
+			   X509_get0_notAfter(x509)) != 1)
+	{
+		goto failure;
+	}
+
+	needed = daydiff >= 180 ? 3 : 2;
+
+	trusted = 0;
+	for (int i = 0; i < sk_SCT_num(scts); i++) {
+		sct = sk_SCT_value(scts, i);
+		switch (SCT_get_validation_status(sct)) {
+		case SCT_VALIDATION_STATUS_INVALID:
+			goto failure;
+		case SCT_VALIDATION_STATUS_VALID:
+			trusted++;
+			FALLTHROUGH;
+		default:
+			break;
+		}
+	}
+
+	if (trusted >= needed) {
+		return 1;
+	}
+
+failure:
+	ERR_raise(ERR_LIB_SSL, SSL_R_NO_VALID_SCTS);
+	return 0;
+}
+
+isc_result_t
+isc_tlsctx_require_certificate_transparency(isc_tlsctx_t *ctx) {
+	REQUIRE(ctx != NULL);
+
+	if (SSL_CTX_set_ct_validation_callback(ctx, transparency_cb, NULL) != 1)
+	{
+		return isc_tlserr2result(
+			ISC_LOGCATEGORY_GENERAL, ISC_LOGMODULE_CRYPTO,
+			"SSL_CTX_set_ct_validation_callback", ISC_R_TLSERROR);
+	}
+
+	return ISC_R_SUCCESS;
+}
+
 static isc_result_t
 isc__tls_toresult(isc_result_t fallback) {
 	isc_result_t result = fallback;

lib/isccfg/namedconf.c

@@ -3959,6 +3959,7 @@ static cfg_clausedef_t tls_clauses[] = {
 	{ "cipher-suites", &cfg_type_astring, 0 },
 	{ "prefer-server-ciphers", &cfg_type_boolean, 0 },
 	{ "session-tickets", &cfg_type_boolean, 0 },
+	{ "certificate-transparency", &cfg_type_boolean, 0 },
 	{ NULL, NULL, 0 }
 };