LWS_SERVER_OPTION_OPENSSL_AUTO_DH_PARAMETERS takes care of providing a

DH group, aka. finite field group, aka. "DH parameters" (by calling
SSL_CTX_set_dh_auto) in case TLSv1.2 Kx=DH ciphers
(e.g. TLS_DHE_RSA_WITH_AES_256_GCM_SHA384) are selected.

Signed-off-by: Norbert van Bolhuis <nvbolhuis@gmail.com>

Author: nvbolhuis

include/libwebsockets/lws-context-vhost.h

@@ -242,6 +242,12 @@
 #define LWS_SERVER_OPTION_DISABLE_TLS_SESSION_CACHE		 (1ll << 39)
 	/**< (VHOST) Disallow use of client tls caching (on by default) */
 
+#define LWS_SERVER_OPTION_OPENSSL_AUTO_DH_PARAMETERS		 (1ll << 40)
+	/**< Configure openssl to use the default built-in DH parameters
+	 * to support TLSv1.2 Kx=DH ciphers (by calling SSL_CTX_set_dh_auto)
+	 * This is needed when you want to enable TLSv1.2 ephemeral
+	 * Diffie-Hellman (DH) key exchange ciphers
+	 * (e.g. TLS_DHE_RSA_WITH_AES_256_GCM_SHA384). It's not recommended. */
 
 	/****** add new things just above ---^ ******/
 

lib/tls/openssl/openssl-server.c

@@ -520,6 +520,10 @@ lws_tls_server_vhost_backend_init(const struct lws_context_creation_info *info,
 #ifdef SSL_OP_NO_COMPRESSION
 	SSL_CTX_set_options(vhost->tls.ssl_ctx, SSL_OP_NO_COMPRESSION);
 #endif
+	if (lws_check_opt(info->options,
+	                  LWS_SERVER_OPTION_OPENSSL_AUTO_DH_PARAMETERS))
+		SSL_CTX_set_dh_auto(vhost->tls.ssl_ctx, 1);
+
 	SSL_CTX_set_options(vhost->tls.ssl_ctx, SSL_OP_SINGLE_DH_USE);
 	SSL_CTX_set_options(vhost->tls.ssl_ctx, SSL_OP_CIPHER_SERVER_PREFERENCE);
 

test-apps/test-server.c

@@ -579,6 +579,11 @@ int main(int argc, char **argv)
 			       "!DHE-RSA-AES256-SHA256:"
 			       "!AES256-GCM-SHA384:"
 			       "!AES256-SHA256";
+	/*
+	 * This is needed for DHE-RSA-AES256-GCM-SHA384, it does enable all
+	 * TLSv1.2 Kx=DH ciphers though (if the're on the ssl_cipher_list).
+	 */
+	info.options |= LWS_SERVER_OPTION_OPENSSL_AUTO_DH_PARAMETERS;
 #endif
 	info.mounts = &mount;
 #if defined(LWS_WITH_PEER_LIMITS)