Introduce DTLS transport for NNG.

This introduces a new experimental transport for DTLS, that
provides encryption over UDP.  It has a simpler protocol than
the current UDP SP protocol (but we intend to fix that by making
the UDP transport simpler in a follow up!)

There are a few other fixes in the TLS layer itself, and in
the build, that were needed to accomplish this work.

Also there was an endianness bug in the UDP protocol handling, which
is fixed here.

2 files changed, 59 insertions, 5 deletions

diff --git a/src/supplemental/tls/mbedtls/CMakeLists.txt b/src/supplemental/tls/mbedtls/CMakeLists.txt
index acf852bd..466f0a1b 100644
--- a/src/supplemental/tls/mbedtls/CMakeLists.txt
+++ b/src/supplemental/tls/mbedtls/CMakeLists.txt
@@ -13,7 +13,7 @@ if (NNG_TLS_ENGINE STREQUAL "mbed")
         Linking against Mbed TLS may change license terms.
         Consult a lawyer and the license files for details.
         ************************************************************")
-    nng_sources(tls.c)
+    nng_sources(mbedtls.c)
     nng_defines(NNG_TLS_ENGINE_INIT=nng_tls_engine_init_mbed)
     nng_defines(NNG_TLS_ENGINE_FINI=nng_tls_engine_fini_mbed)
     nng_defines(NNG_SUPP_TLS)
diff --git a/src/supplemental/tls/mbedtls/tls.c b/src/supplemental/tls/mbedtls/mbedtls.c
index 7764bbbf..8250740f 100644
--- a/src/supplemental/tls/mbedtls/tls.c
+++ b/src/supplemental/tls/mbedtls/mbedtls.c
@@ -23,6 +23,9 @@
 
 #include "nng/nng.h"
 
+// We use a common cookie for our application.
+#include "mbedtls/ssl_cookie.h"
+
 #include "../tls_engine.h"
 
 // mbedTLS renamed this header for 2.4.0.
@@ -82,6 +85,8 @@ static nni_mtx                  rng_lock;
 struct nng_tls_engine_conn {
 	void               *tls; // parent conn
 	mbedtls_ssl_context ctx;
+	nng_time            exp1;
+	nng_time            exp2;
 };
 
 struct nng_tls_engine_config {
@@ -96,6 +101,8 @@ struct nng_tls_engine_config {
 	nni_list           psks;
 };
 
+static mbedtls_ssl_cookie_ctx mbed_ssl_cookie_ctx;
+
 static void
 tls_dbg(void *ctx, int level, const char *file, int line, const char *s)
 {
@@ -238,15 +245,44 @@ conn_fini(nng_tls_engine_conn *ec)
 	mbedtls_ssl_free(&ec->ctx);
 }
 
+static void
+conn_set_timer(void *arg, unsigned int t1, unsigned int t2)
+{
+	nng_time             now = nng_clock();
+	nng_tls_engine_conn *ec  = arg;
+	ec->exp1                 = t1 ? now + t1 : 0;
+	ec->exp2                 = t2 ? now + t2 : 0;
+}
+
 static int
-conn_init(nng_tls_engine_conn *ec, void *tls, nng_tls_engine_config *cfg)
+conn_get_timer(void *arg)
 {
-	int rv;
+	nng_tls_engine_conn *ec  = arg;
+	nng_time             now = nng_clock();
+	if (ec->exp2 == 0) {
+		return -1;
+	}
+	if (now > ec->exp2) {
+		return 2;
+	}
+	if (now > ec->exp1) {
+		return 1;
+	}
+	return (0);
+}
+
+static int
+conn_init(nng_tls_engine_conn *ec, void *tls, nng_tls_engine_config *cfg,
+    const nng_sockaddr *sa)
+{
+	int  rv;
+	char buf[NNG_MAXADDRSTRLEN];
 
 	ec->tls = tls;
 
 	mbedtls_ssl_init(&ec->ctx);
 	mbedtls_ssl_set_bio(&ec->ctx, tls, net_send, net_recv, NULL);
+	mbedtls_ssl_set_timer_cb(&ec->ctx, ec, conn_set_timer, conn_get_timer);
 
 	if ((rv = mbedtls_ssl_setup(&ec->ctx, &cfg->cfg_ctx)) != 0) {
 		tls_log_warn(
@@ -258,6 +294,12 @@ conn_init(nng_tls_engine_conn *ec, void *tls, nng_tls_engine_config *cfg)
 		mbedtls_ssl_set_hostname(&ec->ctx, cfg->server_name);
 	}
 
+	if (cfg->mode == NNG_TLS_MODE_SERVER) {
+		nng_str_sockaddr(sa, buf, sizeof(buf));
+		mbedtls_ssl_set_client_transport_id(
+		    &ec->ctx, (const void *) buf, strlen(buf));
+	}
+
 	return (0);
 }
 
@@ -484,6 +526,12 @@ config_init(nng_tls_engine_config *cfg, enum nng_tls_mode mode)
 	mbedtls_ssl_conf_rng(&cfg->cfg_ctx, tls_random, cfg);
 	mbedtls_ssl_conf_dbg(&cfg->cfg_ctx, tls_dbg, cfg);
 
+	if (cfg->mode == NNG_TLS_MODE_SERVER) {
+		mbedtls_ssl_conf_dtls_cookies(&cfg->cfg_ctx,
+		    mbedtls_ssl_cookie_write, mbedtls_ssl_cookie_check,
+		    &mbed_ssl_cookie_ctx);
+	}
+
 	return (0);
 }
 
@@ -793,9 +841,14 @@ nng_tls_engine_init_mbed(void)
 #endif
 	// Uncomment the following to have noisy debug from mbedTLS.
 	// This may be useful when trying to debug failures.
-	//	mbedtls_debug_set_threshold(9);
+	// mbedtls_debug_set_threshold(9);
+
+	mbedtls_ssl_cookie_init(&mbed_ssl_cookie_ctx);
+	rv = mbedtls_ssl_cookie_setup(&mbed_ssl_cookie_ctx, tls_random, NULL);
 
-	rv = nng_tls_engine_register(&tls_engine_mbed);
+	if (rv == 0) {
+		rv = nng_tls_engine_register(&tls_engine_mbed);
+	}
 
 #ifdef NNG_TLS_USE_CTR_DRBG
 	if (rv != 0) {
@@ -809,6 +862,7 @@ nng_tls_engine_init_mbed(void)
 void
 nng_tls_engine_fini_mbed(void)
 {
+	mbedtls_ssl_cookie_free(&mbed_ssl_cookie_ctx);
 #ifdef NNG_TLS_USE_CTR_DRBG
 	mbedtls_ctr_drbg_free(&rng_ctx);
 	nni_mtx_fini(&rng_lock);