OpenThread: avoid hardcoded Golioth IPv6 address

Leverage the OpenThread DNS to synthesize the Golioth System Server
IPv6 address avoiding hardcoded IP address in applications.
NAT64 prefix used by the Thred Border Router is set while
synthesizing the address.

Signed-off-by: MarkoPura <[email protected]>

net/golioth/system_client.c

@@ -19,12 +19,41 @@ LOG_MODULE_REGISTER(golioth_system, CONFIG_GOLIOTH_SYSTEM_CLIENT_LOG_LEVEL);
 
 #define RX_BUFFER_SIZE		CONFIG_GOLIOTH_SYSTEM_CLIENT_RX_BUF_SIZE
 
+static char golioth_system_server_host[40] = CONFIG_GOLIOTH_SYSTEM_SERVER_HOST;
+
 static const uint8_t tls_ca_crt[] = {
 #if defined(CONFIG_GOLIOTH_SYSTEM_CLIENT_CA_PATH)
 #include "golioth-systemclient-ca.inc"
 #endif
 };
 
+#ifdef CONFIG_NET_L2_OPENTHREAD
+#include <openthread/nat64.h>
+#include <openthread/dns_client.h>
+#include <zephyr/net/openthread.h>
+#include <openthread/error.h>
+
+static otDnsQueryConfig dns_query;
+K_SEM_DEFINE(ot_dns_resolve, 0, 1);
+
+/* Callback for NAT64 IPv6 translated Golioth System Server address from the DNS query response */
+static void ot_dns_callback(otError aError, const otDnsAddressResponse *aResponse, void *aContext)
+{
+	otIp6Address golioth_ipv6_addr;
+
+	if (aError != OT_ERROR_NONE) {
+		LOG_ERR("Golioth System Server DNS resolving error %d", aError);
+		return;
+	}
+
+	if (otDnsAddressResponseGetAddress(aResponse, 0, &golioth_ipv6_addr, NULL) == OT_ERROR_NONE) {
+		otIp6AddressToString(&golioth_ipv6_addr, golioth_system_server_host, OT_IP6_ADDRESS_STRING_SIZE);
+	}
+
+	k_sem_give(&ot_dns_resolve);
+}
+#endif
+
 #define PING_INTERVAL		(CONFIG_GOLIOTH_SYSTEM_CLIENT_PING_INTERVAL_SEC * 1000)
 #define RECV_TIMEOUT		(CONFIG_GOLIOTH_SYSTEM_CLIENT_RX_TIMEOUT_SEC * 1000)
 
@@ -244,11 +273,50 @@ SYS_INIT(golioth_system_init, APPLICATION,
 
 static int client_connect(struct golioth_client *client)
 {
-	int err;
+	int err = 0;
+
+/*
+ * Get the IPv6 address of GOLIOTH_SYSTEM_SERVER_HOST to avoid hardcoding it
+ * in user application. NAT64 prefix used by the Thread Border Router
+ * is set while synthesizing the IPv6 address.
+ */
+#if defined (CONFIG_NET_L2_OPENTHREAD)
+	struct openthread_context *ot_context = openthread_get_default_context();
+
+	otIp4Address dns_server_addr;
+
+	err = otIp4AddressFromString(CONFIG_DNS_SERVER1, &dns_server_addr);
+	if (err != OT_ERROR_NONE) {
+		LOG_ERR("DNS server IPv4 address error: %d", err);
+		return err;
+	}
+
+	err = otNat64SynthesizeIp6Address(ot_context->instance,
+					  &dns_server_addr,
+					  &dns_query.mServerSockAddr.mAddress);
+	if (err != OT_ERROR_NONE) {
+		LOG_ERR("Synthesize DNS server IPv6 address error: %d", err);
+		return err;
+	}
+
+	err = otDnsClientResolveIp4Address(ot_context->instance,
+					   CONFIG_GOLIOTH_SYSTEM_SERVER_HOST,
+					   ot_dns_callback,
+					   ot_context,
+					   &dns_query);
+	if (err != OT_ERROR_NONE) {
+		LOG_ERR("Golioth System Server address resolution DNS query error: %d", err);
+		return err;
+	}
+
+	k_sem_take(&ot_dns_resolve, K_FOREVER);
+
+#endif
 
 	err = golioth_connect(client,
-			      CONFIG_GOLIOTH_SYSTEM_SERVER_HOST,
+			      golioth_system_server_host,
 			      CONFIG_GOLIOTH_SYSTEM_SERVER_PORT);
+
 	if (err) {
 		LOG_ERR("Failed to connect: %d", err);
 		return err;