net: mqtt: Allow to bind client to a specific interface

Add a new "if_name" pointer to the transport configuration structure,
allowing the application to bind MQTT client to a specific network
interface.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

Author: rlubos

include/zephyr/net/mqtt.h

@@ -809,6 +809,11 @@ struct mqtt_transport {
 	 */
 	enum mqtt_transport_type type;
 
+	/** Name of the interface that the MQTT client instance should be bound to.
+	 *  Leave as NULL if not specified.
+	 */
+	const char *if_name;
+
 	/** Use either unsecured TCP or secured TLS transport */
 	union {
 		/** TCP socket transport for MQTT */

subsys/net/lib/mqtt/mqtt_transport_socket_tcp.c

@@ -29,6 +29,26 @@ int mqtt_client_tcp_connect(struct mqtt_client *client)
 		return -errno;
 	}
 
+	NET_DBG("Created socket %d", client->transport.tcp.sock);
+
+	if (client->transport.if_name != NULL) {
+		struct ifreq ifname = { 0 };
+
+		strncpy(ifname.ifr_name, client->transport.if_name,
+			sizeof(ifname.ifr_name) - 1);
+
+		ret = zsock_setsockopt(client->transport.tcp.sock, SOL_SOCKET,
+				       SO_BINDTODEVICE, &ifname,
+				       sizeof(struct ifreq));
+		if (ret < 0) {
+			NET_ERR("Failed to bind ot interface %s error (%d)",
+				ifname.ifr_name, -errno);
+			goto error;
+		}
+
+		NET_DBG("Bound to interface %s", ifname.ifr_name);
+	}
+
 #if defined(CONFIG_SOCKS)
 	if (client->transport.proxy.addrlen != 0) {
 		ret = setsockopt(client->transport.tcp.sock,
@@ -41,8 +61,6 @@ int mqtt_client_tcp_connect(struct mqtt_client *client)
 	}
 #endif
 
-	NET_DBG("Created socket %d", client->transport.tcp.sock);
-
 	size_t peer_addr_size = sizeof(struct sockaddr_in6);
 
 	if (broker->sa_family == AF_INET) {

subsys/net/lib/mqtt/mqtt_transport_socket_tls.c

@@ -32,6 +32,24 @@ int mqtt_client_tls_connect(struct mqtt_client *client)
 
 	NET_DBG("Created socket %d", client->transport.tls.sock);
 
+	if (client->transport.if_name != NULL) {
+		struct ifreq ifname = { 0 };
+
+		strncpy(ifname.ifr_name, client->transport.if_name,
+			sizeof(ifname.ifr_name) - 1);
+
+		ret = zsock_setsockopt(client->transport.tls.sock, SOL_SOCKET,
+				       SO_BINDTODEVICE, &ifname,
+				       sizeof(struct ifreq));
+		if (ret < 0) {
+			NET_ERR("Failed to bind ot interface %s error (%d)",
+				ifname.ifr_name, -errno);
+			goto error;
+		}
+
+		NET_DBG("Bound to interface %s", ifname.ifr_name);
+	}
+
 #if defined(CONFIG_SOCKS)
 	if (client->transport.proxy.addrlen != 0) {
 		ret = setsockopt(client->transport.tls.sock,