net: context: Set target network interface in send if needed

If we are sending a network packet and if the remote address
is not set in the context (which means that connect() has not
been called), then we must set the target network interface
to a proper value.
This is done so that when we select the local source address,
we might select the wrong interface if we have multiple network
interfaces in the system. In this case the packet would be always
assigned to first network interface regardless of the destination
address.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

subsys/net/ip/net_context.c

@@ -1462,6 +1462,20 @@ static int context_sendto(struct net_context *context,
 		if (net_ipv6_is_addr_unspecified(&addr6->sin6_addr)) {
 			return -EDESTADDRREQ;
 		}
+
+		/* If application has not yet set the destination address
+		 * i.e., by not calling connect(), then set the interface
+		 * here so that the packet gets sent to the correct network
+		 * interface. This issue can be seen if there are multiple
+		 * network interfaces and we are trying to send data to
+		 * second or later network interface.
+		 */
+		if (addr6 && net_ipv6_is_addr_unspecified(
+				&net_sin6(&context->remote)->sin6_addr)) {
+			iface = net_if_ipv6_select_src_iface(&addr6->sin6_addr);
+			net_context_set_iface(context, iface);
+		}
+
 	} else if (IS_ENABLED(CONFIG_NET_IPV4) &&
 		   net_context_get_family(context) == AF_INET) {
 		const struct sockaddr_in *addr4 =
@@ -1488,6 +1502,19 @@ static int context_sendto(struct net_context *context,
 		if (!addr4->sin_addr.s_addr) {
 			return -EDESTADDRREQ;
 		}
+
+		/* If application has not yet set the destination address
+		 * i.e., by not calling connect(), then set the interface
+		 * here so that the packet gets sent to the correct network
+		 * interface. This issue can be seen if there are multiple
+		 * network interfaces and we are trying to send data to
+		 * second or later network interface.
+		 */
+		if (addr4 && net_sin(&context->remote)->sin_addr.s_addr == 0U) {
+			iface = net_if_ipv4_select_src_iface(&addr4->sin_addr);
+			net_context_set_iface(context, iface);
+		}
+
 	} else if (IS_ENABLED(CONFIG_NET_SOCKETS_PACKET) &&
 		   net_context_get_family(context) == AF_PACKET) {
 		struct sockaddr_ll *ll_addr = (struct sockaddr_ll *)dst_addr;