aws_iot example without network offloading

Question

Hello, 
 I would like to know how to build the aws_iot example without network offloading to the modem. I need to make AWS connections through both the nRF9160’s LTE-M connection as well as through an Ethernet ENC424J600 controller ( PHYTEC link board ETH or equivalent ), only one of these two interfaces will be enabled at a time. 
 As I understand it, if I want to use more than one interface in my app, I have to disable sockets offloading ( CONFIG_NET_SOCKETS_OFFLOAD =n ), but doing that prevents enabling LTE connectivity ( CONFIG_LTE_CONNECTIVITY =y). Hence my issues of having everything somehow use MbedTLS and being transparent to the AWS IoT library. 
 I have spent weeks trying to make the examples work independently and together (aws_iot, aws_iot_mqtt (which I cannot get to compile for the nRF9160DK because of build errors with TF-M), https_client and others) and cannot get anywhere close to where I need… I can make the dns_resolve work over Ethernet for example, by disabling the sockets offloading so that getaddrinfo() properly calls dns_get_addr_info() and such but putting everything together is a nightmare because of configuration incompatibilities with nRF Modem’s libraries… 
 I hope someone has any idea to help me here, thanks!

GilDev · Accepted Answer

I have made some advancements on the matter. Turns out you can indeed keep the offloaded sockets enabled but still use the native implementation by using the SOCK_NATIVE flag when opening a socket. SOCK_NATIVE_TLS gives issues but that makes some sense considering the documentation: 
 When CONFIG_NET_SOCKETS_OFFLOAD is enabled, offloaded sockets take precedence
when creating a new socket. Combine this flag with a socket type when
creating a socket, to enforce native socket creation (e. g. SOCK_STREAM | SOCK_NATIVE).
If it's desired to create a native TLS socket, but still offload the
underlying TCP/UDP socket, use e. g. SOCK_STREAM | SOCK_NATIVE_TLS. 
 The thing is that this doesn&rsquo;t seem to work for some functions like getaddrinfo() that is used kind of everywhere to resolve host names. Instead those only statically rely on the definition of CONFIG_NET_SOCKETS_OFFLOAD. I had to patch zsock_getaddrinfo() and zsock_freeaddrinfo() with a global &ldquo;ethernet&rdquo; flag like this to make it changeable at runtime: 
 #include "global.h"
int zsock_getaddrinfo(const char *host, const char *service,
 const struct zsock_addrinfo *hints,
 struct zsock_addrinfo **res)
{
	if (!ethernet) {
 if (IS_ENABLED(CONFIG_NET_SOCKETS_OFFLOAD)) {
 return socket_offload_getaddrinfo(host, service, hints, res);
 }
	}

int ret = DNS_EAI_FAIL;

…

void zsock_freeaddrinfo(struct zsock_addrinfo *ai)
{
	if (!ethernet) {
 if (IS_ENABLED(CONFIG_NET_SOCKETS_OFFLOAD)) {
 return socket_offload_freeaddrinfo(ai);
 }
	}

free(ai);
} 
 I also patched mqtt_client_tls_connect() so the type given to the socket is SOCK_NATIVE instead of SOCK_NATIVE_TLS, making the entire socket going through MbedTLS: 
 int mqtt_client_tls_connect(struct mqtt_client *client)
{
	const struct sockaddr *broker = client->broker;
	struct mqtt_sec_config *tls_config = &client->transport.tls.config;
	int type = SOCK_STREAM;
	int ret;

if (tls_config->set_native_tls) {
 type |= SOCK_NATIVE;
	}

client->transport.tls.sock = zsock_socket(broker->sa_family,
 type, IPPROTO_TLS_1_2);
	if (client->transport.tls.sock < 0) {
 return -errno;
	}

NET_DBG("Created socket %d", client->transport.tls.sock);

… 
 And finally this flag is enabled when Ethernet is needed by patching connect_client() of the aws_iot library: 
 #include "global.h"
static int connect_client(struct aws_iot_config *const config)
{
	int err;

err = client_broker_init(&client);
	if (err) {
 LOG_ERR("client_broker_init, error: %d", err);
 return err;
	}

if (ethernet) {
 client.transport.tls.config.set_native_tls = true;
	}

err = mqtt_connect(&client);
	if (err) {
 LOG_ERR("mqtt_connect, error: %d", err);
 err = connect_error_translate(err);
 perror("mqtt_connect");
 return err;
	}

… 
 This way, by changing the value of this ethernet flag on startup, I can up one interface or the other (LTE-M or Ethernet), start the DHCPv4 client on the Ethernet interface in case it is the selected one and the rest of the sample works properly by updating the AWS shadow. This is not made to have both interfaces up and working at the same time, in my case I just enable one or the other on startup depending on some local conditions (Ethernet cable plugged in or SIM card inserted). 
 This is not a clean fix, it will probably require some adaptations to be made as I go along in my development (I&rsquo;m not sure it will work as is with aws_fota for example). I&rsquo;ll try to post the whole project with the SDK&rsquo;s patch at some point. 
 It seems crazy to me that I have to go through that much and alter the Zephyr networking libraries in this way, so I hope someone will be able to look at this and give some opinion, I may have missed something&hellip;

aws_iot example without network offloading

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