How to select which network interface to use in a ESP32 - wifi

Question

I'm currently using a Esp32 which presents, in addition to wifi, an ethernet interface.
I'm using the esp-idf v3.3 with FreeRTOS.
To use it I included the "esp_eth.h" library (https://docs.espressif.com/projects/esp-idf/en/release-v3.1/api-reference/ethernet/esp_eth.html#api-reference-phy-lan8720).
I'd like to use both Wifi and ethernet interfaces basing on what I want to do but selecting which one to use, is there a way?
The practical use is to receive a command through the ethernet interface (for example a site to ping), ping the server through the wifi interface and answer back to the ethernet interface the ping result.
How can I select which interface to use (i dont want that the ping is made through the ethernet or that the response go through the wifi).
#edit: i found the method netif_set_default(struct netif * netif), but i dont know if it is the best way to select the interface to use for the specific action(i should swap from one interface to another)

Answer 1

Just get the IP of the Ethernet interface, then bind a socket to that IP address.
struct in_addr iaddr = {0};
#if USE_ANY_IF
// Bind the socket to any address
iaddr.s_addr = htonl(INADDR_ANY);
#else
// bind only to the Ethernet interface
tcpip_adapter_ip_info_t ip_info = {0};
tcpip_adapter_get_ip_info(TCPIP_ADAPTER_IF_ETH, &ip_info);
inet_addr_from_ip4addr(&iaddr, &ip_info.ip);
#endif
Then use it to bind a socket, for example:
static int create_multicast_ipv4_socket(struct in_addr bind_iaddr)
{
struct sockaddr_in saddr = {0};
int sock = -1;
int err = 0;
char addrbuf[32] = {0};
sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);
if (sock < 0)
{
ESP_LOGE(V4TAG, "Failed to create socket. Error %d", errno);
return -1;
}
saddr.sin_addr.s_addr = bind_iaddr.s_addr; // what interface IP to bind to. Can be htonl(INADDR_ANY)
saddr.sin_family = PF_INET;
saddr.sin_port = htons(UDP_PORT);
inet_ntoa_r(saddr.sin_addr.s_addr, addrbuf, sizeof(addrbuf) - 1);
ESP_LOGI(TAG, "Binding to interface %s...", addrbuf);
err = bind(sock, (struct sockaddr *)&saddr, sizeof(struct sockaddr_in));
if (err < 0)
{
ESP_LOGE(V4TAG, "Failed to bind socket. Error %d", errno);
goto err;
}
// Assign multicast TTL (set separately from normal interface TTL)
uint8_t ttl = MULTICAST_TTL;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(uint8_t));
if (err < 0)
{
ESP_LOGE(V4TAG, "Failed to set IP_MULTICAST_TTL. Error %d", errno);
goto err;
}
// All set, socket is configured for sending
return sock;
err:
close(sock);
return -1;
}