Wifi + Ethernet workable at the same time on Android

Android 6.0 + kernel 4.4 on Upboard
(Thank Bruce and Solar for help)
Wifi + Ethernet can't workable at the same time on Android, even we use "ifconfig" to setting.

Upboard Setting:

ip :
eth0    inet addr:192.168.1.20  Bcast:192.168.1.255  Mask:255.255.255.0
wlan0   inet addr:172.20.10.5   Bcast:172.20.10.15   Mask:255.255.255.240

route :
待補

PC(linux) Setting:

ip :
eth0    inet addr:192.168.1.21  Bcast:192.168.1.255  Mask:255.255.255.0

We can use "$ping -I eth0 192.168.1.21" to get response from Upboard to PC, but if we use "$ping 192.168.1.21" it can't get any response.
According the above description here we come to the conclusion. We need to specify the interface so it can transmit the package successfully.

sample code: ref : stackoverflow
(sender.c and listener.c)

#include <netinet/in.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define PORT  12345
#define SERVER_ADDR "192.168.1.20"

int main(int argc, char *argv[])
{
    int fd, nbytes;
    char message[16];
    struct sockaddr_in addr;

    if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
     perror("socket");
  return -1;
    }

    strncpy(message, argv[1], sizeof(message));

    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr(SERVER_ADDR);
    addr.sin_port = htons(PORT);

    const char device[] = "eth0";
    if (setsockopt(fd, SOL_SOCKET, SO_BINDTODEVICE, device, sizeof(device)) < 0) {
        perror("SO_BINDTODEVICE");
  return -1;
    }
    while (1) {
        if ((nbytes = sendto(fd, message, strlen(message), 0, 
                   (struct sockaddr *) &addr, sizeof(addr))) < 0) {
         perror("sendto");
   return -1;
     }
     usleep(1000*1000);
    }
}

Android/PC run the program, PC/Android run tcpdump, it should get packages. But...the program need root permission if you want to run Android App. = ="