Unable to receive custom Ethernet frame

Donnie_Song

I created a socket_raw, the custom protocol type is 0x2328, and I send 0x2328 type data on another machine. The capture packet can be captured, but recvfrom cannot receive it. What should I do? Here is the code received.

struct sockaddr_ll sll;
struct ifreq ifr;

if ((sd = socket (PF_PACKET, SOCK_RAW, htons (0x2328))) < 0) {
perror ("socket() failed to get socket descriptor for using ioctl() ");
exit (EXIT_FAILURE);
}

// Use ioctl() to look up interface name and get its MAC address.
memset (&ifr, 0, sizeof (ifr));
sprintf (ifr.ifr_name, "eth1");

if (ioctl (sd, SIOCGIFINDEX, &ifr) < 0) {
perror ("ioctl() failed");
return (EXIT_FAILURE);
}

memset (&sll, 0, sizeof (sll));
int sll_len;
sll.sll_family = AF_PACKET;
sll.sll_ifindex = ifr.ifr_ifindex;
sll_len = sizeof(sll);

char buffer[1024];
recvfrom(sd, buffer, sizeof(buffer), 0, (struct sockaddr *)&sll, &sll_len);

Lost User

You must check the return value from the recvfrom call to find out why it is not working. See recvfrom function | Microsoft Docs[^].

Donnie_Song

recvfrom has no return value, because there is no data, so it is blocking. And if I change 0x2328 to 0x0003, I can receive it.

Lost User

Try making it a non-blocking socket. You should then get a status return.

Donnie_Song

After I set it to non-blocking, I always returned error code 11, but I didn't find the error message corresponding to 11.

Victor Nijegorodov

Could you show the actual code used to obtain the error?

Lost User

See recvfrom function | Microsoft Docs[^] for possible status returns.

Jochen Arndt

It won't help you with your problem but error code 11 is EGAIN defined in errno.h. It indicates that you should execute the function again until you get data or an error, or give up (time out). It is returned by non-blocking functions when the corresponding blocking function would block.

Donnie_Song

int main(int argv, char *argc[])
{
struct ifreq ifr;
struct sockaddr_ll sll;
int sd, sll_len;
sll_len = sizeof(sll);
if ((sd = socket(PF_PACKET, SOCK_RAW, htons(0x2328))) < 0)
printf("create socket failed!\n");

    sll.sll\_halen = ETH\_ALEN;



    strcpy(ifr.ifr\_name, "ens33");


    ioctl(sd, SIOCGIFFLAGS, &ifr);
    ifr.ifr\_flags |= IFF\_PROMISC;

    if(fcntl(sd, F\_SETFL, O\_NONBLOCK) == -1) {
            perror("fcntl");
            exit(errno);
    }
    char recvbuf\[2048\];
    sleep(5);
    int n\_read = recvfrom(sd, recvbuf, 2048, 0, (struct sockaddr \*)&sll, &sll\_len);
    if (n\_read <= 0)
    {
            printf("%d\\n", errno);
    }

}

I started to cycle data before I hibernate, but I still can't receive it

Donnie_Song

I found the reason. Previously, because send and recv were sent and received on the same network card, it could not be received.

Jochen Arndt

Fine to hear that. I do know that it does not work on the same interface. But you wrote in your initial post that you are sending from another machine so that I thought it must be something else.

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