C++ problem with Datagram (UDP)winsocket to sendto and recvfrom on the same socket through the loopback adapter

Question

I am trying to send some data with a udp socket and receive them back on the same socket through windows loopback adatper. In my network properties I set the loopback adapter to have the following ip 192.168.1.1

the recvfrom function returns -1 indicating an error. I also monitor the traffic on the loopback adapter with wireshark and nothing seem to be sent to the loopback adapter, I see no trafic.

Is it true that on windows we can't use the loopback address(127.0.0.1) ? I saw that on some forums, that is why I try to use the loopback adapter. I also tried to send directly to my own ip, but it gives no better results. Btw it is possible to send to his own ip and get the data back?

I would appreciate any help and just in case, I am new to socket programming.

Below is my code:

#define DST "192.168.1.1"

int _tmain(int argc, char* argv[])
{


int    numbytes;
int    bytes_sent;
int    server_sock;
char   send_msg[100];
int    send_msg_length = 100;
char   rcv_msg[100] = { 0 };
int    rcv_msg_length = 100;
int    i;

WSADATA wsaData;   
if(WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
{
    fprintf(stderr, "WSAStartup failed.\n");
    return 1;
}

sockaddr_in to_addr;
sockaddr_in me;

unsigned short Port = 27015;
to_addr.sin_family      = AF_INET;
to_addr.sin_port        = htons(Port);
to_addr.sin_addr.s_addr = inet_addr(DST);

me.sin_family           = AF_INET;
me.sin_port             = 0;
me.sin_addr.s_addr      = htonl(INADDR_ANY);
memset( &(me.sin_zero), '\0', 8 );

if ((server_sock = socket(AF_INET, SOCK_DGRAM, 0)) == -1)
{
    perror("talker: socket");
}

if ( bind( server_sock, (SOCKADDR *)&me, sizeof( me ) ) == -1)
{
    printf("Error binding/n");
    return 1;
};

int length = sizeof( to_addr );
bytes_sent = sendto(server_sock, send_msg, send_msg_length, 0, (SOCKADDR *)&to_addr, length);
if (bytes_sent == -1)
{
    perror("talker: sendto");
    exit(1);
}

printf("Sent %d bytes to %s\n", bytes_sent, DST);
printf("listener: waiting to recvfrom...\n");

if ( numbytes = recvfrom(server_sock, rcv_msg, rcv_msg_length, 0, (SOCKADDR *)&to_addr, &length ) ) 
{
    perror("recvfrom");
    exit(1);
}

closesocket(server_sock);
WSACleanup();

return 0;
}

Varipapa

Answer 1

You are binding your server socket to port 0, i.e. asking the OS to assign random port number to it. That has to be the same port you are sending to, 27015 in your case.

Also, you don't have to byte-swap INADDR_ANY.

Gerta

Yea thx a lot, now I get my data back. I still don't see any traffic on wireshark when i connect to the loopback adapter, any idea why? – Varipapa

I don't think you have to change loopback address for this to work, 127.0.0.1 should be fine. Don't know about traffic not showing up on local interface - haven't touched Windows for some time. – Gerta

Eeeh, no sorry I don't know where I can accept answer, I try to upvote you but I can't as I have not enough points. – Varipapa

Ok, I found it now , not so eaay to see that on iphone :) – Varipapa

@Nikolai: While you can get away with not swapping INADDR_ANY it's a poor habit which will confuse you when you use INADDR_LOOPBACK: #6082392 – Hermosa

Note as well that Wireshark can't snoop traffic from the loopback adaptor on Windows (or couldn't last time I checked). – Binford