Select MAC address for binding to a specific bluetooth adapter

Question

The following piece of code binds a Bluetooth socket. My expectation is that the call to bind() (or to listen()) with that MAC address as argument fails, because when using AF_INET sockets, I cannot bind() to an IP address that I don't have. This answer suggests that it's possible to select your adapter, but I cannot reproduce that in my experiments.

/* From https://people.csail.mit.edu/albert/bluez-intro/x502.html */
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <bluetooth/bluetooth.h>
#include <bluetooth/rfcomm.h>

int main(int argc, char **argv)
{
    struct sockaddr_rc loc_addr = { 0 }, rem_addr = { 0 };
    char buf[1024] = { 0 };
    int s, client, bytes_read;
    socklen_t opt = sizeof(rem_addr);

    // allocate socket
    s = socket(AF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);

    // bind socket to port 1 of the first available 
    // local bluetooth adapter
    loc_addr.rc_family = AF_BLUETOOTH;
    // loc_addr.rc_bdaddr = *BDADDR_ANY;
    bdaddr_t my_bdaddr = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06};
    loc_addr.rc_bdaddr = my_bdaddr;
    loc_addr.rc_channel = (uint8_t) 1;
    {
        int res = bind(s, (struct sockaddr *)&loc_addr, sizeof(loc_addr));
        char baddr[128];
        ba2str(&loc_addr.rc_bdaddr, baddr);
        fprintf(stderr, "bound to %s: %d\n", baddr, res);
    }

    // put socket into listening mode
    {
        int res = listen(s, 1);
        fprintf(stderr, "Listening: %d\n", res);
    }

    // accept one connection
    client = accept(s, (struct sockaddr *)&rem_addr, &opt);

    ba2str( &rem_addr.rc_bdaddr, buf );
    fprintf(stderr, "accepted connection from %s\n", buf);
    memset(buf, 0, sizeof(buf));

    // read data from the client
    bytes_read = read(client, buf, sizeof(buf));
    if( bytes_read > 0 ) {
        printf("received [%s]\n", buf);
    }

    // close connection
    close(client);
    close(s);
    return 0;
}

So I guess my question is: Is my expectation of the bind() having to fail wrong?

The main reason I am concerned with the MAC address is that I want another party to connect to me and send it some data. For the other party to establish a connection, it needs my MAC address and port. Now I can find my MAC address via the DBus API (or probably via some other means) but then I'm afraid that it's racy, e.g. I cannot be sure that the MAC address I queried is the same that I've bound to, because in the time between the query and the bind the user may have removed their Bluetooth adapter and inserted a new one. And before I jump through the hoops of subscribing to BlueZ DBus signals I thought I'd use the MAC for binding the socket.

Select MAC address for binding to a specific bluetooth adapter

0 Answers0