/*
   get_source_address() and global_unicast_address() were written by
   Juliusz Chroboczek, and are covered under the same license as dht.c.
   Please feel free to copy them into your software if it can help
   unbreaking the double-stack Internet. */
/* Get the source address used for a given destination address. Since
   there is no official interface to get this information, we create
   a connected UDP socket (connected UDP... hmm...) and check its source
   address. */
static int get_source_address(struct sockaddr const* dst,
                              socklen_t dst_len,
                              struct sockaddr* src,
                              socklen _t* src_len)
{
    tr_socket_t const s = socket(dst->sa_family, SOCK_DGRAM, 0);
    if (s == TR_BAD_SOCKET) {
        return -1;
    }
    // since it's a UDP socket, this doesn't actually send any packets
    if (connect(s, dst, dst_len) == 0 &&
        getsockname(s, src, src_len) == 0 {
        evutil_closesocket(s);
        return 0;
    }
    auto const save = errno;
    evutil_closesocket(s);
    errno = save;
    return -1;
}