Difference between INADDR_ANY in Linux and Windows socket programming

Question

My Winsock Delphi application should listen on all network interfaces for multicast UDP/IP stream. It listened normally until I tried it on another PC with different network adapters' priority order.

Then I started to research problem and found on some forums that INADDR_ANY (or 0.0.0.0) has different meaning in Windows and Linux:

• In Linux it means "listen on all interfaces" and for sending - send through default interface
• In Windows it means "listen on default interface" (0.0.0.1 for second one). Citation: "If this member specifies an IPv4 address of 0.0.0.0, the default IPv4 multicast interface is used" - without mentioning whether it is for listening or for sending.

Could you confirm or deny this?

How to listen really on all interfaces?

Here is a little piece of my code:

TMulticastListener = class(TThread)
private
  mreq: ip_mreq;
  ............
end;

constructor TMulticastListener.Create;
var err: Integer;
    wData: WsaData;
    reuse: Integer;
begin
  inherited Create(true);

  err := WSAStartup(MAKEWORD(2, 2), wData);
  if err = SOCKET_ERROR then begin
    // Tell the user that we could not find a usable Winsock DLL
    perror('WSAStartup');
    Exit;
  end;

  // create what looks like an ordinary UDP socket
  fd := socket(AF_INET, SOCK_DGRAM, 0);
  if fd = INVALID_SOCKET then begin
    perror('socket');
      Exit;
  end;

  reuse := 1;

  // allow multiple sockets to use the same PORT number
  if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, Pointer(@reuse), SizeOf(reuse)) < 0) then begin
    perror('Reusing ADDR failed');
    Exit;
  end;

  // set up destination address
  FillChar(addr, sizeof(addr), 0);
  addr.sin_family := AF_INET;
  addr.sin_addr.s_addr := htonl(INADDR_ANY); // N.B.: differs from sender
  addr.sin_port := htons(HELLO_PORT);

  // bind to receive address
  if (bind(fd, addr, sizeof(addr)) < 0) then begin
      perror('bind');
      Exit;
  end;

  // use setsockopt() to request that the kernel join a multicast group
  mreq.imr_multiaddr.s_addr := inet_addr(HELLO_GROUP);
  mreq.imr_interface.s_addr := htonl(INADDR_ANY); //inet_addr('0.0.0.0');
  if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, @mreq, sizeof(mreq)) < 0) then begin
      perror('setsockopt');
      Exit;
  end;
end;

Answer 1

Windows and Linux actually behave the same regarding the use of INADDR_ANY. The confusion here is because the two links you provide are being used in different contexts.

When using the bind function to bind to an address/port, specifying INADDR_ANY means that the socket will be able to receive packets on the given port from any interface. However, doing so does not set up anything regarding multicast.

In the context of the IP_ADD_MEMBERSHIP call to setsockopt, setting the interface to INADDR_ANY will have the system join the given multicast group on the default network interface.

The Linux link you gave refers to bind, while the Windows link refers to setsockopt and IP_ADD_MEMBERSHIP.

If you want to join the multicast group on all interfaces, you need to retrieve the list of interfaces on the system and join each one. On Windows, the GetAdaptersAddresses() function will give you the list of interfaces. On Linux, use the getifaddrs() function.

Here's an example of how to use the GetAdaptersAddresses() function in C:

struct iflist {
    char name[50];
    struct sockaddr_in sin;
    int isloopback;
    int ismulti;
    int ifidx;
};

void getiflist(struct iflist *list, int *len)
{
    IP_ADAPTER_ADDRESSES *head, *curr;
    IP_ADAPTER_UNICAST_ADDRESS *uni;
    char *buf;
    int buflen, err, i;

    buflen = 100000;
    buf = calloc(buflen, 1);
    head = (IP_ADAPTER_ADDRESSES *)buf;
    if ((err = GetAdaptersAddresses(AF_UNSPEC, 0, NULL, head,
                                    &buflen)) != ERROR_SUCCESS) {
        char errbuf[300];
        FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, NULL, err,
                      0, errbuf, sizeof(errbuf), NULL);
        printf("GetAdaptersAddresses failed: (%d) %s", err, errbuf);
        free(buf);
        return;
    }
    for (*len = 0, curr = head; curr; curr = curr->Next) {
        if (curr->IfType == IF_TYPE_TUNNEL) continue;
        for (uni = curr->FirstUnicastAddress; uni; uni = uni->Next) {
            if (curr->OperStatus == IfOperStatusUp) {
                memset(&list[*len], 0, sizeof(struct iflist));
                strncpy(list[*len].name, (char *)curr->AdapterName,
                        sizeof(list[i].name) - 1);
                memcpy(&list[*len].sin, uni->Address.lpSockaddr,
                        uni->Address.iSockaddrLength);
                list[*len].isloopback =
                        (curr->IfType == IF_TYPE_SOFTWARE_LOOPBACK);
                list[*len].ismulti =
                        ((curr->Flags & IP_ADAPTER_NO_MULTICAST) == 0);
                if (uni->Address.lpSockaddr->sa_family == AF_INET6) {
                    list[*len].ifidx = curr->Ipv6IfIndex;
                } else {
                    list[*len].ifidx = curr->IfIndex;
                }
                (*len)++;
            }
        }
    }
    free(buf);
}