How to Multicast (send) to first NIC?

Question

I found recently that if I have a dial-up connection (this is for a kiosk) and a local area network connection, when the dial-up connection is established (with internet acc

Answer 1

Apparently setsockopt and IP_MULTICAST_IF don't work if wsock32.dll is used instead of ws2_32.dll. I thought I was doing it wrong when I kept getting 1.0.0.0 as the IP address even when it was something else that I've set with setsockopt. Funny thing is, before the call to IP_MULTICAST_IF, it would return 0.0.0.0, so setsockopt` did change something, just not correctly.

Someone else who had this same problem way back in 2004 - http://us.generation-nt.com/ip-multicast-problem-help-37595922.html. When we #include "winsock2.h" we need to use ws2_32.dll. However, with C++ Builder, it's impossible to use ws2_32.dll when we use winsock2.h - the RTL implicitly links in wsock32.dll and you can't link ws2_32.dll even if you explicitly specify #pragma comment(lib, "ws2_32.lib"). Embarcadero really need to fix this! Someone in the RTL team must've decided it's clever to implicitly include wsock32.dll. The only 'clever' thing it did was users didn't have to include one line in their code - #pragma comment(lib, "wsock32.lib"). While they're at that, they might as well include every single DLL files known to mankind.

Answer 2

Use GetAdaptersAddresses() to enumerate all available interface IP addresses.

IP_MULTICAST_IF is for IPv4 addresses only. It expects you to pass a DWORD value containing the desired IPv4 address (in network byte order) to setsockopt(), eg:

DWORD dwIP = ...;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF, (char *)&dwIP, sizeof(dwIP));