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Receiving Multicast Messages on a Multihomed Windows PC

Answer 1

A:

You may want to check/change your routing table. There will be a route for multicast (224.0.0.0, subnet 240.0.0.0) traffic in there with its appropriate metric:

C:\Users\Cetra>netstat -rn

*****

IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0   192.168.80.254    192.168.80.99     20
        127.0.0.0        255.0.0.0         On-link         127.0.0.1    306
        127.0.0.1  255.255.255.255         On-link         127.0.0.1    306
  127.255.255.255  255.255.255.255         On-link         127.0.0.1    306
     192.168.80.0    255.255.255.0         On-link     192.168.80.99    276
    192.168.80.99  255.255.255.255         On-link     192.168.80.99    276
   192.168.80.255  255.255.255.255         On-link     192.168.80.99    276

        224.0.0.0        240.0.0.0         On-link         127.0.0.1    306 
        224.0.0.0        240.0.0.0         On-link     192.168.80.99    276

  255.255.255.255  255.255.255.255         On-link         127.0.0.1    306
  255.255.255.255  255.255.255.255         On-link     192.168.80.99    276

******

Cetra 2010-06-07 10:49:26

Thanks for your reply. Adding/changing a multicast entry to the routing table is a workarkound that works. But most probably I won't be able to change the routing table of customer PCs.

Basti 2010-06-07 11:02:11

I don't regard it as a workaround, it's just how networking works. Would your customer's PCs be multihomed also?

Cetra 2010-06-07 11:06:51

As the application is a product, I can't exclude that.I already thought about changing the routing table by a windows API function... But I'm unsure if that couldn't result to some side effects on the customer PC. So I dropped the thought.

Basti 2010-06-07 11:13:37

Well it would not harbour much trust if you changed it automatically. If you had something in your settings dialogue that listed all the interfaces and had a disclaimer when they changed it, i.e, "this will change the routes for multicast traffic, are you sure? y/n".

Cetra 2010-06-07 11:16:24

Yes, that would be a better approach :-)However, I still can hardly believe that there is no way to force a socket to send the Multicast IGMP Join message using a specific NIC. That doesn't make any sense to me.

Basti 2010-06-07 11:20:56

Answer 2

A:

The problem was a simple typo. Instead of using the structure struct ip_mreq_source, the structure struct ip_mreq must be used if using the option IP_MULTICAST_IF. (The other structure is needed for the IP_ADD_SOURCE_MEMBERSHIP option)

Using the wrong structure had most probably the result that the setsockeopt function found a zero where the NIC IP address was expected. Zero is also the value of the INADDR_ANY constant, which choose the default NIC of the system. :-)

Basti 2010-06-09 14:40:25

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Receiving Multicast Messages on a Multihomed Windows PC

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