Loopback Pseudo Interface: A Potential Cause of Multicast Disablement

In a specific network configuration, it has been observed that the use of multicast can be aggressively disabled if there is an interface present that does not support it. This issue arises when multiple network interfaces are used by a server, such as in the case of having two NICs, Ethernet 2 and Ethernet 3.

When examining the network configuration using the command `netsh int ipv4 show config`, only the Loopback Pseudo-Interface 1 is displayed. It can be inferred that this might be the cause of multicast disablement, as the interface type is IF_TYPE_SOFTWARE_LOOPBACK.

To overcome this issue, it is suggested to override the condition that checks for interfaces of type IF_TYPE_SOFTWARE_LOOPBACK and assume that multicast works. This can be achieved by using the deprecated AssumeMulticast setting or the current multicast attribute on a network interface definition.

In addition, another potential solution is to remove the condition `iface->IfType != IF_TYPE_SOFTWARE_LOOPBACK` from the code. However, it should be noted that this approach may not always work and may require further testing.

For developers building from source, they can verify these findings by implementing a similar workaround in their own code. It's also worth noting that there is an existing implementation of this workaround for Linux, although it has not been merged yet.

In conclusion, the Loopback Pseudo-Interface 1 can be a potential cause of multicast disablement when multiple network interfaces are used by a server. By understanding and addressing this issue, developers can ensure that their applications can effectively use multicast in these environments.

Loopback pseudo interface 1