What is the difference between MULTICAST and ALLMULTI ifconfig ifconfig flags

Question

What is the precise difference between the ifconfig flags MULTICAST and ALLMULTI?

Also How do I change the default flags under Ubuntu?

Answer 1

In Linux the driver has three multicast operating modes.

• Multicast can be disabled.
• It can be enabled, IFF_MULTICAST. The driver looks at multicast packets to see if they match *dev->mc_list which is a list of currently configured multicast addresses (usually managed by the applications that need the group).
• Then the last mode, IFF_ALLMULTI. In this mode the driver forwards all multicast packets to the kernel for handling. This is commonly used for routing since you don't usually want the router to be a client of the stream/group but a courier for it. Here is some more information on the internals of that:

http://www.makelinux.com/ldd3/chp-17-sect-14#chp-17-sect-14.1

In Ubuntu you use ifconfig:

 # ifconfig eth0 allmulti