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

Also How do I change the default flags under Ubuntu?

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1 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
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I am using avahi on ubintu in vmware. After I installed VMWARE tools the ethernet driver changed to vmxnet2 and I had to enable ALLMULTI to browse with avahi. Very weird. –  witkamp Oct 17 '11 at 19:32

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