What is the purpose of net_device.uc_promisc field?

why struct net_device has a field uc_promisc? How this field开发者_Go百科 is used?

Thank you all!

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When a device that doesn't support unicast filtering has to listen to several unicast addresses, it is put on promiscous mode, according to dev->uc_count and dev->uc_promisc. Check the __dev_set_rx_mode() function.

Many devices implement ndo_set_rx_mode(), and set their unicast (and multicast) filters via ndo_set_rx_mode(). For devices that don't implement that, Linux sets the device to promiscuous mode, and keeps track of that fact with dev->uc_promisc.

So there are several flags for promiscuous mode:

• dev->flags & IFF_PROMISC means the device is in promiscuous mode.
• dev->gflags & IFF_PROMISC means the user has requested promiscuous mode.
• dev->uc_promisc means promiscuous mode has been enabled (actually, its reference count has been incremented) due to the need to listen to additional unicast address in a device that doesn't implement ndo_set_rx_mode().

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It sounds like it could be a way of enabling (or tracking the enabled/disabled status of) promiscuous mode on the device.