Re: [PATCHv2 net] bonding: fix multicast MAC address synchronization

[Paolo Abeni]

On 8/5/25 10:09 AM, Hangbin Liu wrote:
> There is a corner case where the NS (Neighbor Solicitation) target is set to
> an invalid or unreachable address. In such cases, all the slave links are
> marked as down and set to *backup*. This causes the bond to add multicast MAC
> addresses to all slaves. The ARP monitor then cycles through each slave to
> probe them, temporarily marking as *active*.
> 
> Later, if the NS target is changed or cleared during this probe cycle, the
> *active* slave will fail to remove its NS multicast address because
> bond_slave_ns_maddrs_del() only removes addresses from backup slaves.
> This leaves stale multicast MACs on the interface.
> 
> To fix this, we move the NS multicast MAC address handling into
> bond_set_slave_state(), so every slave state transition consistently
> adds/removes NS multicast addresses as needed.
> 
> We also ensure this logic is only active when arp_interval is configured,
> to prevent misconfiguration or accidental behavior in unsupported modes.

As noted by Jay in the previous revision, moving the handling into
bond_set_slave_state() could possibly impact a lot of scenarios, and
it's not obvious to me that restricting to arp_interval != 0 would be
sufficient.

I'm wondering if the issue could/should instead addressed explicitly
handling the mac swap for the active slave at NS target change time. WDYT?

Thanks,

Paolo