Re: [PATCH net-next v4 0/7] bonding: Extend arp_ip_target format to allow for a list of vlan tags.

[Hangbin Liu <liuhangbin@gmail.com>]

On Thu, Aug 07, 2025 at 11:10:20AM -0700, Jay Vosburgh wrote:
> >> This is a key difference between IPv6 and IPv4:
> >> In IPv4, it's possible to get a destination route via the bond even when the
> >> source IP is configured on a different interface. But in IPv6, the routing
> >> mechanism is stricter in requiring the source address to be valid on the
> >> outgoing interface.
> >>
> >> I'm not sure how to fix this yet, as it's fundamentally tied to how IPv6
> >> routing behaves.
> >
> >I am thinking that we don't need to do a route lookup as if we are sending
> >from the bonding interface.  We only need to find the interface we should
> >send the packet through.  As if we ran "ip route get <dest addr>".
> 
> 	Assuming I'm following correctly, the whole point of the route
> lookup is to determine which interface the ARP (or NS for IPv6) should
> nominally sent through (based on the destination address).  This serves
> two purposes:
> 
> 	- collecting the VLAN tags,
> 
> 	- insuring that the ARP / NS won't be sent on a logically
> incorrect interface (e.g., its address corresponds to some totally
> unrelated interface).
> 
> 	So, really, I'm agreed that what we're really looking for is
> "what is the proper output interface to use to send to destination X,"
> which we can then check to see if that interface is logically connected
> to the bond (e.g., a VLAN atop the bond).
> 
> 	Is the solution to call ip6_route_output() with the flowi6_oif
> set to zero?  That seems to be what happens for the "ip route get" case
> in inet6_rtm_getroute() (he says, looking at the code but not running
> actual tests).

Looks reasonable to me. We can find the dst interface first and check
if it's an upper link of the bond interface.

Thanks
Hangbin