Re: [PATCH net] net: bridge: locally receive all multicast packets if IFF_ALLMULTI is set

[Felix Fietkau]

On 17.02.25 12:54, Nikolay Aleksandrov wrote:
> On 2/17/25 13:26, Felix Fietkau wrote:
> > If multicast snooping is enabled, multicast packets may not always end up on
> > the local bridge interface, if the host is not a member of the multicast
> > group. Similar to how IFF_PROMISC allows all packets to be received locally,
> > let IFF_ALLMULTI allow all multicast packets to be received.
> >
> > Signed-off-by: Felix Fietkau <nbd@xxxxxxxx>
> > ---
> >  net/bridge/br_input.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
> > index 232133a0fd21..7fa2da6985b5 100644
> > --- a/net/bridge/br_input.c
> > +++ b/net/bridge/br_input.c
> > @@ -155,6 +155,8 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
> >  			pkt_type = BR_PKT_MULTICAST;
> >  			if (br_multicast_rcv(&brmctx, &pmctx, vlan, skb, vid))
> >  				goto drop;
> > +			if (br->dev->flags & IFF_ALLMULTI)
> > +				local_rcv = true;
> >  		}
> >  	}
> >  
>
> This doesn't look like a bug fix, IMO it should be for net-next.
>
> Also you might miss a mcast stat increase, see the multicast code
> below, the only case that this would cover is the missing "else"
> branch of:
>                         if ((mdst && mdst->host_joined) ||
>                              br_multicast_is_router(brmctx, skb)) {
>                                  local_rcv = true;
>                                  DEV_STATS_INC(br->dev, multicast);
>                          }
>
> So I'd suggest to augment the condition and include this ALLMULTI check there,
> maybe with a comment to mention that all other cases are covered by the current
> code so people are not surprised.
Will do, thanks.

> By the way what is the motivation for supporting this flag? I mean you can
> make the bridge mcast router and it will receive all mcast anyway.

OpenWrt uses a user space daemon for DHCPv6/RA/NDP handling, and in 
relay mode it sets the ALLMULTI flag in order to receive all relevant 
queries on the network.
This works for normal network interfaces and non-snooping bridges, but 
not snooping bridges (unless, as you pointed out, multicast routing is 
enabled).

- Felix