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

[Shengyu Qu]

Ping for next version.

在 2025/2/17 20:58, Felix Fietkau 写道:

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

let IFF_ALLMULTI allow all multicast packets to be received.

Signed-off-by: Felix Fietkau <n...@nbd.name>
---
 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

             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;
         }
     }

group. Similar to how IFF_PROMISC allows all packets to be received locally, @@ -155,6 +155,8 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb

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);
                         }


code so people are not surprised.

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

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.

- Felix

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).

OpenPGP_0xE3520CC91929C8E7.asc
Description: OpenPGP public key

OpenPGP_signature.asc
Description: OpenPGP digital signature