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diff --git a/share/man/man4/bridge.4 b/share/man/man4/bridge.4
--- a/share/man/man4/bridge.4
+++ b/share/man/man4/bridge.4
@@ -615,6 +615,19 @@
listener state is known, for non-IP multicast, and for exempt link-local
control traffic.
.Pp
+The link-local
+.Li 224.0.0.0/24
+block is never snooped and never pruned, as required by RFC 4541.
+Membership reports naming a group in that block are parsed but create no
+listener state, and traffic addressed to it is always forwarded to every
+port.
+Without this exemption a single reporting host could cause link-local
+control protocols, such as mDNS on
+.Li 224.0.0.251
+or OSPF on
+.Li 224.0.0.5 ,
+to be pruned away from ports whose own membership reports were suppressed.
+.Pp
IGMPv3 group records are reduced conservatively to any-source
.Pq (*,G)
listener state.
diff --git a/sys/net/if_bridge.c b/sys/net/if_bridge.c
--- a/sys/net/if_bridge.c
+++ b/sys/net/if_bridge.c
@@ -1409,6 +1409,25 @@
return (IN_MULTICAST(ntohl(group.s_addr)));
}
+/*
+ * Whether listener state may be learned for a group. RFC 4541 section
+ * 2.1.1 requires that traffic addressed to the link-local 224.0.0.0/24
+ * block always be forwarded on all ports, so membership in those groups is
+ * never recorded. Learning them would let one reporting host cause the
+ * bridge to prune link-local control protocols -- mDNS on 224.0.0.251, OSPF
+ * on 224.0.0.5 and 224.0.0.6 -- away from ports whose own reports were
+ * suppressed by IGMP report suppression. This is reachable in practice:
+ * FreeBSD reports these groups by default (net.inet.igmp.sendlocal=1).
+ *
+ * A report for such a group is well-formed, just not actionable, so callers
+ * treat it as accepted-with-no-effect rather than as a parse rejection.
+ */
+static bool
+bridge_mdb_igmp_group_snoopable(struct in_addr group)
+{
+ return (!IN_LOCAL_GROUP(ntohl(group.s_addr)));
+}
+
static void
bridge_mdb_input(struct bridge_softc *sc, struct bridge_iflist *bif,
struct mbuf *m)
@@ -1499,6 +1518,8 @@
!bridge_mdb_igmp_group_valid(igmp.igmp_group) ||
!in_hosteq(igmp.igmp_group, ip.ip_dst))
return (EINVAL);
+ if (!bridge_mdb_igmp_group_snoopable(igmp.igmp_group))
+ return (0);
return (bridge_mdb_add_member(sc, AF_INET, &igmp.igmp_group,
vlan, bif, BRIDGE_MDB_IGMP_MAXAGE));
case IGMP_HOST_LEAVE_MESSAGE:
@@ -1549,7 +1570,8 @@
return (EINVAL);
if (pass == 1 &&
- bridge_mdb_igmp_group_valid(rec.ig_group)) {
+ bridge_mdb_igmp_group_valid(rec.ig_group) &&
+ bridge_mdb_igmp_group_snoopable(rec.ig_group)) {
/*
* Reduce each record conservatively to ASM
* (*,G) listener state. Any record that
@@ -1740,6 +1762,16 @@
if (!IN_MULTICAST(ntohl(ip.ip_dst.s_addr)))
return (false);
+ /*
+ * Never prune the link-local 224.0.0.0/24 block (RFC 4541 2.1.1):
+ * flood it exactly as the bridge did before snooping existed.
+ * Learning already refuses these groups, but checking on the
+ * forwarding side too means listener state recorded by an earlier
+ * kernel cannot prune control traffic after an upgrade.
+ */
+ if (IN_LOCAL_GROUP(ntohl(ip.ip_dst.s_addr)))
+ return (false);
+
if (bridge_mdb_key_init(&key, AF_INET, &ip.ip_dst, vlan) != 0)
return (false);
diff --git a/tests/sys/net/if_bridge_test.sh b/tests/sys/net/if_bridge_test.sh
--- a/tests/sys/net/if_bridge_test.sh
+++ b/tests/sys/net/if_bridge_test.sh
@@ -1386,6 +1386,67 @@
vnet_cleanup
}
+atf_test_case "mdb_link_local_exempt" "cleanup"
+mdb_link_local_exempt_head()
+{
+ atf_set descr 'bridge never snoops or prunes the link-local 224.0.0.0/24 block'
+ atf_set require.user root
+ atf_set require.progs "cc tcpdump timeout"
+}
+
+mdb_link_local_exempt_body()
+{
+ vnet_init
+ vnet_init_bridge
+
+ bridge_mdb_vnet_topology_create
+
+ atf_check -s exit:0 -o ignore \
+ sysctl net.link.bridge.igmp_snooping=1
+
+ bridge_mdb_compile_join_helper
+
+ # 224.0.0.251 (mDNS) is inside the link-local block. FreeBSD reports
+ # it by default (net.inet.igmp.sendlocal=1), so the bridge does see a
+ # membership report for it -- but RFC 4541 2.1.1 requires that this
+ # traffic still reach every port, so no listener state may be created.
+ jexec mdbl /tmp/mcast_join_bridge_test 224.0.0.251 192.0.2.2 30 &
+ joiner_pid=$!
+ sleep 2
+
+ atf_check -o match:'^0$' \
+ sysctl -n net.link.bridge.mdb_entries
+
+ jexec mdbs route add -net 224.0.0.0/4 \
+ -interface ${mdb_sender_epair}b >/dev/null 2>&1 || true
+
+ capture="${PWD}/mdb_link_local.tcpdump"
+ rm -f "${capture}"
+
+ # The uninterested port must still receive it.
+ jexec mdbi sh -c "timeout 4 tcpdump --immediate-mode -n \
+ -i ${mdb_idle_epair}b -c 1 \
+ 'icmp and dst host 224.0.0.251' > ${capture} 2>&1" &
+ tcpdump_pid=$!
+ sleep 1
+
+ jexec mdbs ping -c 3 -t 1 224.0.0.251 >/dev/null 2>&1 || true
+ wait ${tcpdump_pid}
+ idle_status=$?
+
+ kill ${joiner_pid} 2>/dev/null || true
+
+ if [ ${idle_status} -ne 0 ]; then
+ atf_fail "link-local 224.0.0.251 was pruned from an uninterested port"
+ fi
+}
+
+mdb_link_local_exempt_cleanup()
+{
+ rm -f /tmp/mcast_join_bridge_test /tmp/mcast_join_bridge_test.c
+ vnet_cleanup
+}
+
atf_test_case "member_ifaddrs_enabled" "cleanup"
member_ifaddrs_enabled_head()
{
@@ -2075,6 +2136,7 @@
atf_add_test_case "mdb_counters"
atf_add_test_case "mdb_router_forward"
atf_add_test_case "mdb_igmpv3_source_join"
+ atf_add_test_case "mdb_link_local_exempt"
atf_add_test_case "member_ifaddrs_enabled"
atf_add_test_case "member_ifaddrs_disabled"
atf_add_test_case "member_ifaddrs_vlan"

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