Fix a long-standing limitation in IPv4 multicast group membership.

By making the imo_membership array a dynamically allocated vector,
this minimizes disruption to existing IPv4 multicast code. This
change breaks the ABI for the kernel module ip_mroute.ko, and may
cause a small amount of churn for folks working on the IGMPv3 merge.

Previously, sockets were subject to a compile-time limitation on
the number of IPv4 group memberships, which was hard-coded to 20.
The imo_membership relationship, however, is 1:1 with regards to
a tuple of multicast group address and interface address. Users who
ran routing protocols such as OSPF ran into this limitation on machines
with a large system interface tree.

1 files changed, 3 insertions, 2 deletions

diff --git a/sys/netinet/ip_var.h b/sys/netinet/ip_var.h
index 6d0a122..a051825 100644
--- a/sys/netinet/ip_var.h
+++ b/sys/netinet/ip_var.h
@@ -85,11 +85,12 @@ struct ipoption {
 struct ip_moptions {
 	struct	ifnet *imo_multicast_ifp; /* ifp for outgoing multicasts */
 	struct in_addr imo_multicast_addr; /* ifindex/addr on MULTICAST_IF */
+	u_long	imo_multicast_vif;	/* vif num outgoing multicasts */
 	u_char	imo_multicast_ttl;	/* TTL for outgoing multicasts */
 	u_char	imo_multicast_loop;	/* 1 => hear sends if a member */
 	u_short	imo_num_memberships;	/* no. memberships this socket */
-	struct	in_multi *imo_membership[IP_MAX_MEMBERSHIPS];
-	u_long	imo_multicast_vif;	/* vif num outgoing multicasts */
+	u_short	imo_max_memberships;	/* max memberships this socket */
+	struct	in_multi **imo_membership;	/* group memberships */
 };
 
 struct	ipstat {