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, 8 insertions, 1 deletions

diff --git a/sys/netinet/in.h b/sys/netinet/in.h
index 1476bf4..570ed33 100644
--- a/sys/netinet/in.h
+++ b/sys/netinet/in.h
@@ -424,7 +424,14 @@ __END_DECLS
  */
 #define	IP_DEFAULT_MULTICAST_TTL  1	/* normally limit m'casts to 1 hop  */
 #define	IP_DEFAULT_MULTICAST_LOOP 1	/* normally hear sends if a member  */
-#define	IP_MAX_MEMBERSHIPS	20	/* per socket */
+
+/*
+ * The imo_membership vector for each socket is now dynamically allocated at
+ * run-time, bounded by USHRT_MAX, and is reallocated when needed, sized
+ * according to a power-of-two increment.
+ */
+#define	IP_MIN_MEMBERSHIPS	31
+#define	IP_MAX_MEMBERSHIPS	4095
 
 /*
  * Argument structure for IP_ADD_MEMBERSHIP and IP_DROP_MEMBERSHIP.