This patch adds support for allowing BGP to create and bring up neighbor

sessions dynamically. The operator configures a range of neighbor addresses
to which peering is allowed. The ranges are configured as subnets and
multiple ranges are allowed. Each range is associated with a peer-group
so that additional parameters can be configured.

BGP neighbor sessions are dynamically created when connections are initiated
by remote neighbors whose addresses fall within a configured range. The
sessions are deleted when the BGP connection terminates.

A limit on the number of neighbors allowed from each range of addresses
can be specified.

IPv4 and IPv6 peering is supported. Over the peering, any of the address
families configured for the peer-group can be negotiated.

bgpd/bgp_fsm.c

@@ -951,6 +951,15 @@ bgp_stop (struct peer *peer)
   char orf_name[BUFSIZ];
   int ret = 0;
 
+  if (peer_dynamic_neighbor(peer) &&
+      !(CHECK_FLAG(peer->flags, PEER_FLAG_DELETE)))
+    {
+      if (bgp_debug_neighbor_events(peer))
+        zlog_debug ("%s (dynamic neighbor) deleted", peer->host);
+      peer_delete (peer);
+      return -1;
+    }
+
   /* Can't do this in Clearing; events are used for state transitions */
   if (peer->status != Clearing)
     {
@@ -1117,6 +1126,14 @@ bgp_stop_with_error (struct peer *peer)
   if (peer->v_start >= (60 * 2))
     peer->v_start = (60 * 2);
 
+  if (peer_dynamic_neighbor(peer))
+    {
+      if (bgp_debug_neighbor_events(peer))
+        zlog_debug ("%s (dynamic neighbor) deleted", peer->host);
+      peer_delete (peer);
+      return -1;
+    }
+
   return(bgp_stop (peer));
 }
 
@@ -1128,6 +1145,14 @@ bgp_stop_with_notify (struct peer *peer, u_char code, u_char sub_code)
   /* Send notify to remote peer */
   bgp_notify_send (peer, code, sub_code);
 
+  if (peer_dynamic_neighbor(peer))
+    {
+      if (bgp_debug_neighbor_events(peer))
+        zlog_debug ("%s (dynamic neighbor) deleted", peer->host);
+      peer_delete (peer);
+      return -1;
+    }
+
   /* Clear start timer value to default. */
   peer->v_start = BGP_INIT_START_TIMER;
 
@@ -1180,6 +1205,14 @@ bgp_connect_success (struct peer *peer)
 static int
 bgp_connect_fail (struct peer *peer)
 {
+  if (peer_dynamic_neighbor(peer))
+    {
+      if (bgp_debug_neighbor_events(peer))
+        zlog_debug ("%s (dynamic neighbor) deleted", peer->host);
+      peer_delete (peer);
+      return -1;
+    }
+
   return (bgp_stop (peer));
 }
 
@@ -1737,9 +1770,11 @@ bgp_event_update (struct peer *peer, int event)
   int ret = 0;
   struct peer *other;
   int passive_conn = 0;
+  int dyn_nbr;
 
   other = peer->doppelganger;
   passive_conn = (CHECK_FLAG(peer->sflags, PEER_STATUS_ACCEPT_PEER)) ? 1 : 0;
+  dyn_nbr = peer_dynamic_neighbor(peer);
 
   /* Logging this event. */
   next = FSM [peer->status -1][event - 1].next_state;
@@ -1772,7 +1807,7 @@ bgp_event_update (struct peer *peer, int event)
       bgp_timer_set (peer);
 
     }
-  else if (!passive_conn && peer->bgp)
+  else if (!dyn_nbr && !passive_conn && peer->bgp)
     {
       /* If we got a return value of -1, that means there was an error, restart
        * the FSM. If the peer structure was deleted

bgpd/bgp_network.c

@@ -224,11 +224,30 @@ bgp_accept (struct thread *thread)
 
   /* Check remote IP address */
   peer1 = peer_lookup (NULL, &su);
+
+  if (! peer1)
+    {
+      peer1 = peer_lookup_dynamic_neighbor (NULL, &su);
+      if (peer1)
+        {
+          /* Dynamic neighbor has been created, let it proceed */
+          peer1->fd = bgp_sock;
+          bgp_fsm_change_status(peer1, Active);
+          BGP_TIMER_OFF(peer1->t_start); /* created in peer_create() */
+
+          if (peer_active (peer1))
+              BGP_EVENT_ADD (peer1, TCP_connection_open);
+
+          return 0;
+        }
+    }
+
   if (! peer1)
     {
       if (bgp_debug_neighbor_events(peer))
 	{
-	  zlog_debug ("[Event] BGP connection IP address %s is not configured",
+	  zlog_debug ("[Event] %s connection rejected - not configured"
+                      " and not valid for dynamic",
 		      inet_sutop (&su, buf));
 	}
       close (bgp_sock);

bgpd/bgp_route.c

@@ -2508,6 +2508,10 @@ bgp_maximum_prefix_overflow (struct peer *peer, afi_t afi,
                                   BGP_NOTIFY_CEASE_MAX_PREFIX, ndata, 7);
       }
 
+      /* Dynamic peers will just close their connection. */
+      if (peer_dynamic_neighbor (peer))
+        return 1;
+
       /* restart timer start */
       if (peer->pmax_restart[afi][safi])
 	{