Fix invalid nexthop advertisement with nexthop-unchanged

When we apply nexthop-unchanged action to the locally-originated route
with unspecified nexthop (0.0.0.0 or ::), we advertise the path with
unspecified nexthop. This may ends up with the session shutdown with
Invalid Update Message Notification (only tested with FRR 10.2.1).

Fix this issue by setting table.PolicyOptions.OldNextHop to the local
peering address when the path is locally-originated and has an
unspecified nexthop.

Signed-off-by: Yutaro Hayakawa <[email protected]>

Author: YutaroHayakawa

pkg/server/server.go

@@ -781,8 +781,17 @@ func (s *BgpServer) prePolicyFilterpath(peer *peer, path, old *table.Path) (*tab
 
 	peer.fsm.lock.RLock()
 	options := &table.PolicyOptions{
-		Info:       peer.fsm.peerInfo,
-		OldNextHop: path.GetNexthop(),
+		Info: peer.fsm.peerInfo,
+	}
+	if path.IsLocal() && path.GetNexthop().IsUnspecified() {
+		// We need a special treatment for the locally-originated path
+		// with unspecified nexthop (0.0.0.0 or ::). In this case, the
+		// OldNextHop option should be set to the local address.
+		// Otherwise, we advertise the unspecified nexthop as is when
+		// nexthop-unchanged is configured.
+		options.OldNextHop = peer.fsm.peerInfo.LocalAddress
+	} else {
+		options.OldNextHop = path.GetNexthop()
 	}
 	path = table.UpdatePathAttrs(peer.fsm.logger, peer.fsm.gConf, peer.fsm.pConf, peer.fsm.peerInfo, path)
 	peer.fsm.lock.RUnlock()