Add logic to handle implicit route table associations to main route t…

…able (#835)

* Add implicit association logic

* run go fmt

* remove unnecessary if statement

* add nil check

pkg/cloud/aws/services/ec2/routetables.go

@@ -29,7 +29,8 @@ import (
 )
 
 const (
-	anyIPv4CidrBlock = "0.0.0.0/0"
+	anyIPv4CidrBlock       = "0.0.0.0/0"
+	mainRouteTableInVPCKey = "main"
 )
 
 func (s *Service) reconcileRouteTables() error {
@@ -109,6 +110,9 @@ func (s *Service) describeVpcRouteTablesBySubnet() (map[string]*ec2.RouteTable,
 	res := make(map[string]*ec2.RouteTable)
 	for _, rt := range rts {
 		for _, as := range rt.Associations {
+			if as.Main != nil && *as.Main {
+				res[mainRouteTableInVPCKey] = rt
+			}
 			if as.SubnetId == nil {
 				continue
 			}

pkg/cloud/aws/services/ec2/subnets.go

@@ -202,6 +202,10 @@ func (s *Service) describeVpcSubnets() (v1alpha1.Subnets, error) {
 
 		// ... or if it has an internet route
 		rt := routeTables[*ec2sn.SubnetId]
+		if rt == nil {
+			// If there is no explicit association, subnet defaults to main route table as implicit association
+			rt = routeTables[mainRouteTableInVPCKey]
+		}
 		if rt != nil {
 			spec.RouteTableID = rt.RouteTableId
 			for _, route := range rt.Routes {