# File lib/fog/aws/requests/compute/attach_classic_link_vpc.rb, line 35
        def attach_classic_link_vpc(instance_id, vpc_id, security_group_ids, dry_run=false)
          response = Excon::Response.new
          vpc = self.data[:vpcs].find{ |v| v['vpcId'] == vpc_id }
          instance = self.data[:instances][instance_id]
          if vpc && instance
            if instance['instanceState']['name'] != 'running' || instance['vpcId']
              raise Fog::Compute::AWS::Error.new("Client.InvalidInstanceID.NotLinkable => Instance #{instance_id} is unlinkable")
            end
            if instance['classicLinkVpcId']
              raise Fog::Compute::AWS::Error.new("Client.InvalidInstanceID.InstanceAlreadyLinked => Instance #{instance_id} is already linked")
            end

            response.status = 200
            response.body = {
              'requestId' => Fog::AWS::Mock.request_id,
              'return'    => true
            }
            unless dry_run
              instance['classicLinkSecurityGroups'] = security_group_ids
              instance['classicLinkVpcId'] = vpc_id
            end
            response
          elsif !instance
            raise Fog::Compute::AWS::NotFound.new("The instance ID '#{instance_id}' does not exist.")
          elsif !vpc
            raise Fog::Compute::AWS::NotFound.new("The VPC '#{vpc_id}' does not exist.")
          end

        end