https://zenn.dev/coord_e/articles/poor-mans-private-subnet

handler.rb

require 'aws-sdk-ec2'
require 'aws-sdk-autoscaling'

ROUTE_TABLE_ID = ENV.fetch('ROUTE_TABLE_ID')

class Handler
  def initialize
    @ec2_client = Aws::EC2::Client.new
    @autoscaling_client = Aws::AutoScaling::Client.new
  end

  def handle_launch(instance_id)
    @ec2_client.modify_instance_attribute(
      instance_id:,
      source_dest_check: {
        value: false,
      },
    )
    @ec2_client.create_route(
      destination_cidr_block: '0.0.0.0/0',
      route_table_id: ROUTE_TABLE_ID,
      instance_id: instance_id,
    )
  end

  def handle_terminate(instance_id)
    @ec2_client.delete_route(
      destination_cidr_block: '0.0.0.0/0',
      route_table_id: ROUTE_TABLE_ID,
    )
  rescue Aws::EC2::Errors::InvalidRouteNotFound
    nil
  end

  def handle_event(event:)
    detail = event.fetch('detail')
    instance_id = detail.fetch('EC2InstanceId')
    case event.fetch('detail-type')
    when 'EC2 Instance-launch Lifecycle Action'
      handle_launch(instance_id)
    when 'EC2 Instance-terminate Lifecycle Action'
      handle_terminate(instance_id)
    end
    @autoscaling_client.complete_lifecycle_action(
      auto_scaling_group_name: detail.fetch('AutoScalingGroupName'),
      lifecycle_action_result: 'CONTINUE',
      lifecycle_action_token: detail.fetch('LifecycleActionToken'),
      lifecycle_hook_name: detail.fetch('LifecycleHookName'),
    )
  end
end

def lambda_handler(event:, context:)
  Handler.new.handle_event(event:)
end