Network ACL block connection within private subnet

Lambda and RDS are in the same private subnet. I try both RDS Endpoint and Private IP, none of them work Here is the code in lambda function

import pkg from 'pg'; const {Client} = pkg;

export const handler = async(event, context) => { console.log("Event -->",JSON.stringify(event)); context.callbackWaitsForEmptyEventLoop = false; const client = new Client({ user: 'postgres', host: "Private IP", database: 'demo', password: 'password', port: 5432 }); try { await client.connect(); const res = await client.query('SELECT $1::text as message', [ 'Hello world!', ]); console.log(res.rows[0].message); // Hello world! await client.end(); } catch (err) { console.log('error while trying to connect to db'); } };

Does this mean that there are two subnets on which Lambda is running? In that case, is there any possibility of access to RDS from a different subnet? In other words, I feel the following image is occurring. Could it be possible that the timeout is communicating from the Lambda ENI to the RDS subnet (different subnet)?

VPC: 100.0.0.0/24 Subnet 1c: 100.0.0.144/28 Subnet 1a: 100.0.0.128/28 RDS Publicly accessible: No My RDS required 2 different subnets ( 4 in total - 2 public and 2 private). Lambda function VPC configuration I edit in order is 1st subnet, 2nd subnet, both subnet but no luck

Access from Subnet 1c to Subnet 1a will fail if all connections are blocked by the network ACL. In other words, edit the network ACL as follows.
Subnet 1c inbound

Subnet 1c outbound

Subnet 1a inbound

Subnet 1a outbound

Hi @Riku_Kobayashi If RDS is in a single zone (free tier), is NACL still configured as above?

Interface Type | Description | Instance ID | Status lambda | AWS Lambda VPC ENI-createFileUpload-57197d84-7eee-4d1b-bfd1-5e08d4ebc398 | – | In-use Did you mean this ?

Yes, this ENI sends the data back and forth between your private subnet and AWS Lambda service backend. This traffic is not public and it is guaranteed by PrivateLink. You can refer to this documentation to get to know more about PrivateLink : https://docs.aws.amazon.com/vpc/latest/privatelink/what-is-privatelink.html