Access Private RDS DB From Another VPC

Question

I'm trying to access a private RDS Instance from a different VPC using a Peering Connection. I have two VPCs:

1. VPC-K8S (172.20.0.0/16) with one public subnet
2. VPC-RDS (172.17.0.0/16) with one public subnet (172.17.0.0/24) and 3 private subnets (172.17.{1,2,3}.0/24)

VPC-RDS has 2 security groups (not actual names):

1. default, which accepts SSH from my IP
2. db, which accepts TCP over port 5432 from the default security group.

I deploy my DB instances in VPC-RDS after creating a DB Subnet Group in the private subnets, and configure it to not be publicly accessible. To access it from my workstation, I create a small instance in the public subnet of VPC-RDS with the default security group, and create an SSH tunnel:

ssh -L 5432:rds-host-name.us-east-1.rds.amazonaws.com:5432  -i "KeyName.pem"  [email protected]

I can access the RDS from my workstation via localhost.

I want to be able to access my RDS instance from my Kubernetes cluster (VPC-K8S). I set up a peering connection between the two, and configure the route tables appropriately (in VPC-K8S: 172.17.0.0/16 -> pcx-112233; VPC-RDS: 172.20.0.0/16 -> pcx-112233)

I cannot connect to RDS from one of my K8S nodes, or any instance in the K8S VPC. I suspected that it had something to do with the db security group, but even when I opened port 5432 to all IPs (0.0.0.0/0) it didn't help.

Any ideas how to do this, or is this only possible via a publicly accessible RDS instance or a Bastion host that is in VPC-RDS and the default SG?

Answer 1

Stupid oversight, but I'll leave this up if it helps anyone.

My private subnets in VPC-RDS use a different route table than the public subnet. This is done so that internet addresses (for the catch all rule 0.0.0.0/0) point to the NAT gateway as opposed to the internet gateway in the public subnet.

I added a rule to the private subnets' route table for the peering connection (172.20.0.0/16 -> pcx-112233), and then configured the db security group to accept TCP traffic on port 5432 from 172.20.0.0/16.