AWS CDK requires a "public" subnet to deploy

Question

I am new to AWS CDK and I am trying to deploy resources like Lambda function, 2 Security Groups in an existing VPC.
I use this code in order to use the existing VPC:

const vpc = ec2.Vpc.fromLookup(this, 'ExistingVPC', {
      vpcName: 'TT-DD-vpc01',
    });

So when I run cdk synth , I get the error:

Error: There are no 'Public' subnet groups in this VPC. Available types:
    at LookedUpVpc.selectSubnetObjectsByType (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject
ode_modules\aws-cdk-lib\aws-ec2\lib\vpc.js:1:6197)
    at LookedUpVpc.selectSubnetObjects (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject
ode_modules\aws-cdk-lib\aws-ec2\lib\vpc.js:1:4915)
    at LookedUpVpc.selectSubnets (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject
ode_modules\aws-cdk-lib\aws-ec2\lib\vpc.js:1:2603)
    at Function.configureVpc (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject
ode_modules\aws-cdk-lib\aws-lambda\lib\function.js:1:23508)
    at new Function (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject
ode_modules\aws-cdk-lib\aws-lambda\lib\function.js:1:12479)
    at new MyCdkProjectStack (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject\lib\my_cdk_project-stack.js:85:30)
    at Object. (C:\Users\dimit\Projects\awscdk\AWSCDK\MyCdkProject\bin\my_cdk_project.js:6:1)
    at Module._compile (node:internal/modules/cjs/loader:1256:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1310:10)
    at Module.load (node:internal/modules/cjs/loader:1119:32)

why cdk needs to have a "public" subnet configured ?
I created a subnet in AWS UI but I dont know how to make it "Public".

Any help would be appreciated !

Answer

Hi,

A subnet is called a Public subnet if it has a direct route to an internet gateway. Resources in a public subnet can access the public internet. To read more about Subnets in VPC https://docs.aws.amazon.com/vpc/latest/userguide/configure-subnets.html

If the subnet has a route to anywhere through Internet gateway. It's a Public subnet. Here is how the route table will look like https://docs.aws.amazon.com/vpc/latest/userguide/route-table-options.html#route-tables-internet-gateway

To resolve your issue:

1. VPC Configuration: Check if your VPC has the necessary subnets (public, private, etc.) created.
2. Subnet Group Names: Ensure that you’re using the correct subnet group names in your configuration.
3. Internet Gateway: If you’re expecting a public subnet, make sure it has an associated internet gateway.
4. Route Tables: Check the route tables associated with your subnets. Public subnets should have a route to the internet gateway (0.0.0.0/0).

Additionally, you can refer here: https://stackoverflow.com/questions/56980500/aws-cdk-error-there-are-no-public-subnets-in-this-vpc-use-a-different-vpc-su

```
tag: Key: aws-cdk:subnet-type Value: Public
```

AWS CDK requires a "public" subnet to deploy

Relevant content