AWS Lambda Endpoint URL Construction in a VPC

Question

I have a technical question about how the AWS Lambda client determines the endpoint URL when invoking a function using client.invoke() in Java SDK 1.x. My Lambda function is deployed inside a VPC, and I want to understand how the SDK constructs the URL used for DNS resolution and how it maps to a specific IP.

Specifically, I’m trying to determine whether there is an equivalent to the private S3 bucket VPC endpoint (e.g., https://bucket.<VPC_endpoint_DNS_name> ) for Lambda functions. I’d also like to know how the SDK resolves the endpoint for both public and private Lambda functions and whether there is a way to retrieve the exact URL being used during invocation.

Any insights on how Lambda endpoints are structured, especially for private functions within a VPC, would be helpful.

Thanks

I tried looking into the AWSClientBuilder library & found this:

AWSLambdaClient(AwsSyncClientParams clientParams, boolean endpointDiscoveryEnabled) {
    super(clientParams);
    this.awsCredentialsProvider = clientParams.getCredentialsProvider();
    this.advancedConfig = clientParams.getAdvancedConfig();
    this.init();
  }

  private void init() {
    this.setServiceNameIntern("lambda");
    this.setEndpointPrefix("lambda");
    this.setEndpoint("lambda.us-east-1.amazonaws.com");
    HandlerChainFactory chainFactory = new HandlerChainFactory();
    this.requestHandler2s.addAll(chainFactory.newRequestHandlerChain("/com/amazonaws/services/lambda/request.handlers"));
    this.requestHandler2s.addAll(chainFactory.newRequestHandler2Chain("/com/amazonaws/services/lambda/request.handler2s"));
    this.requestHandler2s.addAll(chainFactory.getGlobalHandlers());
  }

But here the endpoint is hardcoded & it doesn't mention anything about how it constructs endpoint when lambda function is inside a VPC

I'd expect the invoke call to target the regular in-region AWS Lambda service DNS name e.g. lambda.us-east-2.amazonaws.com. If your Lambda function is in your VPC, you have a Lambda VPC endpoint, and you've enabled private DNS then that endpoint will resolve to the private IP of the ENI associated with the Lambda VPC endpoint. Otherwise it will resolve to the public IP(s) of the Lambda service endpoints. — jarmod
– jarmod, Commented Feb 26 at 13:40
Thanks for the explanation, jarmod. To clarify: I don't have a Lambda VPC endpoint, but I do have a "PrivateLink Ready partner services" endpoint allowing an external service to connect to AWS resources in my VPC privately. In this case, would the Lambda SDK still automatically resolve to the private IP? Or do I need to configure something special for this PrivateLink setup to work with Lambda function calls? I just have my Lambda function behind the VPC — Jainam Jain
– Jainam Jain, Commented Feb 26 at 13:52
Just to clarify, where is this client that invokes the Lambda function actually running? Your comment "allowing an external service to connect to AWS resources in my VPC privately" makes me wonder if you're thinking that invoking a Lambda function requires network access to that Lambda function inside a VPC. It does not. It simply requires access to the AWS Lambda service endpoint (the control plane of the Lambda service). I'm not sure that's what you're thinking, but just want to rule it out. — jarmod
– jarmod, Commented Feb 26 at 14:58
@jarmod Yes, I was able to resolve it—really appreciate your inputs here! As per AWS, when invoking a Lambda function privately, the endpoint remains the same, but it's essential to specify a DNS hostname. This ensures that DNS resolution routes the request through private connectivity rather than public internet. So your point about it hitting the Lambda control plane was spot on. The missing piece was just configuring the appropriate DNS hostname on our end to keep the traffic private. — Jainam Jain
– Jainam Jain, Commented Apr 9 at 6:26

jarmod · Accepted Answer · 2025-02-26 15:29:38Z

0

When a client invokes an AWS Lambda function, that client sends a signed API request to the AWS Lambda service's control plane via its public API endpoints (e.g. lambda.us-east-1.amazonaws.com).

It doesn't matter if the invoked Lambda function itself is attached to your custom VPC ("runs in your VPC") or not ("runs in an Amazon-managed VPC").

The client that invoked the Lambda function is never connected directly to the Lambda function (it's connected to the Lambda service's control plane) so the Lambda function's networking configuration (VPC, security groups, network routing, etc.) are all immaterial to the act of invoking the Lambda function.

answered Feb 26 at 15:29

jarmod

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