AWS Lambda invocation of internal ALB getting timed out

Question

I have to call ALB using its DNS from a Lambda function. Both of them are under the same VPC and have the same set of private subnets associated with them. All the private subnets have a Route table with the route to nat-gateway configured.
The target of the ALB is service running in the EC2 instances.
The call from the Lambda function to ALB is getting timed out. What could be the reason for the same ?

Answer

Hello,

For the ALB, was it set up as Internal or Public Facing?  If it's public facing, you will need to add a NAT Gateway to the VPC to allow the Lambda to access it even though both are in the same VPC.

If the ALB is set up as Internal, ensure that the Lambda is able to access the ALB's security group.  The security group assigned to the ALB may not allow the Lambda access and it will need to be added to the SG.

Answer

I did a mistake on my end, under terraform for LB the internal was set as false. Which made it internet-facing instead of internal and under private subnets.

AWS Lambda invocation of internal ALB getting timed out

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