How to Use data aws_vpc in Terraform for VPC Lookup

Use the data "aws_vpc" block in Terraform to look up an existing AWS VPC by specifying its id or filter criteria like tags. This lets you reference VPC attributes in your Terraform configuration without creating a new VPC.

Syntax

The data "aws_vpc" block fetches information about an existing AWS VPC. You can specify either the id of the VPC or use filter blocks to find it by tags or other attributes.

• id: The unique identifier of the VPC (e.g., vpc-123abc).
• filter: One or more filters to find the VPC by attributes like tag:Name.
• most_recent: Optional boolean to select the most recent VPC if multiple match filters.

terraform

data "aws_vpc" "example" {
  id = "vpc-123abc"
}

# Or using filter

data "aws_vpc" "example" {
  filter {
    name   = "tag:Name"
    values = ["my-vpc"]
  }
  most_recent = true
}

Example

This example shows how to look up an existing VPC by its tag Name and then use its ID to create a subnet inside it.

terraform

provider "aws" {
  region = "us-east-1"
}

data "aws_vpc" "selected" {
  filter {
    name   = "tag:Name"
    values = ["my-vpc"]
  }
  most_recent = true
}

resource "aws_subnet" "example" {
  vpc_id            = data.aws_vpc.selected.id
  cidr_block        = "10.0.1.0/24"
  availability_zone = "us-east-1a"
}

Output

Terraform will read the existing VPC with tag Name=my-vpc and create a subnet in that VPC with CIDR 10.0.1.0/24 in us-east-1a.

Common Pitfalls

• Not specifying most_recent = true when multiple VPCs match filters can cause errors.
• Using id and filter together is not allowed; choose one.
• Forgetting to set the correct AWS region in the provider can cause lookup failures.
• Assuming the data source creates resources; it only reads existing ones.

terraform

data "aws_vpc" "wrong" {
  id = "vpc-123abc"
  filter {
    name   = "tag:Name"
    values = ["my-vpc"]
  }
}

# Correct usage:
data "aws_vpc" "correct" {
  filter {
    name   = "tag:Name"
    values = ["my-vpc"]
  }
  most_recent = true
}

Quick Reference

Remember these key points when using data aws_vpc:

• Use id to specify a VPC directly.
• Use filter to find a VPC by tags or attributes.
• Set most_recent = true if filters match multiple VPCs.
• Access attributes like data.aws_vpc.example.id or data.aws_vpc.example.cidr_block.

Key Takeaways

Use data aws_vpc to read existing VPC info without creating new resources.

Specify either id or filter, never both, to identify the VPC.

Set most_recent = true when filters match multiple VPCs to avoid errors.

Access VPC attributes via data source references for use in other resources.

Ensure your AWS provider region matches the VPC location for successful lookup.