Let’s walk through why your test setup isn’t routing traffic through the NAT gateway as expected, even with the internet gateway (igw-4d4b212a) attached to your VPC (vpc-77049811). Here are the key areas to validate step by step:

1. Confirm NAT Gateway Deployment & Basics #

• First off: make sure you’ve actually provisioned a NAT gateway in your VPC—an internet gateway alone doesn’t handle NAT routing. The NAT gateway needs to live in a public subnet (one with a route pointing directly to igw-4d4b212a).
• Double-check that the NAT gateway has an Elastic IP (EIP) assigned. Without an EIP, it can’t translate private IPs to a public address for outbound traffic.

2. Audit Private Subnet Route Tables #

Your Elastic Beanstalk instances and test EC2 should be in private subnets. For these subnets:

• The associated route table must have a default route (0.0.0.0/0) pointing to the NAT gateway’s ID (e.g., nat-xxxxxx), not the internet gateway.
• Ensure the private subnets are explicitly linked to this route table—subnets often default to the VPC’s main route table, which might still route traffic directly to the IGW.

3. Validate Elastic Beanstalk VPC Configuration #

When launching your Beanstalk environment in the test VPC:

• Confirm you selected private subnets for application instances. If you picked public subnets, instances will bypass the NAT gateway and route directly through the IGW.
• Check the environment’s "VPC Configuration" settings to ensure the NAT gateway is selected as the outbound traffic handler (some setups let you choose between NAT gateway and internet gateway).

4. Security Group & NACL Rule Checks #

• Security Groups: Make sure your EC2/Beanstalk instances have outbound rules allowing traffic to the target API’s IP/port (e.g., tcp/443 to x.x.x.x/32). A broad 0.0.0.0/0 outbound rule works too, since the NAT gateway will handle IP translation.
• Network Access Control Lists (NACLs): For both the public subnet (where the NAT gateway resides) and private subnets:
  • Inbound rules should allow return traffic from the API (use ephemeral ports, typically 1024-65535).
  • Outbound rules should permit traffic to the API’s IP/port and any necessary traffic to the NAT gateway.

5. Test Traffic Flow Directly from the EC2 Instance #

Log into your test EC2 instance and run these diagnostic commands:

• Check the active route table: ip route show—the default route should point to the NAT gateway’s private IP, not the IGW.
• Verify your public IP: curl ifconfig.me—this should match the NAT gateway’s EIP (if it shows the instance’s own public IP, you’re still routing through the IGW).
• Test API connectivity: curl -v https://your-target-api.com to see if the connection fails due to IP whitelisting or routing issues.

6. Check the VPC’s Main Route Table #

If your private subnets are using the VPC’s main route table, make sure it doesn’t have a route to the internet gateway. The main table should only include the local VPC route (10.0.0.0/16) plus the route to the NAT gateway.

内容的提问来源于stack exchange，提问作者synkyo