AWS: How to Troubleshoot Private Instance Ping Issues in Subnet

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If you’re having trouble pinging private instances inside an AWS VPC, don’t worry—you can troubleshoot the issue step by step.

First, check the security group attached to your non-responsive instance. Even if you think it’s the same as others, make sure that inbound ICMP traffic is allowed from your source IP address. Sometimes, a small change here can cause connectivity problems.

Next, look at the Network Access Control Lists (ACLs) for the subnet where the instance lives. While default ACLs usually let all traffic through, custom rules might be blocking ICMP requests. Verify that there’s nothing in these rules that could be causing the blockage.

Then, review your route tables. Confirm that both private subnets are associated with the correct route table that has a route for 0.0.0.0/0 pointing to the NAT Gateway. Proper routing is essential for private instances to communicate outside their subnet.

Ensure your NAT Gateway is up and running. It should be in an “Available” state and associated with an Elastic IP address. If it’s not active, your instances won’t be able to access the internet or respond to ping requests.

Also, verify that your VPC has an Internet Gateway attached. The public subnet that hosts the NAT Gateway needs a route for 0.0.0.0/0 pointing to this Internet Gateway. Without this, external communications won’t be possible.

Remember, for your ping to work, the target you are pinging must support ICMP traffic. Test this from your local computer first to make sure the target responds to ICMP requests.

Finally, check your VPC’s DNS settings. Make sure that DNS hostnames and resolution are enabled so that your instances can properly resolve domain names.

If you’re still having trouble, enabling VPC Flow Logs can help. These logs show detailed information about the traffic flowing in and out of your VPC, helping you identify where the problem occurs—be it security settings, routing, or network ACLs.

Following these steps should help you identify and fix the ping connectivity issue in your AWS VPC.