Amazon EC2 instance unable to resolve host

Question

I have a bunch of amazon ec2 instances. Three instances became unable to resolve a specific url. When I try curl <url>, those three instances say "curl: (6) Could not resolve host", while all the other instances are able to resolve the url and return the correct data. It's only that specific url, as well - all other urls work fine.

What could have caused this? I haven't changed anything on any of the instances, so it's not something I did. How do I find out why it happened?

Answer 1

Here is my checklist of things you need  to verify. Are you sure you havent’t changed the following:

• DNS Settings on the instance? (check with dig to see if name resolution is OK)

• NACL rules on the subnet where your instance is running?

• Security Group rules of the SG attached to your instances?

Check if you have made any changes.