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?