App Transport Security is a feature that improves the security of connections between an app and web services. The feature consists of default connection requirements that conform to best practices for secure connections. Apps can override this default behavior and turn off transport security.
Transport security is available on iOS 9.0 or later, and on OS X 10.11 and later.
Source: https://developer.apple.com/library/prerelease/ios/technotes/App-Transport-Security-Technote/
That unless you change something in your iOS app' plist, your app will not be able to communicate with unsecure HTTP servers, when it runs on iOS 9.
That's a good thing really; Apple is trying to force people to update their HTTP servers to use the latest HTTPS protocols & recommendations: TLS 1.2, SHA256 or better, forward secrecy.
Where this could cause you issues is if you use third-party services that have not yet adhered to those standards.
They might have a good reason not to (doubtful!), or they are working on implementing them, but are not there yet.
Right now, the best workaround is to whitelist all servers on a case-by-case basis, per the service providers own recommendations.
Thus the need for the list in this repository.
In the ats.plist file in this repo, you'll find the exceptions that you need to add to your app .plist, if you use some of the third-party services that published recommendations regarding ATS.
If you know of any other services that published recommendations for exceptions that should be used in iOS 9 apps, for their service to work as expected, please fork and create a pull request.
nscurl --ats-diagnostics https://api.your-server.com
In no particular order: