How can I figure out which URL is being blocked by App Transport Security?

I'm upgrading an iOS app to iOS 9, and I have some URLs that are not secure, and I need a few exceptions to App Transport Security. I've added the two that I know about, but there are some warnings happening now stating:

App Transport Security has blocked a cleartext HTTP (http://) resource load since it is insecure. Temporary exceptions can be configured via your app's Info.plist file.

I need to know what URLs are being blocked so I can add them to the Info.plist, they're probably images or media files. How can I make the system log the URLs it decided not to load?

Solution 1:

Editor Note: @jessedc points out in the comments below the link to the official Apple documentation on how to do this: https://developer.apple.com/library/content/qa/qa1887/_index.html

Okay, I have an answer I don't like! I still very much want a better one.

In my application:didFinishLaunchingWithOptions: method I added the line

setenv("CFNETWORK_DIAGNOSTICS", "3", 1);

When I ran the app then, in the log I can find an entry that looks like this:

2015-07-02 15:27:56.152 MyApp[45041:9125662] CFNetwork diagnostics log file
created at: /Users/micah.hainline/Library/Developer/CoreSimulator/Devices/
11BCA581-5F5F-494D-932A-2ECFCA33EA93/data/Containers/Data/Application/
9ACC6941-8039-4B86-B5E8-A6C66E2AD520/Library/Logs/CrashReporter/CFNetwork_com
.myapp.MyApp_45041.nwlrb.log

When I open that file I have a huge number of log entries about everything that's happened on the network. I search for kCFErrorDomainCFNetwork in that file and get logs for failed network requests. I can use that to see what URLs the system was trying to hit, and then can add that URL to the exceptions for App Transport Security.

Solution 2:

Accepted answer is correct but If you're doing this on Xamarin iOS, you will need to use;

Environment.SetEnvironmentVariable("CFNETWORK_DIAGNOSTICS", "3", EnvironmentVariableTarget.Process);

Also, if you're looking for the log file use something like Simulator manager to find the correct location easier (https://github.com/tue-savvy/SimulatorManager).