NSURLErrorFailingURLKey

NSURLErrorFail... is not a single official constant, but a common shorthand seen in logs and error descriptions that points to failures reported under the NSURLErrorDomain used by Apple’s URL loading system. This domain covers errors produced by APIs such as NSURLSession and NSURLConnection when a network request cannot be completed. An NSError in this domain carries a numeric code that indicates the specific failure reason, along with a localized description and a potentially helpful recovery suggestion in the userInfo dictionary.

Typical causes of NSURLErrorDomain failures include network connectivity problems, DNS resolution failures, inability to reach a host, timeouts, and issues with servers or security configurations. Examples of common failure themes are not being connected to the Internet, the request timing out, the host or URL being unavailable, or problems establishing a secure connection. The exact code and accompanying userInfo help developers diagnose the underlying problem.

For handling these errors, developers typically inspect the error’s domain and code, then adapt behavior accordingly. Useful steps include displaying a user-friendly message, attempting retries for transient conditions (such as network interruptions or timeouts), validating the requested URL, and inspecting the failing URL in the userInfo. In cases of SSL/TLS or certificate errors, appropriate user-facing guidance or stricter security handling may be required.

Best practices involve leveraging the error’s localizedDescription, recoverySuggestion, and any relevant userInfo keys (for example, the failing URL) to inform users or automate recovery where feasible. Testing across different network conditions and ensuring robust error handling in completion handlers help improve resilience of network-enabled apps.

See also NSURLErrorDomain, NSURLSession, and common URL loading error handling guidance.