# URL Loading System error info keys Recognize these keys from the user info dictionary of error objects produced by URL Loading APIs. ## Discussion These keys are only present in the [`NSURLErrorDomain`](/documentation/Foundation/NSURLErrorDomain). ## Topics ### Keys [`NSURLErrorFailingURLErrorKey`](/documentation/Foundation/NSURLErrorFailingURLErrorKey) The URL which caused a load to fail. [`NSURLErrorFailingURLPeerTrustErrorKey`](/documentation/Foundation/NSURLErrorFailingURLPeerTrustErrorKey) The state of a failed SSL handshake. [`NSURLErrorFailingURLStringErrorKey`](/documentation/Foundation/NSURLErrorFailingURLStringErrorKey) The URL which caused a load to fail. [`NSURLErrorBackgroundTaskCancelledReasonKey`](/documentation/Foundation/NSURLErrorBackgroundTaskCancelledReasonKey) A key in the error dictionary that provides the reason for canceling a background task. [URL Session Background Task Cancellation Reasons](/documentation/Foundation/url-session-background-task-cancellation-reasons) Reasons that indicate why the system canceled a background task. [`NSURLErrorNetworkUnavailableReasonKey`](/documentation/Foundation/NSURLErrorNetworkUnavailableReasonKey) The reason the network was unavailable for a task. [`NetworkUnavailableReason`](/documentation/Foundation/URLError/NetworkUnavailableReason-swift.enum) An enumeration of reasons explaining why a task couldn’t satisfy networking constraints. [`NSURLErrorNetworkUnavailableReason`](/documentation/Foundation/NSURLErrorNetworkUnavailableReason) An enumeration of reasons why a task couldn’t satisfy networking constraints. ### Deprecated [`NSErrorFailingURLStringKey`](/documentation/Foundation/NSErrorFailingURLStringKey) The URL that caused the error. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)