NSURLErrorCannotFindHost

NSURLErrorCannotConnectToHost is a Foundation networking error constant used on Apple platforms (iOS, macOS, watchOS, tvOS) to indicate that a network connection to a resolved host could not be established. It appears as an NSError in the NSURLErrorDomain with numeric code -1004 and originates from higher-level APIs such as NSURLSession, NSURLConnection and underlying CFNetwork.

The error is typically raised after DNS resolution succeeds but no TCP connection can be made to the target host and port. Common causes include the remote service being offline, the server not listening on the expected port, firewall or network routing blocking the connection, intermediate proxies or VPN interference, or incorrect URL/port configuration. It differs from related errors like NSURLErrorCannotFindHost (DNS failure) and NSURLErrorTimedOut (no response within a timeout interval).

Handling strategies include verifying the URL and port, testing connectivity with tools like curl or ping, checking server status and logs, and ensuring firewalls and proxies permit the traffic. In client code, developers commonly check error.domain == NSURLErrorDomain and error.code == NSURLErrorCannotConnectToHost to implement retries with exponential backoff, present user-friendly messages, or trigger alternate flows. Monitoring and diagnostic tools (server logs, packet captures, reachability or NWPathMonitor) help isolate whether the issue is client-side, network-level, or server-side.