What does errno 118 (ENETUNREACH) mean?

ENETUNREACH

ENETUNREACH means the selected destination cannot be reached through a usable network path. It is primarily a routing or network-availability observation, whereas EHOSTUNREACH is more specific to the target host after a network path has been considered.

For Winsock, capture WSAENETUNREACH immediately with WSAGetLastError(). A high-level errno mapping alone loses whether the failing call was a route-sensitive connection attempt, a send, or a helper that selected a destination from multiple DNS answers.

Route-selection evidence

• Log all candidate addresses, the one selected, address family, source interface, default route, and proxy or tunnel configuration.
• Check whether IPv6 was selected while the active route exists only for IPv4, or vice versa.
• Inspect VPN split routes and policy routing; a destination can be reachable on one adapter but forbidden on another.
• Use address fallback only when the resolver or protocol policy permits it, and retain per-address failures rather than collapsing them into one timeout.

References

• Microsoft: connect()
• Microsoft: WSAENETUNREACH
• POSIX: ENETUNREACH

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