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EHOSTUNREACH
EHOSTUNREACH denotes host-level reachability failure after an attempt to use the selected route. It is distinct from name resolution: a hostname can resolve successfully while the resulting address cannot be reached from this host, interface, address family, or network policy domain.
Native Winsock code uses WSAEHOSTUNREACH via WSAGetLastError(). This UCRT value is the portability-layer name, so collect the original error channel and the final numeric destination before applying a POSIX-style classification.
Route-focused diagnostics
- Record the resolved address list, the selected address, interface, source address, family, and any proxy or tunnel that changed the route.
- Compare with
ENETUNREACH: the latter points to network reachability, while this status is more specific to the host destination. - Check policy routes, VPN split tunneling, firewall rules, and IPv4/IPv6 preference changes before treating it as a remote server outage.
- Do not retry a fixed unreachable route in a tight loop; allow routing or address-selection state to change and use bounded backoff.
References
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