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ERROR_HOST_DOWN
The remote system is unavailable.
ERROR_HOST_DOWN is Win32 error 1256 (0x4E8). A network-dependent operation could identify its remote host but could not use that system. The result does not reveal whether the machine is powered off, isolated by routing or firewall changes, rebooting, or reachable only through a failed service path.
Separate name, route, host, and service failures
- resolve the host name and record every returned address
- inspect the route and interface chosen for the attempted address
- test whether the host responds through another expected protocol
- verify the target service and port independently of general host reachability
- check VPN, proxy, firewall, and network-compartment changes
Evidence to capture
Log the canonical host name, numeric destination, address family, port or protocol, local interface, start and failure times, timeout, and lower-layer error if one exists. Preserve DNS answers and connection attempts separately. A later successful retry can otherwise hide that the first attempt selected a stale IPv6 or VPN address.
Diagnostic sequence
Begin from the client context that failed. Confirm DNS resolution, inspect the routing table, and test the exact destination address rather than a different alias. Check whether the host is in planned maintenance or still booting. If ICMP is blocked, lack of ping response is not proof that the host is down; test the application protocol and review server-side events when available.
For multihomed systems, compare which address each client selected. A service may be available on one interface but not another. Repeated 1256 from one network segment can indicate filtering or incorrect subnet registration rather than a failed server.
Recovery strategy
Use bounded backoff and refresh name resolution before retrying. Fail over only to endpoints that are known to represent the same service and data consistency domain. For state-changing requests, determine whether any part reached the remote service before submitting again. Administrators should restore routing, firewall policy, or server health instead of changing credentials.
Comparison with nearby network codes
ERROR_NETWORK_UNREACHABLE identifies a routing-level inability to reach the network. ERROR_CONNECTION_REFUSED indicates an actively rejected transport endpoint. ERROR_ONLY_IF_CONNECTED concerns the client’s logical session state. Error 1256 broadly says the remote host is unavailable.
Example
A client resolves a server to an address on a retired data-center subnet and returns 1256. The server is healthy at a new address, but DNS cache and stale registration direct only some clients to the old route. Refreshing registration and expiring the stale record fixes the host-down reports.
References
- Microsoft: System Error Codes (1000–1299)
- Microsoft: Name Resolution
- Microsoft: getaddrinfo function
- Microsoft: Network troubleshooting with netsh
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