What does errno 64 (EHOSTDOWN) mean?

EHOSTDOWN

Host-level unavailability, not a missing network route

EHOSTDOWN reports that the destination host is considered down or unavailable for the socket operation. It is narrower than ENETDOWN, which concerns the local network path, and different from ENETUNREACH, which concerns reaching the destination network at all.

This error should be preserved as a host-specific networking result rather than rewritten as “the service is stopped.” A host can be unavailable because of link, routing, neighbor, tunnel, or policy conditions that occur below the remote application.

What provides useful context

• Record the resolved numeric destination, address family, selected route, source interface, and any host-specific reachability or neighbor events available from the system.
• Compare multiple hosts in the same destination network. A shared failure points toward network scope; a single-host failure points toward host reachability or endpoint configuration.
• Keep name-resolution results with the error. A hostname that changes between attempts can otherwise make a host-specific condition appear intermittent.

Related errors

EHOSTUNREACH is often used when no route to the specific host is available. The practical distinction is valuable in logs: retain both codes instead of collapsing them into a generic “offline” status.

References

• XNU: Darwin errno definitions
• FreeBSD manual: errno(2)
• POSIX: connect()

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