What does errno 128 (ENETUNREACH) mean?

ENETUNREACH

No usable path to the destination network

ENETUNREACH describes an operation attempted toward an unreachable network. The failure is broader than a single application service refusing a connection and is different from ECONNREFUSED, which requires an active rejection from the target side.

Solaris also distinguishes EHOSTUNREACH for an unreachable host. In practice, routing or gateway status information may not always make the network/host boundary obvious, so retaining the exact errno and destination prefix is useful when comparing failures.

Route-oriented investigation

• Capture the destination address, selected route, next hop, and outgoing interface at the failure time.
• Check route-table or zone changes and whether the address family has an appropriate default or specific route.
• Do not treat repeated ENETUNREACH as a server-port problem until basic path selection succeeds.

References

• Oracle Solaris intro(2) errno catalog
• POSIX connect() error conditions
• POSIX Issue 8 errno.h

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