What does errno 123 (ENOPROTOOPT) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A UCRT POSIX-supplement status for an option unsupported by the selected protocol or socket type.

Could be also:
Constant	Type	OS
ERROR_INVALID_NAME	Win32 error	Windows
EPFNOSUPPORT	errno	Solaris
INACCESSIBLE_BOOT_DEVICE	BugCheck Code	Windows

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EPFNOSUPPORT	EAFNOSUPPORT

ENOPROTOOPT

ENOPROTOOPT means the caller requested a protocol option that the selected socket does not provide. The option name can be valid in another family, socket type, or transport, so the failure is not necessarily a spelling or value error.

On Windows, native Winsock APIs return WSAENOPROTOOPT through WSAGetLastError(). A common source is applying a TCP-only control to a datagram socket or using a family-specific IOCTL where the current provider does not expose it. Keep the option level, name, socket family, type, and protocol in the log.

Validate the option contract

Record the option level and name separately; a valid option at the wrong level is a different mistake from an unsupported option.
Record socket family, type, protocol, and provider before the call, not only the final descriptor value.
Check whether the intended feature belongs to setsockopt, ioctlsocket, or WSAIoctl.
Do not silently ignore an unsupported security or keepalive option; document the fallback semantics explicitly.

References

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