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ENOPROTOOPT
ENOPROTOOPT is about an option lookup, not a remote-network failure. With getsockopt() or setsockopt(), Linux reports it when the option is unknown at the supplied level. A valid option name at SOL_SOCKET is not automatically valid at IPPROTO_IP, IPPROTO_IPV6, IPPROTO_TCP, or another protocol level.
The same source code can therefore work on one operating system, kernel configuration, protocol family, or socket type and fail on another. Linux also documents concrete option-specific cases: for example, attempting to change SO_SNDLOWAT returns ENOPROTOOPT. Do not replace the error with an unverified fallback; first establish whether the application selected the intended level, option name, socket family, and feature.
Useful checks
- Log the numeric
level,optname, socket domain, type, protocol, and the exact call that failed. - Validate the option against the documentation for the selected protocol rather than only against a generic socket-options list.
- For portable code, make unsupported tuning optional and retain a visible capability decision instead of silently applying a different option.
- Do not confuse this with
EOPNOTSUPP: that often describes an unsupported operation or flag, whereasENOPROTOOPTidentifies an option at the wrong or unsupported level.
Linux getsockopt(2) · Linux socket options · POSIX error definitions
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