What does errno 43 (EPROTONOSUPPORT) mean?

EPROTONOSUPPORT

The protocol is unavailable before a connection can begin

EPROTONOSUPPORT means that the requested protocol cannot be used with the selected socket family on this system. The error is local: it does not say that a remote server rejected the connection, and it can occur before any packet is sent.

It is important to retain the protocol number as well as any high-level protocol name. Libraries often translate a configuration such as “TCP” or “raw” into a family, type, and protocol triple; the conversion layer can choose an invalid triple even when each component is valid in another combination.

Distinguish the three layers of compatibility

• EPFNOSUPPORT concerns the protocol family itself.
• ESOCKTNOSUPPORT concerns the requested socket type.
• EPROTOTYPE concerns an incompatible type/protocol pairing.

What to verify

Start with the exact socket() arguments and any platform-specific feature gate. Only after a compatible socket has been created do address, route, service, TLS, or application-protocol diagnostics become relevant.

References

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

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