What does errno 47 (EAFNOSUPPORT) mean?

EAFNOSUPPORT

The address does not fit the socket's family

EAFNOSUPPORT means that the address presented to a socket operation is not valid for that socket's address family. It is often visible in bind(), connect(), sendto(), or sendmsg() when a sockaddr was built for the wrong family.

The family field and the address length must be treated as one object. A valid-looking port number or byte sequence is not enough: an IPv4 address, IPv6 address, and local-domain pathname have different structure and interpretation rules.

Checks that produce evidence

• Log the socket's family, the sa_family field, supplied address length, and the API that consumed the address.
• Verify that a dual-stack or proxy layer did not reuse an IPv4 address structure for an IPv6 socket, or pass a destination object after its storage was overwritten.
• For local-domain sockets, record the exact pathname or abstract-address representation separately from Internet-address logs.

Not the same as unreachable

The kernel rejected the address representation before it could decide whether a network or host was reachable. Compare this with ENETUNREACH and EHOSTUNREACH, which require a usable destination address.

References

• XNU: Darwin errno definitions
• POSIX: bind()
• POSIX: connect()

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