EC
ENOPROTOOPT
Linux / POSIXERRORCommonNetworkHIGH confidence

Protocol Not Available

Production Risk

Programming error; check option-to-protocol compatibility in the man page.

What this means

ENOPROTOOPT (errno 92) is returned by getsockopt() or setsockopt() when the requested socket option is not supported by the protocol.

Why it happens
  1. 1Setting TCP_NODELAY on a UDP socket
  2. 2Using SO_KEEPALIVE with a protocol that does not support it
  3. 3Setting an IPv6-only option on an IPv4 socket
How to reproduce

setsockopt() with a TCP option on a UDP socket.

trigger — this will error
trigger — this will error
int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
int one = 1;
// TCP_NODELAY is a TCP option, not valid for UDP
setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));
// Returns -1, errno = ENOPROTOOPT

expected output

setsockopt: Protocol not available (ENOPROTOOPT)

Fix

Use the correct level and option for the socket type

WHEN When setting socket options

Use the correct level and option for the socket type
// For UDP, use SOL_SOCKET level options
setsockopt(sockfd, SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));

// TCP_NODELAY is only for TCP sockets:
int tcp_sockfd = socket(AF_INET, SOCK_STREAM, 0);
setsockopt(tcp_sockfd, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));

Why this works

Each option belongs to a specific protocol level (SOL_SOCKET, IPPROTO_TCP, IPPROTO_UDP, etc.).

Sources

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