EC
EAFNOSUPPORT
Linux / POSIXERRORCommonNetworkHIGH confidence

Address Family Not Supported by Protocol

Production Risk

Common during IPv4/IPv6 migration; ensure socket and address families match.

What this means

EAFNOSUPPORT (errno 97) is returned when a socket address of the wrong address family is passed to an operation, or when the kernel does not support IPv6.

Why it happens
  1. 1Connecting an AF_INET socket to an AF_INET6 address
  2. 2IPv6 not compiled into the kernel
  3. 3Passing an AF_INET6 sockaddr to a bind() on an AF_INET socket
How to reproduce

Connecting an IPv4 socket to an IPv6 address.

trigger — this will error
trigger — this will error
int sockfd = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in6 addr6 = { .sin6_family = AF_INET6, ... };
// Wrong address family for this socket
connect(sockfd, (struct sockaddr*)&addr6, sizeof(addr6));
// Returns -1, errno = EAFNOSUPPORT

expected output

connect: Address family not supported by protocol (EAFNOSUPPORT)

Fix

Match socket and address address families

WHEN When EAFNOSUPPORT is returned on connect/bind

Match socket and address address families
// Create IPv6 socket for IPv6 addresses
int sockfd = socket(AF_INET6, SOCK_STREAM, 0);
struct sockaddr_in6 addr6 = {
  .sin6_family = AF_INET6,
  .sin6_port = htons(80),
};
inet_pton(AF_INET6, "::1", &addr6.sin6_addr);
connect(sockfd, (struct sockaddr*)&addr6, sizeof(addr6));

Why this works

The socket address family (AF_INET vs AF_INET6) must match the socket created.

Sources
Official documentation ↗

Linux Programmer Manual connect(2)

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