class Socket::ResolutionError

static VALUE
sock_resolv_error_code(VALUE self)
{
    return rb_attr_get(self, id_error_code);
}

Returns the raw error code indicating the cause of the hostname resolution failure.

begin
  Addrinfo.getaddrinfo("ruby-lang.org", nil)
rescue Socket::ResolutionError => e
  if e.error_code == Socket::EAI_AGAIN
    puts "Temporary failure in name resolution."
  end
end

Note that error codes depend on the operating system.