CURLOPT_KRBLEVEL man page
CURLOPT_KRBLEVEL — set FTP kerberos security level
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_KRBLEVEL, char *level);
Pass a char * as parameter. Set the kerberos security level for FTP; this also enables kerberos awareness. This is a string that should match one of the following: 'clear', 'safe', 'confidential' or 'private'. If the string is set but doesn't match one of these, 'private' will be used. Set the string to NULL to disable kerberos support for FTP.
The application does not have to keep the string around after setting this option.
This option was known as CURLOPT_KRB4LEVEL up to 7.16.3
Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space.