k5_try_realm_txt_rr
(void)k5_try_realm_txt_rr(context, "_kerberos", NULL, &realm);
if (k5_try_realm_txt_rr(context, "_kerberos", fqdn, &realm) == 0)
krb5_error_code k5_try_realm_txt_rr(krb5_context context, const char *prefix,