how to force dhclient not to use DUID-type 4?

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how to force dhclient not to use DUID-type 4?

Hello all,
I hope I'm using the connect list for this dhclient (or admin?) issue.
If not, I'm sorry for that.
Which one it the correct one?
My question in short words:
which words we have to put in a line in /etc/dhcp/dhclient.conf (or somewhere else) to force dhclient to use DUID-TTL (client-id=1) oder DUID-LL (client-id=3) for DHCPv6 solicitate message.
We don't want to define the whole client-id, just the type of it.

Now the long version of my issue:
We want to use a IPv6 Linux client with DHCP. The client is connected to a network where the first hop is a Cisco ASA. The ASA is configured and is working as a DHCPv6 relay system, too.
The DHCPv6-Server is attached to another interface of the ASA firewall.
We see a working relay config for IPv6, but mainly for Windows client. Most of today debian Linux clients doesn't get any DHCPv6 information even if other and/or managed flag is set in RA packets.
Debuging DHCP on the ASA, we see dhclient is using a dhcp client id with type 4 (new in RFC 6355) and default type when using network manager.
This type of client id is most likely not implemented in the relay agent and the Cisco bug-report (CSCvr63051) recommends to change the type of client-id to 1 (if we know this, we can interpret the bug-report like that)
OK, looks like we have to force the dhcp6 client to use another type of the client-id...
Looking to RFC 3315, the client-id of 1 is DUID-LLT. So we added the following line in /etc/dhcp/dhclient.conf
 option dhcp6.client-id duid-llt;
To have the same situation than a clear system we did a
  rm /var/lib/dhcp/dhclient6.leases
and after that we run
  dhclient -6 -r ens192
Now we checked log file:
  journalctl | grep dhc
dhclient[839]: /etc/dhcp/dhclient.conf line 14: expecting "code" keyword.
dhclient[839]: option dhcp6.client-id =
dhclient[839]:                         ^

We change the added line to
   option dhcp6.client-id code=1;
and got another message:
dhclient[2321]: /etc/dhcp/dhclient.conf line 14: expecting option code number.
dhclient[2321]: option dhcp6.client-id code=
dhclient[2321]:                             ^

we tried many different things like
 option dhcp6.client-id code="\000\001";
 option dhcp6.client-id code="1"
but we never got rid of a error message like above. We deleted the lease-file and then we run dhclient every time
Now the question is:
can anyone help how we have to add the line to force dhclient to use DUID-LLT or DUID-LL without a error message?
Or is there something totally wrong in our way to realize this?
Any other ideas to force the client to sent the DHCPv6 solicit-message with a client-id type of 1 or type of 3?
Looking forward for interesting answers.

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