.. ischooklib:: libdhcp_ddns_tuning.so
.. _hooks-ddns-tuning:

``libdhcp_ddns_tuning.so``: DDNS Tuning
=======================================

This hook library adds support for fine-tuning various DNS update aspects.
It currently supports procedural host-name generation and the ability to skip
performing DDNS updates for select clients.

.. note::

    :ischooklib:`libdhcp_ddns_tuning.so` is part of the open source code and is
    available to every Kea user.
    It was previously available only to ISC customers with a paid support contract.

The library, which was added in Kea 2.1.5, can be loaded by the :iscman:`kea-dhcp4`
and :iscman:`kea-dhcp6` daemons by adding it to the ``hooks-libraries`` element of the
server's configuration:

.. code-block:: javascript

    {
        "hooks-libraries": [
            {
                "library": "/usr/local/lib/libdhcp_ddns_tuning.so",
                "parameters": {
                    ...
                }
            },
            ...
        ],
        ...
    }

Procedural Host-Name Generation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This hook library provides the ability to generate host names procedurally, based on
an expression. The expression can be defined globally in the hook parameters, using
`hostname-expr`. If defined globally, it applies to all hosts in all subnets. The
expressions can use all tokens defined in :ref:`classify`. An example of a global
expression is shown below:

.. code-block:: javascript

    {
        "hooks-libraries": [
            {
                "library": "/usr/local/lib/libdhcp_ddns_tuning.so",
                "parameters": {
                    "hostname-expr": "'host-'+hexstring(pkt4.mac,'-')",
                    ...
                }
            },
            ...
        ],
        ...
    }

The default behavior of :iscman:`kea-dhcp4` is to prefer the FQDN option (code 81) over the
host name option (code 12) when a client sends both.   The following example shows
the ``hostname-expr`` one would use to reverse this rule:

.. code-block:: javascript

    {
        "hooks-libraries": [
            {
                "library": "/usr/local/lib/libdhcp_ddns_tuning.so",
                "parameters": {
                    "hostname-expr" : "ifelse(option[12].exists, option[host-name].text, option[81].text)"
                }
            }
        ]
    }

It is also possible to define this parameter in a subnet, using the user-context mechanism.
If defined at the subnet level, the expression applies to a specific subnet only. If the
subnet expression is defined as empty, ``""``, it suppresses (or disables) the use of a
global expression for that subnet. An example subnet expression is shown below:

.. code-block:: javascript

   {
    "subnet4": [{
        "subnet": "192.0.2.0/24",
        "pools": [{
            "pool": "192.0.2.10 - 192.0.2.20"
        } ],

        "user-context": {
            "ddns-tuning": {
                "hostname-expr": "'guest-'+int8totext(substring(pkt4.yiaddr, 0,1))+'-' \
                                          +int8totext(substring(pkt4.yiaddr, 1,2))+'-' \
                                          +int8totext(substring(pkt4.yiaddr, 2,3))+'-' \
                                          +int8totext(substring(pkt4.yiaddr, 3,4))"
            }
        }
    }],
    ...
   }

.. note::

   The expression value above uses a backslash, ``\``, to show line continuation. This is for
   clarity only and is not valid JSON supported by Kea parsing. The actual value must
   be expressed on a single line.

.. note::

   Privacy should be taken into consideration when generating a host name. The host name
   is usually inserted into the DNS, which is a public system. Exposing identifiers that
   can be used to track devices, such as a MAC address, are usually a very bad idea.
   The global expression example here used a MAC address for simplicity.

DHCPv4 Host-Name Generation
---------------------------

With this library installed, the behavior for :iscman:`kea-dhcp4` when forming host names in
response to a client query (e.g. DISCOVER, REQUEST) is as follows:

  1. If a host name is supplied via a host reservation, use it with the DDNS
  behavioral parameters to form the final host name. Go to step 4.

  2. If the client supplied an FQDN option (option 81), use the domain name value
  specified within it, with the DDNS behavioral parameters, to form the final
  host name. Go to step 4.

  3. If the client supplied a host-name option (option 12), use the host name specified
  within it, with the DDNS behavioral parameters, to form the final host name.

  4. If there is a ``ddns-tuning`` in-scope host-name expression (either global or subnet),
  calculate the host name using the expression. If the calculated value is not a fully
  qualified name and there is an in-scope ``ddns-qualifying-suffix``, append the suffix.

  5. If the value calculated by the hook is not an empty string and is different than
  the host name formed in steps 1 or 2, the calculated value becomes the
  final host name.

DHCPv6 Host-Name Generation
---------------------------

With this library installed, the behavior for :iscman:`kea-dhcp6` when forming host names in
response to a client query (e.g. SOLICIT, REQUEST, RENEW, REBIND) is as follows:

  1. If the client supplied an FQDN option (option 39), use the domain name value
  specified within it, with the DDNS behavioral parameters, to form the final
  host name. Go to step 4.

  2. If the client did not supply an FQDN but ``ddns-replace-client-name`` is either
  ``always`` or ``when-not-present``, then calculate the final form of the host
  name and use it to create an outbound FQDN. Go to step 4.

  3. If there is no outbound FQDN at this point, client-name processing for this
  packet stops. Without an outbound FQDN there is no way to communicate a host
  name to the client.

  4. If a host name is supplied via a host reservation, use it along with the DDNS
  behavioral parameters to form the final host name; it supersedes the FQDN value
  calculated in steps 1 or 2.

  5. If there is a ``ddns-tuning`` in-scope host name expression (either global or subnet),
  calculate the host name using the expression. If the calculated value is not a fully
  qualified name and there is an in-scope ``ddns-qualifying-suffix``, append the suffix.

  6. If the value calculated by the hook is not an empty string and is different than
  the host name formed in steps 1 or 2, the calculated value becomes the
  final host name.


Skipping DDNS Updates
~~~~~~~~~~~~~~~~~~~~~

:ischooklib:`libdhcp_ddns_tuning.so` also provides the ability to skip DDNS updates on a
per-client basis. The library recognizes a special client class, "SKIP_DDNS"; when a
client is matched to this class, the Kea servers (:iscman:`kea-dhcp4` and :iscman:`kea-dhcp6`) do not
send DDNS update requests (NCRs) to :iscman:`kea-dhcp-ddns`. A common use case would be
to skip DDNS updates for fixed-address host reservations. This is done easily by
simply assigning the class to the host reservation as shown below:

.. code-block:: javascript

    {
        "reservations": [
        {
            "hw-address": "01:02:03:04:05:06",
            "ip-address": "192.0.2.1",
            "client-classes": [ "SKIP_DDNS", "foo", "bar" ]
        }]
    }

The :ischooklib:`libdhcp_ddns_tuning.so` hook library notes the
presence of the ``"SKIP_DDNS"`` class in the
client's class list each time the client requests, renews, or releases its lease,
and instructs :iscman:`kea-dhcp4` to bypass sending DDNS updates. A similar workflow is
supported for :iscman:`kea-dhcp6`:

.. code-block:: javascript

    {
        "reservations": [
        {
            "duid": "01:02:03:04:05:06",
            "ip-address": "2001:db8::1",
            "client-classes": [ "SKIP_DDNS", "foo", "bar" ]
        }]
    }

Although "SKIP_DDNS" is a special class, it can be defined with a test
expression. Defining it as shown below would omit DDNS updates for all KNOWN
clients:

.. code-block:: javascript

    {
        "client-classes":[
        {
            "name": "SKIP_DDNS",
            "test": "member('KNOWN')"
        }]
    }

.. note::

    The :ischooklib:`libdhcp_ddns_tuning.so` hook library must be
    loaded for the ``"SKIP_DDNS"`` class to have an effect.
