.. _classify:

*********************
Client Classification
*********************

Client Classification Overview
==============================

In certain cases it is useful to differentiate among different types
of clients and treat them accordingly. Common reasons include:

-  The clients represent different pieces of topology, e.g. a cable
   modem is not the same as the clients behind that modem.

-  The clients have different behavior, e.g. a smartphone behaves
   differently from a laptop.

-  The clients require different values for some options, e.g. a
   docsis3.0 cable modem requires different settings from a docsis2.0
   cable modem.

To make management easier, different clients can be grouped into a
client class to receive common options.

An incoming packet can be associated with a client class in several
ways:

-  Implicitly, using a vendor class option or another built-in condition.

-  Using an expression which evaluates to ``true``.

-  Using static host reservations, a shared network, a subnet, etc.

-  Using a hook.

Client classification can be used to change the behavior of almost any
part of the DHCP message processing. There are currently nine
mechanisms that take advantage of client classification:

- Dropping queries.

- Subnet selection.

- Pool selection.

- Lease limiting.

- Rate limiting.

- DDNS tuning.

- Defining DHCPv4 private (codes 224-254) and code 43 options.

- Assigning different options.

- Setting specific options for use with the TFTP server address
  and the boot file field for DHCPv4 cable modems.

.. _classify-classification-steps:

Classification Steps
--------------------

The classification process is conducted in several steps:

1.  The ``ALL`` class is associated with the incoming packet.

2.  Vendor class options are processed.

3.  Classes with matching expressions and not marked for later evaluation ("on
    request" or depending on the ``KNOWN``/``UNKNOWN`` built-in classes)
    are processed in the order they are defined in the
    configuration; the boolean expression is evaluated and, if it
    returns ``true`` (a match), the incoming packet is associated with the
    class.

4.  If a private or code 43 DHCPv4 option is received, it is decoded
    following its client-class or global (or, for option 43,
    last-resort) definition.

5.  When the incoming packet belongs to the special class ``DROP``, it is
    dropped and an informational message is logged with the packet
    information.

.. note::

    The ``pkt4_receive`` and ``pkt6_receive`` callouts are called here.

6.  When the ``early-global-reservations-lookup`` global parameter is
    configured to ``true``, the process looks up global reservations and
    partially performs steps 8, 9, and 10. The lookup is limited to
    global reservations; if one is found the ``KNOWN`` class is set,
    but if none is found the ``UNKNOWN`` class is **not** set.

7.  A subnet is chosen, possibly based on the class information when
    some subnets are reserved. More precisely: when choosing a subnet,
    the server iterates over all of the subnets that are feasible given
    the information found in the packet (client address, relay address,
    etc.). It uses the first subnet it finds that either has no
    class associated with it, or has a class which matches one of the
    packet's classes.

.. note::

    The ``subnet4_select`` and ``subnet6_select`` callouts are called here.

8.  The server looks for host reservations. If an identifier from the
    incoming packet matches a host reservation in the subnet or shared
    network, the packet is associated with the ``KNOWN`` class and all
    classes of the host reservation. If a reservation is not found, the
    packet is assigned to the ``UNKNOWN`` class.

9.  Classes with matching expressions - directly, or indirectly using the
    ``KNOWN``/``UNKNOWN`` built-in classes and not marked for later evaluation
    ("on request") - are processed in the order they are defined
    in the configuration; the boolean expression is evaluated and, if it
    returns ``true`` (a match), the incoming packet is associated with the
    class. After a subnet is selected, the server determines whether
    there is a reservation for a given client. Therefore, it is not
    possible to use the ``UNKNOWN`` class to select a shared network or
    a subnet. For the ``KNOWN`` class, only global reservations are used and the
    ``early-global-reservations-lookup`` parameter must be configured to
    ``true``.

10. When the incoming packet belongs to the special class ``DROP``, it is
    dropped and an informational message is logged with the packet
    information. Since Kea version 1.9.8, it is permissible to make the ``DROP``
    class dependent on the ``KNOWN``/``UNKNOWN`` classes.

11. If needed, addresses and prefixes from pools are assigned, possibly
    based on the class information when some pools are reserved for
    class members.

.. note::

    The ``lease4_select``, ``lease4_renew``, ``lease6_select``, ``lease6_renew``,
    and ``lease6_rebind`` callouts are called here.

12. The ``evaluate-additional-classes`` lists (if any) are evaluated first
    for the pool, then the subnet, and the finally shared-network to which the
    assigned resources belong. Classes are evaluated in the order they appear
    within each list.

13. Options are assigned, again possibly based on the class information
    in the order that classes were associated with the incoming packet.
    The first class matched to the packet which specifies a given
    option wins. For DHCPv4 private and code 43 options, this includes
    option definitions specified within classes.  The order of precedence
    for options is as follows:

    1. Host options
    2. Pool options
    3. Subnet options
    4. Shared network options
    5. Options from classes assigned during normal classification:

        a. ``ALL`` class
        b. Vendor classes
        c. Host classes
        d. Evaluated classes that do not depend on ``KNOWN`` class
        e. ``KNOWN`` and ``UNKNOWN`` classes
        f. Evaluated classes that depend on ``KNOWN`` class

    6. Options from classes in ``evaluate-additional-classes`` from:

        a. Pool
        b. Subnet
        c. Shared network

    7. Global options

.. note::

   Care should be taken with client classification, as it is easy for
   clients that do not meet any class criteria to be denied service
   altogether.

.. _built-in-client-classes:

Built-in Client Classes
=======================

Some classes are built-in, so they do not need to be explicitly defined. They
can be defined if there is a need to associate lease lifetimes, option data,
etc. with them.

Vendor class information is the primary example: the server checks whether an
incoming DHCPv4 packet includes the vendor class identifier option (60)
or an incoming DHCPv6 packet includes the vendor class option (16). If
it does, the content of that option is prepended with ``VENDOR_CLASS_``
and the result is interpreted as a class for that packet. The content that is
considered is the whole class identifier for DHCPv4, and the first vendor class
data field for DHCPv6. The enterprise number and subsequent vendor class data
fields are not used for the purpose of classification. For example, modern cable
modems send such options with value ``docsis3.0``, so the packet belongs to
class ``VENDOR_CLASS_docsis3.0``.

The ``HA_`` prefix is used by :ischooklib:`libdhcp_ha.so` to
designate certain servers to process DHCP packets as a result of load
balancing. The class name is constructed by prepending the ``HA_`` prefix
to the name of the server which should process the DHCP packet. This
server uses an appropriate pool or subnet to allocate IP addresses
(and/or prefixes), based on the assigned client classes. The details can
be found in :ref:`hooks-high-availability`.

The ``SPAWN_`` prefix is used by template classes to generate spawned class
names at runtime. The spawned class name is constructed by prepending the
``SPAWN_`` prefix to the template class name and the evaluated value:
``SPAWN_<template-class-name>_<evaluated-value>``.
More details can be found in :ref:`classification-configuring`.

The ``BOOTP`` class is used by :ischooklib:`libdhcp_bootp.so` to classify and
respond to inbound BOOTP queries.

The ``SKIP_DDNS`` class is used by the DDNS-tuning hook library to suppress
DDNS updates on a per client basis.

Other examples are the ``ALL`` class, to which all incoming packets belong,
and the ``KNOWN`` class, assigned when host reservations exist for a
particular client. By convention, the names of built-in classes begin with all
capital letters.

Currently recognized built-in class names are ``ALL``, ``KNOWN`` and ``UNKNOWN``,
and the prefixes ``VENDOR_CLASS_``, ``HA_``, ``AFTER_``, ``EXTERNAL_``,
``SKIP_DDNS``. Although the ``AFTER_`` prefix is a provision for an
as-yet-unwritten hook, the ``EXTERNAL_`` prefix can be freely used; built-in
classes are implicitly defined so they never raise warnings if they do not
appear in the configuration.

.. _classification-using-expressions:

Using Expressions in Classification
===================================

The expression portion of a classification definition contains operators
and values. All values are currently strings; operators take a string or
strings and return another string. When all the operations have
completed, the result should be a value of ``true`` or ``false``. The packet
belongs to the class (and the class name is added to the list of
classes) if the result is ``true``. Expressions are written in standard
format and can be nested.

Expressions are pre-processed during the parsing of the configuration
file and converted to an internal representation. This allows certain
types of errors to be caught and logged during parsing. Examples of
these errors include an incorrect number or type of argument to an
operator. The evaluation code also checks for this class of error and
generally throws an exception, though this should not occur in a
normally functioning system.

Other issues, such as the starting position of a substring being
outside of the substring or an option not existing in the packet, result
in the operator returning an empty string.

Dependencies between classes are also checked. For instance, forward
dependencies are rejected when the configuration is parsed; an
expression can only depend on already-defined classes (including built-in
classes) which are evaluated in a previous or the same evaluation phase.
This does not apply to the ``KNOWN`` or ``UNKNOWN`` classes.

.. table:: List of classification values

   +-----------------------+-------------------------------+-----------------------+
   | Name                  | Example expression            | Example value         |
   +=======================+===============================+=======================+
   | String literal        | 'example'                     | 'example'             |
   +-----------------------+-------------------------------+-----------------------+
   | Hexadecimal string    | 0x5a7d                        | 'Z}'                  |
   | literal               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | IP address literal    | 10.0.0.1                      | 0x0a000001            |
   +-----------------------+-------------------------------+-----------------------+
   | Integer literal       | 123                           | '123'                 |
   +-----------------------+-------------------------------+-----------------------+
   | Binary content of the | option[123].hex               | '(content of the      |
   | option                |                               | option)'              |
   +-----------------------+-------------------------------+-----------------------+
   | Option existence      | option[123].exists            | 'true'                |
   +-----------------------+-------------------------------+-----------------------+
   | Binary content of the | option[12].option[34].hex     | '(content of the      |
   | sub-option            |                               | sub-option)'          |
   +-----------------------+-------------------------------+-----------------------+
   | Sub-Option existence  | option[12].option[34].exists  | 'true'                |
   +-----------------------+-------------------------------+-----------------------+
   | Client class          | member('foobar')              | 'true'                |
   | membership            |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Known client          | known                         | member('KNOWN')       |
   +-----------------------+-------------------------------+-----------------------+
   | Unknown client        | unknown                       | not member('KNOWN')   |
   +-----------------------+-------------------------------+-----------------------+
   | DHCPv4 relay agent    | relay4[123].hex               | '(content of the RAI  |
   | sub-option            |                               | sub-option)'          |
   +-----------------------+-------------------------------+-----------------------+
   | DHCPv6 Relay Options  | relay6[nest].option[code].hex | (value of the option) |
   +-----------------------+-------------------------------+-----------------------+
   | DHCPv6 Relay Peer     | relay6[nest].peeraddr         | 2001:DB8::1           |
   | Address               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | DHCPv6 Relay Link     | relay6[nest].linkaddr         | 2001:DB8::1           |
   | Address               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Interface name of     | pkt.iface                     | eth0                  |
   | packet                |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Source address of     | pkt.src                       | 10.1.2.3              |
   | packet                |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Destination address   | pkt.dst                       | 10.1.2.3              |
   | of packet             |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Length of packet      | pkt.len                       | 513                   |
   +-----------------------+-------------------------------+-----------------------+
   | Hardware address in   | pkt4.mac                      | 0x010203040506        |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Hardware length in    | pkt4.hlen                     | 6                     |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Hardware type in      | pkt4.htype                    | 6                     |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | ciaddr field in       | pkt4.ciaddr                   | 192.0.2.1             |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | giaddr field in       | pkt4.giaddr                   | 192.0.2.1             |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | yiaddr field in       | pkt4.yiaddr                   | 192.0.2.1             |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | siaddr field in       | pkt4.siaddr                   | 192.0.2.1             |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Message type in       | pkt4.msgtype                  | 1                     |
   | DHCPv4 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Transaction ID (xid)  | pkt4.transid                  | 12345                 |
   | in DHCPv4 packet      |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Message type in       | pkt6.msgtype                  | 1                     |
   | DHCPv6 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Transaction ID in     | pkt6.transid                  | 12345                 |
   | DHCPv6 packet         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Vendor option         | vendor[*].exists              | 'true'                |
   | existence (any        |                               |                       |
   | vendor)               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Vendor option         | vendor[4491].exists           | 'true'                |
   | existence (specific   |                               |                       |
   | vendor)               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Enterprise-id from    | vendor.enterprise             | 4491                  |
   | vendor option         |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Vendor sub-option     | vendor[4491].option[1].exists | 'true'                |
   | existence             |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Vendor sub-option     | vendor[4491].option[1].hex    | docsis3.0             |
   | content               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Vendor class option   | vendor-class[*].exists        | 'true'                |
   | existence (any        |                               |                       |
   | vendor)               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Vendor class option   | vendor-class[4491].exists     | 'true'                |
   | existence (specific   |                               |                       |
   | vendor)               |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Enterprise-id from    | vendor-class.enterprise       | 4491                  |
   | vendor class option   |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | First data chunk from | vendor-class[4491].data       | docsis3.0             |
   | vendor class option   |                               |                       |
   +-----------------------+-------------------------------+-----------------------+
   | Specific data chunk   | vendor-class[4491].data[3]    | docsis3.0             |
   | from vendor class     |                               |                       |
   | option                |                               |                       |
   +-----------------------+-------------------------------+-----------------------+

Notes:

-  Hexadecimal strings are converted into a string as expected. The
   starting ``0X`` or ``0x`` is removed, and if the string is an odd number
   of characters a "0" is prepended to it.

-  IP addresses are converted into strings of length 4 or 16. IPv4,
   IPv6, and IPv4-embedded IPv6 (e.g. IPv4-mapped IPv6) addresses are
   supported.

-  Integers in an expression are converted to 32-bit unsigned integers
   and are represented as four-byte strings; for example, 123 is
   represented as ``0x0000007b``. All expressions that return numeric values
   use 32-bit unsigned integers, even if the field in the packet is
   smaller. In general, it is easier to use decimal notation to
   represent integers, but it is also possible to use hexadecimal
   notation. When writing an integer in hexadecimal, care should be
   taken to make sure the value is represented as 32 bits, e.g. use
   ``0x00000001`` instead of ``0x1`` or ``0x01``. Also, make sure the value is
   specified in network order, e.g. 1 is represented as ``0x00000001``.

-  ``option[code].hex`` extracts the value of the option with the code
   ``code`` from the incoming packet. If the packet does not contain the
   option, it returns an empty string. The string is presented as a byte
   string of the option payload, without the type code or length fields.

-  ``option[code].exists`` checks whether an option with the code ``code``
   is present in the incoming packet. It can be used with empty options.

-  ``member('foobar')`` checks whether the packet belongs to the client
   class ``foobar``. To avoid dependency loops, the configuration file
   parser verifies whether client classes were already defined or are
   built-in, i.e., beginning with ``VENDOR_CLASS_``, ``AFTER_`` (for the
   to-come "after" hook) and ``EXTERNAL_`` or equal to ``ALL``, ``KNOWN``,
   ``UNKNOWN``, etc.

   ``known`` and ``unknown`` are shorthand for ``member('KNOWN')`` and ``not
   member('KNOWN')``. Note that the evaluation of any expression using
   the ``KNOWN`` class (directly or indirectly) is deferred after the host
   reservation lookup (i.e. when the ``KNOWN`` or ``UNKNOWN`` partition is
   determined).

-  ``relay4[code].hex`` attempts to extract the value of the sub-option
   ``code`` from the option inserted as the DHCPv4 Relay Agent Information
   (82) option. If the packet does not contain a RAI option, or the RAI
   option does not contain the requested sub-option, the expression
   returns an empty string. The string is presented as a byte string of
   the option payload without the type code or length fields. This
   expression is allowed in DHCPv4 only.

-  ``relay4`` shares the same representation types as ``option``; for
   instance, ``relay4[code].exists`` is supported.

-  ``relay6[nest]`` allows access to the encapsulations used by any DHCPv6
   relays that forwarded the packet. The ``nest`` level specifies the
   relay from which to extract the information, with a value of 0
   indicating the relay closest to the DHCPv6 server. Negative values
   allow relays to be specified counting from the DHCPv6 client, with -1 indicating
   the relay closest to the client. If the requested
   encapsulation does not exist, an empty string ``""`` is returned. This
   expression is allowed in DHCPv6 only.

-  ``relay6[nest].option[code]`` shares the same representation types as
   ``option``; for instance, ``relay6[nest].option[code].exists`` is
   supported.

-  Expressions starting with ``pkt4`` can be used only in DHCPv4. They
   allow access to DHCPv4 message fields.

-  ``pkt6`` refers to information from the client request. To access any
   information from an intermediate relay, use ``relay6``. ``pkt6.msgtype``
   and ``pkt6.transid`` output a 4-byte binary string for the message type
   or transaction ID. For example, the message type ``SOLICIT`` is
   ``0x00000001`` or simply 1, as in ``pkt6.msgtype == 1``.

-  "Vendor option" means the Vendor-Identifying Vendor-Specific Information
   option in DHCPv4 (code 125; see `Section 4 of RFC
   3925 <https://tools.ietf.org/html/rfc3925#section-4>`__) and the
   Vendor-Specific Information Option in DHCPv6 (code 17, defined in
   `Section 21.17 of RFC
   8415 <https://tools.ietf.org/html/rfc8415#section-21.17>`__). "Vendor
   class option" means the Vendor-Identifying Vendor Class Option in DHCPv4
   (code 124; see `Section 3 of RFC
   3925 <https://tools.ietf.org/html/rfc3925#section-3>`__) in DHCPv4 and
   the Class Option in DHCPv6 (code 16; see `Section 21.16 of RFC
   8415 <https://tools.ietf.org/html/rfc8415#section-21.16>`__). Vendor
   options may have sub-options that are referenced by their codes.
   Vendor class options do not have sub-options, but rather data chunks,
   which are referenced by index value. Index 0 means the first data
   chunk, index 1 is for the second data chunk (if present), etc.

-  In the vendor and vendor-class constructs an asterisk (*) or 0 can be
   used to specify a wildcard ``enterprise-id`` value, i.e. it will match
   any ``enterprise-id`` value.

-  Vendor Class Identifier (option 60 in DHCPv4) can be accessed using the
   option[60] expression.

-  `RFC 3925 <https://tools.ietf.org/html/rfc3925>`__ and `RFC
   8415 <https://tools.ietf.org/html/rfc8415>`__ allow for multiple
   instances of vendor options to appear in a single message. The client
   classification code currently examines the first instance if more
   than one appear. For the ``vendor.enterprise`` and ``vendor-class.enterprise``
   expressions, the value from the first instance is returned. Please
   submit a feature request on the
   `Kea GitLab site <https://gitlab.isc.org/isc-projects/kea>`__ to request
   support for multiple instances.

.. table:: List of classification expressions

   +-----------------------+-------------------------+-----------------------+
   | Name                  | Example                 | Description           |
   +=======================+=========================+=======================+
   | Equal                 | 'foo' == 'bar'          | Compare the two       |
   |                       |                         | values and return     |
   |                       |                         | ``true`` or ``false`` |
   +-----------------------+-------------------------+-----------------------+
   | Match                 | match('foo.*', 'foobar')| Match a regular       |
   |                       |                         | expression with a     |
   |                       |                         | value.                |
   +-----------------------+-------------------------+-----------------------+
   | Not                   | not ('foo' == 'bar')    | Logical negation      |
   +-----------------------+-------------------------+-----------------------+
   | And                   | ('foo' == 'bar') and    | Logical and           |
   |                       | ('bar' == 'foo')        |                       |
   +-----------------------+-------------------------+-----------------------+
   | Or                    | ('foo' == 'bar') or     | Logical or            |
   |                       | ('bar' == 'foo')        |                       |
   +-----------------------+-------------------------+-----------------------+
   | Substring             | substring('foobar',0,3) | Return the requested  |
   |                       |                         | substring             |
   +-----------------------+-------------------------+-----------------------+
   | Concat                | concat('foo','bar')     | Return the            |
   |                       |                         | concatenation of the  |
   |                       |                         | strings               |
   +-----------------------+-------------------------+-----------------------+
   | Concat (operator +)   | 'foo' + 'bar'           | Return the            |
   |                       |                         | concatenation of the  |
   |                       |                         | strings               |
   +-----------------------+-------------------------+-----------------------+
   | Ifelse                | ifelse('foo' ==         | Return the branch     |
   |                       | 'bar','us','them')      | value according to    |
   |                       |                         | the condition         |
   +-----------------------+-------------------------+-----------------------+
   | Hexstring             | hexstring('foo', '-')   | Converts the value to |
   |                       |                         | a hexadecimal string, |
   |                       |                         | e.g. 66-6F-6F         |
   +-----------------------+-------------------------+-----------------------+
   | Lcase                 | lcase('LoWeR')          | Converts the value of |
   |                       |                         | a string expression   |
   |                       |                         | to lower case, e.g.   |
   |                       |                         | 'lower'               |
   +-----------------------+-------------------------+-----------------------+
   | Ucase                 | ucase('uPpEr')          | Converts the value of |
   |                       |                         | a string expression   |
   |                       |                         | to upper case, e.g.   |
   |                       |                         | 'UPPER'               |
   +-----------------------+-------------------------+-----------------------+
   | Split                 | split('foo.bar', '.', 2)| Return the second     |
   |                       |                         | field, splitting on   |
   |                       |                         | dots.                 |
   +-----------------------+-------------------------+-----------------------+

.. table:: List of conversion-to-text expressions

   +-----------------------+---------------------------+------------------------+
   | Name                  | Example                   | Description            |
   +=======================+===========================+========================+
   | AddressToText         | addrtotext (192.10.0.1)   | Represent the 4 bytes  |
   |                       | addrtotext (2003:db8::)   | of an IPv4 address or  |
   |                       |                           | the 16 bytes of an     |
   |                       |                           | IPv6 address in human  |
   |                       |                           | readable format        |
   +-----------------------+---------------------------+------------------------+
   | Int8ToText            | int8totext (-1)           | Represents the 8-bit   |
   |                       |                           | signed integer in text |
   |                       |                           | format                 |
   +-----------------------+---------------------------+------------------------+
   | Int16ToText           | int16totext (-1)          | Represents the 16-bit  |
   |                       |                           | signed integer in text |
   |                       |                           | format                 |
   +-----------------------+---------------------------+------------------------+
   | Int32ToText           | int32totext (-1)          | Represents the 32-bit  |
   |                       |                           | signed integer in text |
   |                       |                           | format                 |
   +-----------------------+---------------------------+------------------------+
   | UInt8ToText           | uint8totext (255)         | Represents the 8-bit   |
   |                       |                           | unsigned integer in    |
   |                       |                           | text format            |
   +-----------------------+---------------------------+------------------------+
   | UInt16ToText          | uint16totext (65535)      | Represents the 16-bit  |
   |                       |                           | unsigned integer in    |
   |                       |                           | text format            |
   +-----------------------+---------------------------+------------------------+
   | UInt32ToText          | uint32totext (4294967295) | Represents the 32-bit  |
   |                       |                           | unsigned integer in    |
   |                       |                           | text format            |
   +-----------------------+---------------------------+------------------------+

Notes:

The conversion operators can be used to transform data from binary to the text
representation. The only requirement is that the input data type length matches
an expected value.

The ``AddressToText`` token expects 4 bytes for IPv4 addresses or 16 bytes for IPv6
addresses. The ``Int8ToText`` and ``UInt8ToText`` tokens expect 1 byte, the ``Int16ToText`` and
``UInt16ToText`` tokens expect 2 bytes, and ``Int32ToText`` and ``UInt32ToText`` expect 4 bytes.
For all conversion tokens, if the data length is 0, the result string is empty.

Predicates
----------

The two predicates are Equal and Match. They can be used to build other
common predicates, for instance:
::

           not (substring('foobar', 3, 3) == 'bar')
           match('foo.*', lcase('FooBar'))
           match('.*foo.*', 'is it foo or bar')
           match('^.*foo.*$', 'is it foo or bar')

So inequality, case insensitive pattern matching or pattern search.

.. note::

   Detected invalid regular expressions are considered as syntax errors,
   runtime exceptions during match are handled as no match.

.. warning::

   Be careful with the match operator as it can result in very poor
   performance leading to regular expression denial of service (ReDoS).


Logical Operators
-----------------

The Not, And, and Or logical operators are the common operators. Not has
the highest precedence and Or the lowest. And and Or are (left)
associative. Parentheses around a logical expression can be used to
enforce a specific grouping; for instance, in "A and (B or C)". Without
parentheses, "A and B or C" means "(A and B) or C".

Substring
---------

The substring operator ``substring(value, start, length)`` accepts both
positive and negative values for the starting position and the length.
For ``start``, a value of 0 is the first byte in the string while -1 is
the last byte. If the starting point is outside of the original string
an empty string is returned. ``length`` is the number of bytes to extract.
A negative number means to count towards the beginning of the string but
does not include the byte pointed to by ``start``. The special value ``all``
means to return all bytes from start to the end of the string. If the length
is longer than the remaining portion of the string, then the entire
remaining portion is returned. Some examples may be helpful:
::

           substring('foobar', 0, 6) == 'foobar'
           substring('foobar', 3, 3) == 'bar'
           substring('foobar', 3, all) == 'bar'
           substring('foobar', 1, 4) == 'ooba'
           substring('foobar', -5, 4) == 'ooba'
           substring('foobar', -1, -3) == 'oba'
           substring('foobar', 4, -2) == 'ob'
           substring('foobar', 10, 2) == ''


Concat
------

The concat function ``concat(string1, string2)`` returns the concatenation
of its two arguments. For instance:
::

           concat('foo', 'bar') == 'foobar'

For user convenience, Kea version 1.9.8 added an associative operator
version of the concat function. For instance:
::

           'abc' + 'def' + 'ghi' + 'jkl' + '...'

is the same as:
::

           concat(concat(concat(concat('abc', 'def'), 'ghi'), 'jkl'), '...')

or:
::

           concat('abc', concat('def', concat('ghi', concat('jkl', '...'))))

or:
::

           'abcdefghijkl...'

Split
---------

The split operator ``split(value, delimiters, field-number)`` accepts a list
of characters to use as delimiters and a positive field number of the
desired field when the value is split into fields separated by the delimiters.
Adjacent delimiters are not compressed out, rather they result in an empty
string for that field number. If value is an empty string, the result will be an
empty string. If the delimiters list is empty, the result will be the original
value. If the field-number is less than one or larger than the number of
fields, the result will be an empty string. Some examples follow:
::

           split ('one.two..four', '.', 1) == 'one'
           split ('one.two..four', '.', 2) == 'two'
           split ('one.two..four', '.', 3) == ''
           split ('one.two..four', '.', 4) == 'four'
           split ('one.two..four', '.', 5) == ''

.. note::

   To use a hard-to-escape character as a delimiter, use its ASCII hex value.
   For example, split by ``single quote`` using ``0x27``:
   ``split(option[39].text, 0x27, 1)``

Ifelse
------

The ifelse function ``ifelse(cond, iftrue, ifelse)`` returns the ``iftrue``
or ``ifelse`` branch value following the boolean condition ``cond``. For
instance:
::

            ifelse(option[230].exists, option[230].hex, 'none')


Hexstring
---------

The hexstring function ``hexstring(binary, separator)`` returns the binary
value as its hexadecimal string representation: pairs of hexadecimal
digits separated by the separator, e.g ``':'``, ``'-'``, ``''`` (empty separator).
::

             hexstring(pkt4.mac, ':')


.. note::

   The expression for each class is executed on each packet received. If
   the expressions are overly complex, the time taken to execute them
   may impact the performance of the server. Administrators who need complex or
   time-consuming expressions should consider writing a
   :ref:`hook <hooks-libraries>` to perform the necessary work.

.. _classification-configuring:

Configuring Classes
===================

A client class definition can contain the following properties:
 - The ``name`` parameter is mandatory and must be unique among all classes.
 - The ``test`` expression is not mandatory and represents a string containing the
   logical expression used to determine membership in the class. The entire
   expression is included in double quotes (``"``). The result should evaluate
   to a boolean value (``true`` or ``false``).
 - The ``template-test`` expression is not mandatory and represents a string
   containing the logical expression used to generate a spawning class. The
   entire expression is included in double quotes (``"``). The result should
   evaluate to a string value representing the variable part of the spawned
   class name. If the resulting string is empty, no spawning class is generated.
   The resulting spawned class has the following generated name format:
   ``SPAWN_<template-class-name>_<evaluated-value>``.
   After classes are evaluated and a spawned class is generated, the corresponding
   template class name is also associated with the packet. An Option specified in a
   spawned class will take precedence over the same option if set in its template class.
 - The ``option-data`` list is not mandatory and contains options that should be
   assigned to members of this class. In the case of a template class, these
   options are assigned to the generated spawned class.
 - The ``option-def`` list is not mandatory and is used to define custom options.
 - The ``only-if-required`` has been replaced with ``only-in-additional-list`` and
   is now deprecated. It will still be accepted as input for a time to allow users
   to migrate but will eventually be unsupported.
 - The ``only-in-additional-list`` flag is not mandatory; when its value is set to
   ``false`` (the default), membership is determined during classification and is
   available for subnet selection, for instance. When the value is set to
   ``true``, membership is evaluated only if the class appears in an
   ``evaluate-additional-classes`` list and is usable only for option configuration.
 - The ``user-context`` is not mandatory and represents a map with user-defined data
   and possibly configuration options for hook libraries.
 - The ``next-server`` parameter is not mandatory and configures the ``siaddr`` field in
   packets associated with this class. It is used in DHCPv4 only.
 - The ``server-hostname`` is not mandatory and configures the ``sname`` field in
   packets associated with this class. It is used in DHCPv4 only.
 - The ``boot-file-name`` is not mandatory and configures the ``file`` field in
   packets associated with this class. It is used in DHCPv4 only.
 - The ``valid-lifetime``, ``min-valid-lifetime``, and ``max-valid-lifetime`` are
   not mandatory and configure the valid lifetime fields for this client class.
 - The ``preferred-lifetime``, ``min-preferred-lifetime``, and
   ``max-preferred-lifetime`` are not mandatory and configure the preferred
   lifetime fields for this client class. It is used in DHCPv6 only.


.. note::

   ``test`` and ``template-test`` are mutually exclusive in a client class
   definition. Use either one, or neither, but not both. If both are provided,
   the configuration is rejected.

In the following example, the class named ``Client_foo`` is defined. It is
comprised of all clients whose client IDs (option 61) start with the string
``foo``. Members of this class will be given 192.0.2.1 and 192.0.2.2 as their
domain name servers.

::

   "Dhcp4": {
       "client-classes": [
           {
               "name": "Client_foo",
               "test": "substring(option[61].hex,0,3) == 'foo'",
               "option-data": [
                   {
                       "name": "domain-name-servers",
                       "code": 6,
                       "space": "dhcp4",
                       "csv-format": true,
                       "data": "192.0.2.1, 192.0.2.2"
                   }
               ]
           },
           ...
       ],
       ...
   }

The next example shows a client class named "Client_enterprise" being defined
for use by the DHCPv6 server. It is
comprised of all clients whose client identifiers start with the given
hex string, which would indicate a DUID based on an enterprise ID of
``0x0002AABBCCDD``. Members of this class will be given 2001:db8:0::1 and
2001:db8:2::1 as their domain name servers.

::

   "Dhcp6": {
       "client-classes": [
           {
               "name": "Client_enterprise",
               "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD",
               "option-data": [
                   {
                       "name": "dns-servers",
                       "code": 23,
                       "space": "dhcp6",
                       "csv-format": true,
                       "data": "2001:db8:0::1, 2001:db8:2::1"
                   }
               ]
           },
           ...
       ],
       ...
   }

It is also possible to have both left and right operands of the evaluated
expression processed at runtime. Expressions related to packets can appear in
the expression as many times as needed; there is no limit. However, each token
has a small impact on performance and excessively complex expressions may cause a
bottleneck.

::

   "Dhcp4": {
       "client-classes": [
           {
               "name": "Infrastructure",
               "test": "option[82].option[2].hex == pkt4.mac",
               ...
           },
           ...
       ],
       ...
   }

.. _template-classes:

Template Classes
----------------

The ``template-test`` parameter indicates that the class is a template class.

::

   "Dhcp4": {
       "client-classes": [
           {
               "name": "Client-ID",
               "template-test": "substring(option[61].hex,0,3)",
               ...
           },
           ...
       ],
       ...
   }

If the received DHCPv4 packet contains option 61, then the first three bytes represent
the value ``foo`` in ASCII, and the spawned class uses the
``SPAWN_Client-ID_foo`` name.
Both the ``SPAWN_Client-ID_foo`` and ``Client-ID`` classes are associated with
the packet.

.. note ::

   Template classes can also be used to spawn classes which match regular
   classes, effectively associating the regular class to the packet.
   To achieve this, the regular class must also contain the fixed part of the
   spawned class name:

   ``SPAWN_<template-class-name-used-to-activate-this-regular-class>_<evaluated-value-filtering-this-regular-class>``

::

   "Dhcp6": {
       "client-classes": [
           {
               "name": "SPAWN_Client-ID_foobar",
               "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD",
               ...
           },
           {
               "name": "Client-ID",
               "template-test": "substring(option[1].hex,0,6)",
               ...
           },
           ...
       ],
       ...
   }

If the received DHCPv6 packet contains option 1 (client identifier) with hex
value ``0x0002AABBCCDD``, then the ``SPAWN_Client-ID_foobar`` is associated
with the packet. Moreover, if the first six bytes represent value ``foobar`` in
ASCII, then the spawned class uses the ``SPAWN_Client-ID_foobar`` name,
effectively associating the regular class to the packet. In this second case,
both the ``SPAWN_Client-ID_foobar`` and ``Client-ID`` classes are associated
with the packet.
The ``test`` expression on the regular class ``SPAWN_Client-ID_foobar`` is not
mandatory and can be omitted, but it is used here with a different match
expression for example purposes.

Usually the ``test`` and ``template-test`` expressions are evaluated before
subnet selection, but in some cases it is useful to evaluate it later when the
subnet, shared network, or pools are known but output-option processing has not
yet been done. For this purpose, the ``only-in-additional-list`` flag, which is
``false`` by default, allows the evaluation of the ``test`` expression or the
``template-test`` expression only when it is required by the class's presence
in the ``evaluate-additional-classes`` list of the selected pool, subnet, or
shared network.

The ``evaluate-additional-classes`` list, which is valid for pool, subnet,
and shared-network scope, specifies the classes which are evaluated in
the second pass before output-option processing. The list is built in
same precedence order as the option data, i.e. an option data item in
a subnet takes precedence over one in a shared network. An
additional class in a subnet is added before one in a shared
network. The mechanism is related to the ``only-in-additional-list`` flag but
it is not mandatory that the flag be set to ``true``.

.. note ::

   The ``template-test`` expression can also be used to filter generated spawned
   classes, so that they are created only when needed by using the ``ifelse``
   instruction.

::

   "Dhcp4": {
       "client-classes": [
           {
               "name": "Client-ID",
               "template-test": "ifelse(substring(option[61].hex,4,3) == 'foo', substring(option[12].hex,0,12), '')",
               ...
           },
           ...
       ],
       ...
   }

.. note ::

   The template classes can be used to configure limits which, just like
   options, are associated with the spawned class. This permits the configuration of
   limits that apply to all packets associated with a class spawned at
   runtime, according to the ``template-test`` expression in the parent template
   class. For a more detailed description of how to configure limits using the
   limits hook library, see :ref:`hooks-limits-configuration`.
   For example, using the configuration below, ingress DHCPv6 packets that have
   client ID values (in the format expressed by the Kea evaluator) ``foobar``
   and ``foofoo`` both amount to the same limit of 60 packets per day, while
   other packets that have the first three hextets different than ``foo`` are put
   in separate rate-limiting buckets.

::

   "Dhcp6": {
       "client-classes": [
           {
               "name": "Client-ID",
               "template-test": "substring(option[1].hex,0,3)",
               "user-context" : {
                   "limits": {
                       "rate-limit": "60 packets per day"
                   }
               },
               ...
           },
           ...
       ],
       ...
   }


.. _classification-using-host-reservations:

Using Static Host Reservations in Classification
================================================

Classes can be statically assigned to clients using techniques
described in :ref:`reservation4-client-classes` and
:ref:`reservation6-client-classes`.

Subnet host reservations are searched after subnet selection.
Global host reservations are searched at the same time by default but
the ``early-global-reservations-lookup`` allows to change this behavior
into searching them before the subnet selection.

Pool selection is performed after all host reservations lookups.

.. _classification-subnets:

Configuring Subnets With Class Information
==========================================

.. note:

    As of Kea 2.7.5, ``client-class`` (a single class name) has been replaced
    with ``client-classes`` (a list of one or more class names) and is now
    deprecated. It will still be accepted as input for a time to allow users
    to migrate but will eventually be rejected.

In certain cases it is beneficial to restrict access to certain subnets
only to clients that belong to a given class, using the ``client-classes``
parameter when defining the subnet.  This parameter may be used to specify
a list of one or more classes to which clients must belong in order to
use the subnet. This can be referred to as a class guard for the subnet, or in
other words the subnet is guarded by a client class.

Let's assume that the server is connected to a network segment that uses the
192.0.2.0/24 prefix. The administrator of that network has decided that
addresses from the range 192.0.2.10 to 192.0.2.20 will be managed by the DHCPv4
server. Only clients belonging to client class ``Client_foo`` are allowed to use
this subnet. Such a configuration can be achieved in the following way:

::

   "Dhcp4": {
       "client-classes": [
           {
               "name": "Client_foo",
               "test": "substring(option[61].hex,0,3) == 'foo'",
               "option-data": [
                   {
                       "name": "domain-name-servers",
                       "code": 6,
                       "space": "dhcp4",
                       "csv-format": true,
                       "data": "192.0.2.1, 192.0.2.2"
                   }
               ]
           },
           ...
       ],
       "subnet4": [
           {
               "id": 1,
               "subnet": "192.0.2.0/24",
               "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
               "client-classes": [ "Client_foo" ]
           },
           ...
       ],
       ...
   }

The following example shows how to restrict access to a DHCPv6 subnet. This
configuration restricts use of the addresses in the range 2001:db8:1::1 to
2001:db8:1::FFFF to members of the "Client_enterprise" class.

::

   "Dhcp6": {
       "client-classes": [
           {
               "name": "Client_enterprise",
               "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD",
               "option-data": [
                   {
                       "name": "dns-servers",
                       "code": 23,
                       "space": "dhcp6",
                       "csv-format": true,
                       "data": "2001:db8:0::1, 2001:db8:2::1"
                   }
               ]
           },
           ...
       ],
       "subnet6": [
           {
               "id": 1,
               "subnet": "2001:db8:1::/64",
               "pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
               "client-classes": "Client_enterprise"
           }
       ],
       ...
   }

.. _classification-pools:

Configuring Pools With Class Information
========================================

.. note:

    As of Kea 2.7.5, ``client-class`` (a single class name) has been replaced
    with ``client-classes`` (a list of one or more class names) and is now
    deprecated. It will still be accepted as input for a time to allow users
    to migrate but will eventually be unsupported.

Similar to subnets, in certain cases access to certain address or prefix
pools must be restricted to only clients that belong to at least one of a
list of one or more classes, using the ``client-classes`` when defining
the pool. This can be referred to as a class guard for the pool, or in other
words the pool is guarded by a client class.

Let's assume that the server is connected to a network segment that uses the
192.0.2.0/24 prefix. The administrator of that network has decided that
addresses from the range 192.0.2.10 to 192.0.2.20 are going to be managed by the
DHCPv4 server. Only clients belonging to client class ``Client_foo`` are allowed
to use this pool. Such a configuration can be achieved in the following way:

::

   "Dhcp4": {
       "client-classes": [
           {
               "name": "Client_foo",
               "test": "substring(option[61].hex,0,3) == 'foo'",
               "option-data": [
                   {
                       "name": "domain-name-servers",
                       "code": 6,
                       "space": "dhcp4",
                       "csv-format": true,
                       "data": "192.0.2.1, 192.0.2.2"
                   }
               ]
           },
           ...
       ],
       "subnet4": [
           {
               "id": 1,
               "subnet": "192.0.2.0/24",
               "pools": [
                   {
                       "pool": "192.0.2.10 - 192.0.2.20",
                       "client-classes": [ "Client_foo" ]
                   }
               ]
           },
           ...
       ],
       ...
   }

The following example shows how to restrict access to an address pool. This
configuration restricts use of the addresses in the range 2001:db8:1::1 to
2001:db8:1::FFFF to members of the "Client_enterprise" class.

::

   "Dhcp6": {
       "client-classes": [
           {
               "name": "Client_enterprise_",
               "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD",
               "option-data": [
                   {
                       "name": "dns-servers",
                       "code": 23,
                       "space": "dhcp6",
                       "csv-format": true,
                       "data": "2001:db8:0::1, 2001:db8:2::1"
                   }
               ]
           },
           ...
       ],
       "subnet6": [
           {
               "id": 1,

               "subnet": "2001:db8:1::/64",

               "pools": [
                   {
                       "pool": "2001:db8:1::-2001:db8:1::ffff",
                       "client-classes": [ "Client_foo" ]
                   }
               ]
           },
           ...
       ],
       ...
   }

Class Priority
==============

Client classes in Kea follow the order in which they are specified in the
configuration (vs. alphabetical order). Additional classes are ordered by
pool, subnet, and then shared-network and within each scope by the order in
which they appear in ``evaluate-additional-classes``.

When determining which client-class information (comprised of
options, lease lifetimes, or DHCPv4 field values) is part of the class
definitions to be included in the response, the server examines the union of
options from all of the assigned classes. If two or more classes include the
same class information, the value from the first assigned class is used.
``ALL`` is always the first class, i.e. the class with the highest
priority, and matching required classes are last, so they have the
lowest priority.

Options defined in classes override any global options, and in turn are
overridden by options defined for an individual subnet, shared network, pool, or
reservation.

On the other hand, lease lifetimes and DHCPv4 field values defined at class
scope override any values defined globally, in a subnet scope, or in a
shared-network scope.

.. note::
   Because additional evaluation occurs after lease assignment, parameters
   that would otherwise impact lease life times (e.g. ``valid-lifetime``,
   ``offer-lifetime``) will have no effect when specified in a class that
   also sets ``only-in-additional-list`` true.

As an example, imagine that an incoming packet matches two classes.
Class ``foo`` defines values for an NTP server (option 42 in DHCPv4) and
an SMTP server (option 69 in DHCPv4), while class ``bar`` defines values
for an NTP server and a POP3 server (option 70 in DHCPv4). The server
examines the three options - NTP, SMTP, and POP3 - and returns any that
the client requested. As the NTP server was defined twice, the server
chooses only one of the values for the reply; the class from which the
value is obtained is determined as explained in the previous paragraphs.

.. _option-class-tagging:

Option Class-Tagging
====================

Option class-tagging allows an option value to be conditionally applied
to the response based on the client's class membership.  The effect is
similar to using an if-block in ISC DHCP to conditionally include
options at a given scope.  Class-tagging is done by specifying a list of
one of more class names in the option's ``client-classes`` entry.

Consider a case where members of a given class need the same value for
one option but a subnet-specific value for another option.  The following
example shows class-tagging used to give clients who belong to "GROUP1"
a subnet-specific value for option "bar", while giving all members of "GROUP1"
the same value for option "foo":

.. code-block:: javascript

    {
        "client-classes": [
        {
            "name": "GROUP1",
            "test":"substring(option[123].hex,0,4) == 'ONE'",
            "option-data": [{
                "name": "foo",
                "data": "somefile.txt"
            }]
        }],
        "subnet4": [
        {
            "id": 1,
            "subnet": "178.16.1.0/24",
            "option-data": [{
                "client-classes": ["GROUP1"],
                "name": "bar",
                "data": 123
            }]
        },
        {
            "id": 2,
            "subnet": "178.16.2.0/24",
            "option-data": [
            {
                "client-classes": ["GROUP1"],
                "name": "bar",
                "data": 789
            }]
        }]
    }

The ``client-classes`` list is allowed in an option specification at
any scope.  Option class-tagging is enforced at the time options are
being added to the response which occurs after lease assignment just
before the response is to be sent to the client.

When ``never-send`` for an option is true at any scope, all
``client-classes`` entries for that option are ignored. The
option will not be included.

When ``always-send`` is true at any scope, the option will be
included unless, the option determined by scope specifies
a ``client-classes`` list that does not contain any of the
client's classes.

Otherwise, an option requested by the client will be included in
the response if  the option either does not specify ``client-classes``
or the client belongs to at least one of the classes in ``client-classes``.

When an option's class-tag does not match, it is as though
the option was not specified at that scope.  In the following
example the option "foo" is specified for both the subnet and
the pool.  The pool specification includes a class-tag that limits
the option to members of class "melon":

.. code-block:: javascript

    {
        "id": 100,
        "subnet": "178.16.1.0/24",
        "option-data": [{
            "name": "foo",
            "data": 456
        }],
        "pools": [{
            "pool": "178.16.1.100 - 178.16.1.200",
            "option-data": [{
                "name": "foo",
                "data": 123,
                "client-classes" : [ "melon" ]
            }]
        }]
    }

Clients that match class "melon" will have a value of 123 for option "foo",
while clients that do not match "melon" will have a value of 456 for option
"foo".

It is possible to achieve an if-elseif-else effect but specifying an option
more than once with different class tags and values. Consider the following
configuration for a subnet which provides three possible values for the
"server-str" option:

.. code-block:: javascript

    {
        "id": 1,
        "subnet": "192.0.1.0/24",
        "pools": [{ "pool": "192.0.1.1 - 192.0.1.255" }],
        "option-data": [{
            "client-classes":  [ "class-one" ],
            "name": "server-str",
            "data": "string.one"
        },{
            "client-classes":  [ "class-two" ],
            "name": "server-str",
            "data": "string.two"
        },{
            "name": "server-str",
            "data": "string.other"
       }]
    }

Clients belonging to "class-one" will get a value of "string.one", clients
belonging to "class-two" will get a value of "string.two", and clients that
belong to neither "class-one" nor "class-two" will get a value of
"string.other".

.. note::

    The order that the options are tested is not guaranteed other than an option
    with an empty "client-classes" list is checked last.

.. note::

    Though examples above are for DHCPv4, class-tagging syntax and
    behavior is the same for DHCPv6.

Classes and Hooks
=================

Hooks may be used to classify packets. This may be useful if the
expression would be complex or time-consuming to write, and could be
better or more easily written as code. Once the hook has added the proper class name
to the packet, the rest of the classification system will work as expected
in choosing a subnet and selecting options. For a description of hooks,
see :ref:`hooks-libraries`; for information on configuring classes,
see :ref:`classification-configuring` and :ref:`classification-subnets`.

Debugging Expressions
=====================

While constructing classification expressions, administrators may find
it useful to enable logging; see :ref:`logging` for a more complete
description of the logging facility.

To enable the debug statements in the classification system,
the severity must be set to ``DEBUG`` and the debug level to at least 55.
The specific loggers are ``kea-dhcp4.eval`` and ``kea-dhcp6.eval``.

To understand the logging statements, it is essential to understand a bit about
how expressions are evaluated; for a more complete description, refer to
[the design document](https://gitlab.isc.org/isc-projects/kea/-/wikis/designs/client-classification-design).
In brief, there are two structures used during the evaluation of an
expression: a list of tokens which represent the expressions, and a value
stack which represents the values being manipulated.

The list of tokens is created when the configuration file is processed,
with most expressions and values being converted to a token. The list is
organized in reverse Polish notation. During execution, the list is
traversed in order; as each token is executed, it is able to pop
values from the top of the stack and eventually push its result on the
top of the stack. Imagine the following expression:

::

          "test": "substring(option[61].hex,0,3) == 'foo'",


This will result in the following tokens:

::

          option, number (0), number (3), substring, text ('foo'), equals


In this example, the first three tokens will simply push values onto the
stack. The substring token will then remove those three values and
compute a result that it places on the stack. The text option also
places a value on the stack, and finally the equals token removes the two
tokens on the stack and places its result on the stack.

When debug logging is enabled, each time a token is evaluated it
emits a log message indicating the values of any objects that were popped
off of the value stack, and any objects that were pushed onto the value
stack.

The values are displayed as either text, if the command is known to
use text values, or hexadecimal, if the command either uses binary values
or can manipulate either text or binary values. For expressions that pop
multiple values off the stack, the values are displayed in the order
they were popped. For most expressions this will not matter, but for the
concat expression the values are displayed in reverse order from their
written order in the expression.

Let us assume that the following test has been entered into the
configuration. This example skips most of the configuration to
concentrate on the test.

::

          "test": "substring(option[61].hex,0,3) == 'foo'",


The logging might then resemble this:

::

      2016-05-19 13:35:04.163 DEBUG [kea.eval/44478] EVAL_DEBUG_OPTION Pushing option 61 with value 0x666F6F626172
      2016-05-19 13:35:04.164 DEBUG [kea.eval/44478] EVAL_DEBUG_STRING Pushing text string '0'
      2016-05-19 13:35:04.165 DEBUG [kea.eval/44478] EVAL_DEBUG_STRING Pushing text string '3'
      2016-05-19 13:35:04.166 DEBUG [kea.eval/44478] EVAL_DEBUG_SUBSTRING Popping length 3, start 0, string 0x666F6F626172 pushing result 0x666F6F
      2016-05-19 13:35:04.167 DEBUG [kea.eval/44478] EVAL_DEBUG_STRING Pushing text string 'foo'
      2016-05-19 13:35:04.168 DEBUG [kea.eval/44478] EVAL_DEBUG_EQUAL Popping 0x666F6F and 0x666F6F pushing result 'true'

.. note::

   The debug logging may be quite verbose if there are multiple
   expressions to evaluate; it is intended as an aid in helping
   create and debug expressions. Administrators should plan to disable debug
   logging when expressions are working correctly. Users may also
   wish to include only one set of expressions at a time in the
   configuration file while debugging them, to limit the log
   statements. For example, when adding a new set of expressions, an administrator
   might find it more convenient to create a configuration file that
   only includes the new expressions until they are working
   correctly, and then add the new set to the main configuration file.
