kea/doc/guide/classify.xml

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<chapter id="classify">
  <title>Client Classification</title>

  <section>
    <title>Client Classification Overview</title>
      <para>
      In certain cases it is useful to differentiate between different
      types of clients and treat them accordingly. There are many reasons
      why one might want to treat clients different some common reasons
      include:
      <itemizedlist>
      <listitem><para>
      The clients represent different pieces of topology, for example a cable
      modem vs the clients behind that modem.
      </para></listitem>
      <listitem><para>
      The clients have different behavior, for example a smart phone vs a lapttop
      vs a desktop.
      </para></listitem>
      <listitem><para>
      The clients require different values for some options, for example a docsis3.0
      cable modem vs a docsis2.0 cable modem.
      </para></listitem>
      </itemizedlist>
      </para>

      <para>
      It is envisaged that client classification will be used for changing the
      behavior of almost any part of the DHCP message processing, including assigning
      leases from different pools, assigning different options (or different values of
      the same options) etc. For now, there are only three mechanisms that take
      advantage of client classification: subnet selection, assigning different
      options and for DHCPv4 cable modems there are specific options for use with
      the TFTP server address and the boot file field.
      </para>

      <para>
      The process of doing classification is conducted in three steps. The first step
      is to assess an incoming packet and assign it to zero or more classes.  The
      second step is to choose a subnet, possibly based on the class information.
      The third step is to assign options again possibly based on the class
      information.
      </para>

      <para>
      When determining which options to include in the response the server will
      examine the union of options from all of the assigned classes. In the case two
      or more classes include an option the value from the first class will be used.
      Similarly if two or more classes are associated with a subnet the first subnet
      will be used. In the future the processing order of the various classes may
      be specified but for now it is being left unspecified and may change in
      future releases.
      </para>

      <para>
      For example imagine 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 will examine the three options NTP, SMTP and POP3 and return any
      of them that the client requested.  As the NTP server was defined twice the
      server will choose only one of the values for the reply.
      </para>

      <para>
      There are two methods of doing classification. The first is automatic and relies
      on examining the values in the vendor class options. Information from these
      options is extracted and a class name is constructed from it and added to
      the class list for the packet. The second allows you to specify an expression
      that is evaluated for each packet. If the result is true the packet is
      a member of the class.
      </para>

      <note>
      <para>
        The power of the expressions in the classification step is deliberately
        limited in order to minimize the amount of time required to process each
        expression. The expression for each class must be executed on each packet,
        if they are overly complex or time consuming they may impact the performance
        of the server. If you need complex or time consuming expressions you
        should write a hook to perform the necessary work.
      </para>
      </note>

      <note>
      <para>
        Care should be taken with client classification as it is easy for
        clients that do not meet class criteria to be denied any service altogether.
      </para>
      </note>
  </section>

  <section id="classification-using-vendor">
    <title>Using Vendor Class Information In Classification</title>
      <para>
      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 &quot;VENDOR_CLASS_&quot; then it is interpreted
      as a class. For example, modern cable modems will send this option with
      value &quot;docsis3.0&quot; and as a result the packet will belong to
      class &quot;VENDOR_CLASS_docsis3.0&quot;.
      </para>
  </section>

  <section id="classification-using-expressions">
    <title>Using Expressions In Classification</title>
      <para>
      The expression portion of classification contains operators and values.
      Values are currently strings and operators take a string or strings and
      return another string. When all the operations have completed
      the result should be a value of &quot;true&quot; or &quot;false&quot;.
      The packet belongs to
      the class (and the class name is added to the list of classes) if the result
      is &quot;true&quot;. Expressions are written in standard format and can be nested.
      </para>

      <para>
      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 incorrect number or types of arguments to an operator.  The
      evaluation code will also check for this class of error and generally
      throw an exception, though they should not occur in a normally functioning
      system.
      </para>

      <para>
      Other issues, for example 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.
      </para>

      <para>
      The expressions are a work in progress and the supported operators and
      values are limited. The expectation is that additional operators and values
      will be added over time, however it is expected the basic mechanisms will
      remain the same.
      </para>

      <para>
        <table frame="all" id="classification-values-list">
          <title>List of Classification Values</title>
          <tgroup cols='3'>
          <colspec colname='name' />
          <colspec colname='example' />
          <colspec colname='description' />
          <thead>
            <row>
              <entry>Name</entry>
              <entry>Example</entry>
              <entry>Description</entry>
            </row>
          </thead>
          <tbody>
<row><entry>String</entry><entry>'example'</entry><entry>A string</entry></row>
<row><entry>Hex String</entry><entry>0XABCD</entry><entry>A hexadecimal string</entry></row>
<row><entry>Integer</entry><entry>123</entry><entry>An integer value</entry></row>
<row><entry>Option Text</entry><entry>option[code].text</entry><entry>The value of the option with code "code" from the packet as text</entry></row>
<row><entry>Option Hex</entry><entry>option[code].hex</entry><entry>The value of the option with code "code" from the packet as hex</entry></row>
          </tbody>
          </tgroup>
        </table>
      </para>

      <para>
      Hex Strings are converted into a string as expected.  The starting &quot;0X&quot; or
      &quot;0x&quot; is removed and if the string is an odd number of characters a
      &quot;0&quot; is prepended to it.
      </para>

      <para>
      Option extracts the value of the given option from the incoming packet. If the
      packet doesn't contain the option it returns an empty string.  The string can
      be presented as text or hex with the ".text" or ".hex" modifiers.  In both
      cases only the payload is presented the type code and length fields aren't
      included.
      </para>

      <para>
        <table frame="all" id="classification-expressions-list">
          <title>List of Classification Expressions</title>
          <tgroup cols='3'>
          <colspec colname='name' />
          <colspec colname='example' />
          <colspec colname='description' />
          <thead>
            <row>
              <entry>Name</entry>
              <entry>Example</entry>
              <entry>Description</entry>
            </row>
          </thead>
          <tbody>
<row><entry>Equal</entry> <entry>'foo' == 'bar'</entry><entry>Compare the two values and return "true" or "false"</entry></row>
<row><entry>Substring</entry><entry>substring('foobar',0,3)</entry><entry>Return the requested substring</entry></row>
          </tbody>
          </tgroup>
        </table>
      </para>

      <para>
      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 doesn't 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 length is longer than the remaining portion of the string then
      the entire remaining portion is returned.
      </para>
  </section>

  <section id="classification-configuring">
    <title>Configuring Classes</title>
      <para>
      A class contains three items: a name, a test expression and option data.
      The name must exist and must be unique amongst all classes. The test
      expression and option data are optional.
      </para>

      <para>
      The test expression is a string containing the logical expression used to
      determine membership in the class.  The entire expression is in double
      quotes.
      </para>

      <para>
      The option data is a list which defines any options that should be assigned
      to members of this class.
      </para>

      <para>
        <screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "client-class": "Client_foo"
        }
    ],
    "client-class": [
<userinput>
        {
            "name": "Client_foo",
            "test": "substring(option[61].text,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "doamin-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]
        }
</userinput>
    ...
}</screen>
      </para>

      <para>
      In this example the class named &quot;Client_foo&quot; is defined.  It is comprised
      of all clients who's client ids (option 61) start with the string &quot;foo&quot;.
      They will be given an address from 192.0.2.10 to 192.0.2.20 and 192.0.2.1
      and 192.0.2.2 as their domain name servers.
      </para>
  </section>

  <section id="classification-subnets">
    <title>Configuring Subnets With Class Information</title>
      <para>
        In certain cases it beneficial to restrict access to certain subnets
        only to clients that belong to a given subnet. For details on client
        classes, see <xref linkend="classification-using-vendor"/> and
        <xref linkend="classification-using-expressions"/>
        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 range 192.0.2.10 to 192.0.2.20 are going to be
        managed by the DHCP4 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:
        <screen>
"Dhcp4": {
    "subnet4": [
<userinput>
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "client-class": "Client_foo"
        }
</userinput>
    ],
    "client-class": [
        {
            "name": "Client_foo",
            "test": "substring(option[61].text,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "doamin-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]
        }
    ...
}</screen>
      </para>
  </section>

  <section>
    <title>Using Classes</title>
      <para>
      Currently classes can be used for two functions.  They can supply options
      to the members class and they can choose a subnet for the members of the class.
      </para>

      <para>
      When supplying options class options defined as part of the class definition
      are considred &quot;class globals&quot;.  They will override any global options that
      may be defined and in turn will be overridden by any options defined for an
      individual subnet.
      </para>
  </section>

  <section>
    <title>Classes and Hooks</title>
      <para>
      You may use a hook to classify your packets. This may be useful if the
      expression would either be complex or time consuming and be easier or
      better to write as code.  Once the hook has added the proper class name
      to the packet the rest of the classification system will work as normal
      in choosing a subnet and selecting options.  For a description of the
      hooks see <xref linkend="hooks-libraries"/>, for a description on
      configuring he classes see <xref linkend="classification-configuring"/>
      and <xref linkend="classification-subnets"/>.
      </para>
  </section>

</chapter>