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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. Common reasons include:
<itemizedlist>
<listitem><para>
The clients represent different pieces of topology, e.g. a cable
modem is different to the clients behind that modem.
</para></listitem>
<listitem><para>
The clients have different behavior, e.g. a smart phone behaves
differently to a laptop.
</para></listitem>
<listitem><para>
The clients require different values for some options, e.g. a docsis3.0
cable modem requires different settings to 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 the assignment of
leases from different pools, the assignment of different options (or different values of
the same options) etc. In the current release of the software however, there are
only three mechanisms that take
advantage of client classification: subnet selection, assignment of different
options and, for DHCPv4 cable modems, the setting of 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:
<orderedlist>
<listitem><para>
Assess an incoming packet and assign it to zero or more classes.
</para></listitem>
<listitem><para>
Choose a subnet, possibly based on the class information.
</para></listitem>
<listitem><para>
Assign options, again possibly based on the class information.
</para></listitem>
</orderedlist>
</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 the same option, the value from the first class examined will
be used. When choosing a subnet the server will iterate over all of the
subnets that are feasible given the information found in the packet (client address,
relay address etc). It will use the first subnet it finds that either doesn't
have a class associated with it or that has a class which matches one of
the packet's classes. 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>
As an 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: the class from which the
value is obtained is unspecified.
</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>
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 "VENDOR_CLASS_" and the result is interpreted
as a class. For example, modern cable modems will send this option with
value "docsis3.0" and so the packet will belong to
class "VENDOR_CLASS_docsis3.0".
</para>
</section>
<section id="classification-using-expressions">
<title>Using Expressions In Classification</title>
<para>
The expression portion of classification contains operators and values.
All 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 "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.
</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>
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>
<!-- Text option not fully defined yet, leave it out
<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>
<row><entry>Option Exist</entry><entry>option[code].exist</entry><entry>If the option with code "code" is present in the packet "true" else "false"</entry></row>
<row><entry>DHCPv4 Relay Agent
sub-option</entry><entry>relay4[code].hex</entry><entry>The value of
sub-option with code "code" from the DHCPv4 Relay Agent Information option
(option 82)</entry></row>
</tbody>
</tgroup>
</table>
Hex 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.
</para>
<para>
Integers in the expression are converted to strings
when the expression is read into Kea.
</para>
<para>
"option[code].hex" extracts the value of the option with the given code
from the incoming packet. If the packet doesn't contain the option, it
returns the empty string. The string is presented as a byte string of
the option payload without the type code or length fields.
</para>
<para>
"option[code].exist" checks if an option with the given code is present
in the incoming packet. It can be used with empty options.
</para>
<para>
"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 doesn't contain a RAI option, or the RAI
option doesn't 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.
</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>Not</entry> <entry>not ('foo' == 'bar')</entry><entry>Logical negation</entry></row>
<row><entry>And</entry> <entry>('foo' == 'bar') and ('bar' == 'foo')</entry><entry>Logical and</entry></row>
<row><entry>Or</entry> <entry>('foo' == 'bar') or ('bar' == 'foo')</entry><entry>Logical or</entry></row>
<row><entry>Substring</entry><entry>substring('foobar',0,3)</entry><entry>Return the requested substring</entry></row>
<row><entry>Concat</entry><entry>concat('foo','bar')</entry><entry>Return the concatenation of the strings</entry></row>
</tbody>
</tgroup>
</table>
</para>
<section>
<title>Logical operators</title>
The Not, And and Or logical operators are the common operators. Not
has the highest precedence, 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").
</section>
<section>
<title>Substring</title>
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. Some examples may be helpful:
<screen>
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) == ''
</screen>
</section>
<section>
<title>Concat</title>
The concat function "concat(string1, string2)" returns the
concatenation of its two arguments. For instance:
<screen>
concat('foo', 'bar') == 'foobar'
</screen>
</section>
</section>
<note>
<para>
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. If you need
complex or time consuming expressions you should write a <link
linkend='hooks-libraries'>hook</link> to perform the necessary work.
</para> </note>
<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>
In the following example the class named "Client_foo" is defined.
It is comprised of all clients who's 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.
<screen>
"Dhcp4": {
"client-classes": [<userinput>
{
"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"
}
]
},
...
],</userinput>
...
}</screen>
</para>
<para>
This example shows a client class being defined for use by the DHCPv6 server.
In it the class named "Client_enterprise" is defined. It is comprised
of all clients who's client identifiers start with the given hex string (which
would indicate a DUID based on an enterprise id of 0xAABBCCDD). Members of this
class will be given an 2001:db8:0::1 and 2001:db8:2::1 as their domain name servers.
<screen>
"Dhcp6": {
"client-classes": [<userinput>
{
"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"
}
]
},
...
],</userinput>
...
}</screen>
</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 class using the "client-class"
keyword when defining the subnet.
</para>
<para>
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": {
"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"
}
]
},
...
],<userinput>
"subnet4": [
{
"subnet": "192.0.2.0/24",
"pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
"client-class": "Client_foo"
},
...
],</userinput>,
...
}</screen>
</para>
<para>
The following example shows restricting access to a DHCPv6 subnet. This
configuration will restrict use of the addresses 2001:db8:1::1 to
2001:db8:1::FFFF to members of the "Client_enterprise" class.
<screen>
"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"
}
]
},
...
], <userinput>
"subnet6": [
{
"subnet": "2001:db8:1::/64",
"pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
"client-class": "Client_enterprise"
}
],</userinput>
...
}</screen>
</para>
</section>
<section>
<title>Using Classes</title>
<para>
Currently classes can be used for two functions. They can supply options
to the members of the class and they can be used to choose a subnet from which an
address will be assigned to the class member.
</para>
<para>
When supplying options, options defined as part of the class definition
are considered "class globals". 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>