# Client Classification

## Client Classification Overview

In certain cases it is useful to differentiate between 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 smart phone 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 builtin condition.

- Using an expression which evaluates to true.

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

- Using a hook.

It is envisaged that client classification will be used for changing the behavior of almost any part of the DHCP message processing. There are currently five mechanisms that take advantage of client classification: subnet selection, pool selection, definition of DHCPv4 private (codes 224-254) and code 43 options, 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.

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 ("on request" or depending on the KNOWN/UNKNOWN builtin classes) evaluation are processed in the order they are defined in the configuration; the boolean expression is evaluated and, if it returns true ("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. 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 doesn't have a class associated with it, or has a class which matches one of the packet's classes.

6. 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.

7. Classes with matching expressions - directly or indirectly using the KNOWN/UNKNOWN builtin classes and not marked for later ("on request") evaluation - are processed in the order they are defined in the configuration; the boolean expression is evaluated and, if it returns true ("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 KNOWN/UNKNOWN classes to select a shared network or a subnet.

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

9. Classes marked as "required" are evaluated in the order in which they are listed: first the shared network, then the subnet, and finally the pools that assigned resources belong to.

10. Options are assigned, again possibly based on the class information in the order that classes were associated with the incoming packet. For DHCPv4 private and code 43 options, this includes class local option definitions.

Client classes in Kea follow the order in which they are specified in the configuration (vs. alphabetical order). Required classes follow the order in which they are required.

When determining which options to include in the response, the server examines the union of options from all of the assigned classes. If two or more classes include the same option, the value from the first class examined is used; classes are examined in the order they were associated, so ALL is always the first class and matching required classes are last.

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 unspecified.

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.

## Builtin Client Classes

Some classes are builtin, so they do not need to be defined. The main example uses Vendor Class information: 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 send this option with value "docsis3.0", so the packet belongs to class "VENDOR_CLASS_docsis3.0".

The "HA_" prefix is used by the High Availability hooks library 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 will use an appropriate pool or subnet to allocate IP addresses (and/or prefixes), based on the assigned client classes. The details can be found in .

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

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

## 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, 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.

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 builtin classes) which are evaluated in a previous or the same evaluation phase. This does not apply to the KNOWN or UNKNOWN classes.

### List of Classification Values

Name | Example expression | Example value | Description
--- | --- | --- | ---
String literal | 'example' | 'example' | A string
Hexadecimal string literal | 0x5a7d | 'Z}' | A hexadecimal string
IP address literal | 10.0.0.1 | 0x0a000001 | An IP address
Integer literal | 123 | '123' | A 32-bit unsigned integer value
Binary content of the option | option[123].hex | '(content of the option)' | The value of the option with given code from the packet as hex
Option existence | option[123].exists | 'true' | If the option with given code is present in the packet "true" else "false"
Client class membership | member('foobar') | 'true' | If the packet belongs to the given client class "true" else "false"
Known client | known | member('KNOWN') | If there is a host reservation for the client "true" else "false"
Unknown client | unknown | not member('KNOWN') | If there is a host reservation for the client "false" else "true"
DHCPv4 relay agent sub-option | relay4[123].hex | '(content of the RAI sub-option)' | The value of sub-option with given code from the DHCPv4 Relay Agent Information option (option 82)
DHCPv6 Relay Options | relay6[nest].option[code].hex | (value of the option) | The value of the option with code "code" from the relay encapsulation "nest"
DHCPv6 Relay Peer Address | relay6[nest].peeraddr | 2001:DB8::1 | The value of the peer address field from the relay encapsulation "nest"
DHCPv6 Relay Link Address | relay6[nest].linkaddr | 2001:DB8::1 | The value of the link address field from the relay encapsulation "nest"
Interface name of packet | pkt.iface | eth0 | The name of the incoming interface of a DHCP packet.
Source address of packet | pkt.src | 10.1.2.3 | The IP source address of a DHCP packet.
Destination address of packet | pkt.dst | 10.1.2.3 | The IP destination address of a DHCP packet.
Length of packet | pkt.len | 513 | The length of a DHCP packet (UDP header field), expressed as a 32-bit unsigned integer.
Hardware address in DHCPv4 packet | pkt4.mac | 0x010203040506 | The value of the chaddr field of the DHCPv4 packet, hlen (0 to 16) bytes
Hardware length in DHCPv4 packet | pkt4.hlen | 6 | The value of the hlen field of the DHCPv4 packet padded to 4 bytes
Hardware type in DHCPv4 packet | pkt4.htype | 6 | The value of the htype field of the DHCPv4 packet padded to 4 bytes
ciaddr field in DHCPv4 packet | pkt4.ciaddr | 192.0.2.1 | The value of the ciaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
giaddr field in DHCPv4 packet | pkt4.giaddr | 192.0.2.1 | The value of the giaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
yiaddr field in DHCPv4 packet | pkt4.yiaddr | 192.0.2.1 | The value of the yiaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
siaddr field in DHCPv4 packet | pkt4.siaddr | 192.0.2.1 | The value of the siaddr field of the DHCPv4 packet (IPv4 address, 4 bytes)
Message type in DHCPv4 packet | pkt4.msgtype | 1 | The value of the message type field in the DHCPv4 packet (expressed as a 32-bit unsigned integer).
Transaction ID (xid) in DHCPv4 packet | pkt4.transid | 12345 | The value of the transaction id in the DHCPv4 packet (expressed as a 32-bit unsigned integer).
Message type in DHCPv6 packet | pkt6.msgtype | 1 | The value of the message type field in the DHCPv6 packet (expressed as a 32-bit unsigned integer).
Transaction ID in DHCPv6 packet | pkt6.transid | 12345 | The value of the transaction id in the DHCPv6 packet (expressed as a 32-bit unsigned integer).
Vendor option existence (any vendor) | vendor[*].exists | true | Returns whether a vendor option from any vendor is present ('true') or absent ('false').
Vendor option existence (specific vendor) | vendor[4491].exists | true | Returns whether a vendor option from specified vendor (determined by its enterprise-id) is present ('true') or absent ('false').
Enterprise-id from vendor option | vendor.enterprise | 4491 | If the vendor option is present, it returns the value of the enterprise-id field padded to 4 bytes. Returns "" otherwise.
Vendor sub-option existence | vendor[4491].option[1].exists | true | Returns 'true' if there is vendor option with specified enterprise-id and given sub-option is present. Returns 'false' otherwise.
Vendor sub-option content | vendor[4491].option[1].hex | docsis3.0 | Returns content of the specified sub-option of a vendor option with specified enterprise id. Returns '' if no such option or sub-option is present.
Vendor class option existence (any vendor) | vendor-class[*].exists | true | Returns whether a vendor class option from any vendor is present ('true') or absent ('false').
Vendor class option existence (specific vendor) | vendor-class[4491].exists | true | Returns whether a vendor class option from specified vendor (determined by its enterprise-id) is present ('true') or absent ('false').
Enterprise-id from vendor class option | vendor-class.enterprise | 4491 | If the vendor option is present, it returns the value of the enterprise-id field padded to 4 bytes. Returns "" otherwise.
First data chunk from vendor class option | vendor-class[4491].data | docsis3.0 | Returns content of the first data chunk from the vendor class option with specified enterprise-id. Returns "" if missing.
Specific data chunk from vendor class option | vendor-class[4491].data[3] | docsis3.0 | Returns content of the specified data chunk of a vendor class option with specified enterprise id. Returns '' if no such option or data chunk is present.

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 doesn't 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 builtin, i.e., beginning by "VENDOR_CLASS_", "AFTER__" (for the to come "after" hook) and "EXTERNAL_" or equal to "ALL", "KNOWN", "UNKNOWN"etc.

  "known" and "unknown" are short hands for "member('KNOWN')" and "not member('KNOWN')". Note the evaluation of any expression using directly or indirectly the "KNOWN" class 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 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.

- "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 to specify relays counted from the DHCPv6 client, -1 indicating the relay closest to the client. In general negative "nest" level is the same as the number of relays + "nest" level. If the requested encapsulation doesn't 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 allows 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 will be "0x00000001" or simply 1 as in "pkt6.msgtype == 1".

- Vendor option means Vendor-Identifying Vendor-specific Information option in DHCPv4 (code 125, see Section 4 of RFC 3925) and Vendor-specific Information Option in DHCPv6 (code 17, defined in Section 21.17 of RFC 8415). Vendor class option means Vendor-Identifying Vendor Class Option in DHCPv4 (code 124, see Section 3 of RFC 3925) in DHCPv4 and Class Option in DHCPv6 (code 16, see Section 21.16 of RFC 8415). 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 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 option[60] expression.

- RFC 3925 and RFC 8415 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 vendor.enterprise and vendor-class.enterprise expressions, the value from the first instance is returned. Please submit a feature request on Kea website if you need support for multiple instances.

### List of Classification Expressions

Name | Example | Description
--- | --- | ---
Equal | 'foo' == 'bar' | Compare the two values and return "true" or "false"
Not | not ('foo' == 'bar') | Logical negation
And | ('foo' == 'bar') and ('bar' == 'foo') | Logical and
Or | ('foo' == 'bar') or ('bar' == 'foo') | Logical or
Substring | substring('foobar',0,3) | Return the requested substring
Concat | concat('foo','bar') | Return the concatenation of the strings
Ifelse | ifelse('foo' == 'bar','us','them') | Return the branch value according to the condition
Hexstring | hexstring('foo', '-') | Converts the value to a hexadecimal string, e.g. 0a:1b:2c:3e

### 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 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:

            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'

### 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, ':')

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 hook to perform the necessary work.

## Configuring Classes

A class contains five items: a name, a test expression, option data, option definition and only-if-required flag. The name must exist and must be unique amongst all classes. The test expression, option data and definition, and only-if-required flag are optional.

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

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

The option definition is for DHCPv4 option 43 ( and DHCPv4 private options ().

Usually the test expression is 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 not yet done. The only-if-required flag, false by default, allows to perform the evaluation of the test expression only when it was required, i.e. in a require-client-classes list of the selected subnet, shared-network or pool.

The require-client-classes list which is valid for shared-network, subnet and pool scope specifies the classes which are evaluated in the second pass before output option processing. The list is built in the reversed precedence order of option data, i.e. an option data in a subnet takes precedence on one in a shared-network but required class in a subnet is added after one in a shared-network. The mechanism is related to the only-if-required flag but it is not mandatory that the flag was set to true.

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"
                    }
                ]
            },
            ...
        ],
        ...
    }

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.

    "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"
                    }
                ]
            },
            ...
        ],
        ...
    }

## Using Static Host Reservations In Classification

Classes can be statically assigned to the clients using techniques described in and .

## Configuring Subnets With Class Information

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.

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:

    "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": [
            {
                "subnet": "192.0.2.0/24",
                "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
                "client-class": "Client_foo"
            },
            ...
        ],,
        ...
    }

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.

    "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": [
            {
                "subnet": "2001:db8:1::/64",
                "pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
                "client-class": "Client_enterprise"
            }
        ],
        ...
    }

## Configuring Pools With Class Information

Similar to subnets in certain cases access to certain address or prefix pools must be restricted to only clients that belong to a given class, using the "client-class" when defining the pool.

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 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": [
            {
                "subnet": "192.0.2.0/24",
                "pools": [
                    {
                        "pool": "192.0.2.10 - 192.0.2.20",
                        "client-class": "Client_foo"
                    }
                ]
            },
            ...
        ],,

    }

The following example shows restricting access to an address pool. This configuration will restrict use of the addresses 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": [
            {
                "subnet": "2001:db8:1::/64",

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

## Using Classes

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.

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.

## Classes and Hooks

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 hooks see , for a description on configuring classes see and .

## Debugging Expressions

While you are constructing your classification expressions you may find it useful to enable logging see for a more complete description of the logging facility.

To enable the debug statements in the classification system you will need to set the severity to "DEBUG" and the debug level to at least 55. The specific loggers are "kea-dhcp4.eval" and "kea-dhcp6.eval".

In order to understand the logging statements, one must understand a bit about how expressions are evaluated; for a more complete description refer to the design document at `https://gitlab.isc.org/isc-projects/kea/wikis/design%20documents`. 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 will be traversed in order. As each token is executed it will be 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 will emit 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 will be 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 will be displayed in the order they were popped. For most expressions this won't matter but for the concat expression the values are displayed in reverse order from how they are written 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'

The debug logging may be quite verbose if you have a number of expressions to evaluate. It is intended as an aid in helping you create and debug your expressions. You should plan to disable debug logging when you have your expressions working correctly. You also may wish to include only one set of expressions at a time in the configuration file while debugging them in order to limit the log statements. For example when adding a new set of expressions you might find it more convenient to create a configuration file that only includes the new expressions until you have them working correctly and then add the new set to the main configuration file.