17. Management API¶

A classic approach to daemon configuration assumes that the server’s configuration is stored in configuration files and, when the configuration is changed, the daemon is restarted. This approach has the significant disadvantage of introducing periods of downtime when client traffic is not handled. Another risk is that if the new configuration is invalid for any reason, the server may refuse to start, which will further extend the downtime period until the issue is resolved.

To avoid such problems, the DHCPv4, DHCPv6, and D2 servers in Kea include support for a mechanism that allows online reconfiguration without requiring server shutdown. Both servers can be instructed to open control sockets, which is a communications channel. The server is able to receive commands on that channel, act on them, and report back status.

The DHCPv4, DHCPv6, and D2 servers receive commands over the UNIX domain sockets. For details on how to configure these sockets, see Management API for the DHCPv4 Server and Management API for the DHCPv6 Server. While it is possible to control the servers directly using UNIX domain sockets, that requires that the controlling client be running on the same machine as the server. SSH is usually used to connect remotely to the controlled machine.

Network administrators usually prefer using some form of a RESTful API to control the servers, rather than using UNIX domain sockets directly. Therefore, Kea includes a component called the Control Agent (or CA), which exposes a RESTful API to the controlling clients and can forward commands to the respective Kea services over the UNIX domain sockets. The CA configuration is described in Configuration.

The HTTP requests received by the CA contain the control commands encapsulated within HTTP requests. Simply speaking, the CA is responsible for stripping the HTTP layer from the received commands and forwarding the commands in a JSON format over the UNIX domain sockets to the respective services. Because the CA receives commands for all services, it requires additional “forwarding” information to be included in the client’s messages. This forwarding information is carried within the service parameter of the received command. If the service parameter is not included, or if the parameter is a blank list, the CA will assume that the control command is targeted at the CA itself and will try to handle it on its own.

Control connections over both HTTP and UNIX domain sockets are guarded with timeouts. The default timeout value is set to 10 seconds and is not configurable.

17.1. Data Syntax¶

Communication over the control channel is conducted using JSON structures. If configured, Kea will open a socket and listen for incoming connections. A process connecting to this socket is expected to send JSON commands structured as follows:

{
    "command": "foo",
    "service": [ "dhcp4" ]
    "arguments": {
        "param1": "value1",
        "param2": "value2",
        ...
    }
}

The same command sent over the RESTful interface to the CA will have the following structure:

POST / HTTP/1.1\r\n
Content-Type: application/json\r\n
Content-Length: 147\r\n\r\n
{
    "command": "foo",
    "service": [ "dhcp4" ]
    "arguments": {
        "param1": "value1",
        "param2": "value2",
        ...
    }
}

command is the name of the command to execute and is mandatory. arguments is a map of the parameters required to carry out the given command. The exact content and format of the map are command-specific.

service is a list of the servers at which the control command is targeted. In the example above, the control command is targeted at the DHCPv4 server. In most cases, the CA will simply forward this command to the DHCPv4 server for processing via a UNIX domain socket. Sometimes, the command including a service value may also be processed by the CA, if the CA is running a hooks library which handles such a command for the given server. As an example, the hooks library loaded by the CA may perform some operations on the database, such as adding host reservations, modifying leases, etc. An advantage of performing DHCPv4-specific administrative operations in the CA, rather than forwarding it to the DHCPv4 server, is the ability to perform these operations without disrupting the DHCPv4 service, since the DHCPv4 server doesn’t have to stop processing DHCP messages to apply changes to the database. Nevertheless, these situations are rather rare and, in most cases, when the service parameter contains a name of the service the commands are simply forwarded by the CA. The forwarded command includes the service parameter but this parameter is ignored by the receiving server. This parameter is only meaningful to the CA.

If the command received by the CA does not include a service parameter or this list is empty, the CA simply processes this message on its own. For example, a config-get command which includes no service parameter returns the Control Agent’s own configuration. The config-get command with a service value “dhcp4” is forwarded to the DHCPv4 server and returns the DHCPv4 server’s configuration.

The following list shows the mapping of the values carried within the service parameter to the servers to which the commands are forwarded:

dhcp4 - the command is forwarded to the kea-dhcp4 server.
dhcp6 - the command is forwarded to the kea-dhcp6 server.
d2 - the command is forwarded to the kea-d2 server.

The server processing the incoming command will send a response of the form:

{
    "result": 0|1|2|3,
    "text": "textual description",
    "arguments": {
        "argument1": "value1",
        "argument2": "value2",
        ...
    }
}

result indicates the outcome of the command. A value of 0 means success, while any non-zero value designates an error or a failure to complete the requested action. Currently 1 indicates a generic error, 2 means that a command is not supported, and 3 means that the requested operation was completed, but the requested object was not found. For example, a well-formed command that requests a subnet that exists in a server’s configuration returns the result 0. If the server encounters an error condition, it returns 1. If the command asks for the IPv6 subnet, but was sent to a DHCPv4 server, it returns 2. If the query asks for a subnet-id and there is no subnet with such an id, the result is 3.

The text field typically appears when the result is non-zero and contains a description of the error encountered, but it often also appears for successful outcomes. The exact text is command-specific, but in general uses plain English to describe the outcome of the command. arguments is a map of additional data values returned by the server which are specific to the command issued. The map may be present, but that depends on the specific command.

Note

When sending commands via the Control Agent, it is possible to specify multiple services at which the command is targeted. CA forwards this command to each service individually. Thus, the CA response to the controlling client contains an array of individual responses.

17.2. Using the Control Channel¶

The easiest way to start interacting with the control API is to use common UNIX/Linux tools such as socat and curl.

In order to control the given Kea service via a UNIX domain socket, use socat in interactive mode as follows:

$ socat UNIX:/path/to/the/kea/socket -

or in batch mode, include the “ignoreeof” option as shown below to ensure socat waits long enough for the server to respond:

$ echo "{ some command...}" | socat UNIX:/path/to/the/kea/socket -,ignoreeof

where /path/to/the/kea/socket is the path specified in the Dhcp4/control-socket/socket-name parameter in the Kea configuration file. Text passed to socat is sent to Kea and the responses received from Kea are printed to standard output. This approach communicates with the specific server directly and bypasses the Control Agent.

It is also easy to open a UNIX socket programmatically. An example of a simple client written in C is available in the Kea Developer’s Guide, in the Control Channel Overview chapter, in the Using Control Channel section.

To use Kea’s RESTful API with curl, use the following:

$ curl -X POST -H "Content-Type: application/json" -d '{ "command": "config-get", "service": [ "dhcp4" ] }' http://ca.example.org:8000/

This assumes that the Control Agent is running on host ca.example.org and is running the RESTful service on port 8000.

17.3. Commands Supported by Both the DHCPv4 and DHCPv6 Servers¶

17.3.1. The build-report Command¶

The build-report command returns on the control channel what the command line -W argument displays, i.e. the embedded content of the config.report file. This command does not take any parameters.

{
    "command": "build-report"
}

17.3.2. The config-get Command¶

The config-get command retrieves the current configuration used by the server. This command does not take any parameters. The configuration returned is roughly equal to the configuration that was loaded using the -c command line option during server start-up or later set using the config-set command. However, there may be certain differences, as comments are not retained. If the original configuration used file inclusion, the returned configuration will include all parameters from all the included files.

Note that the returned configuration is not redacted, i.e. it will contain database passwords in plain text if those were specified in the original configuration. Care should be taken not to expose the command channel to unprivileged users.

An example command invocation looks like this:

{
    "command": "config-get"
}

17.3.3. The config-reload Command¶

The config-reload command instructs Kea to load again the configuration file that was used previously. This operation is useful if the configuration file has been changed by some external source; for example, a sysadmin can tweak the configuration file and use this command to force Kea pick up the changes.

Caution should be taken when mixing this with config-set commands. Kea remembers the location of the configuration file it was started with, and this configuration can be significantly changed using the config-set command. When config-reload is issued after config-set, Kea will attempt to reload its original configuration from the file, possibly losing all changes introduced using config-set or other commands.

config-reload does not take any parameters. An example command invocation looks like this:

{
    "command": "config-reload"
}

17.3.4. The config-test Command¶

The config-test command instructs the server to check whether the new configuration supplied in the command’s arguments can be loaded. The supplied configuration is expected to be the full configuration for the target server, along with an optional Logger configuration. As for the -t command, some sanity checks are not performed, so it is possible a configuration which successfully passes this command will still fail in the config-set command or at launch time. The structure of the command is as follows:

{
    "command": "config-test",
    "arguments":  {
        "<server>": {
        }
     }
}

where <server> is the configuration element name for a given server such as “Dhcp4” or “Dhcp6”. For example:

{
    "command": "config-test",
    "arguments":  {
        "Dhcp6": {
            :
        }
     }
}

The server’s response will contain a numeric code, “result” (0 for success, non-zero on failure), and a string, “text”, describing the outcome:

{"result": 0, "text": "Configuration seems sane..." }

or

{"result": 1, "text": "unsupported parameter: BOGUS (<string>:16:26)" }

17.3.5. The config-write Command¶

The config-write command instructs the Kea server to write its current configuration to a file on disk. It takes one optional argument, called “filename”, that specifies the name of the file to write the configuration to. If not specified, the name used when starting Kea (passed as a -c argument) will be used. If a relative path is specified, Kea will write its files only in the directory it is running.

An example command invocation looks like this:

{
    "command": "config-write",
    "arguments": {
        "filename": "config-modified-2017-03-15.json"
    }
}

17.3.6. The leases-reclaim Command¶

The leases-reclaim command instructs the server to reclaim all expired leases immediately. The command has the following JSON syntax:

{
    "command": "leases-reclaim",
    "arguments": {
        "remove": true
    }
}

The remove boolean parameter is mandatory and indicates whether the reclaimed leases should be removed from the lease database (if true), or left in the “expired-reclaimed” state (if false). The latter facilitates lease affinity, i.e. the ability to re-assign an expired lease to the same client that used this lease before. See Configuring Lease Affinity for the details. Also, see Lease Reclamation for general information about the processing of expired leases (lease reclamation).

17.3.7. The libreload Command¶

The libreload command first unloads and then loads all currently loaded hooks libraries. This is primarily intended to allow one or more hooks libraries to be replaced with newer versions without requiring Kea servers to be reconfigured or restarted. Note that the hooks libraries are passed the same parameter values (if any) that were passed when they originally loaded.

{
    "command": "libreload",
    "arguments": { }
}

The server will respond with a result of either 0, indicating success, or 1, indicating failure.

17.3.8. The list-commands Command¶

The list-commands command retrieves a list of all commands supported by the server. It does not take any arguments. An example command may look like this:

{
    "command": "list-commands",
    "arguments": { }
}

The server responds with a list of all supported commands. The arguments element is a list of strings, each of which conveys one supported command.

17.3.9. The config-set Command¶

The config-set command instructs the server to replace its current configuration with the new configuration supplied in the command’s arguments. The supplied configuration is expected to be the full configuration for the target server, along with an optional Logger configuration. While optional, the Logger configuration is highly recommended, as without it the server will revert to its default logging configuration. The structure of the command is as follows:

{
    "command": "config-set",
    "arguments":  {
        "<server>": {
        }
     }
}

where <server> is the configuration element name for a given server such as “Dhcp4” or “Dhcp6”. For example:

{
    "command": "config-set",
    "arguments":  {
        "Dhcp6": {
            :
        }
     }
}

If the new configuration proves to be invalid, the server retains its current configuration. Please note that the new configuration is retained in memory only; if the server is restarted or a configuration reload is triggered via a signal, the server uses the configuration stored in its configuration file. The server’s response contains a numeric code, “result” (0 for success, non-zero on failure), and a string, “text”, describing the outcome:

{"result": 0, "text": "Configuration successful." }

or

{"result": 1, "text": "unsupported parameter: BOGUS (<string>:16:26)" }

17.3.10. The shutdown Command¶

The shutdown command instructs the server to initiate its shutdown procedure. It is the equivalent of sending a SIGTERM signal to the process. This command does not take any arguments. An example command may look like this:

{
    "command": "shutdown"
}

The server responds with a confirmation that the shutdown procedure has been initiated.

17.3.11. The dhcp-disable Command¶

The dhcp-disable command globally disables the DHCP service. The server continues to operate, but it drops all received DHCP messages. This command is useful when the server’s maintenance requires that the server temporarily stop allocating new leases and renew existing leases. It is also useful in failover-like configurations during a synchronization of the lease databases at startup, or recovery after a failure. The optional parameter “max-period” specifies the time in seconds after which the DHCP service should be automatically re-enabled, if the dhcp-enable command is not sent before this time elapses.

{
    "command": "dhcp-disable",
    "arguments": {
        "max-period": 20
    }
}

17.3.12. The dhcp-enable Command¶

The dhcp-enable command globally enables the DHCP service.

{
    "command": "dhcp-enable"
}

17.3.13. The version-get Command¶

The version-get command returns extended information about the Kea version. It is the same information available via the -V command-line argument. This command does not take any parameters.

{
    "command": "version-get"
}

17.4. Commands Supported by the D2 Server¶

The D2 server supports only a subset of DHCPv4 / DHCPv6 server commands:

build-report
config-get
config-reload
config-set
config-test
config-write
list-commands
shutdown
version-get

17.5. Commands Supported by the Control Agent¶

The following commands listed in Commands Supported by Both the DHCPv4 and DHCPv6 Servers are also supported by the Control Agent, i.e. when the service parameter is blank, the commands are handled by the CA and they relate to the CA process itself:

build-report
config-get
config-reload
config-set
config-test
config-write
list-commands
shutdown
version-get