18. 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 (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 assumes that the control command is targeted at the CA itself and attempts to respond.

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

This API can be used by external tools to manage and monitor Kea operation. An example of such a monitoring tool is ISC's Stork. For details, see Monitoring Kea With Stork.

18.1. Data Syntax

Communication over the control channel is conducted using JSON structures. If configured, Kea opens a socket and listens 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 has 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",
        ...
    }
}

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

The service list contains 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 simply forwards 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 hook library which handles such a command for the given server. As an example, the hook 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 does not have to stop processing DHCP messages to apply changes to the database. Nevertheless, these situations are rather rare; 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-dhcp-ddns server.

The server processing the incoming command sends a response of the form:

{
    "result": 0, // 0|1|2|3|4
    "text": "textual description",
    "arguments": {
        "argument1": "value1",
        "argument2": "value2",
        ...
    }
}

The result value is a status code indicating a result of the command. The following general status codes are currently supported:

  • 0 - the command has been processed successfully.
  • 1 - a general error or failure has occurred during the command processing.
  • 2 - the specified command is unsupported by the server receiving it.
  • 3 - the requested operation has been completed but the requested resource was not found. This status code is returned when a command returns no resources or affects no resources.
  • 4 - the well-formed command has been processed but the requested changes could not be applied, because they were in conflict with the server state or its notion of the configuration.

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 an IPv6 subnet, but was sent to a DHCPv4 server, it returns 2. If the query asks for a subnet with subnet-id that has matches, the result is 3. If the command attempts to update a lease but the specified subnet-id does not match the identifier in the server's configuration, the result is 4.

Hook libraries can sometimes return additional status codes specific to their use cases.

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. The arguments map contains 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

Since Kea 1.9.7, it is possible to put comments in commands as in the configuration file. For instance:

{
    "command": "foo",
    // service is a list
    "service": [ "dhcp4" ],
    # command arguments are here.
    "arguments": {
        "param1": "value1",
        ...
        /*
        "param2": "value2",
        ...
        */
    }
}

18.2. Control Agent Command Response Format

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 is always wrapped in an array (JSON list) of individual responses. For example, the response for a command sent to one service would be structured as follows:

[
    {
        "result": 0, // 0|1|2|3|4
        "text": "textual description",
        "arguments": {
            "argument1": "value1",
            "argument2": "value2",
            ...
        }
    }
]

If the command is sent to more than one service, the array would contain responses from each service, in the order they were requested:

[
    {
        "result": 0, // 0|1|2|3|4
        "text": "textual description",
        "arguments": {
            "argument1": "value1",
            "argument2": "value2",
            ...
        }
    },
    {
        "result": 0, // 0|1|2|3|4
        "text": "textual description",
        "arguments": {
            "argument1": "value1",
            "argument2": "value2",
            ...
        }
    },
    ...
]

An exception to this are authentication or authorization errors which cause CA to reject the command entirely. The response to such an error is formatted as a single entry (JSON map) as follows:

{
    "result": 403,
    "text": "Forbidden"
}

These types of errors are possible on systems configured for either basic authentication or agents that load libca_rbac.so.

18.3. 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.

18.4. Commands Supported by Both the DHCPv4 and DHCPv6 Servers

18.4.1. 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"
}

18.4.2. 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 was 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 includes all parameters from all included files. Starting with 2.4.0, the successful response also contains a SHA-256 digest that can be used to easily determine if a configuration has changed or not.

Warning

The returned configuration is not redacted, i.e. it contains 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"
}

18.4.3. The config-hash-get Command

The config-hash-get command retrieves the SHA-256 hash of the current configuration used by the server. This command does not take any parameters. The returned hash can be used to detect configuration changes.

An example command invocation looks like this:

{
    "command": "config-hash-get"
}

And the server's response:

{
    "result": 0,
    "arguments": {
        "hash": "5C3C90EF7035249E2FF74D003C19F34EE0B83A3D329E741B52B2EF95A2C9CC5C"
     }
 }

Starting with 2.4.0, also config-set and config-get return the SHA-256 hash of the new or current configuration. This may be used to later determine if a configuration has changed or not.

18.4.4. 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 system administrator can tweak the configuration file and use this command to force Kea pick up the changes.

Caution should be taken when mixing this with the config-set command. 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 attempts to reload its original configuration from the file, possibly losing all changes introduced using config-set or other commands.

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

{
    "command": "config-reload"
}

If the configuration file is incorrect, reloading it can raise an error which leaves the server in an unusable state. See The config-set Command to learn how to recover from a non-working server.

18.4.5. 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. The configuration is sanity-checked to the extent possible without the server actually attempting to load it; it is possible for a configuration which successfully passes this command to 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 contains 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)" }

18.4.6. 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) is used. If a relative path is specified, Kea writes its files only in the directory where it is running.

An example command invocation looks like this:

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

18.4.7. 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 a returning client that previously used that lease. See Configuring Lease Affinity for details. Also, see Lease Reclamation for general information about the processing of expired leases (lease reclamation).

18.4.8. The libreload Command

This command is now deprecated and will be removed in future Kea versions.

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

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

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

18.4.9. 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.

18.4.10. 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 reverts 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; however, in some cases a fatal error message is logged indicating that the server is no longer providing any service: a working configuration must be loaded as soon as possible. If the control channel is dead, the configuration file can still be reloaded using the SIGHUP signal. If that is unsuccessful, restart the server.

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)" }

Starting with 2.4.0, the successful response from a DHCPv4, DHCPv6, or DHCP-DDNS daemons also contain a SHA-256 digest of the newly set configuration. The digest can be used to easily determine if a configuration has changed or not.

18.4.11. 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",
    "arguments": {
        "exit-value": 3
    }
}

The server responds with a confirmation that the shutdown procedure has been initiated. The optional parameter, exit-value, specifies the numeric value with which the server process exits to the system. The default value is zero.

The DDNS daemon supports an extra parameter, type, which controls the way the process cleans up on exit. The supported shutdown types are:

  • "normal" - stops the queue manager and finishes all current transactions before exiting. This is the default.
  • "drain_first" - stops the queue manager but continues processing requests from the queue until it is empty.
  • "now" - exits immediately.

An example command may look like this:

{
    "command": "shutdown",
    "arguments": {
        "exit-value": 3,
        "type": "drain_first"
    }
}

18.4.12. 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.

Since Kea 1.9.4, there is an additional origin parameter that specifies the command source. A server administrator should typically omit this parameter because the default value "user" indicates that the administrator sent the command. This command can also be sent by the partner server running HA hooks library. In that case, the partner server sets the parameter to "ha-partner". This value is reserved for the communication between HA partners and should not be specified in the administrator's commands, as it may interfere with HA operation. The administrator should either omit this parameter or set it to "user".

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

18.4.13. The dhcp-enable Command

The dhcp-enable command globally enables the DHCP service.

Since Kea 1.9.4, there is an additional origin parameter that specifies the command source. A server administrator should typically omit this parameter because the default value "user" indicates that the administrator sent the command. This command can also be sent by the partner server running the HA hook library. In that case, the partner server sets the parameter to "ha-partner". This value is reserved for the communication between HA partners and should not be specified in the administrator's commands, as it may interfere with HA operation. The administrator should either omit this parameter or set it to "user".

{
    "command": "dhcp-enable",
    "arguments": {
        "origin": "user"
    }
}

18.4.14. The status-get Command

The status-get command returns the server's runtime information:

  • pid: the process ID.

  • uptime: the number of seconds since the start of the server.

  • reload: the number of seconds since the last configuration (re)load.

  • high-availability: HA-specific status information about the DHCP servers configured to use the HA hook library:

    • local: the state, the role (primary, secondary, ...), and the scopes (i.e. what the server is actually processing) of the local server.
    • remote: the remote server's last known state, its served HA scopes, and the role of the remote server in the HA relationship.

  • multi-threading-enabled: a flag indicating whether multi-threading is enabled.

  • thread-pool-size: the number of DHCP service threads.

  • packet-queue-size: the maximum size of the packet queue. There is one queue, regardless of the number of running threads.

  • packet-queue-statistics: the average queue size for the last 10, 100, and 1000 packets, using an approach similar to the UNIX top command. The average queue size for the last 10 packets can be considered an instantaneous value, while the average for the last 1000 packets shows a longer-term trend.

The high-availability information is returned only when the command is sent to the DHCP servers in an HA setup. This parameter is never returned when the status-get command is sent to the Control Agent or DDNS daemon.

The thread-pool-size, packet-queue-size and packet-queue-statistics parameters are returned only when the command is sent to DHCP servers with multi-threading enabled. These three parameters and multi-threading-enabled are never returned when the status-get command is sent to the Control Agent or DDNS daemon.

To learn more about the HA status information returned by the status-get command, please refer to the The status-get Command section.

18.4.15. The server-tag-get Command:

The server-tag-get command returns the configured server tag of the DHCPv4 or DHCPv6 server (Configuration Sharing and Server Tags explains the server tag concept).

18.4.16. The config-backend-pull Command:

The config-backend-pull command triggers the polling of configuration backends (which must be configured for this command to have an effect), explained in Enabling the Configuration Backend.

18.4.17. 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"
}

18.5. Commands Supported by the D2 Server

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

18.6. 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; when the service parameter is blank, the commands are handled by the CA and they relate to the CA process itself: