7. The Kea Control Agent

7.1. Overview of the Kea Control Agent

The Kea Control Agent (CA) is a daemon which exposes a RESTful control interface for managing Kea servers. The daemon can receive control commands over HTTP and either forward these commands to the respective Kea servers or handle these commands on its own. The determination whether the command should be handled by the CA or forwarded is made by checking the value of the service parameter, which may be included in the command from the controlling client. The details of the supported commands, as well as their structures, are provided in Management API.

The CA can use hook libraries to provide support for additional commands or to program custom behavior of existing commands. Such hook libraries must implement callouts for the control_command_receive hook point. Details about creating new hook libraries and supported hook points can be found in the Kea Developer's Guide.

The CA processes received commands according to the following algorithm:

  • Pass command into any installed hooks (regardless of service value(s)). If the command is handled by a hook, return the response.
  • If the service specifies one or more services, forward the command to the specified services and return the accumulated responses.
  • If the service is not specified or is an empty list, handle the command if the CA supports it.

7.2. Configuration

The following example demonstrates the basic CA configuration.

    "Control-agent": {
        "http-host": "",
        "http-port": 8000,
        "trust-anchor": "/path/to/the/ca-cert.pem",
        "cert-file": "/path/to/the/agent-cert.pem",
        "key-file": "/path/to/the/agent-key.pem",
        "cert-required": true,
        "authentication": {
            "type": "basic",
            "realm": "kea-control-agent",
            "clients": [
                "user": "admin",
                "password": "1234"
            } ]

        "control-sockets": {
            "dhcp4": {
                "comment": "main server",
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-v4"
            "dhcp6": {
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-v6",
                "user-context": { "version": 3 }
            "d2": {
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-d2"

        "hooks-libraries": [
            "library": "/opt/local/control-agent-commands.so",
            "parameters": {
                "param1": "foo"
        } ],

        "loggers": [ {
            "name": "kea-ctrl-agent",
            "severity": "INFO"
        } ]

The http-host and http-port parameters specify an IP address and port to which HTTP service will be bound. In the example configuration provided above, the RESTful service will be available at the URL If these parameters are not specified, the default URL is

When using Kea's HA hook library with multi-threading, make sure that the address:port combination used for CA is different from the HA peer URLs, which are strictly for internal HA traffic between the peers. User commands should still be sent via CA.

The trust-anchor, cert-file, key-file, and cert-required parameters specify the TLS setup for HTTP, i.e. HTTPS. If these parameters are not specified, HTTP is used. The TLS/HTTPS support in Kea is described in TLS/HTTPS Support.

As mentioned in Overview of the Kea Control Agent, the CA can forward received commands to the Kea servers for processing. For example, config-get is sent to retrieve the configuration of one of the Kea services. When the CA receives this command, including a service parameter indicating that the client wishes to retrieve the configuration of the DHCPv4 server, the CA forwards the command to that server and passes the received response back to the client. More about the service parameter and the general structure of commands can be found in Management API.

The CA uses UNIX domain sockets to forward control commands and receive responses from other Kea services. The dhcp4, dhcp6, and d2 maps specify the files to which UNIX domain sockets are bound. In the configuration above, the CA connects to the DHCPv4 server via /path/to/the/unix/socket-v4 to forward the commands to it. Obviously, the DHCPv4 server must be configured to listen to connections via this same socket. In other words, the command-socket configuration for the DHCPv4 server and the CA (for that server) must match. Consult Management API for the DHCPv4 Server, Management API for the DHCPv6 Server, and Management API for the D2 Server to learn how the socket configuration is specified for the DHCPv4, DHCPv6, and D2 services.

User contexts can store arbitrary data as long as they are in valid JSON syntax and their top-level element is a map (i.e. the data must be enclosed in curly brackets). Some hook libraries may expect specific formatting; please consult the relevant hook library documentation for details.

User contexts can be specified on either global scope, control socket, basic authentication, or loggers. One other useful feature is the ability to store comments or descriptions; the parser translates a "comment" entry into a user context with the entry, which allows a comment to be attached within the configuration itself.

Basic HTTP authentication was added in Kea 1.9.0; it protects against unauthorized uses of the control agent by local users. For protection against remote attackers, HTTPS and reverse proxy of Secure Connections (in Versions Prior to Kea 1.9.6) provide stronger security.

The authentication is described in the authentication block with the mandatory type parameter, which selects the authentication. Currently only the basic HTTP authentication (type basic) is supported.

The realm authentication parameter is used for error messages when the basic HTTP authentication is required but the client is not authorized.

When the clients authentication list is configured and not empty, basic HTTP authentication is required. Each element of the list specifies a user ID and a password. The user ID is mandatory, must be not empty, and must not contain the colon (:) character. The password is optional; when it is not specified an empty password is used.


The basic HTTP authentication user ID and password are encoded in UTF-8, but the current Kea JSON syntax only supports the Latin-1 (i.e. 0x00..0xff) Unicode subset.

To avoid to expose the password or both the user ID and the associated password these values can be read from files. The syntax was extended by:

  • The directory authentication parameter which handles the common part of file paths. By default the value is the empty string.
  • The``password-file`` client parameter which with the directory parameter specifies the path of a file where the password or when no user ID is given the whole basic HTTP authentication secret before encoding can be read.
  • The user-file client parameter which with the directory parameter specifies the path of a file where the user ID can be read.

When files are used they are read when the configuration is loaded in order to detect configuration errors as soon as possible.

Hook libraries can be loaded by the Control Agent in the same way as they are loaded by the DHCPv4 and DHCPv6 servers. The CA currently supports one hook point - control_command_receive - which makes it possible to delegate processing of some commands to the hook library. The hooks-libraries list contains the list of hook libraries that should be loaded by the CA, along with their configuration information specified with parameters.

Please consult Logging for the details on how to configure logging. The CA's root logger's name is kea-ctrl-agent, as given in the example above.

7.3. Secure Connections (in Versions Prior to Kea 1.9.6)

The Control Agent does not natively support secure HTTP connections, like SSL or TLS, before Kea 1.9.6.

To set up a secure connection, please use one of the available third-party HTTP servers and configure it to run as a reverse proxy to the Control Agent. Kea has been tested with two major HTTP server implementations working as a reverse proxy: Apache2 and nginx. Example configurations, including extensive comments, are provided in the doc/examples/https/ directory.

The reverse proxy forwards HTTP requests received over a secure connection to the Control Agent using unsecured HTTP. Typically, the reverse proxy and the Control Agent are running on the same machine, but it is possible to configure them to run on separate machines as well. In this case, security depends on the protection of the communications between the reverse proxy and the Control Agent.

Apart from providing the encryption layer for the control channel, a reverse proxy server is also often used for authentication of the controlling clients. In this case, the client must present a valid certificate when it connects via reverse proxy. The proxy server authenticates the client by checking whether the presented certificate is signed by the certificate authority used by the server.

To illustrate this, the following is a sample configuration for the nginx server running as a reverse proxy to the Kea Control Agent. The server enables authentication of the clients using certificates.

#   The server certificate and key can be generated as follows:
#   openssl genrsa -des3 -out kea-proxy.key 4096
#   openssl req -new -x509 -days 365 -key kea-proxy.key -out kea-proxy.crt
#   The CA certificate and key can be generated as follows:
#   openssl genrsa -des3 -out ca.key 4096
#   openssl req -new -x509 -days 365 -key ca.key -out ca.crt
#   The client certificate needs to be generated and signed:
#   openssl genrsa -des3 -out kea-client.key 4096
#   openssl req -new -key kea-client.key -out kea-client.csr
#   openssl x509 -req -days 365 -in kea-client.csr -CA ca.crt \
#           -CAkey ca.key -set_serial 01 -out kea-client.crt
#   Note that the "common name" value used when generating the client
#   and the server certificates must differ from the value used
#   for the CA certificate.
#   The client certificate must be deployed on the client system.
#   In order to test the proxy configuration with "curl", run a
#   command similar to the following:
#   curl -k --key kea-client.key --cert kea-client.crt -X POST \
#        -H Content-Type:application/json -d '{ "command": "list-commands" }' \
#         https://kea.example.org/kea
#   curl syntax for basic authentication is -u user:password
#   nginx configuration starts here.

events {

http {
        #   HTTPS server
    server {
        #     Use default HTTPS port.
        listen 443 ssl;
        #     Set server name.
        server_name kea.example.org;

        #   Server certificate and key.
        ssl_certificate /path/to/kea-proxy.crt;
        ssl_certificate_key /path/to/kea-proxy.key;

        #   Certificate Authority. Client certificates must be signed by the CA.
        ssl_client_certificate /path/to/ca.crt;

        # Enable verification of the client certificate.
        ssl_verify_client on;

        # For URLs such as https://kea.example.org/kea, forward the
        # requests to
        location /kea {


The configuration snippet provided above is for testing purposes only. It should be modified according to the security policies and best practices of the administrator's organization.

When using an HTTP client without TLS support, such as kea-shell, it is possible to use an HTTP/HTTPS translator such as stunnel in client mode. A sample configuration is provided in the doc/examples/https/shell/ directory.

7.4. Secure Connections (in Kea 1.9.6 and Newer)

Since Kea 1.9.6, the Control Agent natively supports secure HTTP connections using TLS. This allows protection against users from the node where the agent runs, something that a reverse proxy cannot provide. More about TLS/HTTPS support in Kea can be found in TLS/HTTPS Support.

TLS is configured using three string parameters, giving file names and a boolean parameter:

  • The trust-anchor specifies the Certification Authority file name or directory path.
  • The cert-file specifies the server certificate file name.
  • The key-file specifies the private key file name. The file must not be encrypted.
  • The cert-required specifies whether client certificates are required or optional. The default is to require them and to perform mutual authentication.

The file format is PEM. Either all the string parameters are specified and HTTP over TLS (HTTPS) is used, or none is specified and plain HTTP is used. Configuring only one or two string parameters results in an error.


When client certificates are not required, only the server side is authenticated, i.e. the communication is encrypted with an unknown client. This protects only against passive attacks; active attacks, such as "man-in-the-middle," are still possible.


No standard HTTP authentication scheme cryptographically binds its end entity with TLS. This means that the TLS client and server can be mutually authenticated, but there is no proof they are the same as for the HTTP authentication.

Since Kea 1.9.6, the kea-shell tool supports TLS.

7.5. Starting the Control Agent

The CA is started by running its binary and specifying the configuration file it should use. For example:

$ ./kea-ctrl-agent -c /usr/local/etc/kea/kea-ctrl-agent.conf

It can be started by keactrl as well (see Managing Kea with keactrl).

7.6. Connecting to the Control Agent

For an example of a tool that can take advantage of the RESTful API, see The Kea Shell.