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 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 more or 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": "10.20.30.40",
        "http-port": 8000,
        "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 under the URL of http://10.20.30.40:8000/. If these parameters are not specified, the default URL is http://127.0.0.1:8000/.

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 will connect 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 this 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.

Warning

“dhcp4-server”, “dhcp6-server”, and “d2-server” were renamed to “dhcp4”, “dhcp6”, and “d2” respectively in Kea 1.2. If you are migrating from Kea 1.2, you must modify your CA configuration to use this new naming convention.

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.

The basic HTTP authentication was added by Kea 1.9.0. It protects against not authorized uses of the control agent by local users. For the protection against remote attackers HTTPS and reverse proxy of Secure Connections provide a 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 message when the basic HTTP authentication is required but the client is not authorized.

When the clients authentication list is configured and not empty the 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.

Note

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.

Hooks 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 hooks library. The hooks-libraries list contains the list of hooks libraries that should be loaded by the CA, along with their configuration information specified with parameters.

Please consult Logging for the details 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

The Control Agent does not natively support secure HTTP connections like SSL or TLS. In order to setup 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 certificate 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 http://127.0.0.1:8000.
        location /kea {
            proxy_pass http://127.0.0.1:8000;
        }
    }
}

Note

Note that the configuration snippet provided above is for testing purposes only. It should be modified according to the security policies and best practices of your organization.

When you use an HTTP client without TLS support as kea-shell, you can 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. 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).

7.5. Connecting to the Control Agent

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