Kea: Control Agent Component

| Kea 3.0.3-git

Control Agent Component

Kea 1.2 release has introduced the Control Agent component (CA), which is started by the "kea-ctrl-agent" binary.

The CA exposes a RESTful API which is used by the administrators to manage Kea servers' instances.

In the most typical case, the CA forwards commands received over the RESTful API to the respective Kea servers, e.g. DHCPv4 server, DHCPv6 server etc. The communication between the CA and other servers is established with the use of unix domain sockets. This is possible because the CA is running on the same system as other Kea services to which messages are forwarded.

The CA can forward the same command to multiple Kea services and return an aggregated response (from all services) over the RESTful API. The "service" parameter included in the client's command can contain one or more services at which the command is targeted. The CA will iterate over this list and forward the command to each of them individually.

In some cases, the commands containing the "service" value can be handled directly by the CA. This is usually the case when the CA is running with hooks libraries attached. The hooks libraries must implement callouts for the "control_command_receive" hook point, which will be invoked by the CA when the command is received. If the hooks libraries set the 'skip' status, it is an indication to the CA that the command has been processed by the CA and that it should return the response created by the hooks libraries to the client. An example of the hooks library attached to the CA and handling the commands for other services is a library which stores or retrieves some data from the SQL database.

The "service" parameter is optional. If it is not included in the command (or it is an empty list), this indicates that the command relates to the CA and that the CA should handle it, e.g. return its own configuration in response to a "config-get" command.

# Receiving commands over HTTP

Control Agent uses libkea-http library to establish HTTP connections, receive messages and send responses over HTTP. This library uses boost ASIO for creating TCP connections and asynchronously receive and send the data over the sockets.

The isc::http::HttpListener provides an entry point to this library. It is used by the CA to bind the acceptor to the specific address and port. When the client connects to this address and port, the acceptor's callback function is invoked which opens a new connection and starts receiving data over that socket. The isc::http::HttpConnection implements the logic to read and parse received data. Each new TCP connection is associated with unique instance of the isc::http::HttpConnection When a portion of data is received (asynchronously) over the socket it is provided to the instance of the isc::http::HttpRequestParser object (unique per connection) and data parsing is continued until the parser runs out of data or until the entire HTTP request has been received. The isc::http::HttpRequestParser signals these events using the isc::http::HttpRequestParser::needData and isc::http::HttpRequestParser::httpParseOk respectively.

libkea-http is designed to handle processing messages carrying different content types. The Control Agent uses "application/json" content type which describes messages with JSON structures carried within the message body. The JSON structures represent commands sent to the Kea server(s) by controlling clients. libkea-http provides generic classes (derived from isc::http::HttpRequest) which facilitate validation of messages holding various content types. CA uses isc::http::PostHttpRequestJson, which encapsulate messages sent using HTTP POST and including JSON content, to represent received messages.

# Creating HTTP responses

The isc::http::HttpResponseCreatorFactory is an interface which should be implemented by components using libkea-http to generate instances of the HTTP responses of a desired type. The instance of the factory class is provided to the isc::http::HttpListener via its constructor. The listener calls an implementation of the isc::http::HttpResponseCreatorFactory::create when a new HTTP message has been received and parsed.

The CA component includes the isc::agent::CtrlAgentResponseCreatorFactory class. Its `create()` method implementation returns an instance of the isc::agent::CtrlAgentResponseCreator, which is a derivation of the isc::http::HttpResponseCreator. This creator creates instances of the isc::http::HttpResponseJson, holding responses to the commands in the JSON format.

# Handling commands with Command Manager

The isc::agent::CtrlAgentCommandMgr is a derivation of the isc::config::HookedCommandMgr which adds the capability to forward commands received over HTTP to specific Kea servers. The isc::agent::CtrlAgentCommandMgr forwards commands over a Unix domain socket, using isc::asiolink::UnixDomainSocket class. All responses to a particular command (possibly received from multiple Kea servers) are aggregated within a JSON list and sent back to the controlling client over HTTP.

In some cases the responses may be generated locally (without forwarding). Typically, the command will be generated by the CA when the command sent by the client lacks the "service" parameter, which indicates that the command is targeted at the CA itself. In some cases the commands can also be processed by the hooks libraries attached to the CA.

# Security considerations

The Control Agent doesn't provide any mechanisms to secure the communication over the RESTful API. In the design of the CA we have considered including built-in HTTPS solutions (HTTP + TLS), making use of crypto libraries supported by Kea. It was eventually decided to not implement the secure layer within Kea for the following reasons:

- additional code complexity which requires maintenance, bug fixing and monitoring for security vulnerabilities in the OpenSSL/Botan code,
- OpenSSL/Botan code may be awkward to use and it is likely we wouldn't implement it right,
- need to support two crypto backends: OpenSSL and Botan which puts significant burden on Kea maintenance.

In the installations where securing command channel is critical (most of the installations?), a reverse HTTP proxy can be set up using one of the third party HTTP server implementations, e.g. Apache, nginx etc.