agent-ctl

Agent Control tool

• Overview
• Audience and environment
• Full details
• Code summary
• Running the tool
  • Prerequisites
  • Connect to a real Kata Container
  • Run the tool and the agent in the same environment

Overview

The Kata Containers agent control tool (kata-agent-ctl) is a low-level test tool. It allows basic interaction with the Kata Containers agent, kata-agent, that runs inside the virtual machine.

Unlike the Kata Runtime, which only ever makes sequences of correctly ordered and valid agent API calls, this tool allows users to make arbitrary agent API calls and to control their parameters.

Audience and environment

Warning:

This tool is for advanced users familiar with the low-level agent API calls. Further, it is designed to be run on test and development systems only: since the tool can make arbitrary API calls, it is possible to easily confuse irrevocably other parts of the system or even kill a running container or sandbox.

Full details

For a usage statement, run:

$ cargo run -- --help

To see some examples, run:

$ cargo run -- examples

Code summary

The table below summarises where to look to learn more about both this tool, the agent protocol and the client and server implementations.

Description	File	Example RPC or function	Example summary
Protocol buffers definition of the Kata Containers Agent API protocol	agent.proto	CreateContainer	API to create a Kata container.
Agent Control (client) API calls	src/client.rs	agent_cmd_container_create()	Agent Control tool function that calls the CreateContainer API.
Agent (server) API implementations	rpc.rs	create_container()	Server function that implements the CreateContainers API.

Running the tool

Prerequisites

It is necessary to create an OCI bundle to use the tool. The simplest method is:

$ bundle_dir="bundle"
$ rootfs_dir="$bundle_dir/rootfs"
$ image="busybox"
$ mkdir -p "$rootfs_dir" && (cd "$bundle_dir" && runc spec)
$ sudo docker export $(sudo docker create "$image") | tar -C "$rootfs_dir" -xvf -

Connect to a real Kata Container

1. Start a Kata Container

2. Establish the VSOCK guest CID number for the virtual machine:

   Assuming you are running a single QEMU based Kata Container, you can look at the program arguments to find the (randomly-generated) guest-cid= option value:

   $ guest_cid=$(ps -ef | grep qemu-system-x86_64 | egrep -o "guest-cid=[^,][^,]*" | cut -d= -f2)

3. Run the tool to connect to the agent:

   $ cargo run -- -l debug connect --bundle-dir "${bundle_dir}" --server-address "vsock://${guest_cid}:1024" -c Check -c GetGuestDetails

   This examples makes two API calls:

   • It runs Check to see if the agent's RPC server is serving.
   • It then runs GetGuestDetails to establish some details of the environment the agent is running in.

Run the tool and the agent in the same environment

Warnings:

• This method is only for testing and development!
• Only continue if you are using a non-critical system (such as a freshly installed VM environment).

1. Start the agent, specifying a local socket for it to communicate on:

   $ sudo KATA_AGENT_SERVER_ADDR=unix:///tmp/foo.socket target/x86_64-unknown-linux-musl/release/kata-agent

2. Run the tool in the same environment:

   $ cargo run -- -l debug connect --server-address "unix://@/tmp/foo.socket" --bundle-dir "$bundle_dir" -c Check -c GetGuestDetails

   Note:

   The @ in the server address is required - it denotes an abstract socket which the agent requires (see unix(7)).