Kata Agent

Overview

The Kata agent is a long running process that runs inside the Virtual Machine (VM) (also known as the "pod" or "sandbox").

The agent is packaged inside the Kata Containers guest image which is used to boot the VM. Once the runtime has launched the configured hypervisor to create a new VM, the agent is started. From this point on, the agent is responsible for creating and managing the life cycle of the containers inside the VM.

For further details, see the architecture document.

Audience

If you simply wish to use Kata Containers, it is not necessary to understand the details of how the agent operates. Please see the installation documentation for details of how deploy Kata Containers (which will include the Kata agent).

The remainder of this document is only useful for developers and testers.

Build from Source

Since the agent is written in the Rust language this section assumes the tool chain has been installed using standard Rust rustup tool.

Build with musl

If you wish to build the agent with the musl C library, you need to run the following commands:

bash

$ arch=$(uname -m)
$ rustup target add "${arch}-unknown-linux-musl"
$ sudo ln -s /usr/bin/g++ /bin/musl-g++

Note:

It is not currently possible to build using musl on ppc64le and s390x since both platforms lack the musl target.

Build the agent binary

The following steps download the Kata Containers source files and build the agent:

bash

$ GOPATH="${GOPATH:-$HOME/go}"
$ dir="$GOPATH/src/github.com/kata-containers"
$ git -C ${dir} clone --depth 1 https://github.com/kata-containers/kata-containers
$ make -C ${dir}/kata-containers/src/agent

Change the agent API

The Kata runtime communicates with the Kata agent using a ttRPC based API protocol.

This ttRPC API is defined by a set of protocol buffers files. The protocol files are used to generate the bindings for the following components:

Component	Language	Generation method `[*]`	Tooling required
runtime	Golang	Run, `make generate-protocols`	`protoc`
agent	Rust	Run, `make`

Key:

[*] - All commands must be run in the agent repository.

If you wish to change the API, these files must be regenerated. Although the rust code will be automatically generated by the build script, the Golang code generation requires the external protoc command to be available in $PATH.

To install the protoc command on a Fedora/CentOS/RHEL system:

bash

$ sudo dnf install -y protobuf-compiler

Custom guest image and kernel assets

If you wish to develop or test changes to the agent, you will need to create a custom guest image using the osbuilder tool. You may also wish to create a custom guest kernel.

Once created, configure Kata Containers to use these custom assets to allow you to test your changes.

Note:

To simplify development and testing, you may wish to run the agent stand alone initially.

Tracing

For details of tracing the operation of the agent, see the tracing documentation.

Run the agent stand alone

Although the agent is designed to run in a VM environment, for development and testing purposes it is possible to run it as a normal application.

When run in this way, the agent can be controlled using the low-level Kata agent control tool, rather than the Kata runtime.

For further details, see the agent control tool documentation.

Agent options

The kata agent has the ability to configure agent options in guest kernel command line at runtime. Currently, the following agent options can be configured:

Option	Name	Description	Type	Default
`agent.config_file`	`Configuration file`	Allow to configure options through a configuration file from the root filesystem	string	`""`
`agent.container_pipe_size`	`Container pipe sizes`	Allow to configure the stdout or stderr pipe sizes	integer	`0`
`agent.debug_console`	Debug console flag	Allow to connect guest OS running inside hypervisor Connect using `kata-runtime exec <sandbox-id>`	boolean	`false`
`agent.debug_console_vport`	Debug console port	Allow to specify the `vsock` port to connect the debugging console	integer	`0`
`agent.devmode`	Developer mode	Allow the agent process to coredump	boolean	`false`
`agent.guest_components_rest_api`	`api-server-rest` configuration	Select the features that the API Server Rest attestation component will run with. Valid values are `all`, `attestation`, `resource`	string	`resource`
`agent.guest_components_procs`	guest-components processes	Attestation-related processes that should be spawned as children of the guest. Valid values are `none`, `attestation-agent`, `confidential-data-hub` (implies `attestation-agent`), `api-server-rest` (implies `attestation-agent` and `confidential-data-hub`)	string	`api-server-rest`
`agent.hotplug_timeout`	Hotplug timeout	Allow to configure hotplug timeout(seconds) of block devices	integer	`3`
`agent.cdh_api_timeout`	Confidential Data Hub (CDH) API timeout	Allow to configure CDH API timeout(seconds)	integer	`50`
`agent.image_pull_timeout`	Confidential Data Hub (CDH) Image Pull API timeout	Allow to configure CDH API image pull timeout(seconds)	integer	`1200`
`agent.https_proxy`	HTTPS proxy	Allow to configure `https_proxy` in the guest	string	`""`
`agent.image_registry_auth`	Image registry credential URI	The URI to where image-rs can find the credentials for pulling images from private registries e.g. `file:///root/.docker/config.json` to read from a file in the guest image, or `kbs:///default/credentials/test` to get the file from the KBS	string	`""`
`agent.enable_signature_verification`	Image security policy flag	Whether enable image security policy enforcement. If `true`, the resource indexed by URI `agent.image_policy_file` will be got to work as image pulling policy.	string	`""`
`agent.image_policy_file`	Image security policy URI	The URI to where image-rs Typical policy URIs are like `file:///etc/image.json` to read from a file in the guest image, or `kbs:///default/security-policy/test` to get the file from the KBS	string	`""`
`agent.log`	Log level	Allow the agent log level to be changed (produces more or less output)	string	`"info"`
`agent.log_vport`	Log port	Allow to specify the `vsock` port to read logs	integer	`0`
`agent.no_proxy`	NO proxy	Allow to configure `no_proxy` in the guest	string	`""`
`agent.passfd_listener_port`	File descriptor passthrough IO listener port	Allow to set the file descriptor passthrough IO listener port	integer	`0`
`agent.secure_image_storage_integrity`	Image storage integrity	Allow to use `dm-integrity` to protect the integrity of encrypted block volume	boolean	`false`
`agent.server_addr`	Server address	Allow the ttRPC server address to be specified	string	`"vsock://-1:1024"`
`agent.trace`	Trace mode	Allow to static tracing	boolean	`false`
`systemd.unified_cgroup_hierarchy`	`Cgroup hierarchy`	Allow to setup v2 cgroups	boolean	`false`

Note: Accepted values for some agent options

agent.config_file: If we enable agent.config_file in guest kernel command line, we will generate the agent config from it. The agent will fail to start if the configuration file is not present, or if it can't be parsed properly.

agent.devmode: true | false

agent.hotplug_timeout, agent.image_pull_timeout and agent.cdh_api_timeout: a whole number of seconds

agent.log: "critical"("fatal" | "panic") | "error" | "warn"("warning") | "info" | "debug"

agent.server_addr: "{VSOCK_ADDR}:{VSOCK_PORT}"

agent.trace: true | false

systemd.unified_cgroup_hierarchy: true | false

For instance, you can enable the debug console and set the agent log level to debug by configuring the guest kernel command line in the configuration file:

toml

kernel_params = "agent.debug_console agent.log=debug agent.hotplug_timeout=10"

results in:

bash

{"msg":"announce","level":"INFO","subsystem":"root","version":"0.1.0","pid":"214","source":"agent","name":"kata-agent","config":"AgentConfig { debug_console: true, dev_mode: false, log_level: Debug, hotplug_timeout: 10s, debug_console_vport: 0, log_vport: 0, container_pipe_size: 0, server_addr: "vsock://-1:1024", passfd_listener_port: 0, unified_cgroup_hierarchy: false, tracing: false, supports_seccomp: true }","agent-version":"3.3.0-alpha0"}