Communication between container and host

Introduction

Communication between the hosted workload (container) and its host while not strictly needed is a pretty useful feature.

In LXD, this feature is implemented through a /dev/lxd/sock node which is created and setup for all LXD containers.

This file is a Unix socket which processes inside the container can connect to. It's multi-threaded so multiple clients can be connected at the same time.

Implementation details

LXD on the host binds /var/lib/lxd/devlxd and starts listening for new connections on it.

This socket is then bind-mounted into every single container started by LXD at /dev/lxd/sock.

The bind-mount is required so we can exceed 4096 containers, otherwise, LXD would have to bind a different socket for every container, quickly reaching the FD limit.

Authentication

Queries on /dev/lxd/sock will only return information related to the requesting container. To figure out where a request comes from, LXD will extract the initial socket ucred and compare that to the list of containers it manages.

Protocol

The protocol on /dev/lxd/sock is plain-text HTTP with JSON messaging, so very similar to the local version of the LXD protocol.

Unlike the main LXD API, there is no background operation and no authentication support in the /dev/lxd/sock API.

REST-API

API structure

• /
• /1.0
  • /1.0/config
  • /1.0/config/{key}
  • /1.0/events
  • /1.0/images/{fingerprint}/export
  • /1.0/meta-data

API details

/

GET

• Description: List of supported APIs
• Return: list of supported API endpoint URLs (by default ['/1.0'])

Return value:

[
    "/1.0"
]

/1.0

GET

• Description: Information about the 1.0 API
• Return: dict

Return value:

{
    "api_version": "1.0"
}

/1.0/config

GET

• Description: List of configuration keys
• Return: list of configuration keys URL

Note that the configuration key names match those in the container config, however not all configuration namespaces will be exported to /dev/lxd/sock. Currently only the user.* keys are accessible to the container.

At this time, there also aren't any container-writable namespace.

Return value:

[
    "/1.0/config/user.a"
]

/1.0/config/<KEY>

GET

• Description: Value of that key
• Return: Plain-text value

Return value:

blah

/1.0/events

GET

• Description: websocket upgrade
• Return: none (never ending flow of events)

Supported arguments are:

• type: comma separated list of notifications to subscribe to (defaults to all)

The notification types are:

• config (changes to any of the user.* config keys)
• device (any device addition, change or removal)

This never returns. Each notification is sent as a separate JSON dict:

{
    "timestamp": "2017-12-21T18:28:26.846603815-05:00",
    "type": "device",
    "metadata": {
        "name": "kvm",
        "action": "added",
        "config": {
            "type": "unix-char",
            "path": "/dev/kvm"
        }
    }
}

{
    "timestamp": "2017-12-21T18:28:26.846603815-05:00",
    "type": "config",
    "metadata": {
        "key": "user.foo",
        "old_value": "",
        "value": "bar"
    }
}

/1.0/images/<FINGERPRINT>/export

GET

• Description: Download a public/cached image from the host
• Return: raw image or error
• Access: Requires security.devlxd.images set to true

Return value:

See /1.0/images/<FINGERPRINT>/export in the daemon API.

/1.0/meta-data

GET

• Description: Container meta-data compatible with cloud-init
• Return: cloud-init meta-data

Return value:

#cloud-config
instance-id: abc
local-hostname: abc