python-google-compute-engine

Python Linux Guest Environment for Google Compute Engine

This package contains the Python guest environment which was historically installed on Google supported Compute Engine Linux images. It has been superceded by the guest agent, but is still provided here for interested consumers. The google_compute_engine library is also available as a PyPI package.

Table of Contents

• Overview
• Common Libraries
  • Metadata Watcher
  • Logging
  • Configuration Management
  • File Management
  • Network Utilities
• Daemons
  • Accounts
  • Clock Skew
  • Network
• Instance Setup
• Metadata Scripts
• Configuration

Overview

The Linux guest environment is made up of the following components:

• Accounts daemon to setup and manage user accounts, and to enable SSH key based authentication.
• Clock skew daemon to keep the system clock in sync after VM start and stop events.
• Instance setup scripts to execute VM configuration scripts during boot.
• Network daemon that handles network setup for multiple network interfaces on boot and integrates network load balancing with forwarding rule changes into the guest.
• Metadata scripts to run user-provided scripts at VM startup and shutdown.

The Linux guest environment is written in Python and is version agnostic between Python 2.6 and 3.7. There is complete unittest coverage for every Python library and script. The design of various guest libraries, daemons, and scripts, are detailed in the sections below.

Common Libraries

The Python libraries are shared with each of the daemons and the instance setup tools.

Metadata Watcher

The guest environment relies upon retrieving content from the metadata server to configure the VM environment. A metadata watching library handles all communication with the metadata server.

The library exposes two functions:

• GetMetadata immediately retrieves the contents of the metadata server for a given metadata key. The function catches and logs any connection related exceptions. The metadata server content is returned as a deserialized JSON object.
• WatchMetadata continuously makes a hanging GET, watching for changes to the specified contents of the metadata server. When the request closes, the watcher verifies the etag was updated. In case of an update, the etag is updated and a provided handler function is called with the deserialized JSON metadata content. The WatchMetadata function should never terminate; it catches and logs any connection related exceptions, and catches and logs any exception generated from calling the handler.

Metadata server requests have custom retry logic for metadata server unavailability; by default, any request has one minute to complete before the request is cancelled. In case of a brief network outage where the metadata server is unavailable, there is a short delay between retries.

Logging

The Google added daemons and scripts write to the serial port for added transparency. A common logging library is a thin wrapper around the Python logging module. The library configures appropriate SysLog handlers, sets the logging formatter, and provides a debug options for added logging and console output.

Configuration Management

A configuration file allows users to disable daemons and modify instance setup behaviors from a single location. Guest environment daemons and scripts need a mechanism to integrate user settings into the guest. A configuration management library retrieves and modifies these settings.

The library exposes the following functions:

• GetOptionString retrieves the value for a configuration option. The type of the value is a string if set.
• GetOptionBool retrieves the value for a configuration option. The type of the value is a boolean if set.
• SetOption sets the value of an option in the config file. An overwrite flag specifies whether to replace an existing value.
• WriteConfig writes the configuration values to a file. The function is responsible for locking the file, preventing concurrent writes, and writing a file header if one is provided.

File Management

Guest environment daemons and scripts use a common library for file management. The library provides the following functions:

• SetPermissions unifies the logic to set permissions and simplify file creation across the various Linux distributions. The function sets the mode, UID, and GID, of a provided path. On supported OS configurations that user SELinux, the SELinux context is automatically set.
• LockFile is a context manager that simplifies the process of file locking in Python. The function sets up an flock and releases the lock on exit.

Network Utilities

A network-utilities library retrieves information about a network interface. The library is used for IP forwarding and for setting up an Ethernet interface on boot. The library exposes a GetNetworkInterface function that retrieves the network interface name associated with a MAC address.

Daemons

The guest environment daemons import and use the common libraries described above. Each daemon reads the configuration file before execution. This allows a user to easily disable undesired functionality. Additional daemon behaviors are detailed below.

Accounts

The accounts daemon is responsible for provisioning and deprovisioning user accounts. The daemon grants permissions to user accounts, and updates the list of authorized keys that have access to accounts based on metadata SSH key updates. User account creation is based on adding and remove SSH Keys stored in metadata.

The accounts management daemon has the following behaviors.

• Administrator permissions are managed with a google-sudoers Linux group.
• All users provisioned by the account daemon are added to the google-sudoers group.
• The daemon stores a file in the guest to preserve state for the user accounts managed by Google.
• The authorized keys file for a Google managed user is deleted when all SSH keys for the user are removed from metadata.
• User accounts not managed by Google are not modified by the accounts daemon.

Clock Skew

The clock skew daemon is responsible for syncing the software clock with the hypervisor clock after a stop/start event or after a migration. Preventing clock skew may result in system time has changed messages in VM logs.

Network

The network daemon uses network interface metadata to manage the network interfaces in the guest by performing the following tasks:

• Enabled all associated network interfaces on boot. Network interfaces are specified by MAC address in instance metadata.
• Uses IP forwarding metadata to setup or remove IP routes in the guest.
  • Only IPv4 IP addresses are currently supported.
  • Routes are set on the default Ethernet interface determined dynamically.
  • Google routes are configured, by default, with the routing protocol ID 66. This ID is a namespace for daemon configured IP addresses.

Instance Setup

Instance setup runs during VM boot. The script configures the Linux guest environment by performing the following tasks.

• Optimize for local SSD.
• Enable multi-queue on all the virtionet devices.
• Wait for network availability.
• Set SSH host keys the first time the instance is booted.
• Set the boto config for using Google Cloud Storage.
• Create the defaults configuration file.

The defaults configuration file incorporates any user provided setting in /etc/default/instance_configs.cfg.template and does not override other conflicting settings. This allows package updates without overriding user configuration.

Metadata Scripts

Metadata scripts implement support for running user provided startup scripts and shutdown scripts. The guest support for metadata scripts is implemented in Python with the following design details.

• Metadata scripts are executed in a shell.
• If multiple metadata keys are specified (e.g. startup-script and startup-script-url) both are executed.
• If multiple metadata keys are specified (e.g. startup-script and startup-script-url) a URL is executed first.
• The exit status of a metadata script is logged after completed execution.

Configuration

Users of Google provided images may configure the guest environment behaviors using a configuration file. To make configuration changes, add settings to /etc/default/instance_configs.cfg.template. If you are attempting to change the behavior of a running instance, run /usr/bin/google_instance_setup before reloading the affected daemons.

Linux distributions looking to include their own defaults can specify settings in /etc/default/instance_configs.cfg.distro. These settings will not override /etc/default/instance_configs.cfg.template. This enables distribution settings that do not override user configuration during package update.

The following are valid user configuration options.

Section	Option	Value
Accounts	deprovision_remove	true makes deprovisioning a user destructive.
Accounts	groups	Comma separated list of groups for newly provisioned users.
Accounts	useradd_cmd	Command string to create a new user.
Accounts	userdel_cmd	Command string to delete a user.
Accounts	usermod_cmd	Command string to modify a user's groups.
Accounts	gpasswd_add_cmd	Command string to add a user to a group.
Accounts	gpasswd_remove_cmd	Command string to remove a user from a group.
Accounts	groupadd_cmd	Command string to create a new group.
Daemons	accounts_daemon	false disables the accounts daemon.
Daemons	clock_skew_daemon	false disables the clock skew daemon.
Daemons	ip_forwarding_daemon	false (deprecated) skips IP forwarding.
Daemons	network_daemon	false disables the network daemon.
InstanceSetup	host_key_types	Comma separated list of host key types to generate.
InstanceSetup	optimize_local_ssd	false prevents optimizing for local SSD.
InstanceSetup	network_enabled	false skips instance setup functions that require metadata.
InstanceSetup	set_boto_config	false skips setting up a boto config.
InstanceSetup	set_host_keys	false skips generating host keys on first boot.
InstanceSetup	set_multiqueue	false skips multiqueue driver support.
IpForwarding	ethernet_proto_id	Protocol ID string for daemon added routes.
IpForwarding	ip_aliases	false disables setting up alias IP routes.
IpForwarding	target_instance_ips	false disables internal IP address load balancing.
MetadataScripts	default_shell	String with the default shell to execute scripts.
MetadataScripts	run_dir	String base directory where metadata scripts are executed.
MetadataScripts	startup	false disables startup script execution.
MetadataScripts	shutdown	false disables shutdown script execution.
NetworkInterfaces	setup	false skips network interface setup.
NetworkInterfaces	ip_forwarding	false skips IP forwarding.
NetworkInterfaces	dhcp_command	String path for alternate dhcp executable used to enable network interfaces.

Setting network_enabled to false will skip setting up host keys and the boto config in the guest. The setting may also prevent startup and shutdown script execution.