Azure Arc agent

Άρθρο
09/19/2024

When you enable guest management on VMware VMs, Azure Connected Machine agent is installed on the VMs. This is the same agent Arc-enabled servers use. The Azure Connected Machine agent enables you to manage your Windows and Linux machines hosted outside of Azure on your corporate network or other cloud providers. This article provides an architectural overview of Azure connected machine agent.

Agent components

The Azure Connected Machine agent package contains several logical components bundled together:

The Hybrid Instance Metadata service (HIMDS) manages the connection to Azure and the connected machine's Azure identity.
The guest configuration agent provides functionality such as assessing whether the machine complies with required policies and enforcing compliance.

Note the following behavior with Azure Policy guest configuration for a disconnected machine:
- An Azure Policy assignment that targets disconnected machines is unaffected.
- Guest assignment is stored locally for 14 days. Within the 14-day period, if the Connected Machine agent reconnects to the service, policy assignments are reapplied.
- Assignments are deleted after 14 days and aren't reassigned to the machine after the 14-day period.
The Extension agent manages VM extensions, including install, uninstall, and upgrade. Azure downloads extensions and copies them to the %SystemDrive%\%ProgramFiles%\AzureConnectedMachineAgent\ExtensionService\downloads folder on Windows, and to /opt/GC_Ext/downloads on Linux. On Windows, the extension installs to the path %SystemDrive%\Packages\Plugins\<extension>, and on Linux the extension installs to /var/lib/waagent/<extension>.

Note

The Azure Monitor agent (AMA) is a separate agent that collects monitoring data, and it does not replace the Connected Machine agent; the AMA only replaces the Log Analytics agent, Diagnostics extension, and Telegraf agent for both Windows and Linux machines.

Agent resources

The following information describes the directories and user accounts used by the Azure Connected Machine agent.

Windows agent installation details

The Windows agent is distributed as a Windows Installer package (MSI). Download the Windows agent from the Microsoft Download Center. Installing the Connected Machine agent for Window applies the following system-wide configuration changes:

The installation process creates the following folders during setup.

Directory	Description
%ProgramFiles%\AzureConnectedMachineAgent	azcmagent CLI and instance metadata service executables.
%ProgramFiles%\AzureConnectedMachineAgent\ExtensionService\GC	Extension service executables.
%ProgramFiles%\AzureConnectedMachineAgent\GCArcService\GC	Guest configuration (policy) service executables.
%ProgramData%\AzureConnectedMachineAgent	Configuration, log, and identity token files for azcmagent CLI and instance metadata service.
%ProgramData%\GuestConfig	Extension package downloads, guest configuration (policy) definition downloads, and logs for the extension and guest configuration services.
%SYSTEMDRIVE%\packages	Extension package executables.

Installing the agent creates the following Windows services on the target machine.

Service name	Display name	Process name	Description
himds	Azure Hybrid Instance Metadata Service	himds	Synchronizes metadata with Azure and hosts a local REST API for extensions and applications to access the metadata and request Microsoft Entra managed identity tokens
GCArcService	Guest configuration Arc Service	gc_service	Audits and enforces Azure guest configuration policies on the machine.
ExtensionService	Guest configuration Extension Service	gc_service	Installs, updates, and manages extensions on the machine.

Agent installation creates the following virtual service account.

Virtual Account Description

NT SERVICE\himds Unprivileged account used to run the Hybrid Instance Metadata Service.

Tip

This account requires the Log on as a service right. This right is automatically granted during agent installation, but if your organization configures user rights assignments with Group Policy, you might need to adjust your Group Policy Object to grant the right to NT SERVICE\himds or NT SERVICE\ALL SERVICES to allow the agent to function.
Agent installation creates the following local security group.

Security group name Description

Hybrid agent extension applications Members of this security group can request Microsoft Entra tokens for the system-assigned managed identity
Agent installation creates the following environmental variables

Name Default value Description

IDENTITY_ENDPOINT http://localhost:40342/metadata/identity/oauth2/token

IMDS_ENDPOINT http://localhost:40342

Virtual Account	Description
NT SERVICE\himds	Unprivileged account used to run the Hybrid Instance Metadata Service.

Security group name	Description
Hybrid agent extension applications	Members of this security group can request Microsoft Entra tokens for the system-assigned managed identity

Name	Default value	Description
IDENTITY_ENDPOINT	`http://localhost:40342/metadata/identity/oauth2/token`
IMDS_ENDPOINT	`http://localhost:40342`

There are several log files available for troubleshooting, described in the following table.

Log	Description
%ProgramData%\AzureConnectedMachineAgent\Log\himds.log	Records details of the heartbeat and identity agent component.
%ProgramData%\AzureConnectedMachineAgent\Log\azcmagent.log	Contains the output of the azcmagent tool commands.
%ProgramData%\GuestConfig\arc_policy_logs\gc_agent.log	Records details about the guest configuration (policy) agent component.
%ProgramData%\GuestConfig\ext_mgr_logs\gc_ext.log	Records details about extension manager activity (extension install, uninstall, and upgrade events).
%ProgramData%\GuestConfig\extension_logs	Directory containing logs for individual extensions.

The process creates the local security group Hybrid agent extension applications.
After uninstalling the agent, the following artifacts remain:
- %ProgramData%\AzureConnectedMachineAgent\Log
- %ProgramData%\AzureConnectedMachineAgent
- %ProgramData%\GuestConfig
- %SystemDrive%\packages

Linux agent installation details

The preferred package format for the distribution (.rpm or .deb) that's hosted in the Microsoft package repository provides the Connected Machine agent for Linux. The shell script bundle Install_linux_azcmagent.sh installs and configures the agent.

Installing, upgrading, and removing the Connected Machine agent isn't required after server restart.

Installing the Connected Machine agent for Linux applies the following system-wide configuration changes.

Setup creates the following installation folders.

Directory	Description
/opt/azcmagent/	azcmagent CLI and instance metadata service executables.
/opt/GC_Ext/	Extension service executables.
/opt/GC_Service/	Guest configuration (policy) service executables.
/var/opt/azcmagent/	Configuration, log and identity token files for azcmagent CLI and instance metadata service.
/var/lib/GuestConfig/	Extension package downloads, guest configuration (policy) definition downloads, and logs for the extension and guest configuration services.

Installing the agent creates the following daemons.

Service name	Display name	Process name	Description
himdsd.service	Azure Connected Machine Agent Service	himds	This service implements the Hybrid Instance Metadata service (IMDS) to manage the connection to Azure and the connected machine's Azure identity.
gcad.service	GC Arc Service	gc_linux_service	Audits and enforces Azure guest configuration policies on the machine.
extd.service	Extension Service	gc_linux_service	Installs, updates, and manages extensions on the machine.

There are several log files available for troubleshooting, described in the following table.

Log	Description
/var/opt/azcmagent/log/himds.log	Records details of the heartbeat and identity agent component.
/var/opt/azcmagent/log/azcmagent.log	Contains the output of the azcmagent tool commands.
/var/lib/GuestConfig/arc_policy_logs	Records details about the guest configuration (policy) agent component.
/var/lib/GuestConfig/ext_mgr_logs	Records details about extension manager activity (extension install, uninstall, and upgrade events).
/var/lib/GuestConfig/extension_logs	Directory containing logs for individual extensions.

Agent installation creates the following environment variables, set in /lib/systemd/system.conf.d/azcmagent.conf.

Name Default value Description

IDENTITY_ENDPOINT http://localhost:40342/metadata/identity/oauth2/token

IMDS_ENDPOINT http://localhost:40342
After uninstalling the agent, the following artifacts remain:
- /var/opt/azcmagent
- /var/lib/GuestConfig

Name	Default value	Description
IDENTITY_ENDPOINT	`http://localhost:40342/metadata/identity/oauth2/token`
IMDS_ENDPOINT	`http://localhost:40342`

Agent resource governance

The Azure Connected Machine agent is designed to manage agent and system resource consumption. The agent approaches resource governance under the following conditions:

The Guest Configuration agent can use up to 5% of the CPU to evaluate policies.

The Extension Service agent can use up to 5% of the CPU to install, upgrade, run, and delete extensions. Some extensions might apply more restrictive CPU limits once installed. The following exceptions apply:

Extension type	Operating system	CPU limit
AzureMonitorLinuxAgent	Linux	60%
AzureMonitorWindowsAgent	Windows	100%
AzureSecurityLinuxAgent	Linux	30%
LinuxOsUpdateExtension	Linux	60%
MDE.Linux	Linux	60%
MicrosoftDnsAgent	Windows	100%
MicrosoftMonitoringAgent	Windows	60%
OmsAgentForLinux	Windows	60%

During normal operations, defined as the Azure Connected Machine agent being connected to Azure and not actively modifying an extension or evaluating a policy, you can expect the agent to consume the following system resources:

	Windows	Linux
CPU usage (normalized to 1 core)	0.07%	0.02%
Memory usage	57 MB	42 MB

The performance data above was gathered in April 2023 on virtual machines running Windows Server 2022 and Ubuntu 20.04. The actual agent performance and resource consumption vary based on the hardware and software configuration of your servers.

Instance metadata

Metadata information about a connected machine is collected after the Connected Machine agent registers with Azure Arc-enabled servers, specifically:

Operating system name, type, and version
Computer name
Computer manufacturer and model
Computer fully qualified domain name (FQDN)
Domain name (if joined to an Active Directory domain)
Active Directory and DNS fully qualified domain name (FQDN)
UUID (BIOS ID)
Connected Machine agent heartbeat
Connected Machine agent version
Public key for managed identity
Policy compliance status and details (if using guest configuration policies)
SQL Server installed (Boolean value)
Cluster resource ID (for Azure Local nodes)
Hardware manufacturer
Hardware model
CPU family, socket, physical core, and logical core counts
Total physical memory
Serial number
SMBIOS asset tag
Cloud provider
Amazon Web Services (AWS) metadata, when running in AWS:
- Account ID
- Instance ID
- Region
Google Cloud Platform (GCP) metadata, when running in GCP:
- Instance ID
- Image
- Machine type
- Project ID
- Project number
- Service accounts
- Zone

The agent requests the following metadata information from Azure:

Resource location (region)
Virtual machine ID
Tags
Microsoft Entra managed identity certificate
Guest configuration policy assignments
Extension requests - install, update, and delete.

Note

Azure Arc-enabled servers don't store/process customer data outside the region the customer deploys the service instance in.

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