Azure Container Instances DNS Record Manager

Introduction

Azure Container Instances (ACI) are a great way to run Docker Images in a managed environment with little configuration effort, without the need to deploy a Container runtime like Docker or, even bigger, a Kubernetes Cluster. Despite the ease-of-use ACI has some annoying issues, especially when bound to a private virtual network.

One of these annoyances is the fact that an ACI instance, which is connected to private virtual network, does not propagate a DNS name (FQDN). This is even more problematic because ACI does not support a static IP address and thus an ACI might be bound to a different IP address after a restart.

This repository contains a program (aci-dns-manager) which will ensure that a DNS A-Record is created for an ACI instance, in a startup container, side car or during the startup code (script) of the deployed container.

Note Currently only Private DNS Zones are supported

How to use

Authentication

With ACI (theoretically) two types of principals can be used with aci-dns-manager to authenticate towards the Azure REST API:

• Managed Identity
• Service Principal

Unfortunately ACI only supports Managed Identities only when the ACI is not connected to a private virtual network.

Ref: https://docs.microsoft.com/en-us/azure/container-instances/container-instances-managed-identity#limitations

Due to this, the only way that aci-dns-manager can authenticate at the Azure REST API is using a service principal. The following environment variables must be specified to enable aci-dns-manager to use a service principal for authentication:

• AZURE_CLIENT_ID
• AZURE_CLIENT_SECRET
• AZURE_TENANT_ID

For additional details refer to chapter configuration

Security considerations

Even if an own ACI deployment is trustworthy, the permissions of the aci-dns-manager should be narrowed down to the permissions really required to create (update, delete respectively) the DNS record for the connected ACI.

The ideal way to do this is to use a custom RBAC role definition in Azure, which will then assigned to the principal the aci-dns-manager will use to authenticate towards the Azure REST API.

An example of such a custom role definition could be the following:

{
    "Name": "Private DNS A-Record Writer",
    "Description": "Can read, write DNS A and AAAA records",
    "AssignableScopes": [
        "/subscriptions/{subscriptionId1}"
    ],
    "Actions": [
        "Microsoft.Network/privateDnsZones/A/read",
        "Microsoft.Network/privateDnsZones/A/write",
        "Microsoft.Network/privateDnsZones/read",
        "Microsoft.Network/privateDnsZones/AAAA/read",
        "Microsoft.Network/privateDnsZones/AAAA/write",
        "Microsoft.Network/privateDnsOperationResults/read",
        "Microsoft.Network/privateDnsOperationStatuses/read"
    ],
    "NotActions": [],
    "DataActions": [],
    "NotDataActions": []
}

To allow the deletion of A and AAAA records, e.g. while deprovisioning an ACI, the following Actions must be added to the above shown Role Definition

"Microsoft.Network/privateDnsZones/A/delete",
"Microsoft.Network/privateDnsZones/AAAA/delete"

If the above Role is assigned to a Azure DNS Private Zone resource, the aci-dns-manager could change, or even delete, arbitrary DNS records therein. To mitigate this risk of inadvertently changing DNS entries by aci-dns-manager, the above role should be assigned to a pre-created DNS entry. In that case aci-dns-manager can only change it's "own" DNS record. What is definitely the preferred way to go.

Configuration

The program is configured via various environment variables

Variable	Description	Required
ARM_SUBSCRIPTION_ID	The Azure subscription of the ACI	yes
ACI_RESOURCE_GROUP_NAME	The resource group in which the ACI is deployed	yes
DNS_ZONE_NAME	The name of the DNS zone in which the A record shall be created	yes
AZURE_TENANT_ID	The id (guid) of the Azure Tenant in which the service principal is located used for authentication	no, required if: AZURE_CLIENT_ID is specified
AZURE_CLIENT_ID	The id (guid) of the service principal used for authentication	no
AZURE_CLIENT_SECRET	The secret (password) of the service principal used for authentication	no, required if: AZURE_CLIENT_ID is specified
ACI_INSTANCE_NAME	The name of the ACI for which a a DNS A record shall be created. If not specified the environment variable Fabric_CodePackageName will be used. However, it was found that the ACI runtime does not reliably set this variable. So to prevent any errors the ACI_INSTANCE_NAME variable should always be set	no, if Fabric_CodePackageName is used
ACI_MAX_RETRY	Number of retries to get deployment information (IP address) of the configured ACI	no, default: 10
ACI_TIMEOUT	How long to wait between retries	no, default: 10 seconds
DNS_ZONE_RESOURCE_GROUP_NAME	The resource group of the private DNS Zone	no, default: ACI_RESOURCE_GROUP_NAME
DNS_ZONE_RESOURCE_SUBSCRIPTION_ID	If the private DNS zone is located in a different subscription than the ACI instance, this variable contains the Id (guid) of this subscription.	no, default ARM_SUBSCRIPTION_ID
DNS_A_RECORD_NAME	The name of the DNS A record to create.	no, default container name
DNS_A_RECORD_TTL	The TTL value of the DNS A record	no, default 3600
ARM_TRACE	If this environment variable is set (any value), trace messages will be created during communication with the Azure REST API. Valuable for troubleshooting.	no

Examples

The following examples are provided to show the usage of aci-dns-manager:

1. Scenario: S6-Init

   The S6-Init system will execute aci-dns-manager inside an init script. Details can be found in README

2. Scenario: ACI Init Container

   aci-dns-manager will be run in an Init container defined in the YAML file which describes the ACI. Details can be found in README

Resources

The repository will provider the following type of ways to download aci-dns-manager

• Docker Image
• TAR archive

Please refer to the release section to find the latest versions.

Links

https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/sdk/azidentity#section-readme
https://docs.microsoft.com/en-us/azure/developer/go/manage-resource-groups?tabs=bash%2Cazure-portal
https://docs.microsoft.com/en-us/azure/developer/go/management-libraries#long-running-operations
https://kreuzwerker.de/en/post/managing-multi-process-applications-in-containers-using-s6
https://tutumcloud.wordpress.com/2015/05/20/s6-made-easy-with-the-s6-overlay/
https://docs.microsoft.com/ms-my/azure/service-fabric/service-fabric-environment-variables-reference