Azure App Service Hybrid Connections

Hybrid Connections is both a service in Azure and a feature in Azure App Service. As a service, it has uses and capabilities beyond those that are used in App Service. To learn more about Hybrid Connections and their usage outside App Service, see Azure Relay Hybrid Connections.

Within App Service, Hybrid Connections can be used to access application resources in any network that can make outbound calls to Azure over port 443. Hybrid Connections provides access from your app to a TCP endpoint and does not enable a new way to access your app. As used in App Service, each Hybrid Connection correlates to a single TCP host and port combination. This enables your apps to access resources on any OS, provided it is a TCP endpoint. The Hybrid Connections feature does not know or care what the application protocol is, or what you are accessing. It simply provides network access.

How it works

Hybrid Connections requires a relay agent to be deployed where it can reach both the desired endpoint as well as to Azure. The relay agent, Hybrid Connection Manager (HCM), calls out to Azure Relay over port 443. From the web app site, the App Service infrastructure also connects to Azure Relay on your application's behalf. Through the joined connections, your app is able to access the desired endpoint. The connection uses TLS 1.2 for security and shared access signature (SAS) keys for authentication and authorization.

Diagram of Hybrid Connection high-level flow

When your app makes a DNS request that matches a configured Hybrid Connection endpoint, the outbound TCP traffic will be redirected through the Hybrid Connection.

Note

This means that you should try to always use a DNS name for your Hybrid Connection. Some client software does not do a DNS lookup if the endpoint uses an IP address instead.

App Service Hybrid Connection benefits

There are a number of benefits to the Hybrid Connections capability, including:

  • Apps can access on-premises systems and services securely.
  • The feature does not require an internet-accessible endpoint.
  • It is quick and easy to set up. No gateways required
  • Each Hybrid Connection matches to a single host:port combination, helpful for security.
  • It normally does not require firewall holes. The connections are all outbound over standard web ports.
  • Because the feature is network level, it is agnostic to the language used by your app and the technology used by the endpoint.
  • It can be used to provide access in multiple networks from a single app.
  • It is supported in GA for Windows native apps and is in preview for Linux apps. It is not supported for Windows container apps.

Things you cannot do with Hybrid Connections

Things you cannot do with Hybrid Connections include:

  • Mount a drive.
  • Use UDP.
  • Access TCP-based services that use dynamic ports, such as FTP Passive Mode or Extended Passive Mode.
  • Support LDAP, because it can require UDP.
  • Support Active Directory, because you cannot domain join an App Service worker.

Add and Create Hybrid Connections in your app

To create a Hybrid Connection, go to the Azure portal and select your app. Select Networking > Configure your Hybrid Connection endpoints. Here you can see the Hybrid Connections that are configured for your app.

Screenshot of Hybrid Connection list

To add a new Hybrid Connection, select [+] Add hybrid connection. You'll see a list of the Hybrid Connections that you already created. To add one or more of them to your app, select the ones you want, and then select Add selected Hybrid Connection.

Screenshot of Hybrid Connection portal

If you want to create a new Hybrid Connection, select Create new hybrid connection. Specify the:

  • Hybrid Connection name.
  • Endpoint hostname.
  • Endpoint port.
  • Service Bus namespace you want to use.

Screenshot of Create new hybrid connection dialog box

Every Hybrid Connection is tied to a Service Bus namespace, and each Service Bus namespace is in an Azure region. It's important to try to use a Service Bus namespace in the same region as your app, to avoid network induced latency.

If you want to remove your Hybrid Connection from your app, right-click it and select Disconnect.

When a Hybrid Connection is added to your app, you can see details on it simply by selecting it.

Screenshot of Hybrid connections details

Create a Hybrid Connection in the Azure Relay portal

In addition to the portal experience from within your app, you can create Hybrid Connections from within the Azure Relay portal. For a Hybrid Connection to be used by App Service, it must:

  • Require client authorization.
  • Have a metadata item, named endpoint, that contains a host:port combination as the value.

Hybrid Connections and App Service plans

App Service Hybrid Connections are only available in Basic, Standard, Premium, and Isolated pricing SKUs. There are limits tied to the pricing plan.

Pricing plan Number of Hybrid Connections usable in the plan
Basic 5 per plan
Standard 25 per plan
PremiumV2 200 per app
Isolated 200 per app

The App Service plan UI shows you how many Hybrid Connections are being used and by what apps.

Screenshot of App Service plan properties

Select the Hybrid Connection to see details. You can see all the information that you saw at the app view. You can also see how many other apps in the same plan are using that Hybrid Connection.

There is a limit on the number of Hybrid Connection endpoints that can be used in an App Service plan. Each Hybrid Connection used, however, can be used across any number of apps in that plan. For example, a single Hybrid Connection that is used in five separate apps in an App Service plan counts as one Hybrid Connection.

Pricing

In addition to there being an App Service plan SKU requirement, there is an additional cost to using Hybrid Connections. There is a charge for each listener used by a Hybrid Connection. The listener is the Hybrid Connection Manager. If you had five Hybrid Connections supported by two Hybrid Connection Managers, that would be 10 listeners. For more information, see Service Bus pricing.

Hybrid Connection Manager

The Hybrid Connections feature requires a relay agent in the network that hosts your Hybrid Connection endpoint. That relay agent is called the Hybrid Connection Manager (HCM). To download HCM, from your app in the Azure portal, select Networking > Configure your Hybrid Connection endpoints.

This tool runs on Windows Server 2012 and later. The HCM runs as a service and connects outbound to Azure Relay on port 443.

After installing HCM, you can run HybridConnectionManagerUi.exe to use the UI for the tool. This file is in the Hybrid Connection Manager installation directory. In Windows 10, you can also just search for Hybrid Connection Manager UI in your search box.

Screenshot of Hybrid Connection Manager

When you start the HCM UI, the first thing you see is a table that lists all the Hybrid Connections that are configured with this instance of the HCM. If you want to make any changes, first authenticate with Azure.

To add one or more Hybrid Connections to your HCM:

  1. Start the HCM UI.

  2. Select Configure another Hybrid Connection. Screenshot of Configure New Hybrid Connections

  3. Sign in with your Azure account to get your Hybrid Connections available with your subscriptions. The HCM does not continue to use your Azure account beyond that.

  4. Choose a subscription.

  5. Select the Hybrid Connections that you want the HCM to relay. Screenshot of Hybrid Connections

  6. Select Save.

You can now see the Hybrid Connections you added. You can also select the configured Hybrid Connection to see details.

Screenshot of Hybrid Connection Details

To support the Hybrid Connections it is configured with, HCM requires:

  • TCP access to Azure over port 443.
  • TCP access to the Hybrid Connection endpoint.
  • The ability to do DNS look-ups on the endpoint host and the Service Bus namespace.

Note

Azure Relay relies on Web Sockets for connectivity. This capability is only available on Windows Server 2012 or later. Because of that, HCM is not supported on anything earlier than Windows Server 2012.

Redundancy

Each HCM can support multiple Hybrid Connections. Also, any given Hybrid Connection can be supported by multiple HCMs. The default behavior is to route traffic across the configured HCMs for any given endpoint. If you want high availability on your Hybrid Connections from your network, run multiple HCMs on separate machines. The load distribution algorithm used by the Relay service to distribute traffic to the HCMs is random assignment.

Manually add a Hybrid Connection

To enable someone outside your subscription to host an HCM instance for a given Hybrid Connection, share the gateway connection string for the Hybrid Connection with them. You can see the gateway connection string in the Hybrid Connection properties in the Azure portal. To use that string, select Enter Manually in the HCM, and paste in the gateway connection string.

Manually add a Hybrid Connection

Upgrade

There are periodic updates to the Hybrid Connection Manager to fix issues or provide improvements. When upgrades are released, a popup will show up in the HCM UI. Applying the upgrade will apply the changes and restart the HCM.

Adding a Hybrid Connection to your app programmatically

There is Azure CLI support for Hybrid Connections. The commands provided operate at both the app and the App Service plan level. The app level commands are:

az webapp hybrid-connection

Group
    az webapp hybrid-connection : Methods that list, add and remove hybrid-connections from webapps.
        This command group is in preview. It may be changed/removed in a future release.
Commands:
    add    : Add a hybrid-connection to a webapp.
    list   : List the hybrid-connections on a webapp.
    remove : Remove a hybrid-connection from a webapp.

The App Service plan commands enable you to set which key a given hybrid-connection will use. There are two keys set on each Hybrid Connection, a primary and a secondary. You can choose to use the primary or secondary key with the below commands. This enables you to switch keys for when you want to periodically regenerate your keys.

az appservice hybrid-connection --help

Group
    az appservice hybrid-connection : A method that sets the key a hybrid-connection uses.
        This command group is in preview. It may be changed/removed in a future release.
Commands:
    set-key : Set the key that all apps in an appservice plan use to connect to the hybrid-
                connections in that appservice plan.

Secure your Hybrid Connections

An existing Hybrid Connection can be added to other App Service Web Apps by any user who has sufficient permissions on the underlying Azure Service Bus Relay. This means that if you must prevent others from reusing that same Hybrid Connection (for example when the target resource is a service that does not have any additional security measures in place to prevent unauthorized access), you must lock down access to the Azure Service Bus Relay.

Anyone with Reader access to the Relay will be able to see the Hybrid Connection when attempting to add it to their Web App in the Azure portal, but they will not be able to add it as they lack the permissions to retrieve the connection string which is used to establish the relay connection. In order to successfully add the Hybrid Connection, they must have the listKeys permission (Microsoft.Relay/namespaces/hybridConnections/authorizationRules/listKeys/action). The Contributor role or any other role which includes this permission on the Relay will allow users to use the Hybrid Connection and add it to their own Web Apps.

Troubleshooting

The status of "Connected" means that at least one HCM is configured with that Hybrid Connection, and is able to reach Azure. If the status for your Hybrid Connection does not say Connected, your Hybrid Connection is not configured on any HCM that has access to Azure.

The primary reason that clients cannot connect to their endpoint is because the endpoint was specified by using an IP address instead of a DNS name. If your app cannot reach the desired endpoint and you used an IP address, switch to using a DNS name that is valid on the host where the HCM is running. Also check that the DNS name resolves properly on the host where the HCM is running. Confirm that there is connectivity from the host where the HCM is running to the Hybrid Connection endpoint.

In App Service, the tcpping command line tool can be invoked from the Advanced Tools (Kudu) console. This tool can tell you if you have access to a TCP endpoint, but it does not tell you if you have access to a Hybrid Connection endpoint. When you use the tool in the console against a Hybrid Connection endpoint, you are only confirming that it uses a host:port combination.

If you have a command line client for your endpoint, you can test connectivity from the app console. For example, you can test access to web server endpoints by using curl.