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Migrate an active DNS name
Learn how to migrate a custom DNS domain name that is already assigned to a live site to Azure App Service without any downtime.
top-support-issue
msangapu-msft
msangapu
10da5b8a-1823-41a3-a2ff-a0717c2b5c2d
article
01/31/2023

Migrate an active DNS name to Azure App Service

This article shows you how to migrate an active DNS name to Azure App Service without any downtime.

When you migrate a live site and its DNS domain name to App Service, that DNS name is already serving live traffic. You can avoid downtime in DNS resolution during the migration by binding the active DNS name to your App Service app preemptively.

If you're not worried about downtime in DNS resolution, see Map an existing custom DNS name to Azure App Service.

Prerequisites

To complete the steps, make sure that your App Service app isn't in FREE tier.

1. Get a domain verification ID

When you bind a custom domain preemptively, you accomplish both of the following before making any changes to your existing DNS records:

  • Verify domain ownership by adding a domain verification ID with your domain provider.
  • Enable the domain name in your App service app.

When you finally migrate your custom DNS name from the old site to the App Service app, there will be no downtime in DNS resolution.

  1. In the Azure portal, open the management page of the App Service app.

  2. In the left pane of your app page, select Custom domains.

  3. Copy the ID in the Custom Domain Verification ID box in the Custom Domains page for the next steps.

    :::image type="content" source="./media/manage-custom-dns-migrate-domain/get-domain-verification-id.png" alt-text="Screenshot that shows the ID in the Custom Domain Verification ID box." border="true":::

2. Create the DNS records

[!INCLUDE Access DNS records with domain provider]

Add a TXT record for domain verification. The hostname for the TXT record depends on the type of DNS record type you want to map. See the following table (@ typically represents the root domain):

DNS record example TXT Host TXT Value
@ (root) asuid Domain verification ID shown in the Custom domains management page.
www (sub) asuid.www Domain verification ID shown in the Custom domains management page.
* (wildcard) asuid Domain verification ID shown in the Custom domains management page.

Note

Wildcard * records won't validate subdomains with an existing CNAME's record. You may need to explicitly create a TXT record for each subdomain.

3. Enable the domain for your app

  1. Back in the Custom domains page, select Add custom domain.

    :::image type="content" source="./media/app-service-web-tutorial-custom-domain/add-custom-domain.png" alt-text="A screenshot showing how to open the Add custom domain dialog." border="true":::

  2. For Domain provider, select All other domain services to configure a third-party domain.

  3. For TLS/SSL certificate, select Add certificate later. You can add an App Service managed certificate after you have completed domain migration.

  4. For TLS/SSL type, select the binding type you want.

    [!INCLUDE Certificate binding types]

  5. Type the fully qualified domain name you want to migrate, that corresponds to the TXT record you created, such as contoso.com, www.contoso.com, or *.contoso.com.

    :::image type="content" source="./media/app-service-web-tutorial-custom-domain/configure-custom-domain-preempt.png" alt-text="A screenshot showing how to configure a new custom domain, along with a managed certificate." border="true":::

  6. Select Validate. Although the dialog shows two records you need for the custom domain to be functional for your app, the validation passes with just the domain verification ID (the TXT record).

    :::image type="content" source="./media/app-service-web-tutorial-custom-domain/configure-custom-domain-validate.png" alt-text="A screenshot showing how to validate your DNS record settings in the Add a custom domain dialog." border="true":::

  7. If the Domain validation section shows green check marks, then you've configured the domain verification ID correctly. Select Add. If it shows any red X, fix any errors in your domain provider's website.

    :::image type="content" source="./media/app-service-web-tutorial-custom-domain/configure-custom-domain-preempt-add.png" alt-text="A screenshot showing the Add button activated after validation." border="true":::

  8. You should see the custom domain added to the list. You may also see a red X with No binding.

    Since you selected Add certificate later, you see a red X with No binding. It will remain until you add a private certificate for the domain and configure the binding.

    :::image type="content" source="./media/app-service-web-tutorial-custom-domain/add-custom-domain-preempt-complete.png" alt-text="A screenshot showing the custom domains page with the new secured custom domain." border="true":::

    [!NOTE] Unless you configure a certificate binding for your custom domain, Any HTTPS request from a browser to the domain will receive an error or warning, depending on the browser.

4. Remap the active DNS name

The only thing left to do is remapping your active DNS record to point to App Service. Right now, it still points to your old site.

  1. (A record only) You need the App Service app's external IP address. In the Custom domains page, copy the app's IP address.

    :::image type="content" source="./media/app-service-web-tutorial-custom-domain/mapping-information-migrate.png" alt-text="A screenshot showing how to copy the App Service app's external IP address." border="true":::

  2. Back in the DNS records page of your domain provider, select the DNS record to remap.

  3. Remap the A or CNAME record like the examples in the following table:

    FQDN example Record type Host Value
    contoso.com (root) A @ IP address from Copy the app's IP address
    www.contoso.com (sub) CNAME www <app-name>.azurewebsites.net
    *.contoso.com (wildcard) CNAME * <app-name>.azurewebsites.net

  4. Save your settings.

DNS queries should start resolving to your App Service app immediately after DNS propagation happens.

Frequently asked questions

Can I add an App Service managed certificate when migrating a live domain?

You can add an App Service managed certificate to a migrated live domain, but only after you remap the active DNS name. To add the App Service managed certificate, see Create a free managed certificate.

How do I migrate a domain from another app?

You can migrate an active custom domain in Azure, between subscriptions or within the same subscription. However, such a migration without downtime requires the source app and the target app are assigned the same custom domain at a certain time. Therefore, you need to make sure that the two apps aren't deployed to the same deployment unit (internally known as a webspace). A domain name can be assigned to only one app in each deployment unit.

You can find the deployment unit for your app by looking at the domain name of the FTP/S URL <deployment-unit>.ftp.azurewebsites.windows.net. Check and make sure the deployment unit is different between the source app and the target app. The deployment unit of an app is determined by the App Service plan it's in. It's selected randomly by Azure when you create the plan and can't be changed. When you create two apps in the same resource group and the same region, Azure puts them in the same deployment unit. However, there's no way to make sure that the opposite is true. In other words, the only way to create a plan in a different deployment unit is to keep creating a plan in a new resource group or region until you get a different deployment unit.

Next steps

Learn how to bind a custom TLS/SSL certificate to App Service.

[!div class="nextstepaction"] Purchase an App Service domain.

[!div class="nextstepaction"] Secure a custom DNS name with a TLS binding in Azure App Service