I have a custom domain www.abc.com configured for web app webapp-a, and I'd like to transfer it to webapp-b without downtime.
If I try to add the domain to webapp-b, I am getting
The host name www.abc.com is already assigned to another Azure website: webapp-a
It there a way to let webapp-a continues to serve the requests, until the DNS cache expires?
Edit: The domain has IP based SSL binding, and the DNS is caching the virtual IP.
I faced the same issue, and was able to solve it in a tricky way.
I created new App Service Plan (previously known as Web Hosting Plan), and assigned "new" Web App to it (you can do it via new portal, or you can create Service Plan during Web App creation).
Then, I was able to assign the same host name to Web Apps in different hosting plans.
Thinking about this, I feel it logical: most probably Service Plan is mapped to physical IIS machine, and you cannot have two sites with the same host name in the same IIS for the obvious reason.
After migration from one DNS to another is done, you can remove unused Service Plan (as you basically pay for each separately).
According to a blog post by the Azure App Service Team in June 2017, it would appear that Azure now supports adding the same custom domain to multiple web apps:
There are scenarios where a customer would like to add the same hostname to multiple web apps in the same subscription, having a geo distributed website is one example. Our custom hostname feature allows you to bypass validation for hostnames that have already been validated. You only need to verify domain ownership when you add a hostname for the first time. For all other apps in the same subscription, you can add the same hostname without creating any DNS records.
You can read the entire blog post at https://blogs.msdn.microsoft.com/appserviceteam/2017/06/21/custom-hostnames-with-app-service/. The quote above was taken from the Adding the same custom hostname to multiple web apps section.
This should help in scenarios such as this where you want to transfer a custom domain name from one web app to another. You can simply add the same custom domain name to both web apps, add any required SSL bindings, and then change the DNS to point to the new web app. As the DNS change propagates, traffic should gradually move over to the new web app without any downtime.
I've tested this myself in the North Europe region and had no problems. Both web apps were in different App Service Plans. I have not tested this scenario with both web apps in the same App Service Plan. Bear in mind that if you're using IP addresses/A records in your DNS, you'd need both web apps to have different IP addresses for the DNS to be able to differentiate between your web apps.
Try assigning the domain to Azure Traffic Manager and have the traffic manager forward the request to the second site. Azure Traffic Manager and Web Apps are two different systems so you might be able to assign the same domain name to a web app and a traffic manager.
Once the DNS cache has expired remove the domain from the old Azure web app and add it to the new one, then finally delete the Traffic Manager account.
Option 2
Set the TTL to something very small, say 5 seconds (I believe your hostname provider should let you set that up), wait for the new TTL to propagate through all the caches. Then switch the custom domain from one app to another, and set the TTL back to it's original value.
This will result in just a few seconds of downtime for any customers, but if you do it at a low-traffic time the effect shouldn't be too bad.
