Azure Cloud: can you swap public IP addresses on two VMs?

Intro
I am just beginning to use Microsoft’s Azure cloud environment. Although I am inclined to be a fan of AWS, I haven’t looked at AWS networking for awhile, and the last time I did something I felt totally lost in trying to understand their terminology.

But in spite of my natural inclination to support everything Amazon, I gotta admit that Azure was a good, usable environment for what I needed to do; swap the public IPs on two VMs.

The details
I was not getting any help whatsoever from with my organization. But I did at least get sufficient access to the Resource Group where my Redhat 7.4 VM was running. That was a godsend.

In Azure network interfaces are resources. They have IPs like 10.0.1.4, 10.0.1.7, etc.

Public IP addresses are resources. They have IPs apprporiate for your region. They are typically associated with a network interface.

A network interface in turn is associated to a VM, typically.

For some reason which no one could explain to me, I could no longer patch my RHEL 7.4 server. That began about September 2019. Meantime, I was using an application which relied on a built-in package. Now Redhat always ships with old versions of everything, so running this old version plus lack of patches really put pressure on me to upgrade to a new OS. Can you do an in-place upgrade? As far as I can tell, no. I went with SLES 15 SP1 on a new VM within the same resource group and data center since I have some familiarity with Suse Linux. That OS had a newer version of that open source package, plus it could be patched.

But the IP of the old server was embedded in several places and switching it was not an option. What to do? What to do? Can you even swap IPs on two VMs within the same Resource Group? Who knows?

Well, it turns out you can. The documentation on the topic is pretty good and cleared up some things for me. Particularly the first two links in the references at the bottom.

I took this approach.

Changed the IP from dynamic to Static (go to configure section when looking at this resource). This should have been done from the get-go, but wasn’t. Who knew?

Dissociate the IP from the network interface.

Changed the second IP from dynamic to static.

Dissociate this IP from its network interface.

Associate IP to network interface of the SLES 15 server.

Associate second IP to network interface of the RHEL server.

And that’s it…. It worked like a charm.

Then I cleaned up some old public IP addresses which weren’t being used. You have to remember there is a shortage of IPs. So a lot of the quirk you encounter are due to their utilizing Ips as sparingly as possible. makes sense to me. For instance you can have a “public IP” resource which has no value! it may not get a value until its absolutely needed by virtue of being associated with a network interface on an active server. Stuff like that…

Conclusion
Yes, you can indeed swap public IPs on two servers if they belong to the same Resource Group and I guess the same data center. I know because I did it. As a bonus I found that the Azure documentation is pretty clear and sufficiently detailed.

References and related
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-network-ip-addresses-overview-arm
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-network-network-interface-addresses

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