Quick Fix: Deploying Multiple Ubuntu 18.04 VMs From Template with DHCP Results Same in IP Allocation

In the continuing work I’ve been doing with Terraform, i’ve come across a number of gotchyas when working with VM Templates and deploying them on mass. The nature of the work is that i’m creating and destroying VMs often. Generally speaking I like using Static IP addresses but for the project i’m working on I needed to be able to have an option to deploy and configure the networking with DHCP. Windows and CentOS gave me no issues, however when I went to deploy the Ubuntu 18.04 template I started getting errors on the plan execution.

When I looked at the output of the Terraform where export the VM IP addresses, the json output showed that all the cloned VMs had been assigned the same IP address.

At first I assumed it was due to the same MAC address being assigned by ESXi to the cloned VMs which was resulting in the machines being allocated the same IP, however when I checked the MAC addresses they where all different.

What is Machine-ID:

After some digging online I came across a change in behaviour where Ubuntu uses the machine-id to request DHCP addresses. Ubuntu server default networking goes through cloud-init which by default sends /etc/machine-id in the DHCP request. This leads to the duplicate IP situation.

The /etc/machine-id file contains the unique machine ID of the local system that is set during installation or boot. The machine ID is a single newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from hexadecimal, this corresponds to a 16-byte/128-bit value. This ID may not be all zeros.

The machine ID is usually generated from a random source during system installation or first boot and stays constant for all subsequent boots. Optionally, for stateless systems, it is generated during runtime during early boot if necessary.

Quick Fix:

From a template perspective there is a quick fix that can be applied where the machine-id file is blanked out. This means upon first boot a new ID is generated. You can’t just delete the machine-id file as it needs to exist. If it doesn’t exist the deployment will fail as it expects it to be there in some form.

The simplest way I achieved this was by zero’ing out the file:

Once done, the VM can be saved again as a template and the cloning operation will result in unique IPs being handed out by the DHCP server.

References:

http://manpages.ubuntu.com/manpages/bionic/man5/machine-id.5.html

https://www.freedesktop.org/software/systemd/man/machine-id.html

 

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