Packer is a tool for building operating system images. Packer accepts a template which describes how an image should be built. The installation is done by the operating system vendor’s own tools. Canonical has created templates for CentOS, RHEL, and VMware ESXi. These templates contain post-processing hooks to convert the image into either a tar.gz or dd.gz which can be used by MAAS.
Images can be customized by making changes to the kickstart files. See README.md in the operating system’s directory for more information.