Appendix: Creating Arbitrary CentOS ImagesΒΆ

Creating PXEboot Images in the Installation & Administrator Guide describes how to create PXEboot images from the latest CentOS 7 and 6 repos. This section describes how to create images from CentOS 7 and 6 repos that are not the latest packages.

For example, using the CentOS-7-x86_64-DVD-1908.iso ISO file, you can use scyld-clusterctl repos and scyld-clusterctl distros to create a repo and distro for this Centos version 7.7, then use scyld-modimg to create an image and a boot config.

Or more simply you can use scyld-add-boot-config to perform the same result in fewer steps. Execute the following and accept all the defaults:

scyld-add-boot-config --iso /mnt/isos/CentOS-7-x86_64-DVD-1908.iso

This creates a distro and repo both named CentOS-7-x86_64-1908, and an image and boot config both named CentOS-7-x86_64-1908.

Alternatively, you can avoid the manual acceptance of the defaults by specifying desired names and running the command in batch mode:

scyld-add-boot-config --iso /mnt/isos/CentOS-7-x86_64-DVD-1908.iso \
                      --image CentOS-7.7-Image --boot-config CentOS-7.7-Boot --batch

View the result, which shows the default repo and the new repo, and the default boot config and the new boot config:

[admin@head]$ scyld-clusterctl distros ls -l
    name: CentOS
    package_manager: yum
    release: 7
    name: CentOS-7-x86_64-1908
    package_manager: yum
    release: none

[admin@head]$ scyld-bootctl ls -l
Boot Configurations
    cmdline: enforcing=0
    image: CentOS-7.7-Image
      chksum: a85b01e91c26c52ebf549066c6c5fce544f3c75b
      filename: 3684fbadf53f4c8bb8a3dea24ecf778d
      mtime: 2022-03-18 16:49:06 UTC (1:14:23 ago)
      size: 33.4 MiB (34978448 bytes)
      chksum: 73872862a49ee024bf44c4d796c96bed4d52ee43
      filename: 1d6888add971485395d943df191645c4
      mtime: 2022-03-18 16:49:07 UTC (1:14:23 ago)
      size: 6.4 MiB (6734016 bytes)
    last_modified: 2022-03-18 16:49:07 UTC (1:14:23 ago)
    name: CentOS-7.7-Boot
    release: 3.10.0-1062.el7.x86_64


Creating images from some older ISOs may produce an error message beginning with ERROR: One or more repositories in the newly created image are invalid or unreachable. The scyld-modimg tool will automatically retry the image creation, and if there is no subsequent error reported, then the administrator can assume that the resulting image is useable.


If the CentOS image thus built is subsequently updated using yum update, then by default that updates packages to the latest minor release level, not to newer versions at the image's current minor release level. Also, yum install of additional packages may update dependency packages from their current minor release version level to the latest minor level. Such actions may result in a mixture of packages from different minor releases, which may have unintended consequences.