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The Fedora Live CD Tools David Zeuthen <email@example.com> Jeremy Katz <firstname.lastname@example.org> Last Updated: October 2018 This project concerns tools to generate live CDs on Fedora based systems including derived distributions such as RHEL, CentOS, and others. See the project Wiki at https://fedoraproject.org/wiki/FedoraLiveCD for more details. Discussion of this project takes place at the email@example.com mailing list https://lists.fedoraproject.org/admin/lists/livecd.lists.fedoraproject.org/ This project and its source files are licensed under the GPLv2 license. See the file COPYING for details. 1. LIVE CD DESIGN GOALS The live CD is designed in such a way that when running from a live CD, the system should appear as much as possible as a standard system with all that entails; e.g., read-write rootfs (achieved using dm-snapshot or OverlayFS with the --flat-squashfs option), standard ext4 file system (for extended attributes) or a direct SquashFS, and so on. Another design goal is that the live CD should be ''installable'', i.e., a user should be able to install the bits from the live CD onto a hard disk without this process requiring network access or additional media. Finally, another design goal is that the tool set itself should be separate from configuration; the same unmodified tool should be usable for building various live CD flavors with vastly different configurations, e.g., a GNOME live CD, a KDE live CD, a live CD with music programs, and so on. 2. CREATING A LIVE CD To create a live CD, the livecd-creator tool is used. Super user privileges are needed. The tool is more or less self-documenting, use the --help option to see options. 2.1 HOW THE LIVE CD CREATOR WORKS In a nutshell, the livecd-creator program o Sets up a file for the ext4 file system that will contain all the data comprising the live CD o Loop mounts that file into the file system so there is an installation root o Bind mounts certain kernel file systems (/dev, /dev/pts, /proc, /sys, /selinux) inside the installation root o Uses a configuration file to define the requested packages and default configuration options. The format of this file is the same as is used for installing a system via kickstart. o Installs, using DNF, the requested packages into the installation using the given repositories o Optionally runs scripts as specified by the live CD configuration file. o Relabels the entire installation root (for SELinux) o Creates a live CD specific initramfs that matches the installed kernel o Unmounts the kernel file systems mounted inside the installation root o Unmounts the installation root o Runs resize2fs to minimize and unminimize the ext4 file to remove data from deleted files o Runs resize2fs to minimize on a device-mapper snapshot, to generate a small minimized delta image file which was historically used by anaconda to reduce installation time by not copying unused data to disk o Creates a SquashFS file system containing only the ext4 file (compression) or directly from the installation root (for OverlayFS overlays) o Configures the boot loader o Creates an iso9660 bootable CD 2.2 EXAMPLE: A BAREBONES LIVE CD The command # livecd-creator \ --config=/usr/share/doc/livecd-tools/livecd-fedora-minimal.ks will create a live CD that will boot to a login prompt. Note that in this minimal example, since no configuration is done, the user will not be able to login to the system as the root password is not set or cleared. 2.3 LIVE CD CONFIGURATION FILES The configuration of the live CD is defined by a file that uses the same format as installing a system via kickstart. They can include some basic system configuration items, the package manifest and a script to be run at the end of the build process. For the Fedora project, there are currently a variety of different live CD configuration files. The spin-kickstarts package includes all of the kickstarts used to create the various spins. These include a minimal live image (fedora-minimal-common.ks), a complete workstation image (fedora-live-workstation.ks) and others. 2.4 EXAMPLE: SPINNING THE FEDORA WORKSTATION LIVE CD Assuming that you use the fedora-live-workstation.ks configuration file, then the following command # livecd-creator \ --config=/usr/share/spin-kickstarts/fedora-live-workstation.ks \ --fslabel=Fedora-29-WS-Live-foo will create a live CD called "Fedora-29-WS-Live-foo". The name given by --fslabel is used. o as a file system label on the ext4 and iso9660 file systems (as such it's visible on the desktop as the CD name) o in the isolinux boot loader If you have the repositories available locally and don't want to wait for the download of packages, just substitute the URLs listed in the configuration file to point to your local repositories. 3. LIVE CD INSTALLS As of Fedora 7, Anaconda has support for doing an installation from a live CD. To use this, double click on the "Install to Hard Drive" item on the desktop or run /usr/bin/liveinst if you don't have such an icon. 4. LIVE CD MEDIA VERIFICATION The live CD can incorporate functionality to verify itself. To do so, you need to have isomd5sum installed both on the system used for creating the image and installed into the image. This is so that the implantisomd5 and checkisomd5 utilities can be used. These utilities take advantage of embedding an md5sum into the application area of the iso9660 image. This then gets verified before mounting the real root filesystem. These utilities used to be part of the anaconda-runtime package. 5. LOADING LIVE IMAGES ONTO USB MEDIA USB sticks are becoming increasingly prevalent and are a nice way to use live images. You can take a live CD iso image and transform it so that it can be used on a USB stick. To do so, use the livecd-iso-to-disk script, like the following: livecd-iso-to-disk /path/to/live.iso /dev/sdb1 Replace the '/dev/sdb1' argument above with the (unmounted) partition where you wish to load the live image. This is not a destructive process; any data you currently have on your USB stick will be preserved. Multiple images may be loaded onto a single USB stick. See livecd-iso-to-disk --help for more options and instructions. 6. SECURE IMAGE GENERATION Due to limitations in kickstart, the default invocation of livecd-creator cannot verify RPM package signatures. Package signatures are an essential security function which protects RPMs against tampering during storage or transfer. If possible, you should enable signature checks with the --repo option: livecd-creator \ --config=/usr/share/doc/livecd-tools/livecd-fedora-minimal.ks \ --repo=/etc/yum.repos.d --repo configures RPM sources using DNF ".repo" configuration files instead of kickstart. If you are building a different distro than your host system, be sure to use configuration files for that distro. Your --repo file or directory can use any configuration options available to DNF. The following options must be set in order to verify signatures: gpgcheck=1 gpgkey=path/to/key.asc file:///url/to/another/key.asc In this mode, the %repo directive(s) in the kickstart file, if any, are ignored. Additionally, use --pkgverify-level=all to enforce valid, trusted signatures on all packages installed. 7. MOUNTING LIVE IMAGES A live CD .iso file or an installed live USB device may be mounted with the liveimage-mount script to peer into the live OS filesystem, or even edit it on a device loaded with a persistent storage overlay. liveimage-mount /path/to/live[.iso|device|directory] <mountpoint> See liveimage-mount --help for more options. 7. EDITING LIVE IMAGES Live OS images may be edited using the editliveos script: editliveos [options] <LiveOS_source> This script may be used to merge a persistent overlay, run software updates including kernel and initial RAM filesystem updates, insert files, clone a customized instance, adjust the root or home filesystem or overlay sizes and filesystem or overlay types, seclude private or user-specific files, rebuild the image into a new .iso image distribution file, and refresh the source's persistent filesystem overlay. See editliveos --help for more options and instructions.
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