Dracut is an initramfs infrastructure. The project wiki is at https://github.com/dracutdevs/dracut/wiki (background information about the project's goals and aims can be found at Initrdrewrite). Unlike previous initramfs systems, Dracut aims to have as little as possible hard-coded into the initramfs.
New Kernel Command Line Parameters
Read the dracut.kernel.7 man page.
Quick Debugging Tips
- add "rdshell" to the kernel command line and you are dropped to a shell
- add "rdshell rdinitdebug" to the kernel command line and dracut commands are printed
- you can inspect /init.log and run journalctl from the shell
- mount /boot manually and copy over /init.log or the /run/log/journal files
Technical details
With Dracut, the initramfs has one purpose in life - getting the rootfs mounted so that we can transition to the real rootfs. This is all driven by device availability. Therefore, instead of scripts hard-coded to do various things, we depend on udev to create device nodes for us and then when we have the rootfs's device node, we mount and carry on. This helps to keep the time required in the initramfs as short as possible so that things like a five second boot aren't made impossible as a result of the very existence of an initramfs. It's likely that we'll grow some hooks for running arbitrary commands in the flow of the script, but it's worth trying to resist the urge as much as we can as hooks are guaranteed to be the path to slow-down.
Most of the initrd generation functionality in dracut is provided by a bunch of generator modules that are sourced by the main dracut script to install specific functionality into the initrd. They live in the modules subdirectory, and use functionality provided by dracut-functions to do their work. Some general rules for writing modules:
- Use one of the inst family of functions to actually install files on to the initrd. They handle mangling the pathnames and (for binaries, scripts, and kernel modules) installing dependencies as appropriate so you do not have to.
- Scripts that end up on the initrd should be POSIX compliant. Dracut will try to use /bin/dash as /bin/sh for the initrd if it is available, so you should install it on your system -- dash aims for strict POSIX compliance to the extent possible.
- Hooks MUST be POSIX compliant -- they are sourced by the init script, and having a bashism break your user's ability to boot really sucks.
- Generator modules should have a two digit numeric prefix -- they run in ascending sort order. Anything in the 90-99 range is stuff that dracut relies on, so try not to break those hooks.
- Generator modules and hooks must have a .sh extension.
- We have some breakpoints for debugging your hooks. If you pass 'break' as a kernel parameter, the initramfs will drop to a shell just before switching to a new root. You can pass 'break=hookpoint', and the initramfs will break just before hooks in that hookpoint run.
Also, there is an attempt to keep things as distribution-agnostic as possible. Every distribution has their own tool here and it's not something which is really interesting to have separate across them. So contributions to help decrease the distro dependencies are welcome.
Communicate
- Talk to other Dracut users and developers on our IRC channel
- Newsgroup for the mailing list
- Subscribe to the mailing list
Please read the mailing list guidelines before communicating on the list. See also How to use IRC if the IRC links don't work, or if you need more information.
Using Dracut
See the dracut.8 man page: man dracut
The command dracut exists and will build an image. It is command-line equivalent to most mkinitrd implementations and should be pretty straightforward to use.
To use, just run dracut with an output file name and, optionally, a kernel version (it defaults to using the current). The appropriate modules will be copied over and things should be good to go. If you'd like to customize the list of modules copied in, edit /etc/dracut.conf and set
dracutmodules="foo bar baz"
See also TODO.
Right now, most of the testing is done using a qemu/kvm guest and generating the initramfs on another box but the support is all present to build for the "running" machine. For the former, you can boot the guest using qemu's -kernel and -initrd options.
Getting started
To install on Fedora 11 and onwards run yum install dracut.
To build from source start by downloading the latest tarball from here then do:
tar xf dracut-$version.tar.bz2 cd dracut-$version make su make install sbindir=/sbin sysconfdir=/etc
To generate a initramfs image, run:
dracut <imagename> <kernel version>
To overwrite an existing image, run:
dracut -f <imagename> <kernel version>
To view the generated kernel command line use dracuts helper tool.
dracut-gencmdline
Try to boot from that image by modifying /etc/grub.conf. Be sure to have a fallback entry.
Additional Dracut kernel command line parameters can be found on the options page and in the man page.
When you have become familiar with Dracut take some time to look at the Test Matrix and see if you can contribute to testing.
Getting the Source
The primary methods of distributing the Dracut source are source RPMs in the Fedora development tree and git. To access the current source code in in non-rpm format, you'll need to install git: yum install git.
Note that several related packages will be installed as well. After the git source code management tool has been installed, then you use anonymous git access to the Dracut repository.
git clone git://git.kernel.org/pub/scm/boot/dracut/dracut.git
Once you've committed changes locally, you can push them with git push.
If you would just like to browse the Dracut git repository via the web, then you can use Dracut gitweb.
Reporting Problems
Before filing a bug, please read up on debugging, which will tell you how to fill out useful bug reports that will help us quickly solve your problem. Also take a look at Dracut bugs or try searching Bugzillafor other reports about your problem, as some bugs are often filed by several people.
Dracut Team
In alphabetical order, the following people are the Dracut team and are responsible for the majority of commits.
- Andreas Thienemann
- Bill Nottingham
- Dave Jones
- David Dillow
- Harald Hoyer
- Jeremy Katz
- Peter Jones
- Seewer Philippe
- Victor Lowther
- Warren Togami