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genkernel is a tool created by Gentoo used to automate the build process of the kernel and initramfs. Some of the general features include:

  • Configuring the kernel sources.
  • Building the compressed kernel bzImage and copying it to /boot.
  • Creating an initramfs and copying it to /boot.
  • Creating symlinks in /boot.
  • Adding custom content to the initramfs such as encryption related files, boot splash images, extra modules, and more.
  • Compressing the initramfs.
  • Configuring the bootloader to boot the newly created kernel and initramfs.
It is a common misconception to believe genkernel will 'automatically' generate a custom kernel configuration. genkernel automates the kernel build process and assembles the initramfs, but does not generate a custom kernel configuration file. It uses a generic kernel configuration file that provides support for commonly used subsystems on an architecture-specific basis. Details on the architecture defaults can be seen in the upstream repository. Select the architecture and then choose the kernel config file.


USE flags

USE flags for sys-kernel/genkernel Gentoo automatic kernel building scripts

cryptsetup Enable support for Luks disk encryption using sys-fs/cryptsetup local
firmware Prefer system firmware sys-kernel/linux-firmware over local copy. local
ibm Add support for IBM ppc64 specific systems global
selinux !!internal use only!! Security Enhanced Linux support, this must be set by the selinux profile or breakage will occur global


Initiate the installation of genkernel:

root #emerge --ask sys-kernel/genkernel


The general form of genkernel invocation is as follows:

root #genkernel [options ...] action


The actual behavior of genkernel depends on a large variety of options, the majority of which can be set/unset in the /etc/genkernel.conf file or passed via the genkernel command on each invocation. Options passed over the command line take precedence over options defined in /etc/genkernel.conf. The configuration file is very well documented, however some of the most commonly used options will be explored in this article. The goal is for the reader to be familiar with common genkernel invocations. For a more complete set of explanations refer to the comments in /etc/genkernel.conf itself or to the output of man genkernel.

Some of the options have a variant that triggers a converse behavior. They are figured as --[no-]option_name, and the converse effect is shown in square brackets as in the following example:
CODE Enable or disable a certain option
--[no-]menuconfig : Activates [deactivates] ...

Where options have only a negative effect, the no-, and its effect, which are not optional in this case, are figured without square brackets.

Options acting on user interactivity

The configuration options listed below help the user decide how to interact with the configuration process. Users can even choose whether or not the configuration file created in the process should be saved. The following are considered primary configuration options:

Option Description
--config=/path/to/genkernel.conf Points to the Genkernel configuration file to use (defaults to /etc/genkernel.conf).
--[no-]menuconfig Activates (or deactivates) the make menuconfig command (which invokes an interactive configuration menu) before building the kernel.
--gconfig Provides a kernel configuration utility which depends on the GTK+ libraries. The advantage of this option is that most users find it easier and clearer to configure the kernel using this tool, since it relies on the X-windowing system. The disadvantage of this option is that the X-windowing system is required to use it, so it will not work on the command line.
--xconfig Provides a kernel configuration utility which depends on the QT libraries. The advantage of this option is that most users find it easier and clearer to configure the kernel using this tool, since it relies on the X-windowing system. The disadvantage of this option is that the X-windowing system is needed to use it, so it will not work on the command line.
--[no-]save-config Saves [or does not save] the kernel configuration to a file in the /etc/kernels directory for later use.
--kernname=NickName Allows the modification of the name of the kernel and initrd images in the /boot directory, so that the images produced are kernel-NickName-version and initramfs-NickName-version.

Options acting on the resulting system

The configuration options listed here defines which features will or will not be enabled in the resulting kernel and initrd.

Option Description
--[no-]splash Activates (or deactivates) support for Fbsplash framebuffer splash in the genkernel-built initrd image. To override the default theme used by fbsplash, use --splash=PreferredTheme (where PreferredTheme is the title of one of the directories inside the /etc/splash directory).
--splash-res=PreferredResolution This option allows to select which splash screen resolutions will be supported in the initrd during the start-up of the system. This is useful for two reasons. First, to be able to select only the splash screen resolution(s) relevant to the system. Second, to avoid the unnecessary increase in the disk space required by initrd (since the initrd does not have to support resolutions that are irrelevant for the system's configuration.) However, the option is best omitted if the kernel is being compiled for an Installation CD; this allows splash support for all possible resolutions.
--do-keymap-auto Force keymap selection during the boot sequence.
--lvm Includes support for storage using via Logical Volume Management (LVM2) from static binaries, if available to the system. Relevant (static) LVM2 binaries are compiled if they are unavailable. Be sure to install the sys-fs/lvm2 package on the system (emerge sys-fs/lvm2) before enabling this option, then review the LVM article on the Gentoo wiki.
--dmraid Includes support for DMRAID; the utility which creates RAID mappings using the kernel device-mapper subsystem. DMRAID discovers, activates, deactivates and displays properties of software RAID sets (ATARAID, for example) and contained DOS partitions.
--luks Includes support for Linux Unified Key Setup or LUKS. This will allow to use a device encrypted by LUKS which contains the root filesystem. On the bootloader, set that encrypted device as the value of crypt_root (and root shall be the unencrypted device LUKS creates).
--disklabel Adds support for disk label and UUID support to the initrd.
--iscsi Adds support for iSCSI to the initrd.
--multipath Adds support for Multipath to the initrd.
--linuxrc=/path/to/the/linuxrc_file Specifies a user-created linuxrc — a script that is initialized during the start-up stage of the kernel, prior to the actual boot process. A default linuxrc script can be found in the /usr/share/genkernel/ directory. This script allows booting into a small, modularized kernel; it tries to load the minimum amount of drivers that are needed (as modules) by the system.
--cachedir=/path/to/alt/dir Overrides the default cache location used while compiling the kernel.
--tempdir=/path/to/new/tempdir Specifies the location of the temporary directory used by genkernel while compiling the kernel.
--unionfs Includes support for the Unification File System in the initrd image.
--mountboot Detects whether or not the /boot directory needs to be mounted on a separate partition. It will check /etc/fstab script for instructions on how to mount the boot partition on a file system (if needed).

Options acting on the choice of the tools used for building

The following options are supported by genkernel, and are passed to the relevant applications while the kernel is being assembled. These options affect the compiling tools used for the kernel compilation process, albeit at a much lower level.

Options Description
--kernel-cc=someCompiler Specifies the compiler employed during the kernel compilation process.
--kernel-ld=someLinker Specifies the linker employed during the kernel compilation process.
--kernel-as=someAssembler Specifies the assembler employed during the kernel compilation process.
--kernel-make=someMake Specifies an alternative to the GNU make utility employed during the kernel compilation process.
--utils-cc=someCompiler Specifies the compiler employed during the compilation of support utilities.
--utils-ld=someLinker Specifies the linker employed during the compilation of support utilities.
--utils-as=someAssembler Specifies the assembler employed during the compilation of support utilities.
--utils-make=someMake Specifies an alternative to the GNU make utility employed during the compilation of support utilities.
--makeopts=-jX Specifies the number of concurrent threads that the make utility can implement while the kernel (and utilities) are being compiled. The variable X is a number to be freely chosen, although the most common values are obtained by adding one (1) to the number of cores used by the system, or just use the number of cores on the system. So, for a system with one core, most common option values are -j2 or -j1; a system with two cores most likely uses the -j3 or -j2 options, and so on. (A system with one processor that supports Hyper-Threading™ (HT) Technology can be assumed to have 2 cores, provided Symmetric Multi-Processing (SMP) support is enabled in the kernel.)

Options acting on the compilation process

The following options usually take effect during the actual compilation:

Option Description
--kerneldir=/path/to/sources/ Specifies an alternative kernel source location, rather than the default /usr/src/linux/ location.
--kernel-config=/path/to/config-file Specifies what alternative kernel configuration will be used, rather than the default /path/to/sources/.config file.
--module-prefix=/path/to/prefix-directory/ Specifies a prefix to the directory where kernel modules will be installed (default path is the /lib/modules directory.)
--[no-]clean Activates (or deactivates) the make clean command before compiling the kernel. The make clean command removes all object files and dependencies from the kernel's source tree.
--[no-]mrproper Activates (or deactivates) the make mrproper command before kernel compilation. Like the make clean command listed above, make mrproper removes all object files and dependencies from the kernel's source tree. However, any previous configuration files (in /path/to/sources/.config or /path/to/sources/.config.old) will also be purged from the kernel's source tree. If it is undesirable that the kernel's .config file keeps disappearing, be sure to disable this option!
--oldconfig Issues the make oldconfig command, which attempts to collect configuration information for the system's architecture from a generic script in /usr/share/genkernel. This is a non-interactive process; no user input is entertained. Also, if --oldconfig is used in conjunction with --clean, the latter option is negated, resulting in the activation of the --no-clean option.
--callback="echo hello" Calls the specified arguments (echo hello, in this case) after the kernel and the relevant modules have been built, but before building the initrd image. This may be useful when installing external modules in the initrd image by emerging the relevant item(s) with the callback feature, and then redefining a genkernel module group.
--[no-]install Activates (or deactivates) the make install command, which installs the new kernel image, configuration file, initrd image and system map onto the boot partition. Any compiled modules will be installed as well. By default genkernel will attempt mount /boot if it is on a separate partition before running the install command.
--no-ramdisk-modules Refrains from copying any modules to the genkernel-created initrd image. This option is an exception to the rule about the no- prefix; omission of this prefix creates an invalid genkernel option.
--all-ramdisk-modules Copies all available modules to the genkernel-created initrd image.
--genzimage Creates the initrd image, prior to the kernel image (this hack currently applies only to PPC Pegasos systems).

Debugging options

The use of debugging options during the kernel compilation process controls the amount of information reported, as well as the presentation of said data.

Option Description
--loglevel=<0|1|2|3|4|5> Controls the level of verbosity for information provided by genkernel. The variable <verblevel> is an integer between 0 and 5. The level '0' represents minimal output, while '5' provides as much information as possible about genkernel's activities during the kernel compilation process.
--logfile=/path/to/output_file Ignores the value set by the --loglevel option (above) and sends all debugging data produced by genkernel to the specified output file. Outputs to /var/log/genkernel.log by default.
--[no-]color Activates (or deactivates) colored output of debugging information (reported by genkernel) using escape sequences.
--[no-]debug-cleanup Activates (or deactivates) the full post-run cleanup for debug purposes.


The action passed on the command line with the genkernel [options …] action tells genkernel what action to perform. The following actions are supported:

Action Description
all Builds all stages — the initrd, kernel image and modules.
bzImage Only builds the kernel image.
kernel Only builds the kernel image and modules.
initramfs Only builds the initramfs/ramdisk image.
ramdisk Only builds the initramfs/ramdisk image.


Starting off

Although there are several ways to run genkernel, the least-intrusive approach recommended for most users is provided by genkernel all. Here, a generic configuration which works well for most systems is used. As was mentioned earlier, this approach is not without drawbacks; most of the modules created are useless to the average user and may increase compile time. Below is an illustration of a more efficient approach, achieved by passing certain options to genkernel as root:

root #genkernel --splash --no-install --no-clean --menuconfig all

The above operation causes genkernel to create a framebuffer splash-enabled kernel (--splash) that will have to be manually installed (--no-install). While preparing the kernel source tree, genkernel will refrain from cleaning out any preexisting object files present in the source tree (--no-clean). A menu-driven kernel configuration utility will be displayed that allows the user to select which modules will be built for the system (--menuconfig).

Replacing --no-install with the --install option allows genkernel to automatically install the new kernel in the /boot directory, and will create symlinks if --symlink is specified. Using the --mountboot option allows genkernel to mount the /boot partition automatically, if necessary.

Don't forget that the /etc/genkernel.conf file is sourced by the genkernel command at startup, and that any option defined there, will be applied, except where a command line option takes precedence over it.

Changing the kernel

The first thing that should be done is to allow the triggering of make menuconfig in the /etc/genkernel.conf file:

FILE /etc/genkernel.confForcing the use of the configuration menu in /etc/genkernel.conf
# Run 'make menuconfig' before compiling this kernel?

File management

While using genkernel, the user has to be aware of some aspects relating to kernel configuration and kernel image files management and the way the kernel sources are handled by the system.

Source files

After an emerge -u gentoo-sources, whenever new sources are available, a new kernel source directory is created under /usr/src/ to host them. Normally, the active kernel sources directory is pointed to by the /usr/src/linux symlink.

The /usr/src directory might look like this:

user $ls -l /usr/src
total 16
lrwxrwxrwx  1 root root   19 21 Mar   2013 linux -> linux-3.7.10-gentoo
drwxr-xr-x 24 root root 4096 25 Aug  10:39 linux-3.10.7-gentoo
drwxr-xr-x 20 root root 4096 21 Apr  19:42 linux-3.7.10-gentoo
drwxr-xr-x 21 root root 4096 14 Mar   2013 linux-3.7.9-gentoo

The /usr/src/linux symlink can be changed in different ways.

  • If the symlink USE is set the /usr/src/linux symlink is automatically updated to point to the newly emerged sources.
  • If the symlink USE is not set, the user can change the destination of the symlink using the eselect kernel list followed by the eselect kernel set command.

genkernel will always (exclusively) use the sources pointed by the /usr/src/linux symlink.

Kernel configuration file

If a kernel compilation has already been run with the active kernel sources, there might be a file inside the /etc/kernels directory that contains the kernel configuration that has been applied while creating the last bzimage of the kernel. This file is named, for example kernel-config-x86_64-3.7.9-gentoo-r1 where x86_64 might be substituted with the system's architecture, 3.7.9 might be substituted with the version of the sources used, and r1 with the release of the sources.

It is this kernel-config-x86_64-3.7.9-gentoo-r1 file that is used as a starting configuration when running  genkernel --menuconfig all.

If it is the first time that genkernel is run with the new kernel sources, or if the previous result has not been saved, this file is substituted with a default configuration file that resides at usr/share/genkernel/arch/x86_64/kernel-config where x86_64 is substituted with the actual architecture.

The path to this default configuration file, may be altered by setting the DEFAULT_KERNEL_CONFIG variable in /etc/genkernel.conf file.
Saving the compiled configuration

If the --save-config genkernel option is activated, either from the command line or inside /etc/genkernel.conf, the compiled kernel configuration is saved (with the name given above) into the /etc/kernels directory. At the same time, the configuration is saved in the .config file in /usr/src/linux directory but this file is not reused on the next genkernel all run.

One has to be aware, that each time genkernel is run, with the --save-config genkernel option set, the configuration file in /etc/kernels is overridden. Thus, it is highly recommended to copy this file under a new name before running genkernel in order to preserve it.
Installing the kernel and initramfs into the /boot directory

Specifying the --install option when invoking genkernel, will ask genkernel to install the kernel image and the initramfs into the /boot directory. In order to run --install a convenient manner, set the following in the /etc/genkernel.conf file:

FILE /etc/genkernel.confSetting up kernel and initramfs auto-saving
# Mount BOOTDIR automatically if it is not mounted?
# Save the new configuration in /etc/kernels upon
# successful compilation
# Make symlinks in BOOTDIR automatically?
# Add new kernel to grub2?
  • The first parameter speaks for itself.
  • The second parameter tells genkernel to save the compiled kernel configuration into /etc/kernels
  • The last two options tell genkernel to automatically update the grub configuration. In practice, the following happens:
    • If a previous kernel image with the same name already exist, it is renamed by appending .old to its name. A symlink kernel.old is automatically created that points to it.
    • The new kernel takes the place of any kernel with the same name into /boot. If it is the first time a kernel is compiled, a symlink kernel is automatically created that points to the new kernel.

After running genkernel --menuconfig all, the /boot directory might look like this:

user $ls -al /boot
total 41336
drwxr-xr-x  3 root root    4096 20 avril 17:23 .
drwxr-xr-x 24 root root    4096 15 sept. 12:31 ..
lrwxrwxrwx  1 root root       1 24 févr.  2013 boot -> .
drwxr-xr-x  2 root root    4096 24 févr.  2013 grub
lrwxrwxrwx  1 root root      40 20 avril 17:23 initramfs -> initramfs-genkernel-x86_64-3.7.10-gentoo
-rw-r--r--  1 root root 1314412 20 avril 17:23 initramfs-genkernel-x86_64-3.7.10-gentoo
-rw-r--r--  1 root root 1313548 21 mars   2013 initramfs-genkernel-x86_64-3.7.10-gentoo.old
-rw-r--r--  1 root root 1295344 25 févr.  2013 initramfs-genkernel-x86_64-3.7.9-gentoo
-rw-r--r--  1 root root 3310324 25 févr.  2013 initramfs-genkernel-x86_64-3.7.9-gentoo.old
lrwxrwxrwx  1 root root      44 20 avril 17:23 initramfs.old -> initramfs-genkernel-x86_64-3.7.10-gentoo.old
lrwxrwxrwx  1 root root      37 20 avril 17:23 kernel -> kernel-genkernel-x86_64-3.7.10-gentoo
-rw-r--r--  1 root root 4866656 20 avril 17:23 kernel-genkernel-x86_64-3.7.10-gentoo
-rw-r--r--  1 root root 4866560 21 mars   2013 kernel-genkernel-x86_64-3.7.10-gentoo.old
-rw-r--r--  1 root root 4552288 25 févr.  2013 kernel-genkernel-x86_64-3.7.9-gentoo
-rw-r--r--  1 root root 3400736 25 févr.  2013 kernel-genkernel-x86_64-3.7.9-gentoo.old
lrwxrwxrwx  1 root root      41 20 avril 17:23 kernel.old -> kernel-genkernel-x86_64-3.7.10-gentoo.old

Configuring the bootloader

The symlinks presented above in the bootloader's configuration can be used so that, even if the new kernel is not bootable, the user can always boot on the old one.

To allow the kernel and intird provided by genkernel to run correctly, provide a minimum information in bootloader's configuration file:

  • Add root=/dev/sdaN to the kernel parameters passed to the kernel image, where /dev/sdaN points to the root partition (N is the number of the partition if a partition exists).
  • If splash is used, add a suitable mode line such as vga=0x317 to the parameters passed to the kernel and also add splash=verbose or splash=silent depending on the verboseness required through the boot process.
  • Add the initrd information as required by the bootloader. Consult the Bootloader Configuration Chapter of the Gentoo Handbook for details on how to make the bootloader initrd-aware.

Here is how the grub.conf file might look.

FILE /boot/grub/grub.confAn example of grub.conf file
# This is a sample grub.conf for use with Genkernel, per the Gentoo handbook 
# If you are not using Genkernel and you need help creating this file, you 
# should consult the handbook. Alternatively, consult the grub.conf.sample that 
# is included with the Grub documentation. 
default 0 
timeout 5
title Gentoo Linux 
root (hd0,6) 
kernel /boot/kernel initrd=/dev/ram0 root=/dev/sda7 rootfstype=ext4  
initrd /boot/initramfs
title Gentoo Linux old kernel 
root (hd0,6) 
kernel /boot/kernel.old initrd=/dev/ram0 root=/dev/sda7 rootfstype=ext4 
initrd /boot/initramfs.old

Preserving the working files

The genkernel application automatically saves new changes to the files. If previous changes are to be preserved, then the following actions need to be taken.

  • The first file to preserve is the kernel configuration file in /etc/kernels/ If the source has not changed prior to the recompilation of the kernel, the previously used name for this file will be used. So copying the previous configuration file under a different name helps in preserving the information while keeping the file available as a starting point for a new configuration.
  • The second important thing is to preserve the already bootable kernel and initramfs images. The way to accomplish this depends on the context:
  1. If the last kernel compiled is bootable, running genkernel will rename this kernel (and similarly initramfs) image to kernel-genkernel-ARCH-X.Y.Z-gentoo-rx.old and create a new kernel-genkernel-ARCH-X.Y.Z-gentoo-rx. This mean that even if the new kernel is not bootable, users will always be able to boot the old one.
  2. If the last kernel compiled is not bootable and sources haven't changed since the user compiled a bootable one, prior to running genkernel, first delete the new kernel image and remove the .old suffix from the last bootable one. Without this, if the newly compiled kernel is not bootable for the second time, the bootable kernel-genkernel-ARCH-X.Y.Z-gentoo-rx.old will be kicked out by the renaming of the non bootable kernel-genkernel-ARCH-X.Y.Z-gentoo-rx, giving the user an unbootable system. Use the same reasoning for initramfs.

Using previous kernel configuration while changing the sources

The previous configuration can be used through the MENUCONFIG variable in /etc/genkernel.conf as follows:

FILE /etc/genkernel.confSetting up make menuconfig
# Run 'make menuconfig' before compiling this kernel?
There is no need to run make oldconfig when using genkernel, even if the sources are changed from kernel-genkernel-ARCH-version-gentoo-rx to kernel-genkernel-ARCH-version-gentoo-r(x+1) or from kernel-genkernel-ARCH-version-gentoo to kernel-genkernel-ARCH-(version+1)-gentoo. This is because make menuconfig will try to load the previous configuration into the menu as much as possible. Nevertheless, reviewing each option and new sections carefully is recommended.

Network booting

From an installation CD

The genkernel utility can build kernel and initrd images that provide support for network booting, or netbooting. With any luck, users should be able to netboot any recent computer into the environment provided by the Installation CD.

The magic lies in genkernel's linuxrc script: it will try to netmount the Installation CD using NFS. From there, the init scripts of the Installation CD can take over, as if the CD was present locally.

Building the kernel and initramfs with netboot support

To enable support for netbooting, include the following options while configuring the kernel:

Support for netbooting with genkernel is experimental and may contain bugs.

First, the kernel image must include the drivers for the system's Network Interface Cards (NIC). Normally, drivers for such devices will be compiled as modules. However, it is essential (for netbooting) that such drivers are compiled directly into the kernel image and not as modules.

KERNEL Configuring a 3.x.x series kernel to support various NIC drivers
Device Drivers --->
   Networking Support --->
      Ethernet (10 or 100Mbit)  --->
         [*] Ethernet (10 or 100Mbit)
         <*>   The driver(s) for each network card
Be sure to select <*> and not <M>.

Secondly, it is suggested that IP: kernel level autoconfiguration is enabled as well as IP: DHCP support options. This avoids an unnecessary layer of complexity since the IP address and the NFS path to the Installation CD can be configured on a DHCP server. Of course, this means the kernel command line will remain constant for any machine — which is very important for etherbooting.

KERNEL Configuring a 3.x.x series kernel to support DHCP
Device Drivers --->
   Networking Support --->
      Networking options
         [*] TCP/IP networking--->
         [*]   IP: kernel level autoconfiguration
         [*]     IP: DHCP support

These options tell the kernel to send a DHCP request at bootup.

Additionally, enable SquashFS because most modern Gentoo Installation CDs require it. Support for SquashFS is not included with the generic kernel source tree. To enable SquashFS, apply the necessary patches to the generic kernel source or install gentoo-sources.

KERNEL Configuring the kernel to support SquashFS
File systems--->
   Miscellaneous filesystems --->
      [*] SquashFS 2.X - Squashed file system support

Once the compilation process is completed, create a compressed tarball (tar.gz) that contains the kernel's modules. This step is only necessary if the kernel version does not match the kernel image version on the Installation CD.

To create an archive containing all the modules:

root #cd /
root #tar -cf /tmp/modules-X.Y.Z.tar.gz /lib/modules/X.Y.Z/

Depending on the network boot mechanism, one of the following steps need to be followed:

To create an etherboot image:

root #emerge --ask net-misc/mknbi
root #cd /boot
root #mkelf-linux -params="root=/dev/ram0 init=/linuxrc ip=dhcp" kernel... initrd... > etherboot.img

To create an OpenBoot/SPARC64 TFTP image:

root #emerge --ask sys-apps/sparc-utils
root #cd /boot
root #elftoaout kernel... -o kernel.aout
root #piggyback64 kernel.aout initrd-...
root #mv kernel.aout openboot.img

The openboot.img file is the boot image.

Finally, copy this kernel to the TFTP server. The details are architecture-dependent and are beyond the scope of this guide. Please refer to the documentation for the specific platform of interest.

NFS setup

To setup a NFS share that contains the Installation CD, use the loop device to mount the ISO image and then copy the contents of the CD into the NFS share. As a nice extra, genkernel's initrd scripts will extract all tar.gz files located in the /nfs/livecd/add/ directory. All that needs to be done here is copy the modules-X.Y.Z.tar.gz archive to the /nfs/livecd/add/ directory.

The following assumes that /nfs/livecd is an exported NFS share:

root #mount /tmp/gentoo-livecd.iso /mnt/cdrom -o loop
root #cp -p /mnt/cdrom /nfs/livecd
root #umount /mnt/cdrom

Now copy the modules.tar.gz file into /add:

root #mkdir /nfs/livecd/add
root #cp /tmp/modules-X.Y.Z.tar.gz /nfs/livecd/add

DHCP setup

The netboot images will ask the DHCP server on the network for an IP as well as a root-path option. Both can be specified per host using a MAC address to identify machines:

FILE /etc/dhcpd.confSample client dhcpd.conf setup
# Here, is the NFS server while will be the IP address of the netbooted machine
host netbootableMachine {
         hardware ethernet 11:22:33:44:55:66;
         option root-path "";

Netbooting instructions

Netbooting itself is again very platform-specific. The important part is to specify the ip=dhcp and init=/linuxrc parameters on the kernel command line, as this will bring up the network interface and tell the initrd scripts to mount the Installation CD via NFS. Here are some platform-specific tips.

For etherboot, insert the etherboot disk into the drive and reboot. The kernel command line was specified when the image was constructed.

With Sparc64, press Stop+A at the boot prompt and then enter:

okboot net ip=dhcp init=/linuxrc

For PXE, setup pxelinux (part of syslinux), then create a pxelinux.cfg/default along the lines of:

FILE pxelinux.cfg/defaultDefault entry
DEFAULT gentoo
LABEL gentoo
    KERNEL kernel-X.Y.Z
    APPEND initrd=initrd-X.Y.Z root=/dev/ram0 init=/linuxrc ip=dhcp

Booting a genkernel initramfs


If an initramfs is installed with genkernel, then take a look at the various boot options that can (or should) be defined in the bootloader configuration. The most common ones are added to this guide.

Loading LVM or software-RAID

If the system uses LVM or software-RAID, the initramfs has to be built using the --lvm and --mdadm options. Do not forget to enable support during boot as well. This can be done using the dolvm and domdadm options.

FILE /boot/grub/grub.confEnabling LVM and/or MDADM support
# Example for GRUB 1.x
title Gentoo Linux
root (hd0,0)
kernel /vmlinuz root=/dev/md3 dolvm domdadm
initrd /initramfs-genkernel-x86_64-3.4.3
Booting in single-user mode

If for some reason boot-up fails, rescuing the system by booting in the single-user mode is still possible. This will only load the really necessary services and then drop the user to a rescue (root) shell.

FILE /boot/grub/grub.confBooting in single-user mode
# Example for GRUB 1.x
title Gentoo Linux
root (hd0,0)
kernel /vmlinuz root=/dev/md3 init_opts=S
initrd /initramfs-genkernel-x86_64-3.4.3


compile failed: Kernel not found

This failure can be experienced when attempting to compile a kernel for one architecture with a kernel .config file that has a target of a different architecture than the target. This could be as simple as a machine running an amd64 kernel attempting to compile for a x86 target.

This can be experienced when booting from a x86_64 LiveCD on a system that is capable of operating in 64-bits, however the target to be installed or repaired is 32-bits. After the compilation process begins, genkernel will attempt to automatically detect the architecture for the target based on the currently running kernel. If the currently running kernel is 64-bit, then, unless it is told otherwise, genkernel will presume it should be looking for a 64-bit kernel.

Suppose then that the kernel's .config file has x64 disabled. The make command will execute and the resulting kernel binary will be placed in the associated x86 directory. When genkernel goes to install (move and rename) the kernel it cannot find the kernel because it expected a 64-bit kernel.

The solution to this failure is to set genkernel's --arch-override option to, in this case, the x86 architecture. The final command could look like this:

root #genkernel --kernel-config=/usr/src/linux/.config --btrfs --install --arch-override=x86 all

To override the architecture choice permanently, modify the ARCH_OVERRIDE variable in the /etc/genkernel.conf file.

See also

  • Manual kernel configuration - For the times it is necessary to to do things manually.
  • Dracut - Another initramfs builder available in Gentoo.
    This article is based on a document formerly found on our main website
    The following people contributed to the original document: Tim Yamin, Jimi Ayodele, Thomas Seiler, Joshua Saddler, Sebastian Pipping, José Fournier
    They are listed here as the Wiki history does not allow for any external attribution. If you edit the Wiki article, please do not add yourself here; your contributions are recorded on the history page.