doc/en/genkernel.xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/genkernel.xml,v 1.28 2007/05/31 22:02:14 nightmorph Exp $ -->
<!DOCTYPE guide SYSTEM "/dtd/guide.dtd">

<guide link="/doc/en/genkernel.xml">
<title>Gentoo Linux Genkernel Guide</title>

<author title="Author">
  <mail link="plasmaroo@gentoo.org">Tim Yamin</mail>
</author>

<!-- folajimi@speakeasy.net -->
<author title="Contributor">
  Jimi Ayodele
</author>

<!-- thseiler@gmail.com -->
<author title="NFS Support">
  Thomas Seiler
</author>

<abstract>
This guide intends to provide a reference of all the functions provided by
genkernel.
</abstract>

<license/>

<version>1.5</version>
<date>2007-06-20</date>

<chapter>
<title>Introduction</title>
<section>
<title>Rationale</title>
<body>

<p>
For users who are not privy to kernel compilation, genkernel is a tool to
automate this process. It can help you create a kernel image akin to those
available on Gentoo Installation CDs, which are designed to auto-detect the
hardware configuration of your system. Some users may also be interested in
using genkernel for hardware requiring initialization and a working kernel
before the system starts up. Since genkernel automatically compiles your kernel
modules, you can use hardware that may require certain module parameters to be
loaded for proper operation.
</p>

</body>
</section>
<section>
<title>Target Audience</title>
<body>

<p>
If you are either uncertain about how to compile a kernel, or are just
unfamiliar with your hardware configuration, genkernel is a very handy tool.
It is designed to take the pain out of the kernel compiling process, and
supports most hardware by default.
</p>

<p>
However, if you know what drivers are required by your system, you may be able
to further reduce the time taken to compile the kernel. This is possible since
you can direct genkernel to only build drivers relevant to your hardware.
Oftentimes, the number of drivers required by your system will be fewer
(implying a shorter kernel compilation time) than the default configuration
provides.
</p>

</body>
</section>
<section>
<title>Installing genkernel</title>
<body>

<p>
To obtain genkernel, run <c>emerge genkernel</c> from the command line. If you
are using the <uri
link="/doc/en/handbook/2005.1/hb-install-about.xml#doc_chap2_sect1">Gentoo
Reference Platform</uri> (GRP), remember to install binary packages by passing
the <c>-k</c> flag to emerge. Since the GRP is bundled with an older version of
genkernel, the flags may be different. In any case, consult <c>genkernel
--help</c> for help on how to use the version of genkernel installed on your
system.
</p>

</body>
</section>
</chapter>

<chapter>
<title>Working with genkernel</title>
<section>
<title>How to use genkernel</title>
<body>

<p>
Although there are several ways to run genkernel, the least-intrusive approach
is provided by <c>genkernel all</c>. 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 flags to genkernel as root:
</p>

<pre caption="Running genkernel (with flags)">
# <i>genkernel --bootsplash --no-install --no-clean --menuconfig all</i>
</pre>

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

<p>
There are other flags which alter the result provided by genkernel. For
instance, replacing <c>--no install</c> with the <c>--install</c> flag allows
genkernel to automatically install the new kernel in the <path>/boot</path>
directory. Using the <c>--mountboot</c> flag allows genkernel to mount your
<path>/boot</path> partition automatically, if necessary.
</p>

<p>
Remember, genkernel is designed to make kernel compilation easy and
stress-free. For this reason, genkernel features several flags to ease the
kernel compilation effort. For example, there are flags to help with kernel
configuration, while others affect the actual compilation. Some flags even help
debug the compilation process. For those interested in further optimization,
there are flags that affect kernel assembling, packaging and even kernel
initialization.
</p>

<p>
The rest of this chapter examines the functionality of various flags and
actions available for genkernel. Some of the flags have variants which perform
a converse operation. The converse variants carry the <b><c>no-</c></b> prefix,
and their effects are enclosed within the square brackets, [].
</p>

</body>
</section>
<section>
<title>Configuration Flags</title>
<body>

<p>
The configuration flags listed below exist to help you decide what features
should be enabled or disabled in the kernel prior to compilation. You can even
choose whether or not the configuration file created in the process should be
saved. The following are the primary configuration flags:
</p>

<ul>
  <li>
    <b>--<c>no-</c>menuconfig</b>: Activates <e>[or deactivates]</e> the
    <c>make menuconfig</c> command (which invokes an interactive, menu-based
    kernel configuration utility), before building the kernel.
  </li>
  <li>
    <b>--gconfig</b>: 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 you
    <b>need</b> the X-windowing system to use it, so it will not work on the
    command line.
  </li>
  <li>
    <b>--xconfig</b>: 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 you
    <b>need</b> the X-windowing system to use it, so it will not work on the
    command line.
  </li>
  <li>
    <b>--<c>no-</c>save-config</b>: Saves <e>[or does not save]</e> the kernel
    configuration to a file in the <path>/etc/kernels/</path> directory for
    later use.
  </li>
</ul>

</body>
</section>
<section>
<title>Compilation Flags</title>
<body>

<p>
The following flags usually take effect during the actual compilation:
</p>

<ul>
  <li>
    <b>--kerneldir=<path>/path/to/sources/</path></b>: Specifies an alternative
    kernel source location, rather than the default
    <path>/usr/src/linux/</path> location.
  </li>
  <li>
    <b>--kernel-config=<path>/path/to/config-file</path></b>: Specifies what
    alternative kernel configuration will be used, rather than the default
    <path>/path/to/sources/.config</path> file.
  </li>
  <li>
    <b>--module-prefix=<path>/path/to/prefix-directory/</path></b>: Specifies a
    prefix to the directory where kernel modules will be installed (default
    path is the <path>/lib/modules/</path> directory.)
  </li>
</ul>

<ul>
  <li>
    <b>--<c>no-</c>clean</b>: Activates <e>[or deactivates]</e> the <c>make
    clean</c> command before compiling your kernel. The <c>make clean</c>
    command removes all object files and dependencies from the kernel's source
    tree.
  </li>
  <li>
    <b>--<c>no-</c>mrproper</b>: Activates <e>[or deactivates]</e> the <c>make
    mrproper</c> command before kernel compilation. Like the <c>make clean</c>
    command, <c>make mrproper</c> removes all object files and dependencies
    from the kernel's source tree. However, any previous configuration files
    (in <path>/path/to/sources/.config</path> or
    <path>/path/to/sources/.config.old</path>) will <b>also</b> be purged from
    the kernel's source tree.
  </li>
  <li>
    <b>--oldconfig</b>: Issues the <c>make oldconfig</c> command, which
    attempts to collect configuration information for the system's architecture
    from a generic script in <path>/usr/share/genkernel/</path>.  This is a
    non-interactive process; no user input is entertained. Also, if
    <c>--oldconfig</c> is used in conjunction with <c>--clean</c>, the latter
    flag is negated, resulting in the activation of the <c>--no-clean</c> flag.
  </li>
</ul>

<ul>
  <li>
    <b>--callback="<c>echo hello</c>"</b>: Calls the specified arguments
    (<c>echo hello</c>, in this case) after the kernel and the relevant modules
    have been built, but before building the initrd image. This may be useful
    if you want to install external modules in the initrd image by emerging the
    relevant item(s) with the callback feature, and then redefining a genkernel
    module group.
  </li>
</ul>

<ul>
  <li>
    <b>--<c>no-</c>install</b>: Activates <e>[or deactivates]</e> the <c>make
    install</c> command, which installs your new kernel image, configuration
    file, initrd image and system map onto your mounted boot partition. Any
    compiled modules will be installed as well.
  </li>
  <li>
    <b>--<c>no-</c>initrdmodules</b>: Refrains from copying any modules to the
    genkernel-created initrd image. This flag is an exception to the rule about
    the <c>no-</c> prefix; omission of this prefix creates an invalid genkernel
    flag.
  </li>
  <li>
    <b>--genzimage</b>: Creates the initrd image, prior to the kernel image.
    (This hack currently applies only to PPC Pegasos systems.)
  </li>
</ul>

</body>
</section>
<section>
<title>Compiler Flags</title>
<body>

<p>
The following flags are supported by genkernel, and are passed to the relevant
applications while the kernel is being assembled. These flags affect the
<e>compiler</e> used for the kernel compilation process, albeit at a much lower
level.
</p>

<ul>
  <li>
    <b>--kernel-cc=<c>someCompiler</c></b>: Specifies the compiler employed
    during the kernel compilation process.
  </li>
  <li>
    <b>--kernel-ld=<c>someLinker</c></b>: Specifies the linker employed during
    the kernel compilation process.
  </li>
  <li>
    <b>--kernel-as=<c>someAssembler</c></b>: Specifies the assembler employed
    during the kernel compilation process.
  </li>
  <li>
    <b>--kernel-make=<c>someMake</c></b>: Specifies an alternative to the
    <e>GNU make</e> utility employed during the kernel compilation process.
  </li>
</ul>

<ul>
  <li>
    <b>--utils-cc=<c>someCompiler</c></b>: Specifies the compiler employed
    during the compilation of support utilities.
  </li>
  <li>
    <b>--utils-ld=<c>someLinker</c></b>: Specifies the linker employed during
    the compilation of support utilities.
  </li>
  <li>
    <b>--utils-as=<c>someAssembler</c></b>: Specifies the assembler employed
    during the compilation of support utilities.
  </li>
  <li>
    <b>--utils-make=<c>someMake</c></b>: Specifies an alternative to the <e>GNU
    make</e> utility employed during the compilation of support utilities.
  </li>
</ul>

<ul>
  <li>
    <b>--makeopts=<c>-jX</c></b>: Specifies the number of concurrent threads
    that the make utility can implement while the kernel (and utilities) are
    being compiled. The variable <b>'X'</b> is a number obtained by adding one
    (1) to the number of CPUs used by the system. So, for a system with one
    CPU, the appropriate flag is <c>-j2</c>; a system with two CPUs will use
    the <c>-j3</c> flag, and so on. <e>(A system with one processor that
    supports Hyper-Threading&trade; (HT) Technology can use the
    </e><c>-j3</c><e> flag, provided Symmetric Multi-Processing (SMP) support is
    enabled in the kernel.)</e>
  </li>
</ul>

</body>
</section>
<section>
<title>Debugging Flags</title>
<body>

<p>
The use of debugging flags during the kernel compilation process controls the
amount of information reported, as well as the presentation of said data.
</p>

<ul>
  <li>
    <b>--debuglevel=<c>verblevel</c></b>: Controls the level of verbosity for
    information provided by genkernel. The variable <c>verblevel</c> 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.
  </li>
  <li>
    <b>--debugfile=<path>/path/to/outputfile</path></b>: Ignores the value set
    by the <c>--debuglevel</c> argument, and sends <b>all</b> debugging data
    produced by genkernel to the specified output file, which is located at
    <path>/var/log/genkernel.log</path> by default.
  </li>
  <li>
    <b>--no-color</b>: Activates <e>[or deactivates]</e> colored output of
    debugging information (reported by genkernel) using escape sequences.
  </li>
</ul>

</body>
</section>

<section>
<title>Initialization Flags</title>
<body>

<p>
The flags here are used to create certain effects during system startup. Some
of these flags are primarily for aesthetics, while others may be essential for
enabling certain features on the system.
</p>

<ul>
  <li>
    <b>--<c>no-</c>bootsplash</b>: Activates <e>[or deactivates]</e> support
    for <uri link="http://www.bootsplash.org/">bootsplash</uri> in the
    genkernel-built initrd image. The bootsplash feature is supported on a
    limited number of architectures, and a kernel that supports bootsplash is
    also required.
  </li>
  <li>
    <b>--<c>no-</c>gensplash</b>: Activates <e>[or deactivates]</e> support for
    <uri
    link="http://dev.gentoo.org/~spock/projects/gensplash/">gensplash</uri>
    support in the genkernel-built initrd image. The gensplash utility is
    intended to be a replacement for bootsplash, and is designed for use with
    2.6.x series kernels. To override the default theme used by gensplash, use
    <b>--gensplash=<c>PreferredTheme</c></b> (where <c>PreferredTheme</c> is
    the title of one of the directories inside the <path>/etc/splash/</path>
    directory.
  </li>
  <li>
    <b>--gensplash-res=<c>PreferredResolution</c></b>: This flag allows you 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,
    you are able to select only the splash screen resolution(s) relevant to
    your system. Second, you avoid the unnecessary increase in the disk space
    required by initrd (since the initrd does not have to support resolutions
    that are irrelevant for your system configuration.) However, you may want
    to omit this flag if the kernel is being compiled for an Installation CD;
    this allows gensplash support for all possible resolutions.
  </li>
  <li>
    <b>--do-keymap-auto</b>: Force keymap selection during the boot sequence.
  </li>
  <li>
    <b>--lvm2</b>: Includes support for storage using via <uri
    link="http://sources.redhat.com/lvm2/">Logical Volume Management</uri>
    (LVM2) from <e>static</e> binaries, if available to the system. Relevant
    (static) LVM2 binaries are compiled if they are unavailable. Be sure to
    install the lvm2 package on your system with <c>emerge lvm2</c> before
    enabling this flag, and review the <uri link="/doc/en/lvm2.xml">Gentoo LVM2
    Installation</uri> guide.
  </li>
  <li>
    <b>--evms2</b>: Includes support for storage using the <uri
    link="http://evms.sourceforge.net/">Enterprise Volume Management
    System</uri> (EVMS2), if available. Be sure to install the evms package on
    your system with <c>USE=static emerge evms2</c> before using this
    (genkernel) flag. <e>(Omitting the </e><c>USE=static</c><e> flag during
    package installation will fail to include the necessary static
    binaries.)</e>
  </li>
  <li>
    <b>--dmraid</b>: Includes support for <uri
    link="http://people.redhat.com/~heinzm/sw/dmraid/readme">DMRAID</uri>; 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.
  </li>
  <li>
    <b>--luks</b>: Includes support for <uri
    link="http://luks.endorphin.org/">Linux Unified Key Setup</uri> or LUKS.
    This will allow you to use a device encrypted by LUKS which contains the
    root filesystem. On the bootloader, you then set that encrypted device as
    the value of crypt_root (and real_root shall be the unencrypted device LUKS
    creates).
  </li>
  <li>
    <b>--linuxrc=/path/to/your/linuxrc</b>: Specifies a user-created
    <e>linuxrc</e> &mdash; a script that is initialized during the start-up
    stage of the kernel, prior to the actual boot process. (A default linuxrc
    script should be in the <path>/usr/share/genkernel/</path> directory.) This
    script allows you to boot into a small, modularized kernel and load the
    drivers that are needed (as modules) by the system.
  </li>
  <li>
    <b>--cachedir=/path/to/alt/dir/</b>: Overrides the default cache location
    used while compiling the kernel.
  </li>
  <li>
    <b>--tempdir=/path/to/new/tempdir/</b>: Specifies the location of the
    temporary directory used by genkernel while compiling the kernel.
  </li>
  <li>
    <b>--unionfs</b>: Includes support for the <uri
    link="http://www.fsl.cs.sunysb.edu/project-unionfs.html">Unification File
    System</uri> in the initrd image.
  </li>
</ul>

</body>
</section>
<section>
<title>Miscellaneous Flags</title>
<body>

<p>
The assortment of flags listed below are supported by genkernel, but do not fit
neatly into any of the other categories:
</p>

<ul>
  <li>
    <b>--mountboot</b>: Detects whether or not the <path>/boot/</path>
    directory needs to be mounted on a separate partition. It will check
    <path>/etc/fstab</path> script for instructions on how to mount the boot
    partition on a file system (if needed).
  </li>
  <li>
    <b>--kernname=<c>NickName</c></b>: Allows you to modify the name of the
    kernel and initrd images in the <path>/boot/</path> directory, so that the
    images produced are kernel-<c>NickName</c>-version and
    initramfs-<c>NickName</c>-version.
  </li>
</ul>

</body>
</section>
<section>
<title>Possible Actions</title>
<body>

<p>
An action tells genkernel what to build. Currently, the following actions are
supported:
</p>

<ul>
  <li><c>initrd</c>: Only builds the initrd image</li>
  <li><c>bzImage</c>: Only builds the kernel image</li>
  <li><c>kernel</c>: Only builds the kernel image and modules</li>
  <li>
    <c>all</c>: Builds all stages &mdash; the initrd, kernel image and modules.
  </li>
</ul>

<p>
The last action, <c>all</c>, is recommended for most users since it builds the
stages required for a functional kernel. Remember, an <e>action</e> simply
tells genkernel what to <e>build</e>, not <e>install</e>.
</p>

</body>
</section>
<section>
<title>Bootloader Configuration</title>
<body>

<p>
To set up genkernel to work with your bootloader, three or four changes should
be applied to the bootloader's configuration file:
</p>

<ol>
  <li>
    Add <c>root=/dev/ram0</c> and <c>init=/linuxrc</c> to the kernel parameters
    passed to the kernel image.
  </li>
  <li>
    Add <c>real_root=/dev/hda3</c>, for example, to the kernel parameters
    passed to the kernel image, if <path>/dev/hda3</path> contains your root
    partition.
  </li>
  <li>
    If you are using bootsplash, add a suitable mode line such as
    <c>vga=0x317</c> to the parameters passed to the kernel and also add
    <c>splash=verbose</c> or <c>splash=silent</c> depending on the verboseness
    you require from your bootloader.
  </li>
  <li>
    Add the initrd information as required by the bootloader. Consult the <uri
    link="/doc/en/handbook/handbook-x86.xml?part=1&amp;chap=10">Bootloader
    Configuration Chapter</uri> of the Gentoo Handbook for details on how to
    make your bootloader initrd-aware.
  </li>
</ol>

</body>
</section>
</chapter>

<chapter>
<title>Configuration Options</title>
<section>
<title>Editing /etc/genkernel.conf</title>
<body>

<p>
Passing flags to genkernel from the command line can be cumbersome, especially
if you have about a dozen flags:
</p>

<pre caption="Running genkernel (overloaded with flags)">
# <i>genkernel --debuglevel=5 --no-color --no-mrproper --clean --gensplash \
--kerneldir=/path/to/alternate/kernel/sources --install --menuconfig \
--kernel-config=/path/to/preferred/configfile --save-config --mountboot all</i>
</pre>

<p>
Fortunately, there is a configuration file where most of the basic options can
be set (or changed) as necessary. What follows is a rundown of the more
relevant options:
</p>

<ul>
  <li>
    <b>MENUCONFIG=<c>[yes|no]</c></b>: This option is equivalent to the
    <c>--menuconfig</c> flag used by genkernel, which in turn uses the <c>make
    menuconfig</c> command to invoke a command-line based kernel configuration
    utility. To invoke the utility automatically during kernel configuration
    via this script, set this option to 'yes' here; otherwise, choose 'no'.
  </li>
  <li>
    <b>CLEAN=<c>[yes|no]</c></b>: Setting this option to 'yes' is equivalent to
    the <c>--clean</c> flag used by genkernel, and invokes the <c>make
    clean</c> command to remove all object files and dependencies from the
    kernel's source tree. Setting this option to 'no' creates a cascade effect
    &#8212; it is equivalent to genkernel's <c>--no-clean</c> flag, which
    disables the <c>make clean</c> command and implies genkernel's
    <c>--no-mrproper</c> flag &mdash; essentially nullifying the <c>make
    mrproper</c> command.
  </li>
  <li>
    <b>MRPROPER=<c>[yes|no]</c></b>: Setting this option to 'yes' is equivalent
    to <c>--mrproper</c> flag used by genkernel, and invokes the <c>make
    mrproper</c> command, which purges the kernel source tree of any
    configuration files. Selecting 'no' here is equivalent to genkernel's
    <c>--no-mrproper</c> flag, which disables the <c>make mrproper</c> command.
  </li>
  <li>
    <b>MOUNTBOOT=<c>[yes|no]</c></b>: Setting this option to 'yes' is
    equivalent to the <c>--mountboot</c> flag, and automatically mounts the
    <path>/boot/</path> directory (if needed) at compile time. If the
    <path>/boot/</path> directory is on a separate partition, consider enabling
    this option; it will make for one less (essential) step to remember later.
  </li>
  <li>
    <b>SAVE_CONFIG=<c>[yes|no]</c></b>: After configuring the kernel, the
    selected options are stored as <path>.config</path> in the kernel source
    tree. This script may be overwritten during the next kernel compilation, or
    even purged from the kernel source tree. Choosing 'yes' here is equivalent
    to the <c>--save-config</c> flag, and stores all options selected during
    kernel configuration as a script in the <path>/etc/kernels/</path>
    directory. Choosing 'no' preserves the <e>status quo</e>.
  </li>
  <li>
    <b>USECOLOR=<c>[yes|no]</c></b>: Setting this option to 'yes' is equivalent
    to the <c>--color</c> flag, which colors genkernel's output to ease
    debugging (when needed.)
  </li>
  <li>
    <b>DEBUGLEVEL=<c>[0|1|2|3|4|5]</c></b>: This option is for adjusting the
    verbosity of the output produced by genkernel &mdash; setting this option to
    '0' with <c>--debuglevel=0</c> will suppress all output produced by
    genkernel; setting this option to '5' with <c>--debuglevel=5</c> provides
    the user with all output produced by genkernel.
  </li>
</ul>

<p>
By choosing the appropriate options in <path>/etc/genkernel.conf</path>, you
can halve the number of flags passed to genkernel from the command line:
</p>

<pre caption="Running genkernel (with flags), after employing genkernel.conf">
# <i>genkernel --gensplash --kerneldir=/path/to/alternate/kernel/sources \
--kernel-config=/path/to/preferred/configfile --install all</i>
</pre>

<p>
Identical results are obtained from both approaches, but the latter has most of
the options stored in a script that can be modified at a later date.
</p>

</body>
</section>
</chapter>

<chapter>
<title>Network-Booting with genkernel</title>
<section>
<title>Network Booting from an Installation CD</title>
<body>

<p>
The genkernel utility can build kernel and initrd images that provide support
for network booting, or <e>netboot</e>ing. With any luck, you should be able
to netboot any recent computer into the environment provided by the
Installation CD.
</p>

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

</body>
</section>
<section>
<title>Building Kernel and Initrd Images with Support for Netbooting</title>
<body>

<p>
To enable support for netbooting, include the following options while
configuring the kernel:
</p>

<warn>
Support for netbooting with genkernel is experimental and may contain bugs.
</warn>

<p>
First, the kernel image must include the drivers for your Network Interface
Cards (NIC). Normally, drivers for such devices will be compiled as modules.
However, it is essential (for netbooting) that you have such drivers compiled
directly into the kernel image and <b>not</b> as modules.
</p>

<pre caption="Configuring a 2.6.x series kernel to support your NIC driver">
Device Drivers --->
   Networking Support --->
      Ethernet (10 or 100Mbit)  --->
         [*] Ethernet (10 or 100Mbit)
         &lt;*&gt;   the driver for your network card
<comment>(Be sure to select &lt;*&gt; and not &lt;M&gt;)</comment>
</pre>

<p>
Secondly, we suggest that you enable <c>IP: kernel level autoconfiguration</c>
and the <c>IP: DHCP support</c> 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 &mdash; which is very important for
<e>etherbooting</e>.
</p>

<pre caption="Configuring a 2.6.x series kernel to support DHCP">
Device Drivers --->
   Networking Support --->
      Networking options
         [*] TCP/IP networking--->
         [*]   IP: kernel level autoconfiguration
         [*]     IP: DHCP support
<comment>(These options tell the kernel to send a DHCP request at bootup.)</comment>
</pre>

<p>
Additionally, you should 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 <c>gentoo-sources</c>.
</p>

<pre caption="Configuring the kernel to support SquashFS">
File systems--->
   Miscellaneous filesystems --->
      [*] SquashFS 2.X - Squashed file system support
</pre>

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

<pre caption="Creating a compressed tarball containing the kernel modules">
<comment>(Create a tar.gz containing all the modules)</comment>
# <i>cd /</i>
# <i>tar -cf /tmp/modules-X.Y.Z.tar.gz /lib/modules/X.Y.Z/</i>
</pre>

<p>
Depending on your network boot mechanism, you will need to do some of the
following steps:
</p>

<pre caption="Creating a boot image">
<comment>(Create an etherboot image)</comment>
# <i>emerge mknbi</i>
# <i>cd /boot</i>
# <i>mkelf-linux -params="root=/dev/ram0 init=/linuxrc ip=dhcp" kernel... initrd... > etherboot.img</i>

<comment>(Create a OpenBoot / SPARC64 TFTP image)</comment>
# <i>emerge sparc-utils</i>
# <i>cd /boot</i>
# <i>elftoaout kernel... -o kernel.aout</i>
# <i>piggyback64 kernel.aout System.map-... initrd-...</i>
# <i>mv kernel.aout openboot.img</i> <comment>(This is the boot image)</comment>

<comment>(PXE does not need any more steps, the kernel and initrd can be used as is)</comment>
</pre>

<p>
Finally, copy this kernel to your TFTP server. The details are
architecture-dependent and are beyond the scope of this guide. Please refer to
the documentation for your platform.
</p>

</body>
</section>
<section>
<title>NFS Setup</title>
<body>

<p>
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 <path>/nfs/livecd/add/</path> directory. All you have to do here is copy
the <c>modules-X.Y.Z.tar.gz</c> archive to the <path>/nfs/livecd/add/</path>
directory.
</p>

<pre caption="Preparing the NFS share">
<comment>(This assumes that /nfs/livecd is an exported NFS share)</comment>
# <i>mount /tmp/gentoo-livecd.iso /mnt/cdrom -o loop</i>
# <i>cp -p /mnt/cdrom /nfs/livecd</i>
# <i>umount /mnt/cdrom</i>

<comment>(Copy the modules.tar.gz into /add)</comment>
# <i>mkdir /nfs/livecd/add</i>
# <i>cp /tmp/modules-X.Y.Z.tar.gz /nfs/livecd/add</i>
</pre>

</body>
</section>
<section>
<title>DHCP Setup</title>
<body>

<p>
The netboot images will ask your DHCP server for an IP as well as a root-path
parameter. Both can be specified per host using a MAC address to identify
machines:
</p>

<pre caption="Sample client dhcpd.conf setup">
...

host netbootableMachine {
         hardware ethernet 11:22:33:44:55:66;
         fixed-address 192.168.1.10;
         option root-path "192.168.1.2:/nfs/livecd";
}
<comment># Here, 192.168.1.2 is the NFS server
# While 192.168.1.10 will be the IP address of the netbooted machine</comment>
...
</pre>

</body>
</section>
<section>
<title>Netbooting Instructions</title>
<body>

<p>
Netbooting itself is again very platform-specific. The important part is to
specify the <c>ip=dhcp</c> and <c>init=/linuxrc</c> 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:
</p>

<pre caption="Netbooting Instructions">
<comment># Etherboot - insert the etherboot disk into the drive and reboot
# The kernel command line was specified when the image was constructed</comment>

<comment># Sparc64 - Hit Stop-A at the boot prompt</comment>
ok boot net ip=dhcp init=/linuxrc

<comment># PXE - Setup pxelinux (part of syslinux),
then create a pxelinux.cfg/default along the lines of:</comment>

DEFAULT gentoo
TIMEOUT 40
PROMPT 1

LABEL gentoo
    KERNEL kernel-X.Y.Z
    APPEND initrd=initrd-X.Y.Z root=/dev/ram0 init=/linuxrc ip=dhcp
</pre>

</body>
</section>
</chapter>

<chapter>
<title>Conclusion</title>
<section>
<title>To Automate or not to Automate?</title>
<body>

<p>
The purpose of genkernel is to provide an (easier) alternative to the
time-tested approach to kernel compilation. As always, you are free to decide
on whether or not you want to automate the kernel compilation process.
</p>

</body>
</section>
</chapter>

</guide>