https://github.com/MichielDerhaeg/build-linux

A short tutorial about building Linux based operating systems.
https://github.com/MichielDerhaeg/build-linux

busybox kernel tutorial

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A short tutorial about building Linux based operating systems.

• Host: GitHub
• URL: https://github.com/MichielDerhaeg/build-linux
• Owner: MichielDerhaeg
• License: other
• Created: 2017-04-27T19:24:39.000Z (over 7 years ago)
• Default Branch: master
• Last Pushed: 2024-06-03T16:41:09.000Z (6 months ago)
• Last Synced: 2024-10-14T04:24:29.017Z (about 2 months ago)
• Topics: busybox, kernel, tutorial
• Language: Shell
• Homepage:
• Size: 220 KB
• Stars: 5,146
• Watchers: 188
• Forks: 334
• Open Issues: 7
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
Build yourself a Linux

======================

Introduction

------------

*This started out as a personal project to build a very small Linux based

operating system that has very few moving parts but is still very complete and

useful. Along the way of figuring out how to get the damn thing to boot and

making it do something useful I learned quite a lot. Too much time has been

spent reading very old and hard to find documentation. Or when there was none,

the source code of how other people were doing it. So I thought, why not share

what I have learned.*

[This git repo](https://github.com/MichielDerhaeg/build-linux) contains a

Makefile and scripts that automate everything that will be explained in this

document. But it doesn't necessarily do everything in the same order as it's

explained. You can also use that as reference if you'd like.

The Linux Kernel

----------------

The kernel is the core component of our operating system. It manages the

processes and talks to the hardware on our behalf. You can retrieve a copy of

the source code easily from [kernel.org](https://www.kernel.org/). There are

multiple versions to choose from, choosing one is usually a tradeoff between

stability and wanting newer features. If you look at the

[Releases](https://www.kernel.org/category/releases.html) tab, you can see how

long each version will be supported and keeps receiving updates. So you can

usually just apply the update or use the updated version without changing

anything else or having something break.

So just pick a version and download the tar.xz file and extract it with ``tar

-xf linux-version.tar.xz``. To build the kernel we obviously need a compiler and

some build tools. Installing ``build-essential`` on Ubuntu (or ``base-devel`` on

Arch Linux) will almost give you everything you need. You'll also need to

install ``bc`` for some reason.

The next step is configuring your build, inside the untarred directory you do

``make defconfig``. This will generate a default config for your current cpu

architecture and put it in ``.config``. You can edit it directly with a text

editor but it's much better to do it with an interface by doing ``make nconfig``

(this needs ``libncurses5-dev`` on Ubuntu) because it also deals with

dependencies of enabled features. Here you can enable/disable features

and device drivers with the spacebar. ``*`` means that it will be compiled in

your kernel image. ``M`` means it will be compiled inside a separate kernel

module. This is a part of the kernel that will be put in a separate file and can

be loaded in or out dynamically in the kernel when they are required. The default

config will do just fine for basic stuff like running in a virtual machine. But

in our case, we don't really want to deal with kernel modules so we'll just do

this: ``sed "s/=m/=y/" -i .config``. And we're done, so we can simply do ``make`` to

build our kernel. Don't forget to add ``-jN`` with `N` the number of cores

because this might take a while. When it's done, it should tell you where your

finished kernel is placed. This is usually ``arch/x86/boot/bzImage`` in the

linux source directory for Intel computers.

Other useful/interesting ways to configure the kernel are:

  * ``make localmodconfig`` will look at the modules that are currently

      loaded in the running kernel and change the config so that only those are

      enabled as module. Useful for when you only want to build the things you

      need without having to figure out what that is. So you can install

      something like Ubuntu on the machine first, copy the config to your build

      machine, usually located in /boot, Arch Linux has it in gzipped at

      /proc/config.gz). Do ``lsmod > /tmp/lsmodfile``, transfer this file to you

      build machine and run ``LSMOD=lsmodfile make localmodconfig`` there

      after you created ``.config``. And you end up with a kernel that is

      perfectly tailored to your machine. But this has a huge disadvantage, your

      kernel only supports what you were using at the time. If you insert a

      usb drive it might not work because you weren't using the kernel module

      for fat32 support at the time.

  * ``make localyesconfig``is the same as above but everything gets compiled in

      the kernel instead as a kernel module.

  * ``make allmodconfig`` generates a new config where all options are enabled

      and as much as possible as module.

  * ``make allyesconfig``is same as above but with everything compiled in the

      kernel.

  * ``make randconfig`` generates a random config...

You can check out ``make help`` for more info.

Busybox Userspace

-----------------

All these tools you know and love like ``ls``, ``echo``, ``cat`` ``mv``, and

``rm`` and so on are commonly referred to as the 'coreutils'. Busybox has that

and a lot more, like utilities from ``util-linux`` so we can do stuff like

``mount`` and even a complete init system. Basically, it contains most tools

you expect to be present on a Linux system, except they are a slightly

simplified version of the regular ones.

You can get the source from [busybox.net](https://busybox.net/). They also

provide prebuilt binaries which will do just fine for most use-cases. But just

to be sure we will build our own version.

Configuring busybox is very similar to configuring the kernel. It also uses a

``.config`` file and you can do ``make defconfig`` to generate one and ``make

menuconfig`` to configure it with a GUI. But we are going to use the one I

provided (which I stole from Arch Linux). You can find the config in the git

repo with the name ``bb-config``. Like the ``defconfig`` version, this has most

utilities enabled but with a few differences like statically linking all

libraries.  Building busybox is again done by simply doing ``make``, but before

we do this, let's look into ``musl`` first.

The C Standard Library

----------------------

The C standard library is more important to the operating system than you might

think. It provides some useful functions and an interface to the kernel. But it

also handles DNS requests and provides a dynamic linker. We don't really have to

pay attention to any of this, we can just statically link the one we are using

right now which is probably 'glibc'. This means the following part is optional.

But I thought this would make it more interesting and it also makes us able to

build smaller binaries.

That's because we are going to use [musl](https://www.musl-libc.org/), which is

a lightweight libc implementation. You can get it by installing ``musl-tools``

on Ubuntu or simply ``musl`` on Arch Linux. Now we can link binaries to musl

instead of glibc by using ``musl-gcc`` instead of ``gcc``.

Before we can build busybox with musl, we need sanitized kernel headers for use

with musl. You get that from [this github

repo](https://github.com/sabotage-linux/kernel-headers). And set

``CONFIG_EXTRA_CFLAGS`` in your busybox config to

``CONFIG_EXTRA_CFLAGS="-I/path/to/kernel-headers/x86_64/include"`` to use them.

Obviously change ``/path/to`` to the location where you put the headers repo,

which can be relative from within the busybox source directory.

If you run ``make CC=musl-gcc`` now, the busybox executable will be

significantly smaller because we are statically linking a much smaller libc.

Be aware that even though there is a libc standard, musl is not always a

drop-in replacement for glibc if the application you're compiling uses glibc

specific things.

Building the Disk Image

-----------------------

Installing an OS on a file instead of a real disk complicates things but this

makes development and testing easier.

So let's start by allocating a new file of size 100M by doing ``fallocate -l100M

image``(some distros don't have ``fallocate`` so you can do ``dd if=/dev/zero

of=image bs=1M count=100`` instead). And then we format it like we would format

a disk with ``fdisk image``. It automatically creates an MBR partition table for

us and we'll create just one partition filling the whole image by pressing 'n' and

afterwards just use the default options for everything and keep spamming 'enter'

until you're done. Finally press 'w' exit and to write the changes to the

image.

```bash

$ fdisk image

Welcome to fdisk (util-linux 2.29.2).

Changes will remain in memory only, until you decide to write them.

Be careful before using the write command.

Device does not contain a recognized partition table.

Created a new DOS disklabel with disk identifier 0x319d111f.

Command (m for help): n

Partition type

   p   primary (0 primary, 0 extended, 4 free)

   e   extended (container for logical partitions)

Select (default p):

Using default response p.

Partition number (1-4, default 1):

First sector (2048-204799, default 2048):

Last sector, +sectors or +size{K,M,G,T,P} (2048-204799, default 204799):

Created a new partition 1 of type 'Linux' and of size 99 MiB.

Command (m for help): w

The partition table has been altered.

Syncing disks.

```

In order to interact with our new partition we'll create a loop device for our

image. Loop devices are block devices (like actual disks) that in our case

point to a file instead of real hardware. For this we need root so sudo up

with ``sudo su`` or however you prefer to gain root privileges and afterwards

run:

```bash

$ losetup -P -f --show image

/dev/loop0

```

The loop device probably ends with a 0 but it could be different in your case.

The ``-P`` makes sure the partition also gets a loop device, ``/dev/loop0p1`` in

my case. Let's make a filesystem on it.

```bash

$ mkfs.ext4 /dev/loop0p1

```

If you want to use something other than ext4, be sure to enable it when

configuring your kernel. Now that we have done that, we can mount it and start

putting everything in place.

```bash

$ mkdir image_root

$ mount /dev/loop0p1 image_root

$ cd image_root # it's assumed you do the following commands from this location

$ mkdir -p usr/{sbin,bin} bin sbin boot

```

And while we're at it, we can create the rest of the file system hierarchy. This

is actually standardized and applications often assume this is the way you're

doing it, but you can often do what you want. You can find more info

[here](http://www.pathname.com/fhs/).

```bash

$ mkdir -p {dev,etc,home,lib}

$ mkdir -p {mnt,opt,proc,srv,sys}

$ mkdir -p var/{lib,lock,log,run,spool}

$ install -d -m 0750 root

$ install -d -m 1777 tmp

$ mkdir -p usr/{include,lib,share,src}

```

We'll copy our binaries over.

```bash

$ cp /path/to/busybox usr/bin/busybox

$ cp /path/to/bzImage boot/bzImage

```

You can call every busybox utility by supplying the utility as an argument, like

so: ``busybox ls --help``. But busybox also detects by what name it is called

and then executes that utility. So you can put symlinks for each utility and

busybox can figure out which utility you want by the symlink's name.

```bash

for util in $(./usr/bin/busybox --list-full); do

  ln -s /usr/bin/busybox $util

done

```

These symlinks might be incorrect from outside the system because of the

absolute path, but they work just fine from within the booted system.

Lastly, we'll copy some files from ``../filesystem`` to the image that will be

of some use to us later.

```bash

$ cp ../filesystem/{passwd,shadow,group,issue,profile,locale.sh,hosts,fstab} etc

$ install -Dm755 ../filesystem/simple.script usr/share/udhcpc/default.script

# optional

$ install -Dm644 ../filesystem/be-latin1.bmap usr/share/keymaps/be-latin1.bmap

```

These are the basic configuration files for a UNIX system. The .script file is

required for running a dhcp client, which we'll get to later. The keymap file is

a binary keymap file I use for belgian azerty.

The Boot Loader

---------------

The next step is to install the bootloader - the program that loads our kernel in

memory and starts it. For this we use GRUB, one of the most widely used

bootloaders. It has a ton of features but we are going to keep it very simple.

Installing it is very simple, we just do this:

```bash

grub-install --modules=part_msdos \ 

             --target=i386-pc \

             --boot-directory="$PWD/boot" \

             /dev/loop0

```

Ubuntu users might need to install ``grub-pc-bin`` first if they are on an EFI

system.

The ``--target=i386-pc`` tells grub to use the simple msdos MBR bootloader. This

is often the default, but this can vary from machine to machine so you better

specify it here. The ``--boot-directory`` options tells grub to install the grub

files in /boot inside the image instead of the /boot of your current system.

``--modules=part_msdos`` is a workaround for a bug in Ubuntu's grub. When you

use ``losetup -P``, grub doesn't detect the root device correctly and doesn't

think it needs to support msdos partition tables and won't be able to find the

root partition.

Now we just have to configure grub and then our system should be able to boot.

This basically means telling grub how to load the kernel. This config is located

at ``boot/grub/grub.cfg`` (some distro's use ``/boot/grub2``). This file needs

to be created first, but before we do that, we need to figure something out

first. If you look at ``/proc/cmdline`` on your own machine you might see

something like this:

```bash

$ cat /proc/cmdline

BOOT_IMAGE=/boot/vmlinuz-4.4.0-71-generic root=UUID=83066fa6-cf94-4de3-9803-ace841e5066c ro

```

These are the arguments passed to your kernel when it's booted. The 'root'

option tells our kernel which device holds the root filesystem that needs to be

mounted at '/'. The kernel needs to know this or it won't be able to boot. There

are different ways of identifying your root filesystem. Using a UUID is a

good way because it is a unique identifier for the filesystem generated when you

do ``mkfs``. The issue with using this is that the kernel doesn't really

support it because it depends on the implementation of the filesystem. This

works on your system because it uses an initramfs, but we can't use it now. We

could do ``root=/dev/sda1``, this will probably work but it has some other problems.

The 'a' in 'sda' depends on the order the bios will load the disk and this

can change when you add a new disk, or for a variety of other reasons.

Or when you use a different type of interface/disk it can be something entirely

different. So we need something more robust. I suggest we use the PARTUUID. It's

a unique id for the partition (and not the filesystem like UUID) and this is a

somewhat recent addition to the kernel for msdos partition tables (it's actually

a GPT thing). We'll find the id like this:

```bash

$ fdisk -l ../image | grep "Disk identifier"

Disk identifier: 0x4f4abda5

```

Then we drop the 0x and append the partition number as two digit hexidecimal. An

MBR only has 4 partitions max so that it's hexidecimal or decimal doesn't really

matter, but that's what the standard says. So the grub.cfg should look like this:

```

linux /boot/bzImage quiet init=/bin/sh root=PARTUUID=4f4abda5-01

boot

```

The ``defconfig`` kernel is actually a debug build so it's very verbose, so to

make it shut up you can add the ``quiet`` option. This stops it from being

printed to the console. You can still read it with the ``dmesg`` utility.

``init`` specifies the first process that will be started when the kernel is

booted. For now we just start a shell, we'll configure a real init while it's

running.

So now we should be able to boot the system. You can umount the image, exit root

and start a VM to test it out. The simplest way of doing this is using QEMU.

The Ubuntu package is ``qemu-kvm``, and just ``qemu`` on Arch Linux.

```bash

$ cd ../

$ umount image_root

$ exit # we shouldn't need root anymore

$ qemu-system-x86_64 -enable-kvm image

```

And if everything went right you should now be dropped in a shell in our

homemade operating system.

**Side note:** When using QEMU, you don't actually need a bootloader. You can

tell QEMU to load the kernel for you.

```bash

$ qemu-system-x86_64 -enable-kvm \

                     -kernel bzImage \

                     -append "quiet init=/bin/sh root=/dev/sda1" \

                     image

```

Where ``bzImage`` points to the kernel you built on your system, not the image.

and ``-append`` specifies the kernel arguments (don't forget the quotes). This

could be useful when you would like to try different kernel parameters without

changing ``grub.cfg`` every time.

PID 1: /sbin/init

---------------

The first process started by the kernel (now ``/bin/sh``) has process id 1. This

is not just a number and has some special implications for this process. The

most important thing to note is that when this process ends, you'll end up with

a kernel panic. PID 1 can never ever die or exit during the entire runtime of

your system. A second and less important consequence of being PID 1 is when

another process 'reparents', e.g. when a process forks to the background, PID 1

will become the parent process.

This implies that PID 1 has a special role to fill in our operating system.

Namely that of starting everything, keeping everything running, and shutting

everything down because it's the first and last process to live.

This also makes this ``init`` process very suitable to start and manage services

as is the case with the very common ``sysvinit`` and the more modern

``systemd``. But this isn't strictly necessary and some other process can carry

the burden of service supervision, which is the case with the

[runit](http://smarden.org/runit/)-like ``init`` that is included with

``busybox``.

Unless you passed the ``rw`` kernel parameter the root filesystem is mounted as

read-only. So before we can make changes to our running system we have to

remount it as read-write first. Before we can do any mounting at all we have

to mount the ``proc`` pseudo filesystem that serves as an interface to kernel.

```bash

$ mount -t proc proc /proc

$ mount / -o remount,rw

```

``busybox`` provides only two ways of editing files: ``vi`` and ``ed``. If you

are not confortable using either of those you could always shutdown the VM,

mount the image again, and use your favorite text editor on your host machine.

If you don't use a qwerty keyboard, you might have noticed that the VM uses a

qwerty layout as this is the default. You might want to change it to azerty with

``loadkmap < /usr/share/keymaps/be-latin1.bmap``. You can dump the layout you

are using on your host machine with ``busybox dumpkmap > keymap.bmap`` in a

virtual console (not in X) and put this on your image instead.

First, we'll create a script that handles the initialisation of the system

itself (like mounting filesystems and configuring devices, etc). You could call it

``startup`` and put it in the ``/etc/init.d`` directory (create this first).

Don't forget to ``chmod +x`` this file when you're done.

```bash

#!/bin/sh

# /etc/init.d/startup

# mount the special pseudo filesytems /proc and /sys

mount -t proc proc /proc -o nosuid,noexec,nodev

mount -t sysfs sys /sys -o nosuid,noexec,nodev

# /dev isn't required if we boot without initramfs because the kernel

# will have done this for us but it doesn't hurt

mount -t devtmpfs dev /dev -o mode=0755,nosuid

mkdir -p /dev/pts /dev/shm

# /dev/pts contains pseudo-terminals, gid 5 should be the

# tty user group

mount -t devpts devpts /dev/pts -o mode=0620,gid=5,nosuid,noexec

# /run contains runtime files like pid files and domain sockets

# they don't need to be stored on the disk, we'll store them in RAM

mount -t tmpfs run /run -o mode=0755,nosuid,nodev

mount -t tmpfs shm /dev/shm -o mode=1777,nosuid,nodev

# the nosuid,noexec,nodev options are for security reasons and are not

# strictly necessary, you can read about them in the 'mount'

# man page

# the kernel does not read /etc/hostname on it's own

# you need to write it in /proc/sys/kernel/hostname to set it

# don't forget to create this file if you want to give your system a name

if [[ -f /etc/hostname ]]; then

  cat /etc/hostname > /proc/sys/kernel/hostname

fi

# mdev is a mini-udev implementation that

# populates /dev with devices by scanning /sys

# see the util-linux/mdev.c file in the busybox source

# for more information

mdev -s

echo /sbin/mdev > /proc/sys/kernel/hotplug

# the "localhost" loopback network interface is

# down at boot, we have to set it 'up' or we won't be able to

# make local network connections

ip link set up dev lo

# you could add the following to change the keyboard layout at boot

loadkmap < /usr/share/keymaps/be-latin1.bmap

# mounts all filesystems in /etc/fstab

mount -a

# make the root writable if this hasn't been done already

mount -o remount,rw /

# end of /etc/init.d/startup

```

The next file is the init configuration ``/etc/inittab``. The syntax of this

file is very similar to that of ``sysvinit``'s ``inittab`` but has several

differences. For more information you can look at the ``examples/inittab`` file

in the busybox source.

```inittab

# /etc/inittab

::sysinit:/bin/echo STARTING SYSTEM

::sysinit:/etc/init.d/startup

tty1::respawn:/sbin/getty 38400 tty1

tty2::respawn:/sbin/getty 38400 tty2

tty3::respawn:/sbin/getty 38400 tty3

::ctrlaltdel:/bin/umount -a -r

::shutdown:/bin/echo SHUTTING DOWN

::shutdown:/bin/umount -a -r

# end of /etc/inittab

```

The ``sysinit`` entry is the first command ``init`` will execute. We'll put our

``startup`` script here. You can specify multiple entries of this kind and they

will be executed sequentially. The same goes for the ``shutdown`` entry, which

will obviously be executed at shutdown. The ``respawn`` entries will be executed

after ``sysinit`` and will be restarted when they exit. We'll put some

``getty``'s on the specified tty's. These will ask for your username and execute

``/bin/login`` which will ask for your password and starts a shell for you when

it's correct. If you don't care for user login and passwords, you could instead

of the ``getty``'s do ``::askfirst:-/bin/sh``. ``askfirst`` does the same as

``respawn`` but asks you to press enter first. If no tty is specified it will

figure out what the console is. The ``-`` infront of ``-/bin/sh`` means that

the shell is started as a login shell. ``/bin/login`` usually does this for us

but we have to specify it here. Starting the shell as a login shell means that

it configures certain things it otherwise assumes already to be configured. E.g.

it sources ``/etc/profile``.

We can now start our system with ``init``. You can remove the ``init=/bin/sh``

entry in ``/boot/grub/grub.cfg`` because it defaults to ``/sbin/init``. If

you reboot the system you should see a login screen. But if you run ``reboot``,

you'll notice it won't do anything. This happens because normally ``reboot``

tells the running ``init`` to reboot. You know - the ``init`` that isn't running

right now. So we have two options, we could run ``reboot -f`` which skips the

``init``, or we could do this:

```bash

$ exec init

```

Because the shell we are currently using is PID 1 and you could just replace the

shell process with ``init`` and our system should be properly booted now

presenting you a login prompt.

The root password should be empty so it should only ask for a username.

Service Supervision

-------------------

In the last part of our OS building adventure we'll look into setting up some

services. An important thing to note is that we are using

[runit](http://smarden.org/runit/) for service supervision, which is quite different

from how the more common ``sysvinit`` does things but it'll give you a

feel for which problems it's supposed to solve and how.

A basic service consists of a directory containing a ``run`` executable, usually

a script. This ``run`` script usually starts the daemon and doesn't exit until

the daemon does. If ``run`` exits ``runit`` will think the service itself has

stopped and if it wasn't supposed to stop, ``runit`` will try to restart it. So

be careful with forking daemons. Starting the service is done with ``runsv``.

This is the process that actually monitors the service and restarts it if

necessary. Usually you won't run it manually but doing so is useful for testing

services.

The first service we are going to create is a logging service that collects

messages from other processes and stores them in files.

```bash

$ mkdir -p /etc/init.d/syslogd

$ vi /etc/init.d/syslogd/run

$ cat /etc/init.d/syslog/run

#!/bin/sh

exec syslogd -n

$ chmod +x /etc/init.d/syslog/run

$ runsv /etc/init.d/syslogd & # asynchronous

$ sv status /etc/init.d/syslogd

run: /etc/init.d/syslogd: (pid 991) 1170s

```

It's that simple, but we have to make sure ``syslogd`` doesn't fork or else

``runsv`` will keep trying to start it even though it is already running. That's

what the ``-n`` option is for. The ``sv`` command can be used to control the

service.

To make sure that our new service is started at boot we could create a new

``inittab`` entry for it but this isn't very flexible. A better solution is to

use ``runsvdir``. This runs ``runsv`` for every service in a directory. So

running ``runsvdir /etc/init.d`` would do the trick but this way we can't

disable services at boot. To solve this issue we'll create a separate directory

and symlink the enabled services in there.

```bash

$ mkdir -p /etc/rc.d

$ ln -s /etc/init.d/syslogd /etc/rc.d

$ runsvdir /etc/rc.d & # asynchronous

```

If we add ``::respawn:/usr/bin/runsvdir /etc/rc.d`` to ``/etc/inittab`` all the

services symlinked in ``/etc/rc.d`` will be started at boot. Enabling and

disabling a service now consists of creating and removing a symlink in ``/etc/rc.d``.

Note that ``runsvdir`` monitors this directory and starts the service when the

symlink appears and not just at boot.

### Syslog

``syslogd`` implements the well known ``syslog`` protocol for logging. This

means that it creates a UNIX domain socket at ``/dev/log`` for daemons to

connect and send their logs to. Usually it puts all of the collected logs in

``/var/log/messages`` unless told otherwise. You can specify filters in

``/etc/syslog.conf`` to put certain logs in different files.

```bash

$ vi /etc/syslog.conf

$ cat /etc/syslog.conf

kern.* /var/log/kernel.log

$ sv down /etc/init.d/syslogd # restart

$ sv up /etc/init.d/syslogd

```

This will put everything the kernel has to say in a separate log file

``/var/log/kernel.log``. But ``syslogd`` doesn't read the kernel logs like

``rsyslog`` does. We need a different service for that.

```bash

$ mkdir -p /etc/init.d/klogd

$ vi /etc/init.d/klogd/run

$ cat /etc/init.d/klogd/run

#!/bin/sh

sv up /etc/init.d/syslogd || exit 1

exec klogd -n

$ chmod +x /etc/init.d/klogd/run

$ ln -s /etc/init.d/klogd /etc/rc.d

```

Now we should see kernel logs appearing in ``/var/log/kernel.log``.

The ``sv up /etc/init.d/syslogd || exit 1`` line makes sure ``syslogd`` is

started before ``klogd``. This is how we add dependencies in ``runit``. If

``syslogd`` hasn't been started yet ``sv`` will fail and ``run`` will exit.

``runsv`` will attempt to restart ``klogd`` after a while and will only

succeed when ``syslogd`` has been started. Believe it or not, this is what the

runit documentation says about making dependencies.

### DHCP

The very last thing we will do is provide our system with a network connection.

```bash

$ mkdir -p /etc/init.d/udhcpc

$ vi /etc/init.d/udhcpc/run

$ cat /etc/init.d/udhcpc/run

#!/bin/sh

exec udhcpc -f -S

$ chmod +x /etc/init.d/udhcpc/run

$ ln -s /etc/init.d/udhcpc /etc/rc.d

```

Now we're done. Yes - it's that simple. Note that udhcpc just asks for a lease

from the DHCP server and that's it. When it has a lease it executes

``/usr/share/udhcpc/default.script`` to configure the system. We already copied

this script to this location. This script is included with the busybox source.

These scripts usually use ``ip``, ``route``, and write to ``/etc/resolv.conf``.

If you would like a static ip, you'll have to write a script that does these

things.

Epilogue

--------

That's it! We're done for now. Thanks for reading. I hope you learned something

useful. I certainly did while making this.