Alpine Linux in a chroot

2021-09-17T19:17:45Z

Samekh: use '/bin/ash' instead of 'ash' as chroot command - absolute path is required

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Inside the chroot environment, you can build, debug, and run Alpine packages or develop things. It's the most known way to do so without replacing your system or using a Virtual Machine.
This document explains how to set up an [[Alpine_newbie#Developer|Alpine build environment]] in a chroot under a host Linux distro, that can also be used to install Alpine Linux from a non-Alpine Linux system or live environment.

== Requirements ==

* Working Linux instalation where to perform all the process
* Linux kernel 2.6.22, with <code>curl</code> and <code>chroot</code> binary installed
* target media with at least 100M, 900MB for more complete solution as minimum
* internet connection

== Prerequisites ==

The variables below:

*'''${chroot_dir}''' = Should point to the new root directory
*'''${mirror}''' = Should be replaced with [http://nl.alpinelinux.org/alpine/MIRRORS.txt one of the available Alpine Linux mirrors].
*'''${arch}''' = Should be the cpu architecture like x86 (i386) or amd64(x86_64)..

== Set up APK ==

Download the latest apk static package (replace <tt>${version}</tt> with actual version):

{{Cmd|curl -LO ${mirror}/latest-stable/main/${arch}/apk-tools-static-${version}.apk}}

.apk packages are just gzipped tarballs, you can unpack them using:
{{Cmd|tar -xzf apk-tools-static-*.apk}}

== Install the alpine base installation onto the chroot ==

{{Cmd|./sbin/apk.static -X ${mirror}/latest-stable/main -U --allow-untrusted -p ${chroot_dir} --initdb add alpine-base}}

== Seting up the chroot ==

Before you change root to the new directory, you need to create the required devices

===== Method 1.a Simple way: Using the host's /dev =====

{{Cmd|mount -o bind /dev ${chroot_dir}/dev}}

{{Note|Bind mounts can be made read-only which would limit the chroot from writing to the devices}}

===== Method 1.b Manual way: Creating needed nodes =====

{{Warning|Manually creating devices will only provide the ones that have been created}}

{{Cmd|mknod -m 666 ${chroot_dir}/dev/full c 1 7
mknod -m 666 ${chroot_dir}/dev/ptmx c 5 2
mknod -m 644 ${chroot_dir}/dev/random c 1 8
mknod -m 644 ${chroot_dir}/dev/urandom c 1 9
mknod -m 666 ${chroot_dir}/dev/zero c 1 5
mknod -m 666 ${chroot_dir}/dev/tty c 5 0}}

If you need SCSI disc access you can create the device nodes like this:

{{Note|Every device can have 15 sub-nodes, you should always increment by 16 for every new device}}

{{Cmd|mknod -m 666 ${chroot_dir}/dev/sda b 8 0
mknod -m 666 ${chroot_dir}/dev/sda1 b 8 1
mknod -m 666 ${chroot_dir}/dev/sda2 b 8 2
mknod -m 666 ${chroot_dir}/dev/sda3 b 8 3
mknod -m 666 ${chroot_dir}/dev/sdb b 8 16
mknod -m 666 ${chroot_dir}/dev/sdb1 b 8 17}}

==== Make the process fs and /sys fs available ====

{{Cmd|mount -t proc none ${chroot_dir}/proc
mount -o bind /sys ${chroot_dir}/sys}}

==== Set up name resolution ====

A resolv.conf is needed for name resolution:

You can either copy your host's resolv.conf:
{{Cmd|cp -L /etc/resolv.conf ${chroot_dir}/etc/}}

or instead you can create a new one (this example uses OpenDNS):
{{Cmd|echo -e 'nameserver 8.8.8.8\nnameserver 2620:0:ccc::2' > ${chroot_dir}/etc/resolv.conf}}

==== Prepare the APK repositories ====

Set up APK main repository (replace <tt>${branch}</tt> with the latest stable branch name, e.g. v3.3):

{{Cmd|mkdir -p ${chroot_dir}/etc/apk
echo "${mirror}/${branch}/main" > ${chroot_dir}/etc/apk/repositories}}

{{Warning|The chroot methods are commonly used to have Alpine installed in a existing system. Because of this, the steps for preparing a bootloader are not included.}}

==== Entering your chroot ====

You then can enter your chroot by running this command as the root user (UID 0).

{{Cmd|chroot ${chroot_dir} /bin/ash -l}}

==== Preparing init services ====

If you plan to use your chroot with a init system or setup a new system on another device you should add these services:

{{Cmd|rc-update add devfs sysinit
rc-update add dmesg sysinit
rc-update add mdev sysinit

rc-update add hwclock boot
rc-update add modules boot
rc-update add sysctl boot
rc-update add hostname boot
rc-update add bootmisc boot
rc-update add syslog boot

rc-update add mount-ro shutdown
rc-update add killprocs shutdown
rc-update add savecache shutdown}}


= Troubleshooting =

== Hardened kernels or alpine as chroot host ==

If you are using Alpine as a native build system you will have to make sure that you can run <code>chmod</code> from a chroot. Add the following to <code>/etc/sysctl.conf</code>

<code>kernel.grsecurity.chroot_deny_chmod = 0</code>

Then reload the sysctl configuration

<code>sysctl -p</code>

== chroot: cannot run command ' ... Exec format error ==

This usually indicates that you booted with one architecture (e.g. armf) and are trying to chroot into another (e.g. x86_64). The binaries must be built for the architecture that the host runs!

Note that with '''one exception you can run 32 bit x86 chroot in x86_64, but not viceversa'''!

== WARNING: Ignoring APKINDEX.xxxx.tar.gz ==

Make sure <code>${chroot_dir}/etc/apk/repositories</code> is valid and run:

<code>apk update</code>

= External links =

* You can also use script [https://github.com/alpinelinux/alpine-chroot-install/ alpine-chroot-install]
* https://web.archive.org/web/20190808203313/https://isc.sans.edu/forums/diary/Forensic+use+of+mount+bind/22854/
* Alpine Linux in a chroot on Fedora : http://git.alpinelinux.org/cgit/user/fab/scripts/tree/alpine-chroot.sh script
* Alpine Linux aarch64 in a chroot on AWS Linux : https://gist.github.com/emolitor/0567e51c0ce04f4b025fc78d2cf0b4f1 script

[[Category: Installation]]
[[category: System Administration]]

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Alpine Linux in a chroot