Alpine Linux in a chroot

Inside the chroot environment, you can build, debug, and run alpine packages or develop things. It's the most known way to do so if one wants not to trash their main Alpine system.

This document explains how to set up an Alpine build environment in a chroot under a host Linux distro, can also be used to install Alpine Linux from a non-Alpine Linux livecd.

Requirements

• Working Linux instalation where to perform all the process
• Linux kernel 2.6.22, with wget and chroot 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 chroot directory where you
• ${mirror} = Should be replaced with 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 ${version} with actual version):

wget ${mirror}/latest-stable/main/${arch}/apk-tools-static-${version}.apk

.apk packages are just gzipped tarballs, unpack using:

tar -xzf apk-tools-static-*.apk

Install the alpine base installation onto the chroot

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

Set up the chroot

Before made and enter into the chrooted system must be prepared with device nodes and tempfs :

Method 1.A fast way: using bind mount

Note: Mounts with bind, can mount in read-only the /dev at the alpine chroot so due limited will not touch the access time of the host system

mount /dev/ ${chroot_dir}/dev/ --bind mount -o remount,ro,bind ${chroot_dir}/dev

If you need SCSI or R/W access only do the first command, mounting with "ro" makes more secure your chroot.

Method 1.B manual way: creating need nodes

Warning: Manually creating devices will only provide those representation that you have created.. for auto availability use bind mounts

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:

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/sda4 b 8 4 mknod -m 666 ${chroot_dir}/dev/sda5 b 8 5 mknod -m 666 ${chroot_dir}/dev/sda6 b 8 6 mknod -m 666 ${chroot_dir}/dev/sdb b 8 16 mknod -m 666 ${chroot_dir}/dev/sdb1 b 8 17 mknod -m 666 ${chroot_dir}/dev/sdb2 b 8 18 mknod -m 666 ${chroot_dir}/dev/sdb3 b 8 19 mknod -m 666 ${chroot_dir}/dev/sdb4 b 8 20 mknod -m 666 ${chroot_dir}/dev/sdb5 b 8 21 mknod -m 666 ${chroot_dir}/dev/sdb6 b 8 22

Made available proc and sys fs

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

Make networking resolution access

A resolv.conf is needed for name resolution:

cp -L /etc/resolv.conf ${chroot_dir}/etc/ mkdir -p ${chroot_dir}/root

If you don't want to copy the resolv.conf from the local machine, you can create a new one using OpenDNS servers (or any other):

echo -e 'nameserver 8.8.8.8\nnameserver 2620:0:ccc::2' > ${chroot_dir}/etc/resolv.conf

prepare the apk sources software

Set up APK mirror (replace ${branch} with the latest stable branch name, e.g. v3.3):

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

Mastering your chroot

The chroot methods are commonly used to have alpine in a portion of a already made directory, not forced to be a entire partion, that means Alpine can be in a very minimal directory indise the same partition of a Debian installed linux inclusively, so that why the chroot process does not included the boot method.

So then the following commands will described the need procedures only if the chroot instalation was made to a dedicated partition for and not to a directory inside another linux installed.

Warning: so then by the explained reasons, at this point, Alpine has been succesfully installed onto the chroot directory but still not able to boot it.

Entering your chroot

Take in consideration that the chroot command are only running as root, no stupid sudo tools are recommended for that.

chroot ${chroot_dir} /bin/bash -l

Perform init process

Need to add some minimal initscripts to appropriate runlevels:

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

Make bootable the install

WIP:

Warning: Run only this if Alpine was installed to a dedicated partiton mounted at the ${chroot_dir} directory, becose at this point, Alpine has been succesfully installed onto the chroot directory but still not able to boot it.

dd if=/usr/share/syslinux/mbr.bin of=/dev/sda

Be care of that /dev/sda are the same disk where destination partition was mounted to ${chroot_dir}.

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 chroot can run chmod. Add following to /etc/sysctl.conf

kernel.grsecurity.chroot_deny_chmod = 0

Then run the following command

sysctl -p

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). If you plans to make chroot into another installation must use same arch for both host and hosted chrooted!

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 ${chroot_dir}/etc/apk/repositories is valid and inside the chroot run:

apk update

External links

• You can also use script 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