Alpine Linux in a chroot

This page explains how to set up an Alpine Linux chroot environment under a host Linux distro. 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 chroot can also be used to install Alpine Linux from a non-Alpine Linux system or live environment.

Tip: Use alpine-chroot-install script to setup Alpine Linux chroot build environment on any linux host in few seconds.

Prerequisites

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

alpine-chroot-install script

The official alpine-chroot-install script simplifies installation of Alpine Linux build environment on any x86_64 Linux system using chroot. With this script, it takes seconds to prepare a chroot, install fresh Alpine Linux including gcc build environment and execute first command!

The script provides commands to enter-chroot and destroy the chroot. Refer to the documentation page for more information.

Manual set up of chroot

The below variables are used in this page:

• ${chroot_dir} = Should point to the new root directory(eg./alpine or /var/chroots/alpine)
• ${mirror} = Should be replaced with one of the available Alpine Linux mirrors.
• ${arch} = Should be the cpu architecture like x86_64 for current amd64 or x86 for older i386.

Installation

Download the latest apk static package by replacing the variables like ${version} with actual version found on the chosen Download mirror:

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

.apk packages are just gzipped tarballs, you can unpack them using:

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

Install the Alpine Linux base installation onto the chroot.

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

Create the required devices

Before you can change root to the new directory i.e ${chroot_dir}, you need to create the required devices

Method 1: Using the host's /dev

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 2: Creating needed nodes in the manually

Warning: Manually creating devices will only provide the ones that have been created

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

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

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:

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

or instead you can create a new one (this example uses OpenDNS):

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 ${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

Entering your chroot

Enter your chroot by running this command as the root user (UID 0):

chroot ${chroot_dir} /bin/ash -l

Using chroot for other purposes

The above chroot method is commonly used to have Alpine Linux installed in a existing system. If you plan to use your chroot for other purposes, then the following steps are needed.

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:

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

Installing bootloader

If you plan to use your chroot to setup a new Alpine Linux system and boot from it, you need a bootloader.

Warning: Bootloaders can be installed only if Alpine Linux chroot was installed to a dedicated partiton mounted at the ${chroot_dir} directory.

If you have a dedicated partition in which the chroot is installed, enter your chroot and follow the steps given in the Syslinux page.

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 chmod from a chroot. Add the following to /etc/sysctl.conf

kernel.grsecurity.chroot_deny_chmod = 0

Then reload the sysctl configuration:

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). 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 ${chroot_dir}/etc/apk/repositories is valid and run:

# apk update

See also

• 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 : https://git.alpinelinux.org/cgit/user/fab/scripts/tree/alpine-chroot.sh ^{[Dead Link]} script
• Alpine Linux aarch64 in a chroot on AWS Linux : https://gist.github.com/emolitor/0567e51c0ce04f4b025fc78d2cf0b4f1 script