Please note that what I am about to do isn't actually containerization or isolation. It merely separates trusted code such that it would generally not pollute the host system. Chroot is generally not a security measure; for example, processes inside the chroot could easily escape with /proc/x/root for a PID x that runs outside the chroot.
Often times I want to install a program that is not available in my current distribution (e.g. the Soju IRC bouncer is not available on Debian Bookworm, but I want to use a distribution-packaged version instead of running go build myself because I want to receive updates), or perhaps I want to run something like rustup without the fear of polluting my system. But I don't want to use Docker or LXC because they feel too complicated.
I ended up creating Alpine Linux chroots. Specifically, Alpine Linux Edge, because the edge/testing repository is huge and frequently updated. There's an article called Alpine Linux in a chroot on the Alpine Linux wiki that provided the steps that I use in my scripts. (The official alpine-chroot-install scripts exist but haven't been updated for quite a while.)
The rxchroot scripts
Check the Git repository linked above for the complete set of scripts; here is a short explanation from excerpts of the script.
The first step, of course, is to create the chroot. Basically:
umask 022 # or something else that ensures unprivileged users could access the chroot
curl -LO "https://dl-cdn.alpinelinux.org/alpine/edge/main/${arch}/APKINDEX.tar.gz"
tar xvf APKINDEX.tar.gz
curl -LO "https://dl-cdn.alpinelinux.org/alpine/edge/main/${arch}/apk-tools-static-$(sed -n '/P:apk-tools-static/{n;p;}' APKINDEX | cut -d ':' -f 2).apk"
tar -xzf apk-tools-static-*.apk
./sbin/apk.static -X https://dl-cdn.alpinelinux.org/alpine/edge/main -U --allow-untrusted -p "${chroot_dir}" --initdb add alpine-base
cp -L /etc/resolv.conf ${chroot_dir}/etc/
mkdir -p "${chroot_dir}"/etc/apk
echo "https://dl-cdn.alpinelinux.org/alpine/edge/main" > "${chroot_dir}"/etc/apk/repositories
echo "https://dl-cdn.alpinelinux.org/alpine/edge/community" >> "${chroot_dir}"/etc/apk/repositories
echo "https://dl-cdn.alpinelinux.org/alpine/edge/testing" >> "${chroot_dir}"/etc/apk/repositories
Every time the chroot needs to be used, some special directories must be mounted. Don't double-mount though as that often causes issues.
mount -o bind /dev "${chroot_dir}/dev"
mount -t proc none "${chroot_dir}/proc"
mount -o bind /sys "${chroot_dir}/sys"
Then just enter the chroot:
chroot "${chroot_dir}" /bin/ash -l
And perhaps install stuff:
apk add build-base soju vim
I'm running on a Debian host, and perhaps I want to start services in the Alpine chroot with systemd:
# alpine.service ############################################### [Unit] Description=Alpine chroot mounts Wants=network-online.target After=network-online.target [Service] Type=oneshot RemainAfterExit=true User=root Group=root RuntimeDirectory=/alpine ExecStart=/root/rxchroot/alpine-chroot-activate /alpine ExecReload=/root/rxchroot/alpine-chroot-activate /alpine ExecStop=/root/rxchroot/alpine-chroot-deactivate /alpine [Install] WantedBy=multi-user.target # soju.service ################################################# [Unit] Description=soju in an Alpine chroot Requires=alpine.service After=alpine.service [Service] Type=simple User=root Group=root # Fix the userspec to match the Alpine UID/GUID ExecStart=/sbin/chroot --userspec=101:102 /alpine /usr/bin/soju ExecReload=/bin/kill -HUP $MAINPID Restart=on-failure [Install] WantedBy=multi-user.target
Since a chroot doesn't have its own kernel and all system calls pass through the host kernel, network services open ports as if they just run in the host system.