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.