Understand how linux containers works with practical examples

Iván Moreno
8 min readApr 18, 2021

Nowadays a bast majority of server workloads run using linux containers because of his flexibility and lightweight but have you ever think how does linux containers works. In this tutorial we will demystify how does linux containers works with some practical examples. Linux containers works thanks two kernel features: namespaces and cgroups.

Linux Namespaces

A namespace wraps a global system resource in an abstraction that makes it appear to the processes within the namespace that they have their own isolated instance of the global resource. Changes to the global resource are visible to other processes that are members of the namespace, but are invisible to other processes. One use of namespaces is to implement containers. [1]

Currently the linux kernel have 8 types of namespaces:

Linux control groups (cgroups)

Cgroups allow you to allocate resources — such as CPU time, system memory, network bandwidth, or combinations of these resources — among user-defined groups of tasks (processes) running on a system. You can monitor the cgroups you configure, deny cgroups access to certain resources, and even reconfigure your cgroups dynamically on a running system. [2]

Container Fundamentals (key technologies)

In this section we gonna make some practices with the following key technologies that make possible the usage of containers in linux:

NOTE: This tutorial was made using a VM with 1GB of ram and 1vCPU using debian 10 buster with kernel 4.19.0-16-amd64

Process namespace fundamentals

A process namespace isolate a running command from the host. Let’s see how to implement a process namespace in linux.

List process namespaces

$ lsns -t pid

Get the PID of the current terminal

$ echo $$ # parent PID

Launch a new zsh terminal using namespaces

$ unshare --fork --pid --mount-proc zsh
$ sleep 300 &
$ sleep 300 &
$ sleep 300 &
$ sleep 300 &
$ sleep 300 &
$ top

See the process tree from the parent

$ ps f -g <PPID>

List namespaces

$ lsns -t pid

Filesystem — Overlay FS fundamentals

Containers need to have a filesystem, one of the most used filesystem for containers is overlay who can mount with layes and merge in a single directory, the lower layers are read only and all changes are made on the upper layer. Let's see how does overlay fs works.

Create directories

$ cd /tmp
$ mkdir {lower1,lower2,upper,work,merged}

Create some files in lower directories

$ echo "Lower 1 - original" > lower1/file1.txt
$ echo "Lower 2 - original" > lower2/file2.txt

Create overlay FS

$ mount -t overlay -o lowerdir=/tmp/lower1:/tmp/lower2,upperdir=/tmp/upper,workdir=/tmp/work none /tmp/merged

Create, modify files

$ cd /tmp/merged
$ echo "file created in merged directory" > file_created.txt
$ echo "file 1 modified" > file1.txt

Umount overlay fs

$ cd /tmp
$ umount /tmp/merged

Inspect lower and upper dirs

$ find -name '*.txt' -type f 2>/dev/null | while read fn; do echo ">> cat $fn"; cat $fn; done

Networking — Linux bridge fundamentals

Linux container uses network namespaces to isolate the network from the host, this is possible implementing a bridge interface that acts like network switch, and every container connect to that interface with his own ip address. Let’s see how does linux bridge and network namespaces works.

Create a Network Virtual bridge

$ ip link add br-net type bridge

List Network Interfaces

$ ip link

Assign an IP Address to bridge interface

$ ip addr add brd + dev br-net

Bring UP the bridge interface

$ ip link set br-net up

Create 2 Network Namespaces

$ ip netns add ns1
$ ip netns add ns2

Create a Virtual Ethernet cable pair

$ ip link add veth-ns1 type veth peer name br-ns1
$ ip link add veth-ns2 type veth peer name br-ns2

Assign veth to namespaces

$ ip link set veth-ns1 netns ns1
$ ip link set veth-ns2 netns ns2
$ ip link set br-ns1 master br-net
$ ip link set br-ns2 master br-net

Assign IP address to veth within namespaces

$ ip -n ns1 addr add dev veth-ns1
$ ip -n ns2 addr add dev veth-ns2

Bring UP veth interfaces within Namespaces

$ ip -n ns1 link set veth-ns1 up
$ ip -n ns2 link set veth-ns2 up

Bring UP bridge veth in the local host

$ ip link set dev br-ns1 up
$ ip link set dev br-ns2 up

Configure default route within namespaces

$ ip -n ns1 route add default via dev veth-ns1 
$ ip -n ns2 route add default via dev veth-ns2

Enable IP forward in the host

$ sysctl -w net.ipv4.ip_forward=1

Configure MASQUERADE in the host for subnet

$ iptables -t nat -A POSTROUTING -s ! -o br-net -j MASQUERADE

Control groups (cgroups) fundamentals

Control groups or cgroups are used by containers to limit the usage of resource in the host machine. Let’s see how does cgroups works.

Create cgroups directory

$ mkdir -p /mycg/{memory,cpusets,cpu}

Mount cgroups directory

$ mount -t cgroup -o memory none /mycg/memory
$ mount -t cgroup -o cpu,cpuacct none /mycg/cpu
$ mount -t cgroup -o cpuset none /mycg/cpusets

Create new directories under CPU controller

mkdir -p /mycg/cpu/user{1..3}

Assign CPU shares to every user (This example uses 1vCPU)

# 2048 / (2048 + 512 + 80) = 77%
$ echo 2048 > /mycg/cpu/user1/cpu.shares
# 512 / (2048 + 512 + 80) = 19%
$ echo 512 > /mycg/cpu/user2/cpu.shares
# 80 / (2048 + 512 + 80) = 3%
$ echo 80 > /mycg/cpu/user3/cpu.shares

Create artificial load

$ cat /dev/urandom &> /dev/null &
$ PID1=$!
$ cat /dev/urandom &> /dev/null &
$ PID2=$!
$ cat /dev/urandom &> /dev/null &
$ PID2=$!

Assign process to every user

$ echo $PID1 > /mycg/cpu/user1/tasks
$ echo $PID2 > /mycg/cpu/user2/tasks
$ echo $PID3 > /mycg/cpu/user3/tasks

Monitoring process

$ top

Create a container from scratch

So far we know how does linux namespaces works, now lets create a container using overlayfs, network namespaces, cgroups and process namespaces from scratch. Let’s see how a linux container is created.

Download and extract debian container fs from docker

$ docker pull debian
$ docker save debian -o debian.tar
$ mkdir debian_layer
$ mkdir -p fs/{lower,upper,work,merged}
$ tar xf debian.tar -C debian_layer
$ find debian_layer -name 'layer.tar' -exec tar xf {} -C fs/lower \;

Create bridge interface

$ ip netns add cnt
$ ip link add br-cnt type bridge
$ ip addr add brd + dev br-cnt
$ ip link set br-cnt up
$ sysctl -w net.ipv4.ip_forward=1
$ iptables -t nat -I POSTROUTING 1 -s ! -o br-cnt -j MASQUERADE

Create overlay Filesystem from debian container fs

$ mount -vt overlay -o lowerdir=./fs/lower,upperdir=./fs/upper,workdir=./fs/work none ./fs/merged

Mounting Virtual File Systems

$ mount -v --bind /dev ./fs/merged/dev

Launch process namespace within fs/merged fs

$ unshare --fork --pid --net=/var/run/netns/cnt chroot ./fs/merged \
/usr/bin/env -i PATH=/bin:/usr/bin:/sbin:/usr/sbin TERM="$TERM" \
/bin/bash --login +h
# Mount proc within container
$ mount -vt proc proc /proc

Connect the container with br-cnt

$ ip link add veth-cnt type veth peer name br-veth-cnt
$ ip link set veth-cnt netns cnt
$ ip link set br-veth-cnt master br-cnt
$ ip link set br-veth-cnt up
$ ip -n cnt addr add dev veth-cnt
$ ip -n cnt link set lo up
$ ip -n cnt link set veth-cnt up
$ ip -n cnt route add default via dev veth-cnt
$ ip netns exec cnt ping -c 3

Mount cgroup

$ mkdir /sys/fs/cgroup/memory/cnt
$ echo 10000000 > /sys/fs/cgroup/memory/cnt/memory.limit_in_bytes
$ echo 0 > /sys/fs/cgroup/memory/cnt/memory.swappiness
$ CHILD_PID=$(lsns -t pid | grep "[/]bin/bash --login +h" | awk '{print $4}')
$ echo $CHILD_PID > /sys/fs/cgroup/memory/cnt/tasks

Run commands within container

$ apt update
$ apt install nginx procps curl -y
$ nginx
$ curl
$ curl # from host
$ cat <( </dev/zero head -c 15m) <(sleep 15) | tail

Clean all

$ umount /proc # within container
$ exit # within container
$ umount -R ./fs/merged
$ ip link del br-veth-cnt
$ ip link del br-cnt
$ ip netns del cnt # grep cnt /proc/mounts

Inspect Namespaces within a docker container

Fortunately for us there is a program that simplifies the usage of containers, for us this program is docker who manage the life-cycle of running a container. Let's see how does docker implement the namespaces running a container.

Install docker CE

Install docker community edition from official script in get.docker.com

$ curl -fsSL https://get.docker.com -o install_docker.sh
$ less install_docker.sh # optional
$ sh install_docker.sh
$ usermod -aG docker $USER
$ newgrp docker # Or logout and login

Inspect Docker Network

Create a bridge network using docker

$ docker network create mynet

Inspect bridge network, see subnet using IP

$ BR_NAME=$(ip link | grep -v '@' | awk '/br-/{gsub(":",""); print $2}')
$ ip addr show ${BR_NAME}

Inspect Docker bridge network, see subnet using docker

$ docker network inspect mynet | grep Subnet

Run an nginx web server

$ docker run --name nginx --net mynet -d --rm -p 8080:80 nginx

Inspect network namespace from nginx container

Create symlink from /proc to /var/run/netns

$ CONTAINER_ID=$(docker container ps | awk '/nginx/{print $1}')
$ CONTAINER_PID=$(docker inspect -f '{{.State.Pid}}' ${CONTAINER_ID})
$ mkdir -p /var/run/netns/
$ ln -sfT /proc/${CONTAINER_PID}/ns/net /var/run/netns/${CONTAINER_ID}

Check network interface within namespace

$ ip netns list
$ ip -n ${CONTAINER_ID} link show eth0

Check IP address of nginx container

$ ip -n ${CONTAINER_ID} addr show eth0
$ docker container inspect nginx | grep IPAddress

Check port forwarding from 8080 to 80

$ iptables -t nat -nvL

Inspect cgroups in a docker container

Run a Ubuntu container with limited resources

$ docker run --name test_cg --memory=10m --cpus=.1 -it --rm ubuntu

See cgroup fs hierarchy

$ CONTAINER_ID=$(docker container ps --no-trunc | awk '/test_cg/{print $1}')
$ tree /sys/fs/cgroup/{memory,cpu}/docker/${CONTAINER_ID}

See attached task to container cgroup

$ docker container top test_cg | tail -n 1 | awk '{print $2}' # container parent PID
$ cat /sys/fs/cgroup/{memory,cpu}/docker/${CONTAINER_ID}/tasks # the same as container parent PID

Monitoring the container

$ docker container stats test_cg

Generate CPU load

$ cat /dev/urandom &> /dev/null

Generate Memory load

$ cat <( </dev/zero head -c 50m) <(sleep 30) | tail

Inspect overlay fs in a docker container

Run a ubuntu container with limited resources

$ docker run --name test_overlayfs -it --rm debian

NOTE: The merged layer is the actual container Filesystem

Inspect lower layers with tree and less

$ docker container inspect test_overlayfs -f '{{.GraphDriver.Data.LowerDir}}' | awk 'BEGIN{FS=":"}{for (i=1; i<= NF; i++) print $i}' | while read low; do tree -L 2 $low; done | less

Inspect upper layer (It’s empty)

$ docker container inspect test_overlayfs -f '{{.GraphDriver.Data.UpperDir}}' | while read upper; do tree $upper; done | less

Run command withing the container

$ apt update && apt install nmap -y

Inspect (again) upper layer (now it’s not empty)

$ docker container inspect test_overlayfs -f '{{.GraphDriver.Data.UpperDir}}' | while read upper; do tree $upper; done | less

Inspect docker process namespace

Run docker container

$ docker run --name test_ps -it --rm ubuntu

Launch process within container

$ sleep 600 &
$ sleep 600 &
$ sleep 600 &
$ sleep 600 &
$ sleep 600 &
$ top

See container tree process from container

$ CONTAINER_PID=$(docker container top test_ps | sed -n '2p' | awk '{print $2}')
$ ps f -g ${CONTAINER_PID}

List PID namespaces

$ lsns -t pid

See process using docker

$ docker container top test_ps


In this tutorial we create our first container from scratch understanding what happen behind the scenes when we run a container. I hope this tutorial helps you to understand the technologies behind the linux containers.

Source code