What is Docker?

  • Docker is programme written in Go
  • Docker is an open source community project started in 2013
  • Docker is a company that supports this project. 
  • Docker is a containerization technology/platform for Linux
  • Docker is command line tool
Under the hood

  • Docker is programme written in Go
  • Docker needs Linux kernel version 3.10 or more. 
  • Docker is 2 programme = client + server. The server can be on remote machine
  • Docker manage kernel features (1) cgroups (2) namespace (3) copy-on-write COW file system
  • Cgroups control what a process can use. Whereas namespaces control what a process can see

1. cgroups : limits the amount of resources 

1.1 blkio (Bulk IO device)
1.2 cpu
1.3 cpuacct (CPU accounting OR CPU usage report)
1.4 cpuset (Assigning CPU)
1.5 devices
1.6 freezer : Suspends and resumes tasks in cgroups
1.7 memory
1.8 net_clas : Linux tc (traffic controller) can identify packet from particular cgroups
1.9 net_prio : for bandwidth management
1.10 ns (namespace)
1.11 perf_event : To identify membership of task to specific cgroups for performance analysis. 

/etc/cgconfig.d is daemon process for cgroups

Check all cgroups under folder /sys/fs/cgroups

I found additional cgroups in my syste,


2. namespace = mount, IPC, network, PID, user (user and group) and UTS (=Unix Time Sharing. Host name and domain name). 

2.1 Network Namespace

2.1.1 Network name space includes : network devices, IPv4  and  IPv6  protocol stacks, IP routing tables, firewalls, the /proc/net directory, the /sys/class/net directory, port numbers (sockets), and  so  on

2.1.2 Namespace API has 3 system calls I. clone II. unshare III. setns 

Typically, "ip netns" can be used 

ip netns exec $namespace_id

to run some arbitrary command inside a given namespace.

However, ip netns only knows about namespaces listed in /var/run/netns/, and neither docker nor Kuberenetes have symlink its namespaces there. 

Docker and Kuberenetes can accessed network namespace using "nsenter" and specifying the process ID of the container, e.g.

nsenter -t ${PID} -n ip addr
nsenter -t ${PID} -n route -n
sudo nsenter -t ${PID} -n /bin/sh

nsenter discovers the network namespace at /proc/$PID/ns/net, where the namespace is accessible (by syscalls like setns(2)) via file descriptor.

Docker creates bridges. Docker sets routes. docker uses iptables

Docker Architecture

Docker CLI
dockerd : docker daemon
(docker-)containerd : daemon listening to on UDS, gRPC endpoint
(docker-)containerd-ctr : Light weight CLI to communicate with containerd, for debug purpose
(docker-)runc : lightwieght binary deals with namespace, cgroups etc. 
(docker-)containerd-shim : Between containerd and runc

Docker Dependencies

ca-certificates package contains certificates provided by the Certificate Authorities.

apt-transport-https allows the use of repositories accessed and downloaded via https protocol

software-properties-common provides some useful scripts for adding and removing PPAs + the DBUS backends. Without it, you would need to add and remove repositories (such as PPAs) manually by editing /etc/apt/sources.list and/or any subsidiary files in /etc/apt/sources.list.d

sudo apt install

To add non-root user

sudo groupadd docker

sudo gpasswd -a @USER docker
sudo usermod -aG docker $USER
sudo setfacl -m user::rw /var/run/docker.sock

Docker Flow

docker run puts image to container

1. docker commit creates new image from container
Here commit will also tag image. default tag value is 'latest"

2. docker tag "old name" "new name may contain registry url"
Here "tag" is optional. 

Instead of 1, and 2 use single command
"docker commit "

docker image and container both have IDs. They are different. 

Docker run command

docker run --memory --cpu-share --cpu-quota -p [outside port:]"inside port"[/tcp|udp] --link --name -ti --privileged=true --net="host | some name" --ip "ip address" -v [absolute local path:] --restart=always --pid=host --env KEY=VALUE --volumes-from --rm -d [:tag default is latest]

ti = interactive terminal
rm = remove container, once it is done.
d = detach
-p here outside port is optional
-p :/ default is tcp
-P = --publish-all For exposing all ports
--link = it will add "ip address another container name" at /etc/hosts. It will auto detect IP address of another container. Assume the IP address does not change. 
-v for volume
--privileged to get full access of host OS
--pid will give more privilege to control PID of host OS
--restart : To restart if it dies. 
--net : To specify network name space

docker run = docker create + docker start

^d to exit container. 

Attach and detach 

attach using

docker attach <container name>

to detach, when you are inside container, press ^p, ^q

Run process at container

docker -exec

Cannot add ports, volumes etc.
docker -exec -ti my_container sh -c "echo a && echo b"

Running more than one process or service in a container is discouraged for maximum efficiency & isolation. 


docker logs

Remove and Kill

docker kill

Container moves to stopped state. 

docker rm  

This will remove the container

docker ps -l

l = last running container. 


docker port

It will list out external v/s internal ports. Same like iptables

For dynamic link, first create network
docker network create  

Then use --net while creating both networks.

here a name server will be added to network and it will take care of new ip address. 

If we specify --net=host then we can see, all the bridges inside container also with "brctl show" command

Docker inspect

This command gives various information about docker like author, pid, IP address etc. 

docker inspect --format '{{ .State.Pid }}' "name"
docker inspect --format '{{ .NetworkSettings.IPAddress }}' "name"

Docker images

docker images

List downloaded image. 

Docker Registry 

name = registry FQDN : port /organization name/image name:version tag

short name = organization name/image name

docker rmi

docker login
docker pull
docker push "name as per used in docker tag"
docker search

Amazon's ECR,
Sonatype Nexus,
JFrog Artifactory

port 5000 for push, pull etc.

The docker images can be stored at local host, docker, AWS, Google, Microsoft

docker save -o "tar.gz file" "one or more than one docker image:tag"
docker load -i "tar.gz file"


Volumes are like virtual disc

Two types: (1) persistent (-v) v/s (2) ephemeral (--volumes-from)
Volumes are not part of image

mount -o bind "original folder" "new name of folder. May be existing folder"

Host file system over guest can be mounter. Not visa versa. 


No. Purpose docker file "docker run" command
1 Base image FROM
2 "Author" as output of "docker inspect" command MAINTAINER
3 Debug statement RUN
4 ADD "URL | local path" "container path"
5 During build time and run time ENV --env (or -e)KEY=VALUE
6 It is like CMD. It is first programe to run inside container ENTRYPOINT --entrypoint It will override from DockerFile
7 CMD Last Argument is combination of ENTRYPOINT + CMD
8 Multi project file  COPY
9 sets working dir for build and for container. it is like CD WORKDIR --workdir
10 binary argument Shell Form
11 ["bianry", "argument"] Exec Form
12 Opening outgoing port at iptables firewall EXPOSE portNumber -p [outside port:]"inside port" P=publish. Publish is for outside network. Expose is for inside network.
13 Avoid local path VOLUME ["optional local path" "container path] -v and --volume-from
14 the container will run as user USER sudo

* Dockerfile shall start with FROM
* Dockerfile name shall be Dockerfile. D capital. docker will search for this file, to build Docker image. Now we can have file with any name and specify that file to Docker with -f option. 
* Dockerfile can have multiple lines with "CMD ". The last "CMD " will override all previous lines with "CMD "
* One can pass arguments to Entrypoint
* COPY and ADD are similar. COPY allows to add file from local file system only. ADD allows to add file from URL also.

Sample Docker File



Explore further: 

1. How to run xserver in container ?

Here are the links 4.Lynx

2. How to connect using docker client to remote docker server?
3. how to expose my docker server on network? How to bind it to tcp socket instead of unix socket
4. Is LXD replacement of docker? 

Ansible modules



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