Quick Docker

(Up-to-date source of this post.)

Docker is a container technology. It's a well timed fusion of

• kernel features
• filesystem tricks
• networking hacks

Think of a container not as a virtual machine but a very lighweight wrapper around a single Unix process.

Docker revision controls

1. filesystem layers
2. image tags

Terminology

Docker server - the docker command run in daemon mode on a Linux host:

$ sudo docker -d -H unix:///var/run/docker.sock -H tcp://0.0.0.0:2375

Docker image - one or more filesystem layers and metadata that represent all the files required to run a Dockerized application

Docker container - a Linux container that has been instantiated from a Docker image

Working with Docker images

To launch a container

• download a public image
• create your own

To create a custom image you need a Dockerfile - each line in a Dockerfile creates a new image layer that is stored by Docker

Build an image:

git clone https://github.com/spkane/docker-node-hello.git
cd docker-node-hello
docker build -t example/docker-node-hello:latest .

Run an image (or a container?):

docker run -d -p 80:8080 example/docker-node-hello:latest

• -p 80:8080 tells Docker to proxy the container's port 80 on the host's port 8080 (port binding)
• example/docker-node-hello:latest is a tag

Remove an image:

docker images
docker rmi <image_id>

Remove all images on your Docker host:

docker rmi $(docker images -q -)

Working with Docker containers

A container is a self-contained execution environment that shares the kernel of the host system and which is (optionally) isolated from other containers in the system.

Containers are a Linux only technology.

Create a container (see also "Run an image" above):

docker run --rm -ti ubuntu /bin/bash 

• run - create + start
• --rm - delete the container when it exits
• -t - allocate a pseudo-TTY
• -i - interactive session, e.i. keep STDIN open
• /bin/bash - executable to run within the container

Get into a running container:

docker ps
docker exec -it <container_id> /bin/bash

Stop a container:

docker stop <container_id>

Remove a container:

docker ps -a
docker rm <container_id>

Remove all containers on your Docker host:

docker rm  $(docker ps -a -q)

Sources

• Docker: Up & Running (2015)
• Unix and Linux System Administration Handbook, 5th ed. (2017)

Common Vagrant Tasks

(Up-to-date source of this post.)

Search and add a box (virtual machine image):

vagrant box add https://atlas.hashicorp.com/ubuntu/boxes/trusty64

• added box is global to the vagrant install
• this is the base box (used to start the VM from the clean state)
• base boxes are stored in ~/.vagrant.d/boxes

Initialize vagrant environment:

mkdir ubuntu-trusty64
cd ubuntu-trusty64
vagrant init ubuntu/trusty64

• Vagrantfile is created

Start vagrant environment:

vagrant up

• vagrant "imports" (copies) the base box to provider specific location (ex. ~/.VirtualBox)

Check box(es) status:

vagrant status

Check box(es) SSH configuration:

vagrant ssh-config

Ssh to a box:

vagrant ssh

Clean up:

# save VM's state; fastest to start again; eats most diskspace (hard disk + saved state of RAM)
vagrant suspend

# graceful shutdown; slower to start again, still eats disk space (hard disk)
vagrant halt

# power down and remove all of the guest hard disks; even slower to
# start again (reimport of the base box and reprovisioning)
vagrant destroy

Show status of all vagrant environments on the host (independent of the directory you're in):

vagrant global-status [--prune]

To share a folder from the host on the guest, add following to Vagrantfile:

config.vm.synced_folder "../../eset-repos", "/shared/eset-repos",
  owner: "jreisinger", group: "jreisinger"

Resources

• https://docs.vagrantup.com
• https://sysadmincasts.com/episodes/4-vagrant
• http://docs-v1.vagrantup.com/v1/docs/

Quick Vagrant Guide

See here.

Vagrant - Basic Commands

See here.

Clone and Resize KVM Virtual Machine

(Up-to-date source of this post.)

I needed to upgrade (from Squeeze to Wheezy) some important virtual servers. As I wanted a minimal impact of the upgrade, I chose this procedure:

1. Create identical copy of the server to upgrade
2. Upgrade the copy
3. Upgrade the server if everything worked ok with the copy

The servers to upgrade were virtual machines (VMs) running on KVM. I also discovered that some servers needed more space because their disks had filled up during upgrade. So disk resize was needed. The following steps did the task:

1) Copy the image (.qcow2) and the configuration (.xml) files to some other location. The image file should ideally be copied from a snapshot to avoid data inconsistencies a running machine could create.

2) Edit the following fields in the copied .xml file accordingly

name
uuid
source dev    # make sure you enter the copied image path!
mac address
source bridge # change the VLAN to avoid IP address conflicts

3) Boot the cloned VM and change the hostname and IP address by editing these files:

/etc/network/interfaces
/etc/hostname
/etc/hosts

4) Change back the VLAN and shutdown the cloned VM

5) Increase the disk size

# convert the qcow image to a plain raw file
qemu-img convert system.qcow -O raw system.raw
# create a dummy file (filled with zeros) of the size of extra space you want to add to your image (here 1GB)
dd if=/dev/zero of=zeros.raw bs=1024k count=1024
# add your extra space to your raw system image without fear
cat system.raw zeros.raw > big.raw
# finally convert back your raw image to a qcow file not to waste space
qemu-img convert big.raw -O qcow growed-system.qcow

6) Boot the cloned VM and using cfdisk delete the old small partition and create a new one with the free space

7) Increase the filesystem using:

e2fsck -f
resize2fs

Make sure the VM's image file (.qcow) has the correct access rights, otherwise your system might have disk related problems (I was bitten by this and got helped by my nice colleague).