Working with Docker
I'm rather a keen advocate on Virtualisation technologies and anything that can be used to abstract resources into a pool to be used where needed.
As a result of this interest, I started looking into containerisation, specifically docker. Containerisation is an interesting concept for someone who has come from a VM background. The idea of sharing kernel space between instances of programs is an interesting concept but something quite alien to a VM oriented world.
Here are some arguments for and against the use of containers vs VMs.
- Less overhead required per application - no dedicated OS.
- Portability - You can package containers up very easily into tar archives and move them. While VMs are portable, the storage overhead reduces the portability somewhat.
- Separation of the 3 main data types: configuration, application binaries, application data - This is possible with VMs using different disks, NFS shares etc, but I've found that this is actually easier to accomplish using containers.
- Reduced deployment time - To deploy a new webserver in a container takes around 5 seconds! (Ok configuration takes some time, but configuration for a container vs VM is the same). Cloning an existing VM takes significantly longer, building one from scratch longer still.
- Higher level of separation - No dependency on guest OS at all
- Ability to run any OS, any application
- Deployable OVA templates are common for virtualised services (i.e. Cisco Virtual WLC)
- A more mature technology - HW optimisations have been made, more online resources available when troubleshooting
As a result of these factors I don't think containers replace VMs in all instances, in actual fact the Docker host I am using is itself a VM sitting on my ESXi server!
Docker will run on pretty much ANY OS. I like to use linux (specifically ubuntu) for my servers since it's something I've been using for a long time I am comfortable with it. So we'll look at how we can deploy docker on an ubuntu server 16.04 (xenial) machine to use as our docker host.
- Ubuntu 16.04 Installed and running (VM, or baremetal)
- An active internet connection - We'll be pulling the docker binaries and images from online
- Some knowledge of linux CLI
Always refer to the official documentation to install Docker which can be found here: https://docs.docker.com/engine/installation/linux/ubuntulinux/
However if you want a quick overview of commands to run (specifically for ubuntu 16.04), here's what you can do:
Update current repos:
apt-get update apt-get upgrade
Add repo keys & repository & then update:
sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D sudo apt-add-repository 'deb https://apt.dockerproject.org/repo ubuntu-xenial main' apt-get update
Confirm source of docker-engine is the new repo:
apt-cache policy docker-engine
Install the engine:
apt-get install docker-engine
Check the status of the service:
service docker status
You should now have access to the docker command:
[email protected]:/home/david# docker --help Usage: docker [OPTIONS] COMMAND [arg...] docker [ --help | -v | --version ] A self-sufficient runtime for containers. Options: --config=~/.docker Location of client config files -D, --debug Enable debug mode -H, --host= Daemon socket(s) to connect to -h, --help Print usage -l, --log-level=info Set the logging level --tls Use TLS; implied by --tlsverify --tlscacert=~/.docker/ca.pem Trust certs signed only by this CA --tlscert=~/.docker/cert.pem Path to TLS certificate file --tlskey=~/.docker/key.pem Path to TLS key file --tlsverify Use TLS and verify the remote -v, --version Print version information and quit Commands: attach Attach to a running container build Build an image from a Dockerfile commit Create a new image from a container's changes cp Copy files/folders between a container and the local filesystem create Create a new container diff Inspect changes on a container's filesystem events Get real time events from the server exec Run a command in a running container export Export a container's filesystem as a tar archive history Show the history of an image images List images import Import the contents from a tarball to create a filesystem image info Display system-wide information inspect Return low-level information on a container, image or task kill Kill one or more running containers load Load an image from a tar archive or STDIN login Log in to a Docker registry. logout Log out from a Docker registry. logs Fetch the logs of a container network Manage Docker networks node Manage Docker Swarm nodes pause Pause all processes within one or more containers port List port mappings or a specific mapping for the container ps List containers pull Pull an image or a repository from a registry push Push an image or a repository to a registry rename Rename a container restart Restart a container rm Remove one or more containers rmi Remove one or more images run Run a command in a new container save Save one or more images to a tar archive (streamed to STDOUT by default) search Search the Docker Hub for images service Manage Docker services start Start one or more stopped containers stats Display a live stream of container(s) resource usage statistics stop Stop one or more running containers swarm Manage Docker Swarm tag Tag an image into a repository top Display the running processes of a container unpause Unpause all processes within one or more containers update Update configuration of one or more containers version Show the Docker version information volume Manage Docker volumes wait Block until a container stops, then print its exit code Run 'docker COMMAND --help' for more information on a command. [email protected]:/home/david#
This section only applies if you're running Docker behind a HTTP/S proxy. In one of my environments I am, so I've decided to cover it here just in case. If you're not running behind a proxy server then you can safely ignore this section and move on.
You've probably already had to work around proxy issues given that you've just had to install docker using the apt-get process. This is easy enough using the
export http_proxy=http://proxyip:port command. However this is not something docker can use, so we'll have to dive a little deeper.
Edit this file:
You'll notice a section with the following:
# If you need Docker to use an HTTP proxy, it can also be specified here. #export http_proxy="http://127.0.0.1:3128/"
Uncomment the second line and adjust it accordingly, then restart the docker service:
service docker restart
Create the following file:
/etc/systemd/system/docker.service.d/http-proxy.conf (You may need to create the directory too).
Enter the following:
[Service] Environment="http_proxy=test.blah.com" "https_proxy=test.blah.com"
You'll then want to reload systemd and restart docker:
systemctl daemon-reload systemctl restart docker
Now we've got docker installed it's time for a bit more theory on the terms we'll be using throughout the rest of the post.
Container - This is the big one. Containers are the actual constructs we'll create and manipulate using Docker.
Image - You can think of this as a template of a container. Images are pulled from repositories.
Repository - An online store where images can be found. Docker Hub is the default and most common and can be found here: https://hub.docker.com/explore/
Network - A logical networking instance used to contain and segment containers with respect to addressing and application access.
Just to give you a bit of confidence when managing containers, I've listed pretty much all the commands you'll need below to get started. We'll cover most of them in a bit more depth later.
docker ps -a - Displays containers. The -a attribute shows ALL containers. By default only running containers are displayed.
docker images - Displays images which have been downloaded from repositories or created manually from existing containers or Dockerfiles.
docker start x - Starts container X
docker stop x - Stops container X
docker restart x - Restarts container X
docker run y - Runs the image Y as a container. This container will get an automatically generated name. This command will be elaborated on later.
docker create y - Creates but does not run a container. Similar to run apart from doesn't start the container.
docker stats - Displays real-time statistics for containers on the current host, including CPU, memory, disk and network IO.
docker rm x - Deletes container X
docker logs x - Displays the log for container X
docker exec x z - Executes command Z on container X.
Our first container:
Now we've got a few commands out of the way let's get into creating a container.
It's worth noting that a container only stays running when the process defined within it stays alive. Since busybox is essentially an empty system with no applications we want to run we have to attach to the shell when we run it, like so:
[email protected]:/home/david# docker run -it busybox Unable to find image 'busybox:latest' locally latest: Pulling from library/busybox 4b0bc1c4050b: Pull complete Digest: sha256:817a12c32a39bbe394944ba49de563e085f1d3c5266eb8e9723256bc4448680e Status: Downloaded newer image for busybox:latest / # / #
docker run command we have utilizes the
-it parameters. This means we're going to keep the session interactive, and open a TTY terminal to it. As a result we get the busybox shell.
If we exit from the shell and check the running containers with
docker ps -a we'll notice that we have a busybox container but it's no longer running since the shell process (/bin/sh) exited.
/ # / # exit [email protected]:/home/david# [email protected]:/home/david# [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 9350bc2d3b12 busybox "sh" About a minute ago Exited (0) 3 seconds ago nostalgic_stonebraker [email protected]:/home/david#
We can also see from the above that we can see some basic information about our shiny new container.
The container ID is generated automatically and is a reference to that container.
The Name is also automatically generated (you can specify it manually). The names given to containers are always fairly amusing, in my case nostalgic_stonebraker.
The image we used to create the container.
We can now restart the container using
docker start nostalgic_stonebraker we could also use the container ID instead of the name to start it.
[email protected]:/home/david# docker start nostalgic_stonebraker 9350bc2d3b12 [email protected]:/home/david#
Notice this time we've got no shell, but it's actually running in the background now since the create command allocated a TTY line for the command to run in.
We can attach to the existing process using
docker attach nostalgic_stonebraker:
[email protected]:/home/david# docker attach nostalgic_stonebraker / # / #
If we exit now, it'll stop the container again:
/ # / # exit [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 9350bc2d3b12 busybox "sh" 22 minutes ago Exited (0) 2 seconds ago nostalgic_stonebraker [email protected]:/home/david#
We can avoid this by getting into the container differently, by using
docker exec. First let's start the container again and make sure it's running:
[email protected]:/home/david# docker start nostalgic_stonebraker nostalgic_stonebraker [email protected]:/home/david# [email protected]:/home/david# [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 9350bc2d3b12 busybox "sh" 23 minutes ago Up 2 seconds nostalgic_stonebraker [email protected]:/home/david#
Now we can execute a new command, if we execute a new shell, we can exit from it without stopping the original process. We can also check which processes are running, and we can see two instances of 'sh':
[email protected]:/home/david# docker exec -it nostalgic_stonebraker /bin/sh / # / # / # ps PID USER TIME COMMAND 1 root 0:00 sh 7 root 0:00 /bin/sh 13 root 0:00 ps / # / #
-it does the same thing as when we created the container, we want an interactive terminal so we can execute commands.
Now if we exit, the container continues running:
/ # / # exit [email protected]:/home/david# [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 9350bc2d3b12 busybox "sh" 27 minutes ago Up About a minute nostalgic_stonebraker [email protected]:/home/david#
Voila! You've just successfully run and manipulated your first container!
Now let's remove it!
As you can see it's still running, so we'll stop it and then remove it:
[email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 9350bc2d3b12 busybox "sh" 27 minutes ago Up About a minute nostalgic_stonebraker [email protected]:/home/david# [email protected]:/home/david# [email protected]:/home/david# [email protected]:/home/david# [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 9350bc2d3b12 busybox "sh" 28 minutes ago Up 3 minutes nostalgic_stonebraker [email protected]:/home/david# [email protected]:/home/david# docker stop nostalgic_stonebraker nostalgic_stonebraker [email protected]:/home/david# docker rm nostalgic_stonebraker nostalgic_stonebraker [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES [email protected]:/home/david#
Working with Images:
We can see the images we have locally downloaded using
[email protected]:/home/david# docker images REPOSITORY TAG IMAGE ID CREATED SIZE busybox latest 7968321274dc 45 hours ago 1.11 MB [email protected]:/home/david#
We can remove images using
docker rmi. Let's remove the busybox image we have:
[email protected]:/home/david# docker rmi 7968321274dc Untagged: busybox:latest Untagged: [email protected]:817a12c32a39bbe394944ba49de563e085f1d3c5266eb8e9723256bc4448680e Deleted: sha256:7968321274dc6b6171697c33df7815310468e694ac5be0ec03ff053bb135e768 Deleted: sha256:38ac8d0f5bb30c8b742ad97a328b77870afaec92b33faf7e121161bc78a3fec8 [email protected]:/home/david# [email protected]:/home/david# docker images REPOSITORY TAG IMAGE ID CREATED SIZE [email protected]:/home/david#
We can pull new images using
docker pull. This can be useful to download images before issuing a
docker run. If the image doesn't exist when using
docker run it will be pulled first.
[email protected]:/home/david# docker pull ubuntu Using default tag: latest latest: Pulling from library/ubuntu b3e1c725a85f: Pull complete 4daad8bdde31: Pull complete 63fe8c0068a8: Pull complete 4a70713c436f: Pull complete bd842a2105a8: Pull complete Digest: sha256:7a64bc9c8843b0a8c8b8a7e4715b7615e4e1b0d8ca3c7e7a76ec8250899c397a Status: Downloaded newer image for ubuntu:latest [email protected]:/home/david# [email protected]:/home/david# docker images REPOSITORY TAG IMAGE ID CREATED SIZE ubuntu latest 104bec311bcd 4 weeks ago 129 MB [email protected]:/home/david#
One of my first thoughts on docker was, how on earth does the networking work? As a networking guy it's quite important for me to understand what sort of firewall / NAT rules I need to add to get these things working.
By default docker deploys with 3 networks. bridge, host and none.
Bridge - Do NOT think this means the same as bridged networking in VMware! Bridged networking in this instance means a container will get an IP address in a private range sitting behind the host. This address is then NATted using NAT overload or PAT from a networking term. This means that this container can access the internet and any other resource that the host can reach. This is the default networking method if nothing else is specified.
Host - This is where the container shares the host's networking. So there is no IP on the container and any ports used by the container will be directly exposed. For example, if a webserver is listening on port 80 in a container, that webserver will be accessible by navigating to http://hostip
None - No networking! Easy!
You can create your own networks, this could be useful if you have multiple containers which need to talk to each other but not be accessed by anything else. A single container (e.g. a load balancer) may then talk out of this private network and be accessed by the outside world. Creating your own networks is outside the scope of this post but is covered in detail here: https://docs.docker.com/engine/userguide/networking/
For the most part we'll want to use the bridge network. This allows us to map specific ports to the outside world (essentially port-forwarding) without configuring anything else. We can do this using the
-p src:dst flag when creating or running (if they've not already been created) containers.
E.g. say we wanted to create a webserver application (we'll cover this in more detail in the next section), we might have a create statement which looks like the following:
docker create \ -p 2080:80 \ nginx
This will create an nginx webserver container, which will be accessible from http://hostip:2080
This is exceptionally useful when running multiple copies of the same service on a single machine, since it allows ports to be re-mapped to different external ports and therefore allows the application to have agnostic configuration (i.e. nginx is still listening on port 80).
Something useful which is worth mentioning at this point is sometimes you want to quickly test containerised applications without having to screw around specifying port numbers here there and everywhere. For this what you can do is to leave the container as the default, on the bridged network, and not specify any ports. However next is the clever part: SSH tunnels!
By using a dynamic SSH tunnel to the docker host, you can put your own client (i.e your desktop PC you're sitting at) on the same network as the docker host, and have full access to all of the containers on that host by using their private IP. This dynamic SSH tunnel should be created in something like putty (if you're on windows) and then a SOCKS proxy added using 127.0.0.1:port within the web browser. You can then access a containers' applications directly using the private IP. The private IP can be found using
docker network inspect bridge. This will list each container, it's name, and IP address. This of course only applies if you're running a web application. Some other applications have SOCKS support and can use this method but it's not a complete solution.
As I mentioned in the introduction, one of the benefits (at least I find) in using containers is the ease of which you can keep configuration, application data and application binaries separate.
If we take a look at the nginx example we used in the networking section, we can quickly define these three types of data:
Configuration: Data found in /etc/nginx (i.e. /etc/nginx/nginx.conf and /etc/nginx/sites-enabled/*)
Application Data: The webpages themselves, found in (by default) /var/www/html.
Binary Data: The nginx binaries, and various libraries required by nginx.
So how can we keep these separate? Well the binary data already exists as part of a container so we only have to worry about application data and configuration. Generally it is desirable to keep these persistent, and the nature of containers means they don't exist for very long. It's easier to delete a container and re-create it than manually upgrade binaries if a new version of an application is out.
As such, we have to store the important stuff on persistent storage. We can map directories from the docker host to the container using the
-v /src/dir:/dst/mountpoint flag. In that example, the directory /src/dir is mapped to /dst/mountpoint as a linux filesystem mount. All directories underneath the source will be visable on the destination.
Files can also be mapped using exactly the same syntax, but just specifying a file instead of a directory.
Read-only paths are also possible, and are particularly useful for immutable configuration.
E.g. say we wanted an nginx server which read configuration and application data from the host, we could do something as follows:
docker create \ -v /opt/containers/webserver/config:/etc/nginx/ \ -v /opt/containers/webserver/html:/var/www/html \ nginx
This would map two directories through to the container. We would then have to make sure our configuration existed in the appropriate host directory, and that in the configuration we referred to our content using the path the container sees. I.e. /var/www/html
Applications - The whole point!
One of the main uses for containers is to quickly deploy applications. So far we've looked at busybox, which is cool, but pretty useless by itself. So in this section we'll actually look at some common applications and now to expose them to the outside world.
First, let's download nginx. Nginx if you're not familiar with is it a webserver. It doesn't do it much justice to call it a webserver as it can perform quite a few tasks but it's primary use case is, in fact, a webserver.
Pull the image from the repo:
[email protected]:/home/david# docker pull nginx Using default tag: latest latest: Pulling from library/nginx 75a822cd7888: Pull complete 0aefb9dc4a57: Pull complete 046e44ee6057: Pull complete Digest: sha256:fab482910aae9630c93bd24fc6fcecb9f9f792c24a8974f5e46d8ad625ac2357 Status: Downloaded newer image for nginx:latest [email protected]:/home/david# [email protected]:/home/david# docker images REPOSITORY TAG IMAGE ID CREATED SIZE nginx latest 01f818af747d 2 weeks ago 181.6 MB ubuntu latest 104bec311bcd 4 weeks ago 129 MB [email protected]:/home/david#
Time to create our container, note that I've also given it a meaningful name which is useful when manipulating the container. I'm going to stick to the example I used above for the networking:
docker create \ --name=webserver \ -v /opt/containers/webserver/config:/etc/nginx/ \ -v /opt/containers/webserver/html:/var/www/html/ \ -p 2080:80 \ nginx
[email protected]:/home/david# docker create \ > --name=webserver \ > -v /opt/containers/webserver/config:/etc/nginx/ \ > -v /opt/containers/webserver/html:/var/www/html \ > -p 2080:80 \ > nginx f1d69d3441cf4865df5e86447b0a67136fe9717f60efad4310323e2d63274922 [email protected]:/home/david#
Start the container:
docker start webserver
[email protected]:/home/david# docker start webserver webserver [email protected]:/home/david# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES f1d69d3441cf nginx "nginx -g 'daemon off" 27 seconds ago Up 3 seconds 443/tcp, 0.0.0.0:2080->80/tcp webserver [email protected]:/home/david#
We can see now that we have two ports listed: 443 and 80. Port 80 is redirected from 2080 but port 443 is what's called 'exposed'. This means that a service might be listening on port 443 but we've not mapped it though to any external port.
If we now try and access our host IP on port 2080 via HTTP we get the following:
The docker hub repository provides some excellent images we can use, another of my favourite repos is provided by the folks at http://linuxserver.io and they have a fantastic collection of containers here: http://tools.linuxserver.io/dockers
However sometimes it is desirable to create your own container images. For example, I wanted a webserver (nginx) which also was capable of processing PHP scripts using php7.0-fpm. I wasn't able to find a container with this all bundled in, so decided to make my own.
There are a few different mechanisms you can use to create your own, but two primary methods:
- Create a container, customise it and commit it - This takes an existing container and bundles it into an image you can deploy over and over.
- Use a dockerfile to build the container from scratch all at once - This uses a source container (often a lightweight image, e.g. ubuntu) and installs various packages and configures the system to be ready to run the applications required. It is much more automated than the first option.
To use the first option, you simple need to use
docker commit on the container you've built:
[email protected]:/home/david# docker commit webserver my-webserver sha256:7c870fce360dfded4615a7beb23d7d1982b0d84464fb70f3334726ffb5a87e8e [email protected]:/home/david# docker images REPOSITORY TAG IMAGE ID CREATED SIZE my-webserver latest 7c870fce360d 3 seconds ago 181.6 MB nginx latest 01f818af747d 2 weeks ago 181.6 MB ubuntu latest 104bec311bcd 4 weeks ago 129 MB [email protected]:/home/david#
As you can see, I've created a new image called 'my-webserver' based on the nginx webserver I created above. There is little point in this container, as there's no difference between it and the default nginx container, but it demonstrates how to create your own image.
For the second option, you need an empty directory and inside it contains the bits and pieces your container will use. The most important of these is a file called Dockerfile. For my php-nginx container I used the following definition:
FROM ubuntu:latest RUN apt-get update && apt-get install -y --no-install-recommends php7.0 nginx RUN apt-get clean EXPOSE 80 CMD service php7.0-fpm start && service nginx start && tail -f /dev/null
This essentially takes the latest ubuntu release, installs php7.0 and nginx, tells docker that services running on it will be running on port 80, then issues the command used to run the container. For more information on dockerfiles see the official documentation: https://docs.docker.com/engine/tutorials/dockerimages/
Finally you'll need to build this using
docker build from inside the directory container the dockerfile:
docker build -t php-nginx .
This specifies the tag as php-nginx, and uses the current directory . to build the image (docker will automatically look for the filename 'Dockerfile').
Starting containers on system startup
You can use the
--restart flag to define a restart policy when creating a container. Using
--restart=always will restart a container if it crashes, as well as makes it start on system startup. Personally I don't like this because if a container crashes I want it to stay dead, so I can troubleshoot why it crashed. But I do want containers to start on startup. So I've actually used the approach of starting containers using the linux host itself by editing
/etc/rc.local and adding entries like the following:
sudo docker start syncthing sudo docker start portainer sudo docker start webserver exit 0
Be careful when upgrading docker. I did an apt-get update and apt-get upgrade on my host. Which promptly killed all my running containers, thinking about it this isn't particularly surprising but it's something to be wary of if you need to schedule maintenance windows for the applications which are in containers.
Storing Container Definitions:
I'm not quite sure the best way to do this, but I've taken to the habit of storing my container create statements within a text file, so I can easily re-create containers if I want to change some parameters. E.g. my actual webserver create statement looks like the following:
docker create \ --name=webserver \ -v /mnt/nas/NFS/docker/webserver_config/nginx.conf:/etc/nginx/nginx.conf:ro \ -v /mnt/nas/NFS/docker/webserver_config/logs:/logs \ -v /mnt/nas/NFS/docker/webserver_config/sites:/sites \ -v /mnt/nas/NFS/docker/picserver_config/pics:/pics \ -v /mnt/nas/NFS/docker/webserver_config/www:/www \ -e PGID=0 -e PUID=0 \ -p 2080:80 \ php-nginx
It is worth mentioning that I've only been experimenting with Docker for a few days, and there's still multiple aspects I would like to explore, including:
- Docker Swarm
- Automation using the REST API
- Local repos / registries
- Extended dockerfile configuration
- Much more that I don't even know exists...
I suspect I'll be writing separate posts on these points later down the line, but for now I think I'm done with this post, time to get a bacon sandwich.
Here are a few useful commands I've used for various bits and pieces:
Remove all containers:
docker rm $(docker ps -aq)
Find unused images:
docker images --filter dangling=true
Remove unused images:
docker rmi $(docker images --filter dangling=true)