Module 27 - Custom Containers¶

View the archived source URL.

Introduction¶

We used a prebuilt filesystem in a prior module. Let's make custom filesystems and boot them.

WARNING Here be Dragons: When you build container filesystems with Docker and LXC you are diving into a realm of things I wouldn't expect to be supported.

You may find better luck running Docker and LXC containers in a nested manner in a KVM.

Adding static binaries to containerfs¶

cd ~
wget https://storage.googleapis.com/minimega-files/minimega-2.2-containerfs.tar.bz2
tar xf minimega-2.2-containerfs.tar.bz2

Create a go web server:

package main

import (
    "fmt"
    "net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello World!")
}

func main() {
    http.HandleFunc("/", handler)
    fmt.Println("Server running...")
    http.ListenAndServe(":8080", nil)
}

Compile the go web server to a binary

GOOS=linux GOARCH=386 go build ./webserver.go

Copy this to containerfs:

cp webserver /home/ubuntu/containerfs/

Edit /home/ubuntu/containerfs/init

nano /home/ubuntu/containerfs/init

Before bash run the webserver binary

/webserver &

Booting the container filesystem¶

minimega -attach
$ vm config filesystem /home/ubuntu/containerfs/
$ vm config net 0
$ vm config snapshot true
$ vm launch container custom-cfs
$ vm start custom-cfs

There should be a web server running on port 8080 on the container.

Building a custom containerfs with shared libraries¶

The minirouter container has a clean build script you can tweak for your application.

github.com/sandia-minimega/minimega/blob/master/misc/uminirouter/build.bash

In summary:

• A basic file structure is made
• A 64-bit binary of busybox is downloaded.
• Symbolic links for busy box compiled programs are created
• The PATH variable is updated
• Init and Preinit are copied
• Binaries are copied from the host
• The program ldd is used to locate shared libraries and they are copied over
• Scripts get tweaked to use /bin/sh
• A root user is created with no password
• And a package is made.

LXC¶

apt-get install lxc
lxc-create -t ubuntu -n ubuntu

This downloads and generates a root file system in /var/lib/lxc/ubuntu/rootfs/ of Ubuntu 16.04

Before we can boot this we need to add an init

cat > /var/lib/lxc/ubuntu/rootfs/init << EOF
#!/bin/sh

ifconfig lo up
ifconfig veth0 up
dhclient -v veth0

/usr/sbin/sshd &

bash
EOF

Now we need to fix the permissions on init

chmod 755 /var/lib/lxc/ubuntu/rootfs/init

sshd needs a folder that doesn't already exist, let's fix that:

cd /var/lib/lxc/ubuntu/rootfs/
mkdir -p var/run/sshd
chmod 0755 var/run/sshd

Booting the container filesystem¶

minimega -attach
$ vm config filesystem /var/lib/lxc/ubuntu/rootfs/
$ vm config net 0
$ vm config snapshot false
$ vm launch container ubuntu1604
$ vm start ubuntu1604

The mega_bridge adapter is configured in a bridged adapter manner which you can do following the later networking modules.

Any changes you make will be permanent, including applications you download from apt-get.

In order to get apt-get working you will need to fix the PATH variable with:

export PATH=$PATH:/usr/local/sbin/
export PATH=$PATH:/usr/sbin/
export PATH=$PATH:/sbin
export PATH=$PATH:/usr/local/bin/
export PATH=$PATH:/usr/bin/
export PATH=$PATH:/bin

I also needed to set a nameserver with:

echo "nameserver 8.8.8.8" > resolv.conf

From there apt-get worked and I was able to install packages like python and nano.

I typed exit and changed to snapshot true and booted 50 of these containers using <440mB RAM total

Docker¶

Install Docker

www.digitalocean.com/community/tutorials/how-to-install-and-use-docker-on-ubuntu-16-04

Download the Docker Ubuntu image.

docker pull ubuntu

And run the image

docker run -it ubuntu /bin/bash

From the Docker container's /bin/bash

apt-get update
apt-get install isc-dhcp-client net-tools iputils-ping python

Open another terminal:

docker ps
docker export e50ef793a69d -o ubuntu.tar.gz
docker kill e50ef793a69d

Export the filesystem

mkdir dubuntu
cd dubuntu
tar xf ../ubuntu.tar.gz

Create an init file

cat > init << EOF
#!/bin/sh

ifconfig lo up
ifconfig veth0 up
dhclient -v veth0

bash
EOF

Now we need to fix the permissions on init

chmod 755 /var/lib/lxc/ubuntu/rootfs/init

Booting the container filesystem¶

minimega -attach
$ vm config filesystem /home/ubuntu/dubuntu/
$ vm config net 0
$ vm config snapshot true
$ vm launch container dubuntu1604
$ vm start dubuntu1604

Special Notes¶

There is no /dev/stdout or /dev/stderr so any image using those will need to be modified.

Depending on your base image, it may be difficult to install other packages, like dhclient.

Containers are still pretty experimental.