Container

A simple divertimento for understanding how containers work. This tool allows to execute a command in a very basic container environment.

Rootfs

Container expects to find a rootfs in some local path. One easy way to create a rootfs is by using debootstrap:

$ sudo apt update
$ sudo apt install debootstrap

For example, You can create a ubuntu bionic rootfs easily with:

$ sudo debootstrap --arch amd64 bionic /<a>/<path>

where /a/path points to a path in your local disk where placing the rootfs to be used into the container.

The container searches for the rootfs in the path declared in the file /etc/container/settings.yaml. You must create such file and provide the setttings:

rootfs: /<a>/<path>

Build

Configure go for giving it a go ;)

$ sudo apt install golang
$ echo 'export GOPATH=$HOME/go' >> ~/.bashrc
$ echo 'export PATH=${PATH}:${GOPATH}/bin' >> ~/.bashrc
$ source ~/.bashrc

Checkout and build container tool with:

$ go get -t github.com/rmescandon/container
$ go install -t github.com/rmescandon/container

Run

Setuid

Before launching the container, you need to do a trick for preventing to run it as root. You must set a flag in the file permissions for running the executable as the owner, and set root as the owner. That ways the OS thinks that root is launching the container and thus it is able to configure the network interfaces:

$ sudo chown root $GOBIN/container
$ sudo chmod +s $GOBIN/container

Launch

Launch container tool with:

$ $GOBIN/container run <list_of_commands_to_execute>

for example, listing the files in the rootfs:

$ $GOBIN/container run /bin/sh ls -la

or simply entering into the container for a further cli executions:

$ $GOBIN/container run /bin/sh

All those commands will be executed in a container environment, isolated (more or less) from host.

You can check that

container rootfs is at the settings configured path and host rootfs is not visible from within the container
ls -lah /proc/mounts reports only container mounts but not host's
ip link shows container interfaces but not the host's
id into the container is the root one, but does not have root permissions over the host
container hostname is different from the host one

Settings

Not only rootfs can be configured in the /etc/container/settings.yaml file but the bridge name, virtual devices or the bridge CIDR, like:

rootfs: /data/rootfs/bionic
network:
    bridge: thebridge
    veth : theveth
    cidr: 10.20.30.40/24

TBD

For now only mount, pid, user namespaces work. Network is in progress. It is possible to ping host ips, but not reaching internet from the container

Disclaimer

Tested on Ubuntu Bionic (18.04). This software is delivered as is. No support is provided.

rmescandon/container

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