Gontainer

Gontainer is a container made for fun and curiosity.

The scope of this project was to better understand Linux namespacing, and apply it to create a rudimental container.

Install

If you have a Go environment ready to go, it's as easy as:

go get github.com/alegrey91/Gontainer

Once you retrieved you are ready to build:

go build github.com/alegrey91/Gontainer

Usage

Typing Gontainer -h the following output will be shown:

Usage: ./Gontainer -run -uid [-mnt=/path/rootfs] [-uts [-hostname=new_hostname]] [-ipc] [-net] [-pid]
  -mnt='/path/rootfs'           Enable Mount namespace
  -uts                          Enable UTS namespace
  -hostname='new_hostname'      Set a custom hostname into the container
  -ipc                          Enable IPC namespace
  -net                          Enable Network namespace
  -pid                          Enable PID namespace
  -uid                          Enable User namespace
  -v                            Check Gontainer version

Below there is a full explanation of provided arguments:

• mnt: Mount namespaces control mount points. Upon creation the mounts from the current mount namespace are copied to the new namespace. The clone flag used to create a new namespace of this type is CLONE_NEWNS. [6]
• uts: UTS namespaces allow a single system to appear to have different host and domain names to different processes. The clone flag used to create a new namespace of this type is CLONE_NEWUTS. [6]
• ipc: IPC namespaces isolate processes from SysV style inter-process communication. This prevents processes in different IPC namespaces from using, for example, the SHM family of functions to establish a range of shared memory between the two processes. The clone flag used to create a new namespace of this type is CLONE_NEWIPC. [6]
• net: Network namespaces virtualize the network stack. On creation a network namespace contains only a loopback interface. The clone flag used to create a new namespace of this type is CLONE_NEWNET. [6]
• pid: The PID namespace provides processes with an independent set of process IDs (PIDs) from other namespaces. The first process created in a PID namespace is assigned the process id number 1 and receives most of the same special treatment as the normal init process. The clone flag used to create a new namespace of this type is CLONE_NEWPID. [6]
• uid: User namespaces are a feature to provide both privilege isolation and user identification segregation across multiple sets of processes available since kernel 3.8. With administrative assistance it is possible to build a container with seeming administrative rights without actually giving elevated privileges to user processes. The clone flag used to create a new namespace of this type is CLONE_NEWUSER. [6]

Examples

If you are interested in understanding how a containerized process is isolated from the rest of the system, follow the next step.

User ID isolation

From your terminal run:

Gontainer -run -uid

The result will be:

[user@real-hostname ~]$ ./Gontainer -run -uid
[Gontainer config]
• mnt:  ""
• uts:  disabled
• ipc:  disabled
• net:  disabled
• uid:  enabled

📦 [root@real-hostname] ~/home/user ‣  

What's happened?

We are trying to running Gontainer from the home directory of a non privileged user (user).

Using the flag option -uid we are mapping our local UID with the container's root UID.

For this reason, we are root inside the container. First magic of Linux namespaces!

Mount isolation

Commonly called as chroot this represents the true essence of the system isolation.

First of all, we need a basic root filesystem. If you have docker installed, you can retrieve a rootfs from it:

docker container inspect alpine | grep UpperDir

Just cp -r the resultant path to /tmp/rootfs and then:

Gontainer -run -uid -mnt /tmp/rootfs

As you can see, your OS file system has disappeared, leaving space for a new file system (the alpine fs).

References

1. https://medium.com/@teddyking/linux-namespaces-850489d3ccf
2. https://medium.com/@ssttehrani/containers-from-scratch-with-golang-5276576f9909
3. http://ifeanyi.co/posts/linux-namespaces-part-1/
4. https://klotzandrew.com/blog/container-from-scratch
5. https://www.infoq.com/articles/build-a-container-golang/
6. https://en.wikipedia.org/wiki/Linux_namespaces