RunC - Container Tool

RunC

RunC is a Command line tool for spawning and running containers according to the OCI specification.This is a docker container format and runtime that is being donated to the OCI.

What is a OCI?

Open Container initiative defines specifications for building tools that help in building,

transporting and preparing container images to run.

The OCI consists of 2 specifications,

Runtime Specification ( runtime-spec ) defines how to  run a filesystem bundle that is available on the disk. Generally an OCI implementation will first download the OCI image, unpack the image into a OCI Runtime file system bundle.  At this point we can run this Runtime bundle using a Oci Runtime.

Image specification (image-spec) defines how to create an OCI Image. The image is created

by a build system which will give a image manifest, file system and image configuration.

The manifest file will have details about the content of the file system , dependencies of the

image like link to other file systems etc which will make up the final image.

The Image configuration will have application arguments, env variables etc. All these combined to form an OCI image

How to use RunC to run Containers?

1. Download the runC library based on the platform from here using,

wget https://github.com/opencontainers/runc/releases/download/v1.0.0-rc5/runc.amd64

2. Create a directory structure

mkdir runC

cd runC

mkdir test-container

cd test-container

3. Download a busybox docker container image and export the image to the rootfs filesystem

like,  docker export $(docker create busybox) | tar -C rootfs -xvf -

Now we will see a directory by the name rootfs with multiple files and directories inside

4. Run the runC spec command from the download library using,

[root@manja17-I13330 test-container]# /root/runc/runc.amd64 spec

[root@manja17-I13330 test-container]# ll

total 4

-rw-r--r--  1 root root 2614 Jul 26 07:27 config.json

drwxr-xr-x 12 root root  137 Jul 26 07:09 rootfs

A spec file is created by the name config.json. Check the file to see the configurations details

for the image.

[root@manja17-I13330 test-container]# cat config.json

{

        "ociVersion": "1.0.0",

        "process": {

                "terminal": true,

                "user": {

                        "uid": 0,

                        "gid": 0

                },

                "args": [

                        "sh"

                ],

                "env": [

                        "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",

                        "TERM=xterm"

                ],

                "cwd": "/",

                "capabilities": {

                        "bounding": [

                                "CAP_AUDIT_WRITE",

                                "CAP_KILL",

                                "CAP_NET_BIND_SERVICE"

                        ],

                        "effective": [

                                "CAP_AUDIT_WRITE",

                                "CAP_KILL",

                                "CAP_NET_BIND_SERVICE"

                        ],

                       *********

If you check the config.json, we can see what this container does and how it will run etc.Run the container using the runC command as,

[root@manja17-I13330 test-container]# /root/runc/runc.amd64 run container1

/ # ps ux

PID   USER     TIME  COMMAND

    1 root      0:00 sh

    6 root      0:00 ps ux

/ # exit

Run the container background using,

[root@manja17-I13330 test-container]# /root/runc/runc.amd64 run container1 &

[root@manja17-I13330 test-container]# /root/runc/runc.amd64 list

We will see the containers running listed

All commands that we run is based on the Container ID. lets run some more commands as

[root@manja17-I13330 runc]# ./runc.amd64 ps container1

UID        PID   PPID   C STIME TTY          TIME    CMD

root     21033 21025  0 00:20 ?         00:00:00 sh

[root@manja17-I13330 runc]# ./runc.amd64 exec container1 free

                total          used           free        shared    buffers     cached

Mem:       8175444    8013024     162420          0       2776    5760124

-/+ buffers/cache:     2250124     5925320

Swap:       0             0                 0

More to Come, Happy Learning :-)