docker-cli/components/engine/pkg/libcontainer

libcontainer - reference implementation for containers

background

libcontainer specifies configuration options for what a container is. It provides a native Go implementation for using Linux namespaces with no external dependencies. libcontainer provides many convenience functions for working with namespaces, networking, and management.

container

A container is a self contained directory that is able to run one or more processes without affecting the host system. The directory is usually a full system tree. Inside the directory a container.json file is placed with the runtime configuration for how the processes should be contained and ran. Environment, networking, and different capabilities for the process are specified in this file. The configuration is used for each process executed inside the container.

Sample container.json file:

{
   "hostname" : "koye",
   "networks" : [
      {
         "gateway" : "172.17.42.1",
         "context" : {
            "bridge" : "docker0",
            "prefix" : "veth"
         },
         "address" : "172.17.0.2/16",
         "type" : "veth",
         "mtu" : 1500
      }
   ],
   "cgroups" : {
      "parent" : "docker",
      "name" : "11bb30683fb0bdd57fab4d3a8238877f1e4395a2cfc7320ea359f7a02c1a5620"
   },
   "tty" : true,
   "environment" : [
      "HOME=/",
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
      "HOSTNAME=11bb30683fb0",
      "TERM=xterm"
   ],
   "capabilities_mask" : [
      "SETPCAP",
      "SYS_MODULE",
      "SYS_RAWIO",
      "SYS_PACCT",
      "SYS_ADMIN",
      "SYS_NICE",
      "SYS_RESOURCE",
      "SYS_TIME",
      "SYS_TTY_CONFIG",
      "MKNOD",
      "AUDIT_WRITE",
      "AUDIT_CONTROL",
      "MAC_OVERRIDE",
      "MAC_ADMIN",
      "NET_ADMIN"
   ],
   "context" : {
      "apparmor_profile" : "docker-default"
   },
   "mounts" : [
      {
         "source" : "/var/lib/docker/containers/11bb30683fb0bdd57fab4d3a8238877f1e4395a2cfc7320ea359f7a02c1a5620/resolv.conf",
         "writable" : false,
         "destination" : "/etc/resolv.conf",
         "private" : true
      },
      {
         "source" : "/var/lib/docker/containers/11bb30683fb0bdd57fab4d3a8238877f1e4395a2cfc7320ea359f7a02c1a5620/hostname",
         "writable" : false,
         "destination" : "/etc/hostname",
         "private" : true
      },
      {
         "source" : "/var/lib/docker/containers/11bb30683fb0bdd57fab4d3a8238877f1e4395a2cfc7320ea359f7a02c1a5620/hosts",
         "writable" : false,
         "destination" : "/etc/hosts",
         "private" : true
      }
   ],
   "namespaces" : [
      "NEWNS",
      "NEWUTS",
      "NEWIPC",
      "NEWPID",
      "NEWNET"
   ]
}

Using this configuration and the current directory holding the rootfs for a process, one can use libcontainer to exec the container. Running the life of the namespace, a pid file is written to the current directory with the pid of the namespaced process to the external world. A client can use this pid to wait, kill, or perform other operation with the container. If a user tries to run a new process inside an existing container with a live namespace, the namespace will be joined by the new process.

You may also specify an alternate root place where the container.json file is read and where the pid file will be saved.

nsinit

nsinit is a cli application used as the reference implementation of libcontainer. It is able to spawn or join new containers giving the current directory. To use nsinit cd into a Linux rootfs and copy a container.json file into the directory with your specified configuration.

To execute /bin/bash in the current directory as a container just run:

nsinit exec /bin/bash

If you wish to spawn another process inside the container while your current bash session is running just run the exact same command again to get another bash shell or change the command. If the original process dies, PID 1, all other processes spawned inside the container will also be killed and the namespace will be removed.

You can identify if a process is running in a container by looking to see if pid is in the root of the directory.