Launching Applications Using Docker Containers

Security Warning

IMPORTANT This feature is experimental and is not complete. IMPORTANT Enabling this feature and running Docker containers in your cluster has security implications. With this feature enabled, it may be possible to gain root access to the YARN NodeManager hosts. Given Docker’s integration with many powerful kernel features, it is imperative that administrators understand Docker security before enabling this feature.

Overview

Docker combines an easy-to-use interface to Linux containers with easy-to-construct image files for those containers. In short, Docker enables users to bundle an application together with its preferred execution environment to be executed on a target machine. For more information about Docker, see their documentation.

The Linux Container Executor (LCE) allows the YARN NodeManager to launch YARN containers to run either directly on the host machine or inside Docker containers. The application requesting the resources can specify for each container how it should be executed. The LCE also provides enhanced security and is required when deploying a secure cluster. When the LCE launches a YARN container to execute in a Docker container, the application can specify the Docker image to be used.

Docker containers provide a custom execution environment in which the application’s code runs, isolated from the execution environment of the NodeManager and other applications. These containers can include special libraries needed by the application, and they can have different versions of native tools and libraries including Perl, Python, and Java. Docker containers can even run a different flavor of Linux than what is running on the NodeManager.

Docker for YARN provides both consistency (all YARN containers will have the same software environment) and isolation (no interference with whatever is installed on the physical machine).

Docker support in the LCE is still evolving. To track progress, follow YARN-3611, the umbrella JIRA for Docker support improvements.

Cluster Configuration

The LCE requires that container-executor binary be owned by root:hadoop and have 6050 permissions. In order to launch Docker containers, the Docker daemon must be running on all NodeManager hosts where Docker containers will be launched. The Docker client must also be installed on all NodeManager hosts where Docker containers will be launched and able to start Docker containers.

To prevent timeouts while starting jobs, any large Docker images to be used by an application should already be loaded in the Docker daemon’s cache on the NodeManager hosts. A simple way to load an image is by issuing a Docker pull request. For example:

    sudo docker pull images/hadoop-docker:latest

The following properties should be set in yarn-site.xml:

<configuration>
  <property>
    <name>yarn.nodemanager.container-executor.class</name>
    <value>org.apache.hadoop.yarn.server.nodemanager.LinuxContainerExecutor</value>
    <description>
      This is the container executor setting that ensures that all applications
      are started with the LinuxContainerExecutor.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.linux-container-executor.group</name>
    <value>hadoop</value>
    <description>
      The POSIX group of the NodeManager. It should match the setting in
      "container-executor.cfg". This configuration is required for validating
      the secure access of the container-executor binary.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.linux-container-executor.nonsecure-mode.limit-users</name>
    <value>false</value>
    <description>
      Whether all applications should be run as the NodeManager process' owner.
      When false, applications are launched instead as the application owner.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.allowed-runtimes</name>
    <value>default,docker</value>
    <description>
      Comma separated list of runtimes that are allowed when using
      LinuxContainerExecutor. The allowed values are default, docker, and
      javasandbox.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.docker.allowed-container-networks</name>
    <value>host,none,bridge</value>
    <description>
      Optional. A comma-separated set of networks allowed when launching
      containers. Valid values are determined by Docker networks available from
      `docker network ls`
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.docker.default-container-network</name>
    <value>host</value>
    <description>
      The network used when launching Docker containers when no
      network is specified in the request. This network must be one of the
      (configurable) set of allowed container networks.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.docker.host-pid-namespace.allowed</name>
    <value>false</value>
    <description>
      Optional. Whether containers are allowed to use the host PID namespace.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.docker.privileged-containers.allowed</name>
    <value>false</value>
    <description>
      Optional. Whether applications are allowed to run in privileged
      containers.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.docker.privileged-containers.acl</name>
    <value></value>
    <description>
      Optional. A comma-separated list of users who are allowed to request
      privileged contains if privileged containers are allowed.
    </description>
  </property>

  <property>
    <name>yarn.nodemanager.runtime.linux.docker.capabilities</name>
    <value>CHOWN,DAC_OVERRIDE,FSETID,FOWNER,MKNOD,NET_RAW,SETGID,SETUID,SETFCAP,SETPCAP,NET_BIND_SERVICE,SYS_CHROOT,KILL,AUDIT_WRITE</value>
    <description>
      Optional. This configuration setting determines the capabilities
      assigned to docker containers when they are launched. While these may not
      be case-sensitive from a docker perspective, it is best to keep these
      uppercase. To run without any capabilites, set this value to
      "none" or "NONE"
    </description>
  </property>
</configuration>

In addition, a container-executer.cfg file must exist and contain settings for the container executor. The file must be owned by root with permissions 0400. The format of the file is the standard Java properties file format, for example

`key=value`

The following properties are required to enable Docker support:

The container-executor.cfg must contain a section to determine the capabilities that containers are allowed. It contains the following properties:

Please note that if you wish to run Docker containers that require access to the YARN local directories, you must add them to the docker.allowed.rw-mounts list.

In addition, containers are not permitted to mount any parent of the container-executor.cfg directory in read-write mode.

The following properties are optional:

Part of a container-executor.cfg which allows Docker containers to be launched is below:

yarn.nodemanager.linux-container-executor.group=yarn
[docker]
  module.enabled=true
  docker.privileged-containers.enabled=true
  docker.trusted.registries=centos
  docker.allowed.capabilities=SYS_CHROOT,MKNOD,SETFCAP,SETPCAP,FSETID,CHOWN,AUDIT_WRITE,SETGID,NET_RAW,FOWNER,SETUID,DAC_OVERRIDE,KILL,NET_BIND_SERVICE
  docker.allowed.networks=bridge,host,none
  docker.allowed.ro-mounts=/sys/fs/cgroup
  docker.allowed.rw-mounts=/var/hadoop/yarn/local-dir,/var/hadoop/yarn/log-dir

Docker Image Requirements

In order to work with YARN, there are two requirements for Docker images.

First, the Docker container will be explicitly launched with the application owner as the container user. If the application owner is not a valid user in the Docker image, the application will fail. The container user is specified by the user’s UID. If the user’s UID is different between the NodeManager host and the Docker image, the container may be launched as the wrong user or may fail to launch because the UID does not exist.

Second, the Docker image must have whatever is expected by the application in order to execute. In the case of Hadoop (MapReduce or Spark), the Docker image must contain the JRE and Hadoop libraries and have the necessary environment variables set: JAVA_HOME, HADOOP_COMMON_PATH, HADOOP_HDFS_HOME, HADOOP_MAPRED_HOME, HADOOP_YARN_HOME, and HADOOP_CONF_DIR. Note that the Java and Hadoop component versions available in the Docker image must be compatible with what’s installed on the cluster and in any other Docker images being used for other tasks of the same job. Otherwise the Hadoop components started in the Docker container may be unable to communicate with external Hadoop components.

If a Docker image has a command set, the behavior will depend on whether the YARN_CONTAINER_RUNTIME_DOCKER_RUN_OVERRIDE_DISABLE is set to true. If so, the command will be overridden when LCE launches the image with YARN’s container launch script.

If a Docker image has an entry point set, the entry point will be honored, but the default command may be overridden, as just mentioned above. Unless the entry point is something similar to sh -c or YARN_CONTAINER_RUNTIME_DOCKER_RUN_OVERRIDE_DISABLE is set to true, the net result will likely be undesirable. Because the YARN container launch script is required to correctly launch the YARN task, use of entry points is discouraged.

If an application requests a Docker image that has not already been loaded by the Docker daemon on the host where it is to execute, the Docker daemon will implicitly perform a Docker pull command. Both MapReduce and Spark assume that tasks which take more that 10 minutes to report progress have stalled, so specifying a large Docker image may cause the application to fail.

Application Submission

Before attempting to launch a Docker container, make sure that the LCE configuration is working for applications requesting regular YARN containers. If after enabling the LCE one or more NodeManagers fail to start, the cause is most likely that the ownership and/or permissions on the container-executer binary are incorrect. Check the logs to confirm.

In order to run an application in a Docker container, set the following environment variables in the application’s environment:

The first two are required. The remainder can be set as needed. While controlling the container type through environment variables is somewhat less than ideal, it allows applications with no awareness of YARN’s Docker support (such as MapReduce and Spark) to nonetheless take advantage of it through their support for configuring the application environment.

Once an application has been submitted to be launched in a Docker container, the application will behave exactly as any other YARN application. Logs will be aggregated and stored in the relevant history server. The application life cycle will be the same as for a non-Docker application.

Using Docker Bind Mounted Volumes

WARNING Care should be taken when enabling this feature. Enabling access to directories such as, but not limited to, /, /etc, /run, or /home is not advisable and can result in containers negatively impacting the host or leaking sensitive information. WARNING

Files and directories from the host are commonly needed within the Docker containers, which Docker provides through volumes. Examples include localized resources, Apache Hadoop binaries, and sockets. To facilitate this need, YARN-6623 added the ability for administrators to set a whitelist of host directories that are allowed to be bind mounted as volumes into containers. YARN-5534 added the ability for users to supply a list of mounts that will be mounted into the containers, if allowed by the administrative whitelist.

In order to make use of this feature, the following must be configured.

• The administrator must define the volume whitelist in container-executor.cfg by setting docker.allowed.ro-mounts and docker.allowed.rw-mounts to the list of parent directories that are allowed to be mounted.
• The application submitter requests the required volumes at application submission time using the YARN_CONTAINER_RUNTIME_DOCKER_MOUNTS environment variable.

The administrator supplied whitelist is defined as a comma separated list of directories that are allowed to be mounted into containers. The source directory supplied by the user must either match or be a child of the specified directory.

The user supplied mount list is defined as a comma separated list in the form source:destination:mode. The source is the file or directory on the host. The destination is the path within the contatiner where the source will be bind mounted. The mode defines the mode the user expects for the mount, which can be ro (read-only) or rw (read-write).

The following example outlines how to use this feature to mount the commonly needed /sys/fs/cgroup directory into the container running on YARN.

The administrator sets docker.allowed.ro-mounts in container-executor.cfg to “/sys/fs/cgroup”. Applications can now request that “/sys/fs/cgroup” be mounted from the host into the container in read-only mode.

At application submission time, the YARN_CONTAINER_RUNTIME_DOCKER_MOUNTS environment variable can then be set to request this mount. In this example, the environment variable would be set to “/sys/fs/cgroup:/sys/fs/cgroup:ro”. The destination path is not restricted, “/sys/fs/cgroup:/cgroup:ro” would also be valid given the example admin whitelist.

Privileged Container Security Consideration

Privileged docker container can interact with host system devices. This can cause harm to host operating system without proper care. In order to mitigate risk of allowing privileged container to run on Hadoop cluster, we implemented a controlled process to sandbox unauthorized privileged docker images.

The default behavior is disallow any privileged docker containers. When docker.privileged-containers.enabled is set to enabled, docker image can run with root privileges in the docker container, but access to host level devices are disabled. This allows developer and tester to run docker images from internet without causing harm to host operating system.

When docker images have been certified by developers and testers to be trustworthy. The trusted image can be promoted to trusted docker registry. System administrator can define docker.trusted.registries, and setup private docker registry server to promote trusted images.

Trusted images are allowed to mount external devices such as HDFS via NFS gateway, or host level Hadoop configuration. If system administrators allow writing to external volumes using docker.allow.rw-mounts directive, privileged docker container can have full control of host level files in the predefined volumes.

For YARN Service HTTPD example, container-executor.cfg must define centos docker registry to be trusted for the example to run.

Container Reacquisition Requirements

On restart, the NodeManager, as part of the NodeManager’s recovery process, will validate that a container is still running by checking for the existence of the container’s PID directory in the /proc filesystem. For security purposes, operating system administrator may enable the hidepid mount option for the /proc filesystem. If the hidepid option is enabled, the yarn user’s primary group must be whitelisted by setting the gid mount flag similar to below. Without the yarn user’s primary group whitelisted, container reacquisition will fail and the container will be killed on NodeManager restart.

proc     /proc     proc     nosuid,nodev,noexec,hidepid=2,gid=yarn     0 0

Connecting to a Secure Docker Repository

The Docker client command will draw its configuration from the default location, which is $HOME/.docker/config.json on the NodeManager host. The Docker configuration is where secure repository credentials are stored, so use of the LCE with secure Docker repos is discouraged using this method.

YARN-5428 added support to Distributed Shell for securely supplying the Docker client configuration. See the Distributed Shell help for usage. Support for additional frameworks is planned.

As a work-around, you may manually log the Docker daemon on every NodeManager host into the secure repo using the Docker login command:

  docker login [OPTIONS] [SERVER]

  Register or log in to a Docker registry server, if no server is specified
  "https://index.docker.io/v1/" is the default.

  -e, --email=""       Email
  -p, --password=""    Password
  -u, --username=""    Username

Note that this approach means that all users will have access to the secure repo.

Example: MapReduce

To submit the pi job to run in Docker containers, run the following commands:

    vars="YARN_CONTAINER_RUNTIME_TYPE=docker,YARN_CONTAINER_RUNTIME_DOCKER_IMAGE=hadoop-docker"
    hadoop jar hadoop-examples.jar pi -Dyarn.app.mapreduce.am.env=$vars \
        -Dmapreduce.map.env=$vars -Dmapreduce.reduce.env=$vars 10 100

Note that the application master, map tasks, and reduce tasks are configured independently. In this example, we are using the hadoop-docker image for all three.

Example: Spark

To run a Spark shell in Docker containers, run the following command:

    spark-shell --master yarn --conf spark.executorEnv.YARN_CONTAINER_RUNTIME_TYPE=docker \
        --conf spark.executorEnv.YARN_CONTAINER_RUNTIME_DOCKER_IMAGE=hadoop-docker \
        --conf spark.yarn.AppMasterEnv.YARN_CONTAINER_RUNTIME_DOCKER_IMAGE=hadoop-docker \
        --conf spark.yarn.AppMasterEnv.YARN_CONTAINER_RUNTIME_TYPE=docker

Note that the application master and executors are configured independently. In this example, we are using the hadoop-docker image for both.

Docker Container ENTRYPOINT Support

When Docker support was introduced to Hadoop 2.x, the platform was designed to run existing Hadoop programs inside Docker container. Log redirection and environment setup are integrated with Node Manager. In Hadoop 3.x, Hadoop Docker support extends beyond running Hadoop workload, and support Docker container in Docker native form using ENTRYPOINT from dockerfile. Application can decide to support YARN mode as default or Docker mode as default by defining YARN_CONTAINER_RUNTIME_DOCKER_RUN_OVERRIDE_DISABLE environment variable. System administrator can also set as default setting for the cluster to make ENTRY_POINT as default mode of operation.

In yarn-site.xml, add YARN_CONTAINER_RUNTIME_DOCKER_RUN_OVERRIDE_DISABLE to node manager environment white list:

<property>
        <name>yarn.nodemanager.env-whitelist</name>
        <value>JAVA_HOME,HADOOP_COMMON_HOME,HADOOP_HDFS_HOME,HADOOP_CONF_DIR,HADOOP_YARN_HOME,HADOOP_MAPRED_HOME,YARN_CONTAINER_RUNTIME_DOCKER_RUN_OVERRIDE_DISABLE</value>
</property>

In yarn-env.sh, define:

export YARN_CONTAINER_RUNTIME_DOCKER_RUN_OVERRIDE_DISABLE=true