branch-2.0.4-alpha/hadoop-common-project/hadoop-common/src/site/apt/Superusers.apt.vm

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  Superusers Acting On Behalf Of Other Users
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  ${maven.build.timestamp}

Superusers Acting On Behalf Of Other Users

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* Introduction

   This document describes how a superuser can submit jobs or access hdfs
   on behalf of another user in a secured way.

* Use Case

   The code example described in the next section is applicable for the
   following use case.

   A superuser with username 'super' wants to submit job and access hdfs
   on behalf of a user joe. The superuser has kerberos credentials but
   user joe doesn't have any. The tasks are required to run as user joe
   and any file accesses on namenode are required to be done as user joe.
   It is required that user joe can connect to the namenode or job tracker
   on a connection authenticated with super's kerberos credentials. In
   other words super is impersonating the user joe.

* Code example

   In this example super's kerberos credentials are used for login and a
   proxy user ugi object is created for joe. The operations are performed
   within the doAs method of this proxy user ugi object.

----
    ...
    //Create ugi for joe. The login user is 'super'.
    UserGroupInformation ugi =
            UserGroupInformation.createProxyUser("joe", UserGroupInformation.getLoginUser());
    ugi.doAs(new PrivilegedExceptionAction<Void>() {
      public Void run() throws Exception {
        //Submit a job
        JobClient jc = new JobClient(conf);
        jc.submitJob(conf);
        //OR access hdfs
        FileSystem fs = FileSystem.get(conf);
        fs.mkdir(someFilePath);
      }
    }
----

* Configurations

   The superuser must be configured on namenode and jobtracker to be
   allowed to impersonate another user. Following configurations are
   required.

----
   <property>
     <name>hadoop.proxyuser.super.groups</name>
     <value>group1,group2</value>
     <description>Allow the superuser super to impersonate any members of the group group1 and group2</description>
   </property>
   <property>
     <name>hadoop.proxyuser.super.hosts</name>
     <value>host1,host2</value>
     <description>The superuser can connect only from host1 and host2 to impersonate a user</description>
   </property>
----

   If these configurations are not present, impersonation will not be
   allowed and connection will fail.

   If more lax security is preferred, the wildcard value * may be used to
   allow impersonation from any host or of any user.

* Caveats

   The superuser must have kerberos credentials to be able to impersonate
   another user. It cannot use delegation tokens for this feature. It
   would be wrong if superuser adds its own delegation token to the proxy
   user ugi, as it will allow the proxy user to connect to the service
   with the privileges of the superuser.

   However, if the superuser does want to give a delegation token to joe,
   it must first impersonate joe and get a delegation token for joe, in
   the same way as the code example above, and add it to the ugi of joe.
   In this way the delegation token will have the owner as joe.