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  HDFS Permissions Guide
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HDFS Permissions Guide

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* Overview

   The Hadoop Distributed File System (HDFS) implements a permissions
   model for files and directories that shares much of the POSIX model.
   Each file and directory is associated with an owner and a group. The
   file or directory has separate permissions for the user that is the
   owner, for other users that are members of the group, and for all other
   users. For files, the r permission is required to read the file, and
   the w permission is required to write or append to the file. For
   directories, the r permission is required to list the contents of the
   directory, the w permission is required to create or delete files or
   directories, and the x permission is required to access a child of the
   directory.

   In contrast to the POSIX model, there are no setuid or setgid bits for
   files as there is no notion of executable files. For directories, there
   are no setuid or setgid bits directory as a simplification. The Sticky
   bit can be set on directories, preventing anyone except the superuser,
   directory owner or file owner from deleting or moving the files within
   the directory. Setting the sticky bit for a file has no effect.
   Collectively, the permissions of a file or directory are its mode. In
   general, Unix customs for representing and displaying modes will be
   used, including the use of octal numbers in this description. When a
   file or directory is created, its owner is the user identity of the
   client process, and its group is the group of the parent directory (the
   BSD rule).

   Each client process that accesses HDFS has a two-part identity composed
   of the user name, and groups list. Whenever HDFS must do a permissions
   check for a file or directory foo accessed by a client process,

     * If the user name matches the owner of foo, then the owner
       permissions are tested;
     * Else if the group of foo matches any of member of the groups list,
       then the group permissions are tested;
     * Otherwise the other permissions of foo are tested.

   If a permissions check fails, the client operation fails.

* User Identity

   As of Hadoop 0.22, Hadoop supports two different modes of operation to
   determine the user's identity, specified by the
   hadoop.security.authentication property:

   * <<simple>>

          In this mode of operation, the identity of a client process is
          determined by the host operating system. On Unix-like systems,
          the user name is the equivalent of `whoami`.

   * <<kerberos>>

          In Kerberized operation, the identity of a client process is
          determined by its Kerberos credentials. For example, in a
          Kerberized environment, a user may use the kinit utility to
          obtain a Kerberos ticket-granting-ticket (TGT) and use klist to
          determine their current principal. When mapping a Kerberos
          principal to an HDFS username, all components except for the
          primary are dropped. For example, a principal
          todd/foobar@CORP.COMPANY.COM will act as the simple username
          todd on HDFS.

   Regardless of the mode of operation, the user identity mechanism is
   extrinsic to HDFS itself. There is no provision within HDFS for
   creating user identities, establishing groups, or processing user
   credentials.

* Group Mapping

   Once a username has been determined as described above, the list of
   groups is determined by a group mapping service, configured by the
   hadoop.security.group.mapping property. The default implementation,
   org.apache.hadoop.security.ShellBasedUnixGroupsMapping, will shell out
   to the Unix bash -c groups command to resolve a list of groups for a
   user.

   An alternate implementation, which connects directly to an LDAP server
   to resolve the list of groups, is available via
   org.apache.hadoop.security.LdapGroupsMapping. However, this provider
   should only be used if the required groups reside exclusively in LDAP,
   and are not materialized on the Unix servers. More information on
   configuring the group mapping service is available in the Javadocs.

   For HDFS, the mapping of users to groups is performed on the NameNode.
   Thus, the host system configuration of the NameNode determines the
   group mappings for the users.

   Note that HDFS stores the user and group of a file or directory as
   strings; there is no conversion from user and group identity numbers as
   is conventional in Unix.

* Understanding the Implementation

   Each file or directory operation passes the full path name to the name
   node, and the permissions checks are applied along the path for each
   operation. The client framework will implicitly associate the user
   identity with the connection to the name node, reducing the need for
   changes to the existing client API. It has always been the case that
   when one operation on a file succeeds, the operation might fail when
   repeated because the file, or some directory on the path, no longer
   exists. For instance, when the client first begins reading a file, it
   makes a first request to the name node to discover the location of the
   first blocks of the file. A second request made to find additional
   blocks may fail. On the other hand, deleting a file does not revoke
   access by a client that already knows the blocks of the file. With the
   addition of permissions, a client's access to a file may be withdrawn
   between requests. Again, changing permissions does not revoke the
   access of a client that already knows the file's blocks.

* Changes to the File System API

   All methods that use a path parameter will throw <<<AccessControlException>>>
   if permission checking fails.

   New methods:

     * <<<public FSDataOutputStream create(Path f, FsPermission permission,
       boolean overwrite, int bufferSize, short replication, long
       blockSize, Progressable progress) throws IOException;>>>

     * <<<public boolean mkdirs(Path f, FsPermission permission) throws
       IOException;>>>

     * <<<public void setPermission(Path p, FsPermission permission) throws
       IOException;>>>

     * <<<public void setOwner(Path p, String username, String groupname)
       throws IOException;>>>

     * <<<public FileStatus getFileStatus(Path f) throws IOException;>>>
     
       will additionally return the user, group and mode associated with the
       path.

   The mode of a new file or directory is restricted my the umask set as a
   configuration parameter. When the existing <<<create(path, …)>>> method
   (without the permission parameter) is used, the mode of the new file is
   <<<0666 & ^umask>>>. When the new <<<create(path, permission, …)>>> method
   (with the permission parameter P) is used, the mode of the new file is
   <<<P & ^umask & 0666>>>. When a new directory is created with the existing
   <<<mkdirs(path)>>>
   method (without the permission parameter), the mode of the new
   directory is <<<0777 & ^umask>>>. When the new <<<mkdirs(path, permission)>>>
   method (with the permission parameter P) is used, the mode of new
   directory is <<<P & ^umask & 0777>>>.

* Changes to the Application Shell

   New operations:

     * <<<chmod [-R] mode file …>>>

       Only the owner of a file or the super-user is permitted to change
       the mode of a file.

     * <<<chgrp [-R] group file …>>>

       The user invoking chgrp must belong to the specified group and be
       the owner of the file, or be the super-user.

     * <<<chown [-R] [owner][:[group]] file …>>>

       The owner of a file may only be altered by a super-user.

     * <<<ls file …>>>

     * <<<lsr file …>>>

       The output is reformatted to display the owner, group and mode.

* The Super-User

   The super-user is the user with the same identity as name node process
   itself. Loosely, if you started the name node, then you are the
   super-user. The super-user can do anything in that permissions checks
   never fail for the super-user. There is no persistent notion of who was
   the super-user; when the name node is started the process identity
   determines who is the super-user for now. The HDFS super-user does not
   have to be the super-user of the name node host, nor is it necessary
   that all clusters have the same super-user. Also, an experimenter
   running HDFS on a personal workstation, conveniently becomes that
   installation's super-user without any configuration.

   In addition, the administrator my identify a distinguished group using
   a configuration parameter. If set, members of this group are also
   super-users.

* The Web Server

   By default, the identity of the web server is a configuration
   parameter. That is, the name node has no notion of the identity of the
   real user, but the web server behaves as if it has the identity (user
   and groups) of a user chosen by the administrator. Unless the chosen
   identity matches the super-user, parts of the name space may be
   inaccessible to the web server.

* Configuration Parameters

     * <<<dfs.permissions = true>>>

       If yes use the permissions system as described here. If no,
       permission checking is turned off, but all other behavior is
       unchanged. Switching from one parameter value to the other does not
       change the mode, owner or group of files or directories.
       Regardless of whether permissions are on or off, chmod, chgrp and
       chown always check permissions. These functions are only useful in
       the permissions context, and so there is no backwards compatibility
       issue. Furthermore, this allows administrators to reliably set
       owners and permissions in advance of turning on regular permissions
       checking.

     * <<<dfs.web.ugi = webuser,webgroup>>>

       The user name to be used by the web server. Setting this to the
       name of the super-user allows any web client to see everything.
       Changing this to an otherwise unused identity allows web clients to
       see only those things visible using "other" permissions. Additional
       groups may be added to the comma-separated list.

     * <<<dfs.permissions.superusergroup = supergroup>>>

       The name of the group of super-users.

     * <<<fs.permissions.umask-mode = 0022>>>

       The umask used when creating files and directories. For
       configuration files, the decimal value 18 may be used.

     * <<<dfs.cluster.administrators = ACL-for-admins>>>

       The administrators for the cluster specified as an ACL. This
       controls who can access the default servlets, etc. in the HDFS.