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296 lines
14 KiB
ReStructuredText
=====================================
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Ceph Authentication & Authorization
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=====================================
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Ceph is a distributed storage system where a typical deployment involves a
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relatively small quorum of *monitors*, scores of *metadata servers* (MDSs) and
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many thousands of OSD daemons operating across many hosts/nodes--representing
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the server portion of the Ceph object store. Ceph clients such as CephFS, Ceph
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block device and Ceph Gateway interact with the Ceph object store. All Ceph
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object store clients use the ``librados`` library to interact with the Ceph
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object store. The following diagram illustrates an abstract client/server
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technology stack.
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.. ditaa:: +---------------------------------------------------+
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| client |
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+---------------------------------------------------+
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| librados |
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+---------------------------------------------------+
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+---------------+ +---------------+ +---------------+
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| OSDs | | MDSs | | Monitors |
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+---------------+ +---------------+ +---------------+
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Users are either individuals or system actors such as applications, which
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use Ceph clients to interact with Ceph server daemons.
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.. ditaa:: +-----+
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| {o} |
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+--+--+ /---------\ /---------\
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| | Ceph | | Ceph |
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---+---*----->| |<------------->| |
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| uses | Clients | | Servers |
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| \---------/ \---------/
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/--+--\
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actor
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For additional information, see our `Cephx Guide`_ and `ceph-authtool manpage`_.
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.. _Cephx Guide: ../authentication
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.. _ceph-authtool manpage: ../../../man/8/ceph-authtool
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Ceph Authentication (cephx)
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===========================
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Cryptographic authentication has some computational costs, though they should
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generally be quite low. If the network environment connecting your client and
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server hosts is very safe and you cannot afford authentication, you can use a
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Ceph option to turn it off. **This is not generally recommended**, but should you
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need to do so, details can be found in the `Disable Cephx`_ section.
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.. important:: Remember, if you disable authentication, you are at risk of a
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man-in-the-middle attack altering your client/server messages, which could
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lead to disastrous security effects.
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A key scalability feature of Ceph is to avoid a centralized interface to the
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Ceph object store, which means that Ceph clients must be able to interact with
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OSDs directly. To protect data, Ceph provides its ``cephx`` authentication
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system, which authenticates users operating Ceph clients. The ``cephx`` protocol
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operates in a manner with behavior similar to `Kerberos`_.
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.. _Disable Cephx: ../authentication#disable-cephx
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.. _Kerberos: http://en.wikipedia.org/wiki/Kerberos_(protocol)
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A user/actor invokes a Ceph client to contact a monitor. Unlike Kerberos, each
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monitor can authenticate users and distribute keys, so there is no single point
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of failure or bottleneck when using ``cephx``. The monitor returns an
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authentication data structure similar to a Kerberos ticket that contains a
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session key for use in obtaining Ceph services. This session key is itself
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encrypted with the user's permanent secret key, so that only the user can
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request services from the Ceph monitor(s). The client then uses the session key
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to request its desired services from the monitor, and the monitor provides the
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client with a ticket that will authenticate the client to the OSDs that actually
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handle data. Ceph monitors and OSDs share a secret, so the client can use the
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ticket provided by the monitor with any OSD or metadata server in the cluster.
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Like Kerberos, ``cephx`` tickets expire, so an attacker cannot use an expired
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ticket or session key obtained surreptitiously. This form of authentication will
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prevent attackers with access to the communications medium from either creating
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bogus messages under another user's identity or altering another user's
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legitimate messages, as long as the user's secret key is not divulged before it
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expires.
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To use ``cephx``, an administrator must set up users first. In the following
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diagram, the ``client.admin`` user invokes ``ceph auth get-or-create-key`` from
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the command line to generate a username and secret key. Ceph's ``auth``
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subsystem generates the username and key, stores a copy with the monitor(s) and
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transmits the user's secret back to the ``client.admin`` user. This means that
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the client and the monitor share a secret key.
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.. note:: The ``client.admin`` user must provide the user ID and
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secret key to the user in a secure manner.
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.. ditaa:: +---------+ +---------+
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| Client | | Monitor |
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+---------+ +---------+
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| request to |
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| create a user |
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|-------------->|----------+ create user
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| | | and
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|<--------------|<---------+ store key
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| transmit key |
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To authenticate with the monitor, the client passes in the user name to the
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monitor, and the monitor generates a session key and encrypts it with the secret
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key associated to the user name. Then, the monitor transmits the encrypted
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ticket back to the client. The client then decrypts the payload with the shared
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secret key to retrieve the session key. The session key identifies the user for
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the current session. The client then requests a ticket on behalf of the user
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signed by the session key. The monitor generates a ticket, encrypts it with the
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user's secret key and transmits it back to the client. The client decrypts the
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ticket and uses it to sign requests to OSDs and metadata servers throughout the
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cluster.
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.. ditaa:: +---------+ +---------+
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| Client | | Monitor |
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+---------+ +---------+
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| authenticate |
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|-------------->|----------+ generate and
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| | | encrypt
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|<--------------|<---------+ session key
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| transmit |
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| encrypted |
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| session key |
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|-----+ decrypt |
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| | session |
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|<----+ key |
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| req. ticket |
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|-------------->|----------+ generate and
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| | | encrypt
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|<--------------|<---------+ ticket
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| recv. ticket |
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|-----+ decrypt |
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| | ticket |
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|<----+ |
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The ``cephx`` protocol authenticates ongoing communications between the client
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machine and the Ceph servers. Each message sent between a client and server,
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subsequent to the initial authentication, is signed using a ticket that the
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monitors, OSDs and metadata servers can verify with their shared secret.
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.. ditaa:: +---------+ +---------+ +-------+ +-------+
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| Client | | Monitor | | MDS | | OSD |
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+---------+ +---------+ +-------+ +-------+
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| request to | | |
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| create a user | | |
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|-------------->| mon and | |
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|<--------------| client share | |
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| receive | a secret. | |
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| shared secret | | |
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| |<------------>| |
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| |<-------------+------------>|
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| | mon, mds, | |
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| authenticate | and osd | |
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|-------------->| share | |
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|<--------------| a secret | |
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| session key | | |
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| req. ticket | | |
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|-------------->| | |
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|<--------------| | |
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| recv. ticket | | |
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| make request (CephFS only) | |
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|----------------------------->| |
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|<-----------------------------| |
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| receive response (CephFS only) |
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| make request |
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|------------------------------------------->|
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|<-------------------------------------------|
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receive response
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The protection offered by this authentication is between the Ceph client and the
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Ceph server hosts. The authentication is not extended beyond the Ceph client. If
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the user accesses the Ceph client from a remote host, Ceph authentication is not
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applied to the connection between the user's host and the client host.
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Ceph Authorization (caps)
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=========================
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Ceph uses the term "capabilities" (caps) to describe authorizing an
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authenticated user to exercise the functionality of the monitors, OSDs and
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metadata servers. Capabilities can also restrict access to data within one or
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more pools.
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.. note:: Ceph uses the capabilities discussed here for setting up and
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controlling access between various Ceph client and server instances, and
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are relevant regardless of what type of client accesses the Ceph object
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store. CephFS uses a different type of capability for files and directories
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internal to the CephFS filesystem. CephFS filesystem access controls are
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relevant to CephFS, but not block devices or the RESTful gateway.
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A Ceph ``client.admin`` user sets a user's capabilities when creating
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the user.
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``allow``
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:Description: Precedes access settings for a daemon. Implies ``rw`` for MDS only.
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:Example: ``ceph-authtool -n client.foo --cap mds 'allow'``
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``r``
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:Description: Gives the user read access. Required with monitors to retrieve the CRUSH map.
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:Example: ``ceph-authtool -n client.foo --cap mon 'allow r'``
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``w``
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:Description: Gives the user write access to objects.
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:Example: ``ceph-authtool -n client.foo --cap osd 'allow w'``
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``x``
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:Description: Gives the user the capability to call class methods (i.e., both read and write).
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:Example: ``ceph-authtool -n client.foo --cap osd 'allow x'``
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``class-read``
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:Descriptions: Gives the user the capability to call class read methods. Subset of ``x``.
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:Example: ``ceph-authtool -n client.foo --cap osd 'allow class-read'``
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``class-write``
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:Description: Gives the user the capability to call class write methods. Subset of ``x``.
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:Example: ``ceph-authtool -n client.foo --cap osd 'allow class-write'``
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``*``
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:Description: Gives the user read, write and execute permissions for a particular daemon/pool, and the ability to execute admin commands.
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:Example: ``ceph-authtool -n client.foo --cap osd 'allow *'``
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When setting capabilities for a user, Ceph also supports restricting the
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capabilities to a particular pool. This means you can have full access to some
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pools, and restricted (or no) access to other pools for the same user.
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For example::
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ceph-authtool -n client.foo --cap osd 'allow rwx' pool=customer-pool
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Cephx Limitations
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=================
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The ``cephx`` protocol authenticates Ceph clients and servers to each other. It
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is not intended to handle authentication of human users or application programs
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run on their behalf. If that effect is required to handle your access control
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needs, you must have another mechanism, which is likely to be specific to the
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front end used to access the Ceph object store. This other mechanism has the
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role of ensuring that only acceptable users and programs are able to run on the
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machine that Ceph will permit to access its object store.
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The keys used to authenticate Ceph clients and servers are typically stored in
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a plain text file with appropriate permissions in a trusted host.
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.. important:: Storing keys in plaintext files has security shortcomings, but
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they are difficult to avoid, given the basic authentication methods Ceph
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uses in the background. Those setting up Ceph systems should be aware of
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these shortcomings.
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In particular, arbitrary user machines, especially portable machines, should not
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be configured to interact directly with Ceph, since that mode of use would
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require the storage of a plaintext authentication key on an insecure machine.
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Anyone who stole that machine or obtained surreptitious access to it could
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obtain the key that will allow them to authenticate their own machines to Ceph.
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Rather than permitting potentially insecure machines to access a Ceph object
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store directly, users should be required to sign in to a trusted machine in
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your environment using a method that provides sufficient security for your
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purposes. That trusted machine will store the plaintext Ceph keys for the
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human users. A future version of Ceph may address these particular
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authentication issues more fully.
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At the moment, none of the Ceph authentication protocols provide secrecy for
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messages in transit. Thus, an eavesdropper on the wire can hear and understand
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all data sent between clients and servers in Ceph, even if he cannot create or
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alter them. Further, Ceph does not include options to encrypt user data in the
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object store. Users can hand-encrypt and store their own data in the Ceph
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object store, of course, but Ceph provides no features to perform object
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encryption itself. Those storing sensitive data in Ceph should consider
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encrypting their data before providing it to the Ceph system.
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