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+Application servers
+===================
+
+If you need to install the Kerberos V5 programs on an application
+server, please refer to the Kerberos V5 Installation Guide.  Once you
+have installed the software, you need to add that host to the Kerberos
+database (see :ref:`add_mod_del_princs`), and generate a keytab for
+that host, that contains the host's key.  You also need to make sure
+the host's clock is within your maximum clock skew of the KDCs.
+
+
+Keytabs
+-------
+
+A keytab is a host's copy of its own keylist, which is analogous to a
+user's password.  An application server that needs to authenticate
+itself to the KDC has to have a keytab that contains its own principal
+and key.  Just as it is important for users to protect their
+passwords, it is equally important for hosts to protect their keytabs.
+You should always store keytab files on local disk, and make them
+readable only by root, and you should never send a keytab file over a
+network in the clear.  Ideally, you should run the :ref:`kadmin(1)`
+command to extract a keytab on the host on which the keytab is to
+reside.
+
+
+.. _add_princ_kt:
+
+Adding principals to keytabs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To generate a keytab, or to add a principal to an existing keytab, use
+the **ktadd** command from kadmin.
+
+.. include:: admin_commands/kadmin_local.rst
+   :start-after:  _ktadd:
+   :end-before: _ktadd_end:
+
+
+Examples
+########
+
+Here is a sample session, using configuration files that enable only
+AES encryption::
+
+    kadmin: ktadd host/daffodil.mit.edu@ATHENA.MIT.EDU
+    Entry for principal host/daffodil.mit.edu with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab
+    Entry for principal host/daffodil.mit.edu with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab
+    kadmin:
+
+
+Removing principals from keytabs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To remove a principal from an existing keytab, use the kadmin
+**ktremove** command.
+
+.. include:: admin_commands/kadmin_local.rst
+   :start-after:  _ktremove:
+   :end-before: _ktremove_end:
+
+
+Clock Skew
+----------
+
+A Kerberos application server host must keep its clock synchronized or
+it will reject authentication requests from clients.  Modern operating
+systems typically provide a facility to maintain the correct time;
+make sure it is enabled.  This is especially important on virtual
+machines, where clocks tend to drift more rapidly than normal machine
+clocks.
+
+The default allowable clock skew is controlled by the **clockskew**
+variable in :ref:`libdefaults`.
+
+
+Getting DNS information correct
+-------------------------------
+
+Several aspects of Kerberos rely on name service.  When a hostname is
+used to name a service, the Kerberos library canonicalizes the
+hostname using forward and reverse name resolution.  (The reverse name
+resolution step can be turned off using the **rdns** variable in
+:ref:`libdefaults`.)  The result of this canonicalization must match
+the principal entry in the host's keytab, or authentication will fail.
+
+Each host's canonical name must be the fully-qualified host name
+(including the domain), and each host's IP address must
+reverse-resolve to the canonical name.
+
+Configuration of hostnames varies by operating system.  On the
+application server itself, canonicalization will typically use the
+``/etc/hosts`` file rather than the DNS.  Ensure that the line for the
+server's hostname is in the following form::
+
+    IP address      fully-qualified hostname        aliases
+
+Here is a sample ``/etc/hosts`` file::
+
+    # this is a comment
+    127.0.0.1      localhost localhost.mit.edu
+    10.0.0.6       daffodil.mit.edu daffodil trillium wake-robin
+
+The output of ``klist -k`` for this example host should look like::
+
+    viola# klist -k
+    Keytab name: /etc/krb5.keytab
+    KVNO Principal
+    ---- ------------------------------------------------------------
+       2 host/daffodil.mit.edu@ATHENA.MIT.EDU
+
+If you were to ssh to this host with a fresh credentials cache (ticket
+file), and then :ref:`klist(1)`, the output should list a service
+principal of ``host/daffodil.mit.edu@ATHENA.MIT.EDU``.
+
+
+.. _conf_firewall:
+
+Configuring your firewall to work with Kerberos V5
+--------------------------------------------------
+
+If you need off-site users to be able to get Kerberos tickets in your
+realm, they must be able to get to your KDC.  This requires either
+that you have a slave KDC outside your firewall, or that you configure
+your firewall to allow UDP requests into at least one of your KDCs, on
+whichever port the KDC is running.  (The default is port 88; other
+ports may be specified in the KDC's :ref:`kdc.conf(5)` file.)
+Similarly, if you need off-site users to be able to change their
+passwords in your realm, they must be able to get to your Kerberos
+admin server on the kpasswd port (which defaults to 464).  If you need
+off-site users to be able to administer your Kerberos realm, they must
+be able to get to your Kerberos admin server on the administrative
+port (which defaults to 749).
+
+If your on-site users inside your firewall will need to get to KDCs in
+other realms, you will also need to configure your firewall to allow
+outgoing TCP and UDP requests to port 88, and to port 464 to allow
+password changes.  If your on-site users inside your firewall will
+need to get to Kerberos admin servers in other realms, you will also
+need to allow outgoing TCP and UDP requests to port 749.
+
+If any of your KDCs are outside your firewall, you will need to allow
+kprop requests to get through to the remote KDC.  :ref:`kprop(8)` uses
+the ``krb5_prop`` service on port 754 (tcp).
+
+The book *UNIX System Security*, by David Curry, is a good starting
+point for learning to configure firewalls.