Mod_auth_kerb (http://modauthkerb.sourceforge.net/) is an Apache module
designed to provide Kerberos authentication to the Apache web server. Using the
Basic Auth mechanism, it retrieves a username/password pair from the browser
and checks them against a Kerberos server as set up by your particular
organization. The module also supports the Negotiate authentication method,
which performs full Kerberos authentication based on ticket exchanges, and does
not require users to insert their passwords to the browser. In order to use the
Negotiate method you need a browser supporting it (currently standard IE6.0 or
Mozilla with the negotiateauth extension (http://negotiateauth.mozdev.org/).

The module supports both kerberos4 and kerberos5 protocols for password
verification. The Negotiate mechanism can be only used with Kerberos v5. The
module supports both 1.x and 2.x versions of Apache.

If you are using the Basic Auth mechanism, the module does not do any special
encryption of any sort. The passing of the username and password is done with
the same Base64 encoding that Basic Auth uses. This can easily be converted to
plain text. To counter this, I would suggest also using mod_ssl or Apache-SSL.
The use of SSL encryption is also recommended if you are using the Negotiate
method.

Installing mod_auth_kerb
------------------------

Prerequisites

    * Development enviroment (i.e. libraries and header files) for Kerberos5
      and/or Kerberos4.
      The module is known to work with the MIT Kerberos implementation
      (supporting both krb4 and krb5), the kth-krb Kerberos4 implementation,
      and the Heimdal Kerberos5 implementation. The Kerberos installation on
      your system should contain the krb4-config and/or krb5-config command(s).
    * Apache server installed.
      Both 1.x and 2.x series of Apache are supported (make sure the apache
      installation contains the apxs command)
    * Working C compiler, GNU make.
    * The latest source of the module available from the main project site
      (http://sourceforge.net/project/showfiles.php?group_id=51775). 

You will also need to have an working Kerberos enviroment, of course.

Building and installing the module

Unpack the distribution tarball and run the configure script. The script looks for krb5 and krb4 libraries and headers and then for an Apache installation directory. You can use following flags to specify locations of these files:

    * --with-krb4=
      --with-krb5=
      these options are used to specify locations of the installation
      directories for krb4 and krb5, respectively. If you don't want to
      compile support for one of the method, use no as the appropriate
      parameter.
    * --with-apache=
      use this parameter to specify location where the Apache installation
      resides. 

After the configuration script finishes run make followed by make install. You
will need to have writing permission for the apache directory in order to
install the module. An example of the building stage follows:

  ./configure --with-krb5=/software/krb5-1.3.1		\
              --with-krb4=no				\
              --with-apache=/software/apache-2.0.47
  make
  su
  make install

After installing the module you have to adapt the apache configuration. See
this page for detailed information on configuration.  You can submit any
comment, questions, bugs etc. via the project page.


Configuration
-------------

Before starting configuring the module make sure your Kerberos enviroment is
properly configured (i.e. KDC, /etc/krb5.conf, etc.). The easiest way to check
is using the kinit command from the apache machine to get a ticket for some
known principal (preferably that one who will be used to test the module).

Now you have to create an service key for the module, which is needed to
perform client authentication. Verification of the kerberos password has two
steps. In the first one the KDC is contacted using the password trying to
receive a ticket for the client. After this ticket is sucessfuly acquired, the
module must also verify that KDC hasn't been deliberately faked and the ticket
just received can be trusted. If this check would haven't been done any
attacker capable of spoofing the KDC could impersonate any principal registered
with the KDC. In order to do this check the apache module must verify that the
KDC knows its service key, which the apache shares with the KDC. This service
key must be created during configuration the module. This service key is also
needed when the Negotiate method is used. In this case the module acts as a
standard kerberos service (similarly to e.g. kerberized ssh or ftp servers).
Default name of the service key is HTTP/<fqdn_of_www_server>@REALM, another
name of the first instance can be set using the KrbServiceName option. The key
must be stored in a keytab on a local disk, the Krb5Keytab and Krb4Srvtab
options are used to specify the filename with the keytab. This file should be
only readable for the apache process and contain only the key used for www
authentication.

In order to get the module loaded on start of apache add following line to your
httpd.conf:

	LoadModule	auth_kerb_module	libexec/mod_auth_kerb.so

Summary of Supported Directives

AuthType type
   For Kerberos authentication to work, AuthType must be set to 'Kerberos'. For
   the reasons of backwards compatibility the values KerberosV4 and KerberosV5
   are also supported. Their use is not recommended though, for finer setting
   use following three options.


KrbMethodNegotiate on | off	(set to on by default)
   To enable or disable the use of the Negotiate method. You need a special
   support on the browser side to support this mechanism.

KrbMethodK5Passwd on | off	(set to on by default)
   To enable or disable the use of password based authentication for Kerberos
   v5.

KrbMethodK4Passwd on | off 	(set to on by default)
   To enable or disable the use of password based authentication for Kerberos
   v4.

KrbAuthoritative on | off	(set to on by default)
   If set to off this directive allow authentication controls to be pass on to
   another modules. Use only if you really know what you are doing.

KrbAuthRealms realm1 [realm2 ... realmN]
   This option takes one or more arguments (separated by spaces), specifying
   the Kerberos realm(s) to be used for authentication. This defaults to the
   default realm taken from the local Kerberos configuration.

KrbVerifyKDC on | off		(set to on by default)
   This option can be used to disable the verification tickets against local
   keytab to prevent KDC spoofing atacks. It should be used only for testing
   purposes. You have been warned.

KrbServiceName service		(set to HTTP by default)
   For specification the service name that will be used by Apache for
   authentication. Corresponding key of this name must be stored in the keytab.

Krb4Srvtab /path/to/srvtab
   This option takes one argument, specifying the path to the Kerberos V4
   srvtab. It will simply use the "default srvtab" from Kerberos V4's
   configuration if this option is not specified. The srvtab must be readable
   for the apache process, and should be different from srvtabs containing keys
   for other services.

Krb5Keytab /path/to/keytab
   This option takes one argument, specifying the location of the Kerberos V5
   keytab file. It will use the "default keytab" from Kerberos V5's config if
   it is not specified here. The keytab file must be readable for the apache
   process, and should be different from other keytabs in the system.

KrbSaveCredentials on | off	(set to off by default)
   This option enables credential saving functionality.

Ticket File/Credential Cache Saving
   Sometimes there is need to keep the ticket file or credential cache around
   after a user authenticates, normally for cgi scripts. If you turn on
   KrbSaveCredentials, the tickets will be retrieved into a ticket file or
   credential cache that will be available for the request handler. The ticket
   file will be removed after request is handled.

   A CGI script can use these files by setting the KRB5CCNAME (v5) or KRBTKFILE
   (v4) environment variables to point to the filename as listed above. A
   sample script to use the KRB5CCNAME is here.

Sample .htaccess

	AuthType Kerberos
	AuthName "Kerberos Login"
	KrbAuthRealms KRB.NCSU.EDU NCSU.EDU
	KrbMethodK4Passwd off
	require user principal1@KRB.NCSU.EDU principal2@NCSU.EDU

Reminder, you need to set the appropriate AllowOverride directive in your
server access configuration so that a different AuthType will be allowed.

$Id$