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INFORMATION TECHNOLOGY SERVICES

Installing a WebKDC

These are supplemental installation instructions for installing a WebKDC. Each site running WebAuth only needs one WebKDC for the entire site, so normally users do not need to follow these steps. They are here only for site WebAuth administrators.

If you have installed the Debian or Ubuntu packages, please see /usr/share/doc/libapache2-webkdc/README.Debian.gz and /usr/share/doc/webauth-weblogin/README.Debian.gz instead of these instructions. They are more complete and specific for people using those packages.

It is important to always keep in mind that the security of the entire WebAuth system depends on the security of the WebKDC. A compromise of the WebKDC system would allow an attacker to steal usernames and passwords for anyone using the login server and to be able to forge arbitrary authentication credentials and use them to contact WebAuth servers. This system should be locked down as tightly as possible and proactively monitored for security.

We also recommend giving the WebKDC and WebLogin server its own separate domain name. This will generally require giving it a separate IP address unless you can use wildcard SSL/TLS certificates, but it's useful to separate the cookies used by the WebLogin server from cookies used by applications. This is particularly important if you use Active Directory as your KDC and do ticket delegation. Active Directory Kerberos tickets can be fairly large, and the combination of WebKDC cookies, WebAuth proxy cookies, credential cookies, and id cookies can result in a Cookie header that's longer than the HTTP protocol permits.

  1. Follow the standard WebAuth installation instructions as given in the INSTALL file at the top of the source tree, but also pass the flags --enable-webkdc, --disable-mod_webauth, and --disable-mod_webauthldap to ./configure. This will install only the WebKDC module, and it will also install the Perl bindings (into the site_perl directory for the first version of Perl found on your path).

  2. You'll need to install the following Perl modules from CPAN:

         CGI::Application
         CGI::Application::Plugin::AutoRunmode
         CGI::Application::Plugin::Forward
         CGI::Application::Plugin::Redirect
         CGI::Application::Plugin::TT
         libwww (LWP)
         Crypt::SSLeay -or- IO::Socket::SSL (for LWP https support)
         MIME::Base64 (part of Perl itself since 5.7.3)
         Template (Template Toolkit)
         URI
         XML::Parser

    If you want support for replay detection and rate limiting, you will also need:

         Cache::Memcached
         Digest::SHA (part of Perl itself since 5.9.3)

    You'll probably also want to install and configure mod_fastcgi as well, though it's not a requirement. If you decide to install it, you'll need to configure it like so:

        FastCGIConfig -initial-env LD_LIBRARY_PATH
        LoadModule fastcgi_module modules/mod_fastcgi.so
        <Directory "/usr/local/share/weblogin">
            AllowOverride All
            Options Indexes FollowSymlinks +ExecCGI
            AddHandler fastcgi-script .fcgi
            Order allow,deny
            Allow from all
        </Directory>

    This assumes the WebLogin scripts and templates are installed into /usr/local/share/weblogin, which is the default installation location. You can put them anywhere; just adjust the path above and the paths in Apache configuration options below accordingly.

  3. Add Apache directives for the WebKDC. The basic set is:

        LoadModule webkdc_module \
            /usr/local/lib/apache2/modules/mod_webkdc.so
        WebKdcServiceTokenLifetime 30d
        WebKdcKeyring conf/webkdc/keyring
        WebKdcKeytab conf/webkdc/keytab
        WebKdcTokenAcl conf/webkdc/token.acl

    If you are testing a beta release, please also add:

        WebKdcDebug on

    so that your server will generate more complete logs if anything goes wrong. This may also be useful if this is the first time you've installed a WebKdc. You'll also want to set LogLevel to debug:

        LogLevel debug
  4. Set up a script aliases to run the login, logout, and pwchange CGI services by adding the following lines to your Apache configuration:

        ScriptAlias /login "/usr/local/share/weblogin/login.fcgi"
        ScriptAlias /logout "/usr/local/share/weblogin/logout.fcgi"
        ScriptAlias /pwchange "/usr/local/share/weblogin/pwchange.fcgi"
        Alias /images "/usr/local/share/weblogin/generic/images/"
        Alias /help.html \
            "/usr/local/share/weblogin/generic/templates/help.html"

    You may want to add the first three only inside a VirtualHost block that requires SSL to ensure that they are only accessible over SSL. The pwchange service is optional, without it WebLogin will not be able to correctly handle expired passwords and will instead just not allow users with expired passwords to authenticate.

  5. Set up a <VirtualHost> directive using SSL for the WebKDC service. Inside that directive, set a handler for the URL /webkdc-service. In other words, use a configuration block like:

        <VirtualHost _default_:443>
           SSLEngine on
           ...
           <Location /webkdc-service>
              SetHandler webkdc
           </Location>
           ...
        </VirtualHost>
  6. Create the conf/webkdc directory.

        cd <apache-root>
        mkdir -p conf/webkdc
  7. Install the WebKDC files used by the login server, the portion that runs on the regular SSL port. cd to the root of the WebAuth source tree and then run:

        make install

    This will install Perl modules into the site_perl directory of whatever Perl was found by configure.

  8. If you want to support changing expired passwords and you configured a URL for pwchange.fcgi above, create /etc/webkdc/webkdc.conf and put a line like:

        $EXPIRING_PW_URL = "/pwchange";

    in it, where the URL should be the URL you configured to point to the pwchange.fcgi script. It shouldn't include the scheme or hostname since browsers dislike POST to a URL that's fully-qualified for security reasons.

    Also, if the path and URL in <site-perl>/WebKDC/Config.pm are not correct for your site (see docs/weblogin-config for the defaults), you will also need to create an /etc/webkdc/webkdc.conf for that if you haven't already and, in it, put lines like:

        $KEYRING_PATH = "../conf/webkdc/keyring";
        $URL = "https://localhost/webkdc-service/";

    changing the values to what is appropriate for your site. If you follow the default recommendations on where to place the keyring and CGI scripts, you shouldn't have to change anything other than the $EXPIRING_PW_URL setting. There are other settings you can change; see doc/weblogin-config for all the details.

  9. Install a keytab for the WebKDC. This keytab must be for the same identity as is mentioned in the WebAuthWebKdcPrincipal configuration option in the Apache configuration of individual WebAuth servers. The keytab should for the identity service/webkdc, and should be installed in <apache-root>/conf/webkdc/keytab or in whatever path the Apache configuration option WebKdcKeyring references.

    (Note that you can use some other identity if you prefer, as this is part of the WebAuth module configuration. You'll just then have to configure all of your systems using the WebAuth module to use whatever other identity you chose.)

  10. Create a file named token.acl in <apache-root>/conf/webkdc. This file specifies which tokens various server identities are allowed to create. There is an example file in conf/token.acl, so for an initial configuration, do:

        cp conf/token.acl <apache-root>/conf/webkdc/

    uncomment the first line, and change the ACL pattern to match your realm. This will allow any server with a webauth/* keytab to get an id token (which identifies a user but conveys no other privileges). This is a good place to start. See the mod_webkdc manual for more.

  11. If you want to use replay caching of successful login attempts or rate limiting of failed login attempts, set up a memcached server. If you have a single WebLogin server, configure memcached to listen only to localhost. If you have a pool of WebLogin servers, they will need to share a memcached server. Note that memcached doesn't have much in the way of protocol security, so normally it is protected using firewalls or iptables to disallow connections from systems other than the WebLogin servers.

    Then, set the following variable in /etc/webkdc/webkdc.conf:

        @MEMCACHED_SERVERS = ('127.0.0.1:11211');

    This server setting is for a memcached only listening to localhost, on the default port. Change the IP address and port as necessary for your environment.

    Nothing particularly critical or security-sensitive is stored in memcached; the most an attacker can do is remove an account rate limit or create a denial of service attack.

  12. Configure replay rejection of successful login attempts if desired. This requires setting up a memcached server (step 11).

    Replay rejection prevents using the back button in a browser to replay the authentication to WebLogin and is recommended as partial security protection against attackers with access to browser history. To enable it, set the following variable in /etc/webkdc/webkdc.conf:

        $REPLAY_TIMEOUT = 300;

    The value should be set to whatever the WebKdcTokenMaxTTL setting is for your WebKDC, which by default is 300 seconds (five minutes).

  13. Configure rate limiting of failed logins if desired. This requires setting up a memcached server (step 11).

    If configured, WebLogin will lock out an account after the configured number of failed login attempts, rejecting all attempts to authenticate (successful or unsuccessful) until the timeout interval expires. To enable it, set the following variable in /etc/webkdc/webkdc.conf:

        $RATE_LIMIT_THRESHOLD = 5;

    The rate limit setting allows five failed login attempts before locking out a user for the default interval (five minutes). Adjust this setting based on your local requirements. You may also want to change $RATE_LIMIT_INTERVAL to something other than 300 (five minutes in seconds). See docs/weblogin-config for more information.

  14. Restart the Apache web server. You should now have a working WebKDC and login server, which you can point a WebAuth server at. You will need to install a regular WebAuth server following the instructions in INSTALL to be able to test your WebKDC.

If you want to use let clients use SPNEGO to authenticate to the WebLogin front-end with fallback to standard password authentication, see the separate install-spengo documentation.

If you want to enable multifactor authentication, see the separate install-multifactor documentation.

The WebLogin component of the WebKDC, which handles the login user interface, is configured with /etc/webkdc/webkdc.conf by default. However, the path of this configuration file can be overridden by setting the WEBKDC_CONFIG environment variable to point to some other path. Since one of the configuration settings is the path to the templates, this can be used to provide multiple different WebLogin interfaces at different URLs using the same code.

One convenient way to do that inside Apache is to use mod_env and the SetEnv directive. So, for example, after using the above instructions to set up a /login URL that uses your default local templates, you could add an Apache configuration like:

    ScriptAlias /login-special "/usr/local/share/weblogin/login.fcgi"
    <Location "/login-special">
        SetEnv WEBKDC_CONFIG /etc/webkdc/special.conf
    </Location>

Then, users who go to the /login-special URL instead of /login will launch a WebLogin instance that uses /etc/webkdc/special.conf as the configuration instead. That configuration could then point to different templates, different settings, etc.

Be aware that this won't work if you use FastCGI, since the FastCGI process will load a single configuration and use it to process all requests. If you want to do this in combination with FastCGI, you will need to arrange for multiple FastCGI applications to be spawned with different environments.


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    The Board of Trustees of the Leland Stanford Junior University

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Last modified Wednesday, 01-May-2013 12:24:37 PM

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