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INFORMATION TECHNOLOGY SERVICES

WebAuth SPNEGO Installation

Introduction

The WebLogin front-end for user authentication can optionally support SPNEGO for browsers that support it, with fallback to the standard username/password authentication. SPNEGO is the Microsoft protocol for doing GSSAPI authentication (or NTLMv2 authentication, but that's less interesting) over HTTP. It is supported by, at least, IE on PC, Firefox on all platforms, and Safari on Mac, and probably by other browsers.

Before trying this configuration, set up a basic WebKDC and WebLogin server as described in install-webkdc and make sure that's working first.

These instructions assume that you're using mod_auth_kerb to do the SPNEGO negotiation. Something similar will almost certainly work with mod_gssapi, but I've not tested it myself and the exact name of the configuration options will probably require changes.

Configuration

To use SPNEGO, you need to have two login URLs on your WebLogin server rather than just one (traditionally /login). The following instructions assume that you are using /login as the URL to try SPNEGO first and /login-simple for the password fallback. This also allows clients that don't want to use SPNEGO to point their WebAuthLoginURL directly to the /login-simple version.

Set up an additional ScriptAlias for /login-simple that points to the login.fcgi script. So, for instance, if you currently have:

    ScriptAlias /login "webkdc/login.fcgi"

also add:

    ScriptAlias /login-simple "webkdc/login.fcgi"

Then, add the following Apache configuration block to protect the regular /login URL with SPNEGO:

    <Location "/login">
        AuthType Kerberos
        require valid-user
        KrbMethodNegotiate on
        KrbMethodK5Passwd off
        Krb5Keytab /etc/apache2/keytab
        ErrorDocument 401 /login-simple
    </Location>

Obviously, change the keytab path to something that makes sense on your system.

Create a principal in Kerberos called HTTP/weblogin.example.com, where weblogin.example.com is the name of the system that will be used in the WebAuthLoginURL directives for your WebAuth application servers. Store the key for that principal in the keytab configured above. Some browsers use the name in the URL to construct the principal name and others use the reverse DNS lookup of the IP address to which they're connecting. You may want to patch mod_auth_kerb to support the KrbServiceName Any option and then put both principals in the keytab (the Debian libapache2-mod-auth-kerb package already has this patch applied). The HTTP must be in all caps. For information on how to get a Kerberos principal created and how to download a keytab, contact your local Kerberos administrator.

Finally, edit /etc/webkdc/webkdc.conf or wherever your webkdc.conf file is located (creating it if it doesn't already exist) and put, in it:

    $REMUSER_ENABLED = 1;
    $REMUSER_EXPIRES = 60 * 60 * 8;

The first line tells the WebLogin script to trust REMOTE_USER as set by Apache if present (so make sure that there isn't a way of accessing the script via an Apache authentication mechanism that you don't trust!). The second line says how good authentication via REMOTE_USER should be valid for (see Notes and Cautions below).

By default, whatever appears in REMOTE_USER will be expressed as the authenticated identity, and any user allowed to authenticate by your Kerberos administrative policies (such as cross-realm trust) will be able to authenticate to WebLogin. To change that behavior, you can use the following settings:

    @REMUSER_PERMITTED_REALMS = ("EXAMPLE.COM");
    @REMUSER_LOCAL_REALMS = ("EXAMPLE.COM");

The first setting lists the realms that are permitted to authenticate. If you are happy to allow any realm with which you have cross-realm trust, you can omit this. This is roughly equivalent to the WebKdcPermittedRealms setting in mod_webkdc.

The second setting lists the realms that will be stripped off of the authenticated identity before being passed to WebAuth Application Servers. Depending on local practice, this may be set to your local default realm or left unset. This is roughly equivalent to the WebKdcLocalRealms setting in mod_webkdc.

You will also need to set $KEYRING_PATH if your WebKDC keyring is not located in ../conf/webkdc/keyring relative to your server root. For example, if you are using the Debian or Ubuntu packages, you will need:

    $KEYRING_PATH = "/var/lib/webkdc/keyring";

Now, when the user hits /login with a browser, the server will attempt to do an SPNEGO authentication. If the browser supports it, that authentication information will be passed on to the WebLogin script in REMOTE_USER and it will cobble up a temporary webkdc-proxy token for the WebKDC. If the remote system did not request a Kerberos authenticator or any additional credentials and only needs simple authentication, this will bypass the login prompt. If it does need additional information, the user will get the login screen anyway.

If the SPNEGO authentication fails, /login-simple will be run as the error handler. It understands how to extract the WebAuth information out of the environment and present the regular login page. The user's browser will see the login page as the content of a 401 error, but the browser should handle this correctly. Further interactions will continue on the /login-simple URL, bypassing SPNEGO.

Configuration with Delegation

The above uses only Kerberos authentication. You can also optionally support ticket delegation for browsers that support delegation over SPNEGO. (Normally, this requires additional browser-specific security configuration to enable.) To enable that support, add:

    KrbSaveCredentials on

to the configuration for /login described above and add:

    $WEBKDC_PRINCIPAL = "service/webkdc";

to /etc/webkdc/webkdc.conf or wherever your webkdc.conf file is located, where the value is whatever principal you use for your WebKDC (the same value that you use for WebAuthWebKdcPrincipal). You will also need to configure REMOTE_USER support as described above. Now, if the browser delegates credentials, those credentials will be used to create a proxy token for the WebKDC and the WebKDC will be able to support requests for additional credentials without needing to prompt the user for their password.

If you enable this support, make sure that KRB5CCNAME is not set in the environment of your Apache process except by mod_auth_kerb, since the WebLogin script uses that environment variable to locate the ticket cache to use.

Notes and Cautions

If the user is successfully authenticated with SPNEGO, the WebLogin server will not set a single sign-on cookie the way that it normally does with password authentication. Instead, the next time that the user is redirected to the WebLogin server, the SPNEGO authentication will be repeated. That way, their general WebAuth authentication won't outlast their Kerberos tickets.

However, there is currently no way of extracting from mod_auth_kerb the expiration date of the ticket used to authenticate, so the unless the user delegates credentials, the authentication tokens created by the WebKDC will be valid for whatever period is defined in $REMUSER_EXPIRES in webkdc.conf even if the user's Kerberos credentials are about to expire. This may be improved later by adding a way to export the expiration time of the ticket from mod_auth_kerb to somewhere the WebLogin script can see it.

Implementation Details

Under the hood, if the WebLogin script sees $REMUSER_ENABLED is set and finds REMOTE_USER in the environment, it checks to see if KRB5CCNAME is set. If it is, and $WEBKDC_PRINCIPAL is set in the configuration file, it uses the provided ticket cache to request a proxy token from the WebKDC. That proxy token is then added to the WebKDC login request as if it came from a browser's cookies.

If KRB5CCNAME or $WEBKDC_PRINCIPAL is not set, there are no delegated credentials. In this case, the WebLogin script creates a webkdc-proxy token using the WebKDC keyring, with a proxy subject of "WEBKDC:remuser". It then passes that webkdc-proxy token in to the WebKDC in an initial request and sees if it gets back the desired tokens based on the strength of that webkdc-proxy token. If it does, it proceeds to the confirmation screen; if not, it presents the login form as normal.

The special prefix "WEBKDC:" in a webkdc-proxy token is used to represent single sign-on tokens, so this is a webkdc-proxy token of type remuser instead of the normal krb5. Since it does not contain a TGT or any other useful credential, it cannot be used to fulfill requests for credentials or id tokens with krb5 authenticators; servers that request such things will force a username/password authentication if the user doesn't have a single sign-on cookie.

This webkdc-proxy token is never stored in a cookie, to force reauthentication with SPNEGO on each visit.

License

Copyright 2006, 2008, 2009

The Board of Trustees of the Leland Stanford Junior University

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved. This file is offered as-is, without any warranty.

Last modified Wednesday, 01-May-2013 12:24:36 PM

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