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. Building and installing the module ---------------------------------- see INSTALL 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 server_principal Specifies a principal name to use by Apache when authenticating the clients. By default value of the form HTTP/<FQDN_of_apache>@<realm> is used. The FQDN part can contain any hostname and can be used to work around problems with misconfigured DNS. A corresponding key of this name must be stored in the keytab. If this option is set to 'Any', then any prinicpal from the keytab which matches the client's request may be used. 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. KrbDelegateBasic on | off (set to off by default) If set to 'on' this options causes that Basic authentication is always offered regardless setting the KrbMethodK[45]Pass directives. Then, if a Basic authentication header arrives authentication decision is passed along to another modules. This option is a work-around for insufficient authentication scheme in Apache (Apache 2.1 seems to provide better support for multiple various authentication mechanisms). Note on server principals ------------------------- 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. 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. Constrained Delegation ---------------------- S4U2Proxy, or constrained delegation, enables a service to use a client's ticket to itself to request another ticket for delegation. The KDC checks krbAllowedToDelegateTo to decide if it will issue a new ticket. If KrbConstrainedDelegation is enabled the server will use its own credentials to retrieve a delegated ticket for the user. For this to work the user must have a forwardable ticket (though the delegation flag need not be set). The server needs a valid credentials cache for this to work. The module itself will obtain and manage the necessary credentials. $Id: README,v 1.12 2008/09/17 14:01:55 baalberith Exp $
