Apache module mod_intercept_form_submit
=======================================

Apache module to intercept submission of application login forms. It
retrieves the login and password information from the POST HTTP
request, runs PAM authentication with those credentials, and sets the
REMOTE_USER environment variable if the authentication passes. The
internal r->user field is also set so other modules can use it (even
if the module is invoked very late in the request processing). The
lookup_identity_hook from mod_lookup_identity is explicitly called
after the r->user is set.

If the REMOTE_USER is already set (presumably by some previous module),
no authentication takes place.

If the PAM authentication fails, environment variable
EXTERNAL_AUTH_ERROR is set to the string describing the error.

The assumption is that the application will be amended to trust the
REMOTE_USER value if it is set and skip its own login/password
validation (see the docs/ directory for outline of the interaction).

Module configuration
--------------------

Module mod_authnz_pam needs to be installed and loaded with

    LoadModule authnz_pam_module modules/mod_authnz_pam.so

because mod_intercept_form_submit uses it to do the actual PAM operations.

The mod_intercept_form_submit module needs to be configured for Location
that the application uses to process the login form POST requests. The
configuration has to specify three values:

    InterceptFormPAMService name_of_the_PAM_service

      The PAM service to authenticate against.

    InterceptFormLogin the_login_field_name

      Name of the login field in the login form, and thus the login
      parameter in the POST request.

    InterceptFormPassword the_password_field_name

      Name of the password field in the login form, and thus the
      password parameter in the POST request.

All three parameters above need to be specified or the interception
will not be enabled.

Optional parameters:

    InterceptFormLoginSkip one_login [or_more_logins]

      List of logins to ignore (never attempt to authenticate).

      By default authentication will be attempted for all logins.

    InterceptFormClearRemoteUserForSkipped on|off

      When set to on and authentication is skipped for users
      listed with InterceptFormLoginSkip, clears r->user and
      REMOTE_USER.

      Default is off.

    InterceptFormPasswordRedact on|off

      When set to on and authentication is attempted (no matter
      if it passes or fails), the value of the password will
      be modified in the POST data to string [REDACTED].

      Default is off.

Example:

    LoadModule intercept_form_submit_module modules/mod_intercept_form_submit.so
    LoadModule authnz_pam_module modules/mod_authnz_pam.so
    <Location /users/login>
      InterceptFormPAMService http_application_sss
      InterceptFormLogin login[login]
      InterceptFormPassword login[password]
      InterceptFormLoginSkip admin
      InterceptFormClearRemoteUserForSkipped on
      InterceptFormPasswordRedact on
    </Location>

The PAM service needs to be configured. For the above shown
http_application_sss example, file /etc/pam.d/http_application_sss
could be created with content

    auth    required   pam_sss.so
    account required   pam_sss.so

to authenticate against sssd.

On SELinux enabled systems, boolean allow_httpd_mod_auth_pam needs to
be enabled:

    setsebool -P allow_httpd_mod_auth_pam 1

Building from sources
---------------------

When building from sources, command

    apxs -i -a -c mod_intercept_form_submit.c -Wall -pedantic

should build and install the module.

License
-------

Copyright 2013--2014 Jan Pazdziora

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.