Apache Module mod_auth_form

Basic Configuration

To protect a particular URL with mod_auth_form, you need to decide where you will store your session, and you will need to decide what method you will use to authenticate. In this simple example, the login details will be stored in a session based on mod_session_cookie, and authentication will be attempted against a file using mod_authn_file. If authentication is unsuccessful, the user will be redirected to the form login page.

AuthFormProvider file
AuthUserFile "conf/passwd"
AuthType form
AuthName realm
AuthFormLoginRequiredLocation "http://example.com/login.html"
Session On
SessionCookieName session path=/
SessionCryptoPassphrase secret

The directive AuthType will enable the mod_auth_form authentication when set to the value form. The directives AuthFormProvider and AuthUserFile specify that usernames and passwords should be checked against the chosen file.

The directives Session, SessionCookieName and SessionCryptoPassphrase create an encrypted session stored within an HTTP cookie on the browser. For more information on the different options for configuring a session, read the documentation for mod_session.

In the simple example above, a URL has been protected by mod_auth_form, but the user has yet to be given an opportunity to enter their username and password. Options for doing so include providing a dedicated standalone login page for this purpose, or for providing the login page inline.

Standalone Login

The login form can be hosted as a standalone page, or can be provided inline on the same page.

When configuring the login as a standalone page, unsuccessful authentication attempts should be redirected to a login form created by the website for this purpose, using the AuthFormLoginRequiredLocation directive. Typically this login page will contain an HTML form, asking the user to provide their usename and password.

<form method="POST" action="/dologin.html">
  Username: <input type="text" name="httpd_username" value="" />
  Password: <input type="password" name="httpd_password" value="" />
  <input type="submit" name="login" value="Login" />
</form>

The part that does the actual login is handled by the form-login-handler. The action of the form should point at this handler, which is configured within Apache httpd as follows:

<Location "/dologin.html">
    SetHandler form-login-handler
    AuthFormLoginRequiredLocation "http://example.com/login.html"
    AuthFormLoginSuccessLocation "http://example.com/success.html"
    AuthFormProvider file
    AuthUserFile "conf/passwd"
    AuthType form
    AuthName realm
    Session On
    SessionCookieName session path=/
    SessionCryptoPassphrase secret
</Location>

The URLs specified by the AuthFormLoginRequiredLocation directive will typically point to a page explaining to the user that their login attempt was unsuccessful, and they should try again. The AuthFormLoginSuccessLocation directive specifies the URL the user should be redirected to upon successful login.

Alternatively, the URL to redirect the user to on success can be embedded within the login form, as in the example below. As a result, the same form-login-handler can be reused for different areas of a website.

<form method="POST" action="/dologin.html">
  Username: <input type="text" name="httpd_username" value="" />
  Password: <input type="password" name="httpd_password" value="" />
  <input type="submit" name="login" value="Login" />
  <input type="hidden" name="httpd_location" value="http://example.com/success.html" />
</form>

Inline Login

As an alternative to having a dedicated login page for a website, it is possible to configure mod_auth_form to authenticate users inline, without being redirected to another page. This allows the state of the current page to be preserved during the login attempt. This can be useful in a situation where a time limited session is in force, and the session times out in the middle of the user request. The user can be re-authenticated in place, and they can continue where they left off.

If a non-authenticated user attempts to access a page protected by mod_auth_form that isn't configured with a AuthFormLoginRequiredLocation directive, a HTTP_UNAUTHORIZED status code is returned to the browser indicating to the user that they are not authorized to view the page.

To configure inline authentication, the administrator overrides the error document returned by the HTTP_UNAUTHORIZED status code with a custom error document containing the login form, as follows:

AuthFormProvider file
ErrorDocument 401 "/login.shtml"
AuthUserFile "conf/passwd"
AuthType form
AuthName realm
AuthFormLoginRequiredLocation "http://example.com/login.html"
Session On
SessionCookieName session path=/
SessionCryptoPassphrase secret

The error document page should contain a login form with an empty action property, as per the example below. This has the effect of submitting the form to the original protected URL, without the page having to know what that URL is.

<form method="POST" action="">
  Username: <input type="text" name="httpd_username" value="" />
  Password: <input type="password" name="httpd_password" value="" />
  <input type="submit" name="login" value="Login" />
</form>

When the end user has filled in their login details, the form will make an HTTP POST request to the original password protected URL. mod_auth_form will intercept this POST request, and if HTML fields are found present for the username and password, the user will be logged in, and the original password protected URL will be returned to the user as a GET request.

Inline Login with Body Preservation

A limitation of the inline login technique described above is that should an HTML form POST have resulted in the request to authenticate or reauthenticate, the contents of the original form posted by the browser will be lost. Depending on the function of the website, this could present significant inconvenience for the end user.

mod_auth_form addresses this by allowing the method and body of the original request to be embedded in the login form. If authentication is successful, the original method and body will be retried by Apache httpd, preserving the state of the original request.

To enable body preservation, add three additional fields to the login form as per the example below.

<form method="POST" action="">
  Username: <input type="text" name="httpd_username" value="" />
  Password: <input type="password" name="httpd_password" value="" />
  <input type="submit" name="login" value="Login" />
  
  <input type="hidden" name="httpd_method" value="POST" />
  <input type="hidden" name="httpd_mimetype" value="application/x-www-form-urlencoded" />
  <input type="hidden" name="httpd_body" value="name1=value1&name2=value2" />

</form>

How the method, mimetype and body of the original request are embedded within the login form will depend on the platform and technology being used within the website.

One option is to use the mod_include module along with the KeptBodySize directive, along with a suitable CGI script to embed the variables in the form.

Another option is to render the login form using a CGI script or other dynamic technology.

        AuthFormProvider file
        ErrorDocument 401 "/cgi-bin/login.cgi"
        ...

Logging Out

To enable a user to log out of a particular session, configure a page to be handled by the form-logout-handler. Any attempt to access this URL will cause the username and password to be removed from the current session, effectively logging the user out.

By setting the AuthFormLogoutLocation directive, a URL can be specified that the browser will be redirected to on successful logout. This URL might explain to the user that they have been logged out, and give the user the option to log in again.

SetHandler form-logout-handler
AuthName realm
AuthFormLogoutLocation "http://example.com/loggedout.html"
Session On
SessionCookieName session path=/
SessionCryptoPassphrase secret

Note that logging a user out does not delete the session; it merely removes the username and password from the session. If this results in an empty session, the net effect will be the removal of that session, but this is not guaranteed. If you want to guarantee the removal of a session, set the SessionMaxAge directive to a small value, like 1 (setting the directive to zero would mean no session age limit).

SetHandler form-logout-handler
AuthFormLogoutLocation "http://example.com/loggedout.html"
Session On
SessionMaxAge 1
SessionCookieName session path=/
SessionCryptoPassphrase secret

Usernames and Passwords

Note that form submission involves URLEncoding the form data: in this case the username and password. You should therefore pick usernames and passwords that avoid characters that are URLencoded in form submission, or you may get unexpected results.