docs: more work

[cgi-auth-flexible.git] / caf.pod

# -*- fundamental -*-

=head1 NAME

CGI::Auth::Flexible - web authentication optionally using cookies

=head1 SYNOPSYS

 my $verifier = CGI::Auth::Flexible->new_verifier(setting => value,...);
 my $authreq = $verifier->new_request($cgi);

 # simple applications
 $authreq->check_ok() or return;

 # sophisticated applications
 my $divert_kind = $authreq->check_divert();
 if ($divert_kind) { ... print diversion page and quit ... }

 # while handling the request
 $user = $authreq->get_username();
 $authreq->check_mutate();

=head1 DESCRIPTION

CGI::Auth::Flexible is a library which you can use to add a
forms/cookie-based login facility to a Perl web application.

CGI::Auth::Flexible doesn't interfere with your application's URL path
namespace and just needs a few (configurable) form parameter and
cookie name(s) for its own use.  It tries to avoid making assumptions
about the implementation structure of your application.

Because CGI::Auth::Flexible is licenced under the AGPLv3, you will
probably need to provide a facility to allow users (even ones not
logged in) to download the source code for your web app.  Conveniently
by default CGI::Auth::Flexible provides (for pure Perl webapps) a
mechanism for users to get the source.

CGI::Auth::Flexible is designed to try to stop you accidentally
granting access by misunderstanding the API.  (Also it, of course,
guards against cross-site scripting.)  You do need to make sure to
call CGI::Auth::Flexible before answering AJAX requests as well as
before generating HTML pages, of course, and to call it in every
entrypoint to your system.

=head2 CHECKLIST

As a minimum you need to do all of the things on this checklist, where
applicable.  The items marked SECURITY are the ones that you might
forget: without them your application may appear to work, but will be
insecure.

=over

=item *

Call C<new_verifier> (once at application startup)

=item *

Call C<new_request> (once per request)

=item *

B<SECURITY>: Call C<check_ok> or C<check_divert> on every request, and
honour the return value.

=item *

If you're using C<check_ok>, implement either the
C<username_password_error> or C<login_ok> hook and provide it as
a setting to C<new_verifier>.

=item *

Provide the setting C<dir> (or provide absolute paths for all the
other relevant settings).

=item *

Call C<get_username> when you need to know who's logged in.

=item *

B<SECURITY>: Call C<check_mutate> or C<mutate_ok>, if you specified
C<promise_check_mutate>.

=item *

B<SECURITY>: Call C<check_nonpage> for every request which is not a page load
(if your application has any of those).

=item *

When generating URLs and forms (including AJAX requests), include the
hidden form parameter using C<secret_hidden_val> or
C<secret_hidden_html> when appropriate (see below).

=item *

B<SECURITY>: If you do not override the source provision facility (see
L</SOURCE CODE DOWNLOAD>), check that the assumptions it makes aren't
going to leak security-critical data.

=item *

Set up HTTPS on your webserver, or set the C<encrypted_only> setting
to a false value.

=back

These points will now be covered in more detail.

=head2 INITIALISATION

Your application should, on startup (eg, when it is loaded by
mod_perl) do
C<< $verifier = CGI::Auth::Flexible->new_verifier(settings...) >>.
This call can be expensive and is best amortised.

The resulting verifier object can be used to process individual
requests, in each case with
C<< $authreq = CGI::Auth::Flexible->new_request($cgi) >>.

See L</SETTINGS>.

=head2 CHECKING AND RESPONSE GENERATION

If the user is logged in, your application is to handle the request.
Otherwise, the user needs to be presented with a login form or error
message, as appropriate.  CGI::Auth::Flexible provides two alternative
interfaces for this:

=head3 Simple applications

The simplist usage is to call C<< $request->check_ok() >> which will
check the user's authentication.  If the user is not logged in it will
generate a login form (or redirection or other appropriate page) and
return false; your application should not then processing that request
any further.  If the user is logged in it will return true.

Various hooks are provided to customise the responses generated by
C<check_ok>.

After C<check_ok> returns true you should go ahead and process the
request; you can use C<< $request->get_username >> to find out which
user the request came from.

=head2 Sophisticated applications

If you want to handle the control flow and to generate login forms,
redirections, etc., yourself, you can say
C<< $divert = $request->check_divert >>.  This returns undef if
the user is logged in, or I<divert spec> if some kind of login
page or diversion should be generated.  See L</DIVERT SPEC> below for
details of how to deal with the return value.

=head2 MUTATING OPERATIONS AND EXTERNAL LINKS

=head3 Mutation-ignorant applications

By default CGI::Auth::Flexible does not permit external links into
your site.  All GET requests give a "click to continue" page which
submits a form which loads your app's main page.  In this
configuration all your application's forms and AJAX requests should
use C<POST>.  This restriction arises from complicated deficiencies
in the web's security architecture.

Such applications are also not able to provide user-specific CSS
stylesheets, javascript, favicons, etc.

=head3 Mutation-aware applications

The alternative is for your application to always make a special check
when the incoming request is going to do some kind of action (such as
modifying the user's setup, purchasing goods, or whatever) rather than
just retrieve and/or display information.  We term such requests
"mutating" requests.

Then non-mutating pages can be linked to from other, untrustworthy,
websites.

To support external links, and C<GET> requests, pass
C<< promise_check_mutate => 1 >> in I<settings>, and then call
C<< $authreq->check_mutate() >> before taking any actions.  If the
incoming request is not suitable then C<< $authreq->check_mutate() >>
will call C<die>.

There have to be no mutating C<GET> requests in your application (but
you shouldn't have any of those anyway); if there are, they won't
work.  (CGI::Auth::Flexible will spot them and cause them to fail,
rather than allow them to be insecure.)

=head2 GENERATING URLS, FORMS AND AJAX QUERIES

When you generate a URL, C<POST> form or AJAX request you may need to
include a secret hidden form parameter for the benefit of
CGI::Auth::Generic.  This form parameter will be checked by
C<check_ok>/C<check_divert> and should be ignored by your application.

By default the hidden parameter is called C<caf_assochash>.

After calling C<check_ok> or C<check_divert> the value to put in your
form can be obtained from C<secret_hidden_val>; C<secret_hidden_html>
will generate the whole HTML C<< <input...> >> element.

=head3 Mutation-ignorant applications

For mutation-ignorant applications (see above), all forms etc. should
include the hidden parameter (and as discussed, they must all use
POST rather than GET).

=head3 Mutation-aware applications

For mutation-aware applications, whether to include the secret
parameter depends on the kind of request.  CGI::Auth::Flexible knows
when it is necessary.  You should find out by calling
C<need_add_hidden>.

If it is inconvenient to call C<need_add_hidden> at runtime, you can
rely instead on the following promises:  All POST requests (which
includes all mutating requests) need the parameter.  The return value
of need_add_hidden depends only on the $method and $reqtype
parameters, so you can query it once and remember the answer.
HTML page load GETs do not need the parameter.  It is better to
err on the side of including the parameter.

If you really must, you can call C<need_add_hidden> "on the bench"
during development and bake the answer into your application code
structure.  However, if you do that and a new vulnerability was
discovered which is fixed by changing the answer, updating
CGI::Auth::Flexible wouldn't be sufficient to fix it.

=head3 Mutation-aware applications - non-page requests

If your mutation-aware application supports non-page resources (AJAX
and JSON requests, stylesheets, favicons, etc.) it must inform
CGI::Auth::Flexible when it is handling such a request, by calling
C<check_nonpage>.

Normally C<check_nonpage> will simply return (and you can ignore the
return value).  However, if there is an attack (or, perhaps, a bug) it
will die, stopping the attack.

(You do not need to call C<check_nonpage> for POST requests, but it is
harmless to do so.)

=head3 Mutation-aware applications - novel kinds of request

If you want to support a kind of request perhaps not yet known about
by CGI::Auth::Flexible, you can provide information about that new
request kind using C<update_get_need_add_hidden>.

=head2 DATA STORAGE

CGI::Auth::Flexible needs to store various information in plain files;
it does this in the directory specified by the C<dir> parameter.

=head1 SOURCE CODE DOWNLOAD

By default, CGI::Auth::Flexible provides a facility for users to
download the source code for the running version of your web
application.

This facility makes a number of important assumptions which you need
to check.  Note that if the provided facility is not sufficient
because your application is more sophisticated than it copes with (or
if you disable the builtin facility), you may need to implement a
functioning alternative to avoid violating the AGPLv3 licence.

Here are the most important (default) assumptions:

=over

=item *

Your app's source code is available by looking at @INC, $0 and
S<$ENV{'SCRIPT_FILENAME'}> (the B<source items>).  See
C<srcdump_listitems>.  Where these point to files or directories under
revision control, the source item is the whole containing vcs tree.

=item *

Specifically, there are no compiled or autogenerated Perl
files, Javascript resources, etc., which are not contained in one of
the source item directories.  (Files which came with your operating
system install don't need to be shipped as they fall under the system
library exception.)

=item *

You have not installed any modified versions of system
libraries (including system-supplied Perl modules) in C</usr> outside
C</usr/local>.  See C<srcdump_system_dir>.

=item *

For each source item in a dvcs, the entire dvcs history does
not contain anything confidential (or libellous).  Also, all files which
contain secrets are in the dvcs's I<.ignore> file.  See
C<srcdump_vcsscript_git> et al.

=item *

For each source item NOT in a dvcs, there are no confidential
files with the world-readable bit set (being in a world-inaccessible
directory is not sufficient).  See C<srcdump_excludes>.

=item *

You have none of your app's source code in C</etc>.

=item *

You don't regard pathnames on your server as secret.

=item *

You don't intentionally load Perl code by virtue of C<.>
being in C<@INC> by default.  (See C<srcdump_filter_cwd>.)

=back

=head1 MAIN FUNCTIONS AND METHODS

=over

=item C<< CGI::Auth::Flexible->new_verifier(setting => value, ...) >>

Initialises an instance and returns a verifier object.
The arguments are setting pairs like a hash initialiser.
See L</SETTINGS> below.

=item C<< $verifier->new_request($cgi) >>

Prepares to process a request.  I<$cgi> should normally
be the query object from L<CGI(3perl)>.  Most of the default
hook methods assume that it is; however if you replace enough of
the hook methods then you can pass any value you like and it
will be passed to your hooks.

The return value is the authentication request object (I<$authreq>)
which is used to check the incoming request and will contain
information about its credentials.

=item C<< $authreq->check_divert() >>

Checks whether the user is logged in.  Returns undef if the user is
logged in and we should service the request.  Otherwise returns a
divert spec (see L</DIVERT SPEC>) saying what should happen instead.

This method may die if it doesn't like the request, in which case
the request needs to be rejected.

=item C<< $authreq->check_ok() >>

Checks whether the user is logged in.  Returns true if the user is
logged in and we should service the request.

Otherwise it handles the request itself, generating any appropriate
redirect, login form, or continuation page.  It then returns false and
the application should not process the request further.

=item C<< $verifier->disconnect() >>

Discards the resources (open files, etc.) in the verifier object.

=back

=head REQUEST-RELATED FUNCTIONS AND METHODS

All of these are only valid after C<check_divert> or C<check_ok> has
been called.  (In the case of C<check_ok> it won't normally be sensible
to call these functions unless C<check_ok> returned true.)

=item C<< $authreq->get_divert() >>

Returns the value previously returned by C<check_divert>.

=item C<< $authreq->get_username() >>

Returns the name of the logged-in user.  If the user was not logged
in (or their session had timed out, or something), returns undef.

=item C<< $authreq->check_mutate() >>

Declares to CGI::Auth::Generic that the request being handled will
"mutate".  That is, it will modify some server-side state (eg, adding
items to shopping baskets, posting messages to blogs, sending emails,
or whatever).

If you have set the setting C<promise_check_mutate> you must call
C<check_mutate> whenever appropriate.  If you haven't then it's
irrelevant.  See L<MUTATING OPERATIONS AND EXTERNAL LINKS>.

C<check_mutate> will either return successfully, indicating that all
is well and the request should proceed, or it will die.  If it dies
that means that the request was improper, which can only result from a
bug or an attack.  So an "internal server error" is a suitable
response.

=item C<< $authreq->check_nonpage($method, $reqtype) >>

Declares to CGI::Auth::Generic that the request is not a page request,
but rather a request of type I<$reqtype>.

If your application has set the setting C<promise_check_mutate>,
whenever it is handling anything except an HTML page loads, it must
call this function.  See L</REQUEST TYPES>, and
L<GENERATING URLS, FORMS AND AJAX QUERIES>.

C<check_mutate> will either return successfully, indicating that all
is well and the request should proceed, or it will die, like
C<check_mutate>.

=head RESPONSE-RELATED FUNCTIONS AND METHODS

=item C<< $authreq->url_with_query_params($params, [$nonpagetype]) >>

Convenience function which returns a url for a GET request to this
application.

I<$params> is a hashref specifying the parameters and the PATH_INFO.
The keys are the parameter names, and the values are array refs with
the parameter value(s) (as strings, as yet unquoted).  (They are array
refs because it is possible to pass multiple values for the same
parameter in a single request; normally each arrayref would be a
singleton.)

The request path will be the path to the application.  If a parameter
with name C<< '' >> is supplied, it is taken as the PATH_INFO - its
value will be appended to the application path.  (It should normally
start with C<< / >>, and only one value should be supplied.)

=item C<< something->need_add_hidden($method, $reqtype) >>

Enquires whether a request of type I<$reqtype> using HTTP method
I<$method> needs the hidden form parameter.  See L</REQUEST TYPES>.

=item C<< something->secret_hidden_val() >>

Returns the value of the hidden form parameter.  This should be
included in all POST requests to your application (and thus be a
hidden form parameter in all forms).

It should also be in some (maybe all) GET requests.  If your
application is mutation-ignorant, it should be in all GET requests.
If you are mutation-aware, you need to consult C<need_add_hidden>.

The name of the hidden parameter is the setting C<assoc_param_name>,
C<caf_hassochash> by default.  xxx rename param and setting

=item C<< something->secret_hidden_html() >>

Returns the HTML for an C<INPUT> element specifying the hidden form
parameter.

=item C<< something->secret_cookie_val() >>

Returns the value of the secret cookie.  CGI::Auth::Flexible sets this
cookie in the forms generated by C<check_ok>.  You may also set it
yourself (and indeed you must do so if you use C<check_divert>).

=item C<< $authreq->chain_params() >>

Returns a hash of the "relevant" parameters to this request, in a form
used by XXX.  This is all of the query parameters which are not
related to CGI::Auth::Flexible.  The PATH_INFO from the request is
returned as the parameter C<< '' >>.

xxx why use this function

=back

=head OTHER FUNCTIONS AND METHODS

=over

=item C<< $verifier_or_authreq->hash($data) >>

Hashes the supplied data using the hash function specified by the
C<hash_algorithm> setting, and converts the result to a string of hex
digits.

=item C<< something->update_get_need_add_hidden($reqtype, $value, [$force]) >>

Updates CGI::Auth::Generic's knowledge about the various kinds of
request, and whether they need the hidden form parameter.  This
function applies only to GET requests - POST requests always use the
parameter.

I<$reqtype> is the request type (the value which will be passed to
C<check_nonpage> and C<need_add_hidden>.  If you are supporting a new
I<$reqtype> you shouuld coordinate with CGI::Auth::Flexible upstrea,
or other users, to assign a unique request type name.

This method may be called on an authreq or a verifier, in which case
it will affect all authreqs using the same verifier.  Or it may be
called on the class as a whole, in which case it will affect the
global default list for all verifiers.

If I<$force> is supplied and true, this will override
CGI::Auth::Flexible's existing knowledge.  Otherwise this new setting
will be ignored if CGI::Auth::Flexible already knows about the request
type.  (When called on a verifier or authreq, it will ignore the
update in favour of existing knowledge recorded both globally in the
class or due to previous requests on the same verifier.)

See L</REQUEST TYPES>.

=item C<< CGI::Auth::Flexible::srcdump_dir_cpio($cgi,$verifier,$dumpdir,$dir,$outfn,$how,$script) >>

Helper function for implementing the C<srcdump_process_item> hook.
Generates a tarball using cpio and includes it in the prepared source
code distribution.

The arguments are mostly the same as for that hook.  C<$dir> is the
root directory at which to start the archive.  C<$how> is a short text
string which will be mentioned in the log.

C<$script> is a shell script fragment which must output a
nul-separated list of filenames (e.g. the output of C<find -print0>).
It is textually surrounded by C<( )> and will be executed with C<set -e>
in force.  Its cwd will be C<$dir>.

=item C<< $verifier_or_authreq->($data) | CGI::Auth::Flexible-> >>

Hashes the supplied data using the hash function specified by the
C<hash_algorithm> setting, and converts the result to a string of hex
digits.

=back

=head1 SETTINGS

C<new_verifier> and C<new_request> each take a list of settings, as
a list of pairs C<< key => value >> (like a Perl hash assignment).

The settings supplied to C<new_verifier> are stored in the verifier
and will apply to all authreqs made from it unless overridden in the
call to C<new_request>

When a setting is described as a hook function, it should be a
coderef.  The first argument will be the query object from
L<CGI(3perl)> (strictly, it will be whatever value was passed to
C<new_request>).  The second argument will be the authreq object (the
return value from C<new_request>).
Ie, C<< sub some_hook ($$...) { my ($cgi,$authreq,@stuff) = @_ ... >>

In bullet point headings, the hook functions are shown in the form
C<< some_hook($cgi,$authreq,@stuff) >> even though this would not be
legal syntax.  This should be read to mean that the
%implicit_settings_hash{'some_hook'}($cgi,$authreq,@stuff)
would be a legal call.  (However, the settings hash is not exposed.)

When a hook's default implementation is mentioned and named, that
function won't also be described in the section on the module's
functions.

=over

=head2 GENERAL SETTINGS

=item C<dir>

The directory CGI::Auth::Generic should use for its data storage.
This is actually just a default absolute path used when the other
path settings are relative values.

Must be an absolute filename.

=item C<assocdb_dbh>

CGI::Auth::Flexible needs a database for recording users' login
session.  This database needs to be shared across all instances of the
web application, so in a multi-node cluster it needs to be your actual
database.

CGI::Auth::Flexible will create the table and index it needs if they
don't already exist, and will manage their contents.  You do not need
to integrate them into the rest of your webapp's data storage.  (In
particular, there is no need for transactional integrity across
changes made by CAF and your own application.)

By default, CAF uses a sqlite3 database stored on local disk in the
file named by C<assocdb_path>.  This will be suitable for all
applications which run on a single host.

This value, if supplied, should be a DBI handle for the database.

=item C<assocdb_dsn>

This is the DSN to pass to C<< DBI->connect >>.  Used only if
C<assocdb_dbh> is not supplied.

=item C<assocdb_path>

Path to the sqlite3 database used for CAF's session storage.  The
default is currently C<caf-assocs.db> but will change in the future.

Used only if neither C<assocdb_dbh> or C<assocdb_dsn> are supplied.

If this is a relative path, it is in C<dir>.

=item C<assocdb_table>

Prefix for the SQL tables and indices to use (and to create, if
necessary).

See L</DATABASE TABLES>.

=item C<keys_path>

Path to the keys file used by CAF.  This arrangement will change in
the future.  See L</BUGS>.

=item C<random_source>

Special file to read random numbers from.  Should return
cryptographically secure (pseudo)-random bytes, unpredictable to
adversaries (even ones on the same machine).

On Linux, there is no device which is properly suitable.  This is a
bug in Linux.  You can use C</dev/random> which can block
unnecessarily even though the kernel PRNG has been properly seeded and
is fine, or C</dev/urandom> which might return values which attackers
can predict if the kernel PRNG has not been properly seeded.

The default is C</dev/urandom>.

=item C<secretbits>

Length of the assoc secret.  Defaults to 128.

=item C<hash_algorithm>

Must be a string suitable for use with C<new Digest>.
Defaults to C<SHA-256>.

=item C<login_timeout>

A user will be logged out this many seconds after they first logged
in.  Default: 86400 (one day).

=item C<login_form_timeout>

A login form becomes invalid this many seconds after it has been sent.
Default: 3600 seconds (one hour).

=item C<key_rollover>

The key used for generating assoc secrets is rolled over approximately
this often (in seconds).  Default: 86400.

=item C<assoc_param_name>

Name of the hidden form parameter.  Default: C<caf_assochash>.

=item C<cookie_name>

Name of the cookie used for login sessions.  Default:
C<caf_assocsecret>.

=item C<password_param_name>

Name of the password field in the login form.  Default: C<password>.

Used by C<login_ok_password> (the default C<login_ok> hook),
C<gen_plain_login_form> and the default C<is_login> hook.

=item C<username_param_names>

Arrayref of name(s) of username form parameters.

The first entry is used by C<login_ok_password> (the default
C<login_ok> hook) to pass to the C<username_password_error> hook and
used as the username if all is well.

All the entries are used by C<gen_plain_login_fork> (the default
C<gen_login_form> hook for C<check_ok>) to generate form entry fields.

The default is C<['username']>.

=item C<logout_param_names>

Arrayref of name(s) of form parameters indicating that the request is
a logout request.

Used by the default C<is_logout> hook.

If you want users to be able to explicitly log out, you need to
provide a logout button, something like
C<< <input type="submit" name="caf_logout" ...>>

The default is C<['caf_logout']>

=item C<logged_param_names>

Arrayref of name(s) of form parameters indicating that user has just
logged out.  (During the logout process, the actual logout action is a
POST request, whose response redirects to the "you have been logged
out" page; these form parameters are for this second page.)

Used by the default C<is_loggedout> hook.

The first entry is used by C<check_ok> to generate the redirection.

The default is C<['caf_loggedout']>

=item C<promise_check_mutate>

Boolean.  If true, is a declaration by the application that it is
mutatin-aware.  See L</MUTATING OPERATIONS AND EXTERNAL LINKS>.

The default is 0.

=item C<encrypted_only>

Boolean.  If true, CAF will insist that all transactions be done over
an encrypted http connection.  It will redirect unencrypted requests
to the https instance of the applicattion, and will set the encrypted
only flag on its cookie.

The default is 1.

=back

=item C<< get_url($cgi,$authreq) >>

Hook which returns the URL of this web application.  By default, we
call C<< $cgi->url() >> for each request, but you can fix this if you
prefer.

=item C<< is_login,is_logout,is_loggedout($cgi,$authreq) >>

Hook which returns a boolean indicating whether the request was,
respectively: a login form submission (ie, username and password); a
logout request (submission resulting from the user pressing the
"logout" button); "logged out" page (redirection from the logout
POST).

The default is to check whether any of the corresponding request
parameters (C<< login_param_names >> etc.) was supplied, using the
C<get_param> hook.

=back

=head2 SETTINGS (HOOKS) RELATED TO THE CGI REQUEST OBJECT

=over

=item C<< get_param($cgi,$authreq,$param) >>

Returns the value of a single-valued form parameter.
The default is to call C<< $cgi->param($param) >>.
The semantics are the same as that of C<CGI::param>.

=item C<< get_params($cgi,$authreq) >>

Returns a hash of the parameters.  The return value is a hashref whose
keys are the parameter names and whose values are arrayrefs, one entry
in the arrayref for each value.

The default is to call C<< $cgi->Vars() >>, expect the
results to look like those from C<CGI::Vars>, and massage them into
the required form with split.

=item C<< get_path_info($cgi,$authreq) >>

Returns the PATH_INFO of the request.  The default is to
call C<< $cgi->path_info() >>.

=item C<< get_cookie($cgi,$authreq) >>

Returns the value of the CAF cookie sent with the request, or undef if
none was supplied.  The default is to call C<<
$cgi->cookie($cookie_name) >> (where C<$cookie_name> is from the
setting of the same name).  The return value should be the scalar
value of the cookie.

=item C<< get_method($cgi,$authreq) >>

Returns the HTTP method as a string.  The default is to call
C<< $cgi->request_method() >>.

=back

=item C<< is_https($cgi,$authreq) >>

Returns a boolean indicating whether the request was over an encrypted
channel.  The default is C<< !!$cgi->https() >>.  See C<encrypted_only>.

=back

=head2 SETTINGS RELATED TO HTML GENERATION

These are only used if you call C<check_ok> or xxx some other functions?.

Settings whose names are of the form C<gen_...> are hooks which each
return an array of strings, normally HTML strings, for use by
C<check_ok> (or, in turn, other hooks, or your application).  These
are often documented simply by showing the output produced.  In many
cases parts of the output are in turn obtained from other hooks.  In
some cases the default implementations have been given names for
convenient use by your application.  They will be called in array
context.

We'll write C<gettext(something)> even though actually there is a hook
to control the translation function used.

=over

=item C<handle_divert>($cgi,$authreq,$divert))

C<check_ok> calls this hook before producing output of its own.  If
you want to handle some but not all diversions yourself, you may set
this hook.  The hook should either do nothing and return false, or
return true if it has handled the request (or arrange for the request
to be handled).  If the hook returns true then C<check_ok> simply
returns 0.

=item C<gen_login_form>($cgi,$authreq,$divert))

Default: a table (used mostly for layout) containing input fields for
a login form.  Must be within a C<< <form> >> element, but doesn't
generate it.  Has text fields for every entry in
C<username_param_names> (in each case associated with a description
C<< gettext(ucfirst $parameter_name) >>, a password field (with
description C<gettext("Password")>, and a login submit button (with
description C<gettext("Login")>.

Default is available as the module function C<gen_plain_login_form>.

=item C<gen_login_link>($cgi,$authreq))

Default:

 <a href="http:...">gettext(Log in again to continue.)</a>

Default is available as the module function C<gen_plain_login_link>.

=item C<gen_postmainpage_form>($cgi,$authreq,$params))

Default: form contents (but not the C<< <form> >> element):

C<$params> (in the form returned by the C<get_params> hook) as hidden
fields, and also:

 <input type="submit" ... value=getext('Continue')>

Default is available as the module function C<gen_postmainpage_form>.

=item C<gen_start_html>($cgi,$authreq,$title)

Default: C<$cgi->start_html($title)>

=item C<gen_end_html>($cgi,$authreq,$title)

Default: C<$cgi->end_html($title)>

=item C<gen_footer_html>($cgi,$authreq)>

Default:

 <hr><address>
 Powered by Free / Libre / Open Source Software
 according to the [gen_licence_link_html].
 [gen_source_link_html].
 </address>

Default is available as the module function C<gen_plain_footer_html>.

=item C<gen_licence_link_html>($cgi,$authreq)>

Default: uses C<url_with_query_params> to generate a URL for
downloading the licence, and returns:
  <a href="...">GNU Affero GPL</a>

Default is available as the module function C<gen_plain_licence_link_html>.

=item C<gen_source_link_html>($cgi,$authreq)>

Default: uses C<url_with_query_params> to generate a URL for
downloading the source, and returns:
  <a href="...">Source available</a>

Default is available as the module function C<gen_plain_source_link_html>.

=item C<form_entry_size>

Size of generated text entry fields.  Default is 60.

=item C<dummy_param_name_prefix>

Some of CAF's HTML-generating functions need to invent form parameter
names.  They will all start with this string.  Default: C<caf__>.

=head2 SETTINGS FOR SOURCE CODE DOWNLOAD FACILITY

=over

=item C<srcdump_param_name>

Form parameter name used to indicate that this is a source download
request.  If this parameter is supplied, C<check_ok> and
C<check_divert> will arrange for the applicaton source code to be
delivered as the response (in C<check_ok>'s case by doing it itself
and in C<check_divert>'s case by asking your application to do so.

Default is C<caf_srcdump>.

=item C<srcdump_path>

Path to the directory used for storing pre-prepared source downloads.
Defaults to C<caf-srcdump>.

If this is a relative path, it is in C<dir>.

=item C<srcdump_dump($cgi,$authreq,$srcobj)>

Dump the source code (C<$srcobj='source'> or licence data
(C<$srcobj='licence'>).  The default implementation checks that
C<$srcobj> has reasonable syntax and uses the files C<$srcobj.data>
and C<$srcobj.ctype> with the C<dump> hook.

=item C<dump($cgi,$authreq,$contenttype,$datafilehandle)>

Responds to the request by sending the contents of $datafilehandle
(which should just have been opened) and specifying a content type of
$contenttype.

The default implmentation uses the C<print> hook, and also calls
C<$cgi->header('-type' => $contenttype>, and is available as the
module function C<dump_plain>.

=item C<srcdump_prepare($cgi,$verifier)>

Prepares the source code for download when requested.  Invoked by
C<new_verifier>, always, immediately before it returns the
just-created verifier object.

The default implementation is the module function
C<srcdump_dirscan_prepare>, which prepares a manifest, licence file
and source code tarball of tarballs, as follows:

It processes each entry in the return value from C<srcdump_listitems>.
These are the software's include directories and any other directories
containing source code.  It handles C<.> specially (see
C<srcdump_filter_cwd>).

For each entry it looks, relative to that, for the licence as a file
with a name mentioned in C<srcdump_licence_files>.  The first such
file found is considered to be the licence.  It then calls the hook
C<srcdump_process_item> for the entry.

The licence, a manifest file, and all the outputs generated by the
calls to C<srcdump_process_item>, are tarred up and compressed as a
single source tarball.

It uses the directory named by C<srcdump_path> as its directory for
working and output files.  It uses the filename patterns
C<generate.*>, C<licence.*>, C<s.[a-z][a-z][a-z].*>, C<manifest.*>,
C<source.*> in that directory.

=item C<srcdump_process_item>($cgi,$verifier,$dumpdir,$item,\&outfn,\$needlicence,\%dirsdone)>

Processes a single include directory or software entry, so as to
include the source code found there.  Called only by the default
implementation of C<srcdump_prepare>.

C<$dumpdir> is the directory for working and output files.  C<$item>
is the real (no symlinks) absolute path to the item.

C<\$needlicence> is a ref to a scalar: this scalar is undef if we have
already found the licence file; otherwise it is the filename to which
the licence should be copied.  If the referent is undef on entry,
C<srcdump_process_item> needs to see if it finds the licence; if it
does it should copy it to the named file and then set the scalar to
undef.

C<\%dirsdone> is a ref to the hash used by C<srcdump_prepare> to avoid
including a single directory more than once.  If
C<srcdump_process_item> decides to process a directory other than
C<$item> it should check this hash with the real absolute path of the
other directoy as a key: if the hash entry is true, it has already
been done and should be skipped; otherwise the hash entry should be set.

C<\&outfn> is a coderef which C<srcdump_process_item> should call each
time it wants to generate a file which should be included as part of
the source code.  It should be called using one of these patterns:
   $outfn->("message for log");
   $outfile = $outfn->("message for log", "extension");
The former simply logs this message (along with the associated
C<$item>, so there is no need to mention that).  The latter logs the
message but also generates and returns a filename which should then
be created and filled with some appropriate data.  C<"extension">
should be a string for the file extension, eg C<"txt">.  The output
can be written directly to the named file: there is no need to
write to a temporary file and rename.

The default implementation is the module function
C<srcdump_process_item>.

It skips directories for which C<srcdump_system_dir> returns true.

It then searches the item and its parent
directories for a vcs metadata directory (one of the names in
C<srcdump_vcs_dirs>); if found, it calls the C<srcdump_byvcs> hook
(after checking and updaeing C<%dirsdone>).
Otherwise it calls the C<srcdump_novcs> hook.

=item C<srcdump_novcs($cgi,$verifier,$dumpdir,$item,$outfn)>

Called by the default implementation of C<srcdump_process_item>, with
the same arguments, if it doesn't find vcs metadata.

The default implementation is the module function C<srcdump_novcs>.

If C<$item> is a directory, it uses C<srcdump_dir_cpio> to prepare a
tarball of all the files under C<$item> which have the world read bit
set.  Directories are not included (and their permissions are
disregarded).  The contents of C<srcdump_excludes> are excluded.

If it's a plain file it uses C<srcdump_file> to include the file.

=item C<srcdump_byvcs($cgi,$verifier,$dumpdir,$item,$outfn,$vcs)>

Called by the default implementation of C<srcdump_process_item>, with
the same arguments, if it finds vcs metadata.  The additional argument
C<$vcs> is derived from the entry of C<srcump_vcs_dirs> which was
used: it's the first sequence of word characters, lowercased.

The default implementation is the module function C<srcdump_byvcs>.
It simply calls C<srcdump_dir_cpio> with a script from the setting
C<srcdump_vcsscript>.

=item C<srcdump_vcs_dirs>

Array ref of leaf names of vcs metadata directories.  Used by the
default implementation of C<srcdump_process_item>.  The default value
is C<['.git','.hg','.bzr','.svn']>.

=item C<srcdump_vcs_script>

Hash ref of scripts for generating vcs metadata.  Used by the default
implementation of C<srcdump_byvcs>.  The keys are values of C<$vcs>
(see C<srcdump_byvcs>); the values are scripts as for
C<srcdump_dir_cpio>.

The default has an entry only for C<git>:
  git ls-files -z
  git ls-files -z --others --exclude-from=.gitignore
  find .git -print0

=item C<srcdump_excludes>

Array ref of exclude glob patterns, used by the default implementation
of C<srcdump_novcs>.  The default value is C<['*~','*.bak','*.tmp','#*#']>.

Entries must not contain C<'> or C<\>.

=item C<srcdump_listitems($cgi,$verifier)>

Returns an array of directories which might contain source code of the
web application and which should be therefore be considered for
including in the source code delivery.

Used by the default implementation of C<srcdump_prepare>.

Entries must be directories, plain files, or nonexistent; they may
also be symlinks which resolve to one of those.

If C<.> is included it may be treated specially - see
C<srcdump_filter_cwd>.

The default implementation returns 
C<(@INC, $ENV{'SCRIPT_FILENAME'}, $0)>.

=item C<srcdump_system_dir($cgi,$verifier,$dir)>

Determines whether C<$dir> is a "system directory", in which any
source code used by the application should nevertheless not be
included in the source code dump.

Used by the default implementation of C<srcdump_item>.

The default implementation is as follows: Things in C</etc/> are
system directories.  Things in C</usr/> are too, unless they are in
C</usr/local/> or C</usr/lib/cgi*>.

=item C<srcdump_filter_cwd>

Boolean which controls the handling of C<.> if it appears in the
return value from C<srcdump_listitems>.  Used only by the default
implementation of C<srcdump_prepare>.

If set to false, C<.> is treated normally and no special action is
taken.

However often the current directory may be C</>, or a data directory,
or some other directory containing data which is confidential, or
should not be included in the public source code distribution for
other reasons.  And for historical reasons Perl has C<@INC> containing
C<.> by default (which is arguably dangerous and wrong).

So the default this setting is true, which has the following effects:

C<.> is not searched for source code even if it appears in C<@INC>.
C<.> is removed from C<@INC> and C<%INC> is checked to see if any
modules appear to have already been loaded by virtue of C<.> appearing
in C<@INC> and if they have it is treated as a fatal error.

Only the literal string C<.> is affected.  If the cwd is included by
any other name it is not treated specially regardless of this setting.

=back

=head1 DATABASE TABLES

In a simple application, you do not need to worry about this.  But if
your application runs on multiple frontend hosts with a shared
database, you may need to create for yourself the tables and indices
used by CGI::Auth::Flexible.

xxx document _db_setup_do
xxx make _db_setup_do explicitly overrideable


xxx divert spec
xxx reqtype
xxx settings
xxx html generators
xxx document cookie

xxx bugs wrong default random on Linux
xxx bugs wrong default random on *BSD
xxx bugs keys not shared should be in db
xxx rename caf-assocs.db
xxx rename caf_assocsecret default cookie name
xxx mention relationship between login_timeout and cookies