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/* Part of the SWI-Prolog HTTP package
Author: Jan Wielemaker
E-mail: J.Wielemaker@vu.nl
WWW: http://www.swi-prolog.org
Copyright (c) 2012-2014, University of Amsterdam,
VU University Amsterdam
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in
the documentation and/or other materials provided with the
distribution.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE.
*/
:- module(http_cors,
[ cors_enable/0,
cors_enable/2 % +Requesy, +Options
]).
:- use_module(library(settings)).
:- setting(http:cors, list(atom), [],
'Enable CORS for the listed domains. Use [*] for all domains').
/** <module> Enable CORS: Cross-Origin Resource Sharing
This small module allows for enabling Cross-Origin Resource Sharing
(CORS) for a specific request. Typically, CORS is enabled for API
services that you want to have useable from browser client code that is
loaded from another domain. An example are the LOD and SPARQL services
in ClioPatria.
Because CORS is a security risc (see references), it is disabled by
default. It is enabled through the setting http:cors. The value of this
setting is a list of domains that are allowed to access the service.
Because * is used as a wildcard match, the value [*] allows access from
anywhere.
Services for which CORS is relevant must call cors_enable/0 as part of
the HTTP response, as shown below. Note that cors_enable/0 is a no-op if
the setting http:cors is set to the empty list ([]).
==
my_handler(Request) :-
....,
cors_enable,
reply_json(Response, []).
==
If a site uses a _Preflight_ =OPTIONS= request to find the server's
capabilities and access politics, cors_enable/2 can be used to formulate
an appropriate reply. For example:
==
my_handler(Request) :-
option(method(options), Request), !,
cors_enable(Request,
[ methods([get,post,delete])
]),
format('~n'). % 200 with empty body
==
@see http://en.wikipedia.org/wiki/Cross-site_scripting for understanding
Cross-site scripting.
@see http://www.w3.org/TR/cors/ for understanding CORS
*/
%! cors_enable is det.
%
% Emit the HTTP header =|Access-Control-Allow-Origin|= using
% domains from the setting http:cors. This this setting is []
% (default), nothing is written. This predicate is typically used
% for replying to API HTTP-request (e.g., replies to an AJAX
% request that typically serve JSON or XML).
cors_enable :-
cors_enable_domain,
!.
cors_enable. % CORS not enabled
cors_enable_domain :-
setting(http:cors, List),
List \== [],
!,
format('Access-Control-Allow-Origin: ', []),
write_domains(List),
nl.
write_domains([]).
write_domains([H|T]) :-
write(H),
( T == []
-> true
; write(' '),
write_domains(T)
).
%! cors_enable(+Request, +Options) is det.
%
% CORS reply to a _Preflight_ =OPTIONS= request. Request is the
% HTTP request. Options provides:
%
% - methods(+List)
% List of supported HTTP methods. The default is =GET=, only
% allowing for read requests.
% - headers(+List)
% List of headers the client asks for and we allow. The
% default is to simply echo what has been requested for.
%
% Both methods and headers may use Prolog friendly syntax, e.g.,
% =get= for a method and =content_type= for a header.
%
% @see http://www.html5rocks.com/en/tutorials/cors/
cors_enable(Request, Options) :-
cors_enable_domain,
!,
option(methods(Methods), Options, [get]),
cors_methods(Methods),
( option(headers(ReqHeaders), Options)
-> cors_request_headers(ReqHeaders)
; option(access_control_request_headers(ReqHeader), Request)
-> format('Access-Control-Allow-Headers: ~w~n', [ReqHeader])
; true
).
cors_enable(_, _).
cors_methods([]) :- !.
cors_methods(Methods) :-
format('Access-Control-Allow-Methods: '),
write_methods(Methods),
nl.
write_methods([H|T]) :-
upcase_atom(H, U),
write(U),
( T == []
-> true
; write(', '),
write_methods(T)
).
cors_request_headers([]) :- !.
cors_request_headers(ReqHeaders) :-
phrase(field_names(ReqHeaders), String),
format('Access-Control-Allow-Headers: ~s', String).
field_names([H|T]) -->
http_header:field_name(H),
( {T==[]}
-> ""
; ", ",
field_names(T)
).
|
