Corsica v1.1.0 Corsica
Plug-based swiss-army knife for CORS requests.
Corsica provides facilities for dealing with CORS requests and responses. It provides:
- low-level functions that let you decide when and where to deal with CORS requests and CORS response headers;
- a plug that handles CORS requests and responds to preflight requests;
- a router that can be used in your modules in order to turn them into CORS handlers which provide fine control over dealing with CORS requests.
How it works
Corsica is compliant with the W3C CORS specification. As per this specification, Corsica doesn’t put any CORS response headers in a connection that holds an invalid CORS request. To know what “invalid” CORS request means, have a look at the “Validity of CORS requests” section below.
When some options that are not mandatory and have no default value (such
:max_age
) are not passed to Corsica (in one of the available ways to pass
options to it), the relative header will often not be sent at all. This is
compliant with the specification and at the same time it reduces the size of
the response, even if just by a handful of bytes.
The following is a list of all the CORS response headers supported by Corsica:
Access-Control-Allow-Origin
Access-Control-Allow-Methods
Access-Control-Allow-Headers
Access-Control-Allow-Credentials
Access-Control-Expose-Headers
Access-Control-Max-Age
Using Corsica as a plug
When Corsica
is used as a plug, it intercepts all requests; it only sets a
bunch of CORS headers for regular CORS requests, but it responds (with a 200 OK
and the appropriate headers) to preflight requests.
If you want to use Corsica
as a plug, be sure to plug it in your plug
pipeline before any router: routers like Plug.Router
(or
Phoenix.Router
) respond to HTTP verbs as well as request urls, so if
Corsica
is plugged after a router then preflight requests (which are
OPTIONS
requests) will often result in 404 errors since no route responds to
them.
defmodule MyApp.Endpoint do
plug Head
plug Corsica, max_age: 600, origins: "*", expose_headers: ~w(X-Foo)
plug MyApp.Router
end
Using Corsica as a router generator
When Corsica
is used as a plug, it doesn’t provide control over which urls
are CORS-enabled or with which options. In order to do that, you can use
Corsica.Router
. See the documentation for Corsica.Router
for more
information.
Origins
Allowed origins can be specified by passing the :origins
options either when
using a Corsica-based router or when plugging Corsica
in a plug pipeline.
:origins
can be a single value or a list of values. "*"
can only appear as
a single value. The default value is "*"
. The origin of a request
(specified by the "origin"
request header) will be considered a valid origin
if it “matches” at least one of the origins specified in :origins
. What
“matches” means depends on the type of origin. Origins can be:
- strings - the actual origin and the allowed origin have to be identical
- regexes - the actual origin has to match the allowed regex (as per
Regex.match?/2
) {module, function}
tuples -module.function
is called with the actual origin as its only argument; if it returnstrue
the origin is accepted, if it returnsfalse
the origin is not accepted
For example:
# Matches everything
plug Corsica, origins: "*"
# Matches one of the given origins
plug Corsica, origins: ["http://foo.com", "http://bar.com"]
# Matches the given regex
plug Corsica, origins: ~r{^https?://(.*.?)foo.com$}
The value of the “access-control-allow-origin” header
The :origins
option directly influences the value of the
access-control-allow-origin
response header. When :origins
is "*"
, the
access-control-allow-origin
header is set to *
as well. If the request’s
origin is allowed and :origins
is something different than "*"
, then you
won’t see that value as the value of the access-control-allow-origin
header:
the value of this header will be the request’s origin (which is mirrored).
This behaviour is intentional: it’s compliant with the W3C CORS specification
and at the same time it provides the advantage of “hiding” all the allowed
origins from the client (which only sees its origin as an allowed origin).
The “vary” header
If :origins
is a list with more than one value and the request origin
matches, then a Vary: Origin
header is added to the response.
Options
Besides :origins
, the options that can be passed to the use
macro, to
Corsica.Router.resource/2
and to the Corsica
plug (along with their default
values) are:
:allow_methods
- a list of HTTP methods (as binaries) or:all
. This is the list of methods allowed in theaccess-control-request-method
header of preflight requests. If the method requested by the preflight request is in this list or is a simple method (HEAD
,GET
, orPOST
), then that method is always allowed. The methods specified by this option are returned in theaccess-control-allow-methods
response header. Defaults to["PUT", "PATCH", "DELETE"]
(which means these methods are allowed alongside simple methods). If the value of this option is:all
, all request methods are allowed and only the method inaccess-control-request-method
is returned as the value of theaccess-control-allow-methods
header.:allow_headers
- a list of headers (as binaries) or:all
. This is the list of headers allowed in theaccess-control-request-headers
header of preflight requests. If a header requested by the preflight request is in this list or is a simple header (Accept
,Accept-Language
, orContent-Language
), then that header is always allowed. The headers specified by this option are returned in theaccess-control-allow-headers
response header. Defaults to[]
(which means only the simple headers are allowed). If the value of this option is:all
, all request headers are allowed and only the headers inaccess-control-request-headers
are returned as the value of theaccess-control-allow-headers
header.:allow_credentials
- a boolean. Iftrue
, sends theaccess-control-allow-credentials
with valuetrue
. Iffalse
, prevents that header from being sent at all. If:origins
is set to"*"
and:allow_credentials
is set totrue
, then the value of theaccess-control-allow-origin
header will always be the value of theorigin
request header (as per the W3C CORS specification) and not*
. Defaults tofalse
.:expose_headers
- a list of headers (as binaries). Sets the value of theaccess-control-expose-headers
response header. This option does not have a default value; if it’s not provided, theaccess-control-expose-headers
header is not sent at all.:max_age
- an integer or a binary. Sets the value of theaccess-control-max-age
header used with preflight requests. This option does not have a default value; if it’s not provided, theaccess-control-max-age
header is not sent at all.:log
- see the “Logging” section below. Defaults tofalse
.
Responding to preflight requests
When the request is a preflight request and a valid one (valid origin, valid
request method, and valid request headers), Corsica directly sends a response
to that request instead of just adding headers to the connection (so that a
possible plug pipeline can continue). To do this, Corsica halts the
connection (through Plug.Conn.halt/1
) and sends a response.
Validity of CORS requests
“Invalid CORS request” can mean that a request doesn’t have an Origin
header
(so it’s not a CORS request at all) or that it’s a CORS request but:
- the
Origin
request header doesn’t match any of the allowed origins - the request is a preflight request but it requests to use a method or
some headers that are not allowed (via the
Access-Control-Request-Method
andAccess-Control-Request-Headers
headers)
Logging
Corsica supports basic logging functionalities; it can log whether a CORS request is a valid one, what CORS headers are added to a response and similar information. Corsica distinguishes between three “types” of logs:
- “rejected” logs, for when the request is “rejected” in the CORS perspective, e.g., it’s not allowed
- “invalid” logs, for when the request is not a simple CORS request or not a CORS preflight request
- “accepted” logs, for when the request is a valid and accepted CORS request
It’s possible to configure these logs with the :log
option, which is a
keyword list with the :rejected
, :invalid
, and :accepted
options. These
options specify the logging level of each type of log. The defaults values for
each level are:
rejected: :warn
invalid: :debug
accepted: :debug
false
can be used as the value of a level for a log type to suppress that
type completely. false
can also be used as the value of the :log
option
directly to suppress all logs.
The default value for the :log
option is false
.
For example:
plug Corsica, log: [rejected: :error]
plug Corsica, log: false
plug Corsica, log: [rejected: :info, accepted: false]
Summary
Functions
Callback implementation for Plug.call/2
Checks whether a given connection holds a CORS request
Callback implementation for Plug.init/1
Checks whether a given connection holds a preflight CORS request
Adds CORS response headers to a preflight request to conn
Adds CORS response headers to a simple CORS request to conn
Sends a CORS preflight response regardless of the request being a valid CORS request or not
Functions
Callback implementation for Plug.call/2
.
Checks whether a given connection holds a CORS request.
This function doesn’t check if the CORS request is a valid CORS request: it
just checks that it’s a CORS request, that is, it has an Origin
request
header.
Callback implementation for Plug.init/1
.
Checks whether a given connection holds a preflight CORS request.
This function doesn’t check that the preflight request is a valid CORS
request: it just checks that it’s a preflight request. A request is considered
to be a CORS preflight request if and only if its request method is OPTIONS
and it has a Access-Control-Request-Method
request header.
Note that if a request is a valid preflight request, that makes it a valid
CORS request as well. You can thus call just preflight_req?/1
instead of
preflight_req?/1
and cors_req?/1
.
Adds CORS response headers to a preflight request to conn
.
This function assumes nothing about conn
. If conn
holds an invalid CORS
request or an invalid preflight request, then conn
is returned unchanged;
the absence of CORS headers will be interpreted as an invalid CORS response by
the browser (according to the W3C spec).
If the request is a valid CORS request, the following headers will be added to the response:
Access-Control-Allow-Origin
Access-Control-Allow-Methods
Access-Control-Allow-Headers
and the following headers will optionally be added (based on the value of the corresponding options):
Access-Control-Allow-Credentials
(if the:allow_credentials
option istrue
)Access-Control-Max-Age
(if the:max_age
option is present)
Options
This function accepts the same options accepted by the Corsica
plug
(described in the documentation for the Corsica
module), including :log
for logging.
Examples
put_cors_preflight_resp_headers conn, [
max_age: 86400,
allow_headers: ~w(X-Header),
origins: ~r/w+.foo.com$/
]
Adds CORS response headers to a simple CORS request to conn
.
This function assumes nothing about conn
. If conn
holds an invalid CORS
request or a request whose origin is not allowed, conn
is returned
unchanged; the absence of CORS headers will be interpreted as an invalid CORS
response by the browser (according to the W3C spec).
If the CORS request is valid, the following response headers are set:
Access-Control-Allow-Origin
and the following headers are optionally set (if the corresponding option is present):
Access-Control-Expose-Headers
(if the:expose_headers
option is present)Access-Control-Allow-Credentials
(if the:allow_credentials
option istrue
)
Options
This function accepts the same options accepted by the Corsica
plug
(described in the documentation for the Corsica
module), including :log
for logging.
Examples
conn
|> put_cors_simple_resp_headers(origins: "*", allow_credentials: true)
|> send_resp(200, "Hello!")
Sends a CORS preflight response regardless of the request being a valid CORS request or not.
This function assumes nothing about conn
. If it’s a valid CORS preflight
request with an allowed origin, CORS headers are set by calling
put_cors_preflight_resp_headers/2
and the response is sent with status
status
and body body
. conn
is halted before being sent.
The response is always sent because if the request is not a valid CORS request, then no CORS headers will be added to the response. This behaviour will be interpreted by the browser as a non-allowed preflight request, as expected.
For more information on what headers are sent with the response if the
preflight request is valid, look at the documentation for
put_cors_preflight_resp_headers/2
.
Options
This function accepts the same options accepted by the Corsica
plug
(described in the documentation for the Corsica
module), including :log
for logging.
Examples
This function could be used to manually build a plug that responds to preflight requests. For example:
defmodule MyRouter do
use Plug.Router
plug :match
plug :dispatch
options "/foo",
do: Corsica.send_preflight_resp(conn, origins: "*")
get "/foo",
do: send_resp(conn, 200, "ok")
end