I am building a web API. I found whenever I use Chrome to POST, GET to my API, there is always an OPTIONS request sent before the real request, which is quite annoying. Currently, I get the server to ignore any OPTIONS requests. Now my question is what's good to send an OPTIONS request to double the server's load? Is there any way to completely stop the browser from sending OPTIONS requests?
edit 2018-09-13: added some precisions about this pre-flight request and how to avoid it at the end of this reponse.
OPTIONS requests are what we call pre-flight requests in Cross-origin resource sharing (CORS).
They are necessary when you're making requests across different origins in specific situations.
This pre-flight request is made by some browsers as a safety measure to ensure that the request being done is trusted by the server.
Meaning the server understands that the method, origin and headers being sent on the request are safe to act upon.
Your server should not ignore but handle these requests whenever you're attempting to do cross origin requests.
A good resource can be found here http://enable-cors.org/
A way to handle these to get comfortable is to ensure that for any path with OPTIONS method the server sends a response with this header
Access-Control-Allow-Origin: *
This will tell the browser that the server is willing to answer requests from any origin.
For more information on how to add CORS support to your server see the following flowchart
http://www.html5rocks.com/static/images/cors_server_flowchart.png
edit 2018-09-13
CORS OPTIONS request is triggered only in somes cases, as explained in MDN docs:
Some requests don’t trigger a CORS preflight. Those are called “simple requests” in this article, though the Fetch spec (which defines CORS) doesn’t use that term. A request that doesn’t trigger a CORS preflight—a so-called “simple request”—is one that meets all the following conditions:
The only allowed methods are:
GET
HEAD
POST
Apart from the headers set automatically by the user agent (for example, Connection, User-Agent, or any of the other headers with names defined in the Fetch spec as a “forbidden header name”), the only headers which are allowed to be manually set are those which the Fetch spec defines as being a “CORS-safelisted request-header”, which are:
Accept
Accept-Language
Content-Language
Content-Type (but note the additional requirements below)
DPR
Downlink
Save-Data
Viewport-Width
Width
The only allowed values for the Content-Type header are:
application/x-www-form-urlencoded
multipart/form-data
text/plain
No event listeners are registered on any XMLHttpRequestUpload object used in the request; these are accessed using the XMLHttpRequest.upload property.
No ReadableStream object is used in the request.
Have gone through this issue, below is my conclusion to this issue and my solution.
According to the CORS strategy (highly recommend you read about it) You can't just force the browser to stop sending OPTIONS request if it thinks it needs to.
There are two ways you can work around it:
Make sure your request is a "simple request"
Set Access-Control-Max-Age for the OPTIONS request
Simple request
A simple cross-site request is one that meets all the following conditions:
The only allowed methods are:
GET
HEAD
POST
Apart from the headers set automatically by the user agent (e.g. Connection, User-Agent, etc.), the only headers which are allowed to be manually set are:
Accept
Accept-Language
Content-Language
Content-Type
The only allowed values for the Content-Type header are:
application/x-www-form-urlencoded
multipart/form-data
text/plain
A simple request will not cause a pre-flight OPTIONS request.
Set a cache for the OPTIONS check
You can set a Access-Control-Max-Age for the OPTIONS request, so that it will not check the permission again until it is expired.
Access-Control-Max-Age gives the value in seconds for how long the response to the preflight request can be cached for without sending another preflight request.
Limitation Noted
For Chrome, the maximum seconds for Access-Control-Max-Age is 600 which is 10 minutes, according to chrome source code
Access-Control-Max-Age only works for one resource every time, for example, GET requests with same URL path but different queries will be treated as different resources. So the request to the second resource will still trigger a preflight request.
Please refer this answer on the actual need for pre-flighted OPTIONS request: CORS - What is the motivation behind introducing preflight requests?
To disable the OPTIONS request, below conditions must be satisfied for ajax request:
Request does not set custom HTTP headers like 'application/xml' or 'application/json' etc
The request method has to be one of GET, HEAD or POST. If POST, content type should be one of application/x-www-form-urlencoded, multipart/form-data, or text/plain
Reference:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
When you have the debug console open and the Disable Cache option turned on, preflight requests will always be sent (i.e. before each and every request). if you don't disable the cache, a pre-flight request will be sent only once (per server)
Yes it's possible to avoid options request. Options request is a preflight request when you send (post) any data to another domain. It's a browser security issue. But we can use another technology: iframe transport layer. I strongly recommend you forget about any CORS configuration and use readymade solution and it will work anywhere.
Take a look here:
https://github.com/jpillora/xdomain
And working example:
http://jpillora.com/xdomain/
For a developer who understands the reason it exists but needs to access an API that doesn't handle OPTIONS calls without auth, I need a temporary answer so I can develop locally until the API owner adds proper SPA CORS support or I get a proxy API up and running.
I found you can disable CORS in Safari and Chrome on a Mac.
Disable same origin policy in Chrome
Chrome: Quit Chrome, open an terminal and paste this command: open /Applications/Google\ Chrome.app --args --disable-web-security --user-data-dir
Safari: Disabling same-origin policy in Safari
If you want to disable the same-origin policy on Safari (I have 9.1.1), then you only need to enable the developer menu, and select "Disable Cross-Origin Restrictions" from the develop menu.
As mentioned in previous posts already, OPTIONS requests are there for a reason. If you have an issue with large response times from your server (e.g. overseas connection) you can also have your browser cache the preflight requests.
Have your server reply with the Access-Control-Max-Age header and for requests that go to the same endpoint the preflight request will have been cached and not occur anymore.
I have solved this problem like.
if($_SERVER['REQUEST_METHOD'] == 'OPTIONS' && ENV == 'devel') {
header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Headers: X-Requested-With');
header("HTTP/1.1 200 OK");
die();
}
It is only for development. With this I am waiting 9ms and 500ms and not 8s and 500ms. I can do that because production JS app will be on the same machine as production so there will be no OPTIONS but development is my local.
You can't but you could avoid CORS using JSONP.
After spending a whole day and a half trying to work through a similar problem I found it had to do with IIS.
My Web API project was set up as follows:
// WebApiConfig.cs
public static void Register(HttpConfiguration config)
{
var cors = new EnableCorsAttribute("*", "*", "*");
config.EnableCors(cors);
//...
}
I did not have CORS specific config options in the web.config > system.webServer node like I have seen in so many posts
No CORS specific code in the global.asax or in the controller as a decorator
The problem was the app pool settings.
The managed pipeline mode was set to classic (changed it to integrated) and the Identity was set to Network Service (changed it to ApplicationPoolIdentity)
Changing those settings (and refreshing the app pool) fixed it for me.
OPTIONS request is a feature of web browsers, so it's not easy to disable it. But I found a way to redirect it away with proxy. It's useful in case that the service endpoint just cannot handle CORS/OPTIONS yet, maybe still under development, or mal-configured.
Steps:
Setup a reverse proxy for such requests with tools of choice (nginx, YARP, ...)
Create an endpoint just to handle the OPTIONS request. It might be easier to create a normal empty endpoint, and make sure it handles CORS well.
Configure two sets of rules for the proxy. One is to route all OPTIONS requests to the dummy endpoint above. Another to route all other requests to actual endpoint in question.
Update the web site to use proxy instead.
Basically this approach is to cheat browser that OPTIONS request works. Considering CORS is not to enhance security, but to relax the same-origin policy, I hope this trick could work for a while. :)
you can also use a API Manager (like Open Sources Gravitee.io) to prevent CORS issues between frontend app and backend services by manipulating headers in preflight.
Header used in response to a preflight request to indicate which HTTP headers can be used when making the actual request :
content-type
access-control-allow-header
authorization
x-requested-with
and specify the "allow-origin" = localhost:4200 for example
One solution I have used in the past - lets say your site is on mydomain.com, and you need to make an ajax request to foreigndomain.com
Configure an IIS rewrite from your domain to the foreign domain - e.g.
<rewrite>
<rules>
<rule name="ForeignRewrite" stopProcessing="true">
<match url="^api/v1/(.*)$" />
<action type="Rewrite" url="https://foreigndomain.com/{R:1}" />
</rule>
</rules>
</rewrite>
on your mydomain.com site - you can then make a same origin request, and there's no need for any options request :)
It can be solved in case of use of a proxy that intercept the request and write the appropriate headers.
In the particular case of Varnish these would be the rules:
if (req.http.host == "CUSTOM_URL" ) {
set resp.http.Access-Control-Allow-Origin = "*";
if (req.method == "OPTIONS") {
set resp.http.Access-Control-Max-Age = "1728000";
set resp.http.Access-Control-Allow-Methods = "GET, POST, PUT, DELETE, PATCH, OPTIONS";
set resp.http.Access-Control-Allow-Headers = "Authorization,Content-Type,Accept,Origin,User-Agent,DNT,Cache-Control,X-Mx-ReqToken,Keep-Alive,X-Requested-With,If-Modified-Since";
set resp.http.Content-Length = "0";
set resp.http.Content-Type = "text/plain charset=UTF-8";
set resp.status = 204;
}
}
What worked for me was to import "github.com/gorilla/handlers" and then use it this way:
router := mux.NewRouter()
router.HandleFunc("/config", getConfig).Methods("GET")
router.HandleFunc("/config/emcServer", createEmcServers).Methods("POST")
headersOk := handlers.AllowedHeaders([]string{"X-Requested-With", "Content-Type"})
originsOk := handlers.AllowedOrigins([]string{"*"})
methodsOk := handlers.AllowedMethods([]string{"GET", "HEAD", "POST", "PUT", "OPTIONS"})
log.Fatal(http.ListenAndServe(":" + webServicePort, handlers.CORS(originsOk, headersOk, methodsOk)(router)))
As soon as I executed an Ajax POST request and attaching JSON data to it, Chrome would always add the Content-Type header which was not in my previous AllowedHeaders config.
I want to call web service from another domain using angular2 typescript.
i am using following code to call api
var req_dict = {
method: RequestMethod.Get,
url: 'http//127.0.0.1:5000/users/',
headers: this.head
}
var options = new RequestOptions(req_dict);
var req = new Request(options);
this.http.request(req).subscribe({});
I hosted my application in http//127.0.0.1:8000. I want to use api from port 5000. when i inspected console , request is going to
http://127.0.0.1:8000/http//127.0.0.1:5000/users/
i want to call just http//127.0.0.1:5000/users/. how can call api by absolute url in angular2 ?
In fact, it's not an issue of TypeScript or Angular2 but a CORS issue. These blog posts could help you to understand in details how this works:
http://restlet.com/blog/2015/12/15/understanding-and-using-cors/
http://restlet.com/blog/2016/09/27/how-to-fix-cors-problems/
To be short, in the case of cross domain request, the browser automatically adds an Origin header in the request. There are two cases:
Simple requests. This use case applies if we use HTTP GET, HEAD and POST methods. In the case of POST methods, only content types with the following values are supported: text/plain, application/x-www-form-urlencoded and multipart/form-data.
Preflighted requests. When the "simple requests" use case doesn't apply, a first request (with the HTTP OPTIONS method) is made to check what can be done in the context of cross-domain requests.
So in fact most of work must be done on the server side to return the CORS headers. The main one is the Access-Control-Allow-Origin one.
200 OK HTTP/1.1
(...)
Access-Control-Allow-Origin: *
To debug such issues, you can use developer tools within browsers (Network tab).
Regarding Angular2, simply use the Http object like any other requests (same domain for example):
return this.http.get('https://angular2.apispark.net/v1/companies/')
.map(res => res.json()).subscribe(
...
);
Hope it helps you,
Thierry
Background: I'm running Go on GAE and using Mux for my router. In order to fetch a URL GAE requires that I use its built in urlFetch capability. I want to make this URL fetch happen during my modules init() but as far as I can tell I can only use urlFetch when invoked via a handler.
func init() {
r := mux.NewRouter()
r.HandleFunc("/", homeHandler)
r.HandleFunc("/about", anotherHandler)
http.Handle("/", r)
}
GAE suggests the following code for making a urlFetch:
c := appengine.NewContext(r)
client := urlfetch.Client(c)
... but its argument is an http router, and it doesn't want to work if I pass my mux router. So I'm out of ideas of how to make this urlFetch happen outside the scope of a URL handler.
Error when passing the mux router: "cannot use r (type *mux.Router) as type *http.Request in argument to "appengine".NewContext"
You can't use AppEngine services that require a Context outside of handlers (because the creation of a Context requires an *http.Request value). This by nature means you can't use them in package init() functions either.
Note that you can use them from cron jobs and tasks added to task queues, because tasks and cron jobs are executed by issuing HTTP GET requests.
You have to restructure your code so that the service (urlFetch in your case) gets called from a handler.
A possible solution is to check if init completed in handlers that serve user requests. If not, perform the initialization function you would otherwise put in init() before proceeding to serve the request.
Yes, this may cause first requests to take considerably longer to serve. For this purpose (to avoid this) I recommend you to utilize Warmup requests. A warmup request is issued to a new instance before it goes "live", before it starts serving user requests. In your app.yaml config file you can enable warmup requests by adding -warmup to the inbound_services directive:
inbound_services:
- warmup
This will cause the App Engine infrastructure to first issue a GET request to /_ah/warmup. You can register a handler to this URL and perform initialization tasks. As with any other request, you will have an http.Request in the warmup handler.
But please note that:
..you may encounter loading requests, even if warmup requests are enabled in your app.
Which means that in rare cases it may happen a new instance will not receive a warmup request, so its best to check initialization state in user handlers too.
When I issue a GET request, I get back a 200 OK and the etag header:
etag → "tZIZl_M15FKLVh9sN6Nj0iz1dQE/fA5Fya8Zz6DLGJwPNnIWbruyt30"
In my subsequent request, I send the
If-Not-Modified → "tZIZl_M15FKLVh9sN6Nj0iz1dQE/fA5Fya8Zz6DLGJwPNnIWbruyt30"
header, but the endpoint still sends back 200 OK rather than 304.
How do I get my endpoint to respect the If-Not-Modified header? Documentation on caching using cloud endpoints is non existent :/
Google Cloud Endpoints is a mechanism to call your back-end methods directly.
Hence, they don't follow the normal rules for other requests, like the cache one you're mentioning.
Think of them like AJAX code for App Engine that can be called from your Android/iOS/web code.
You have two options if the cache is important for you:
To use the standard HTTP request/response model, i.e. not to use Cloud Endpoints.
To implement a cache control by yourself inside your own methods.
Using the following two calls for identical resource:
var p1 = Restangular.one('accounts', 123).one('buildings', 456).get();
var p2 = Restangular.one('accounts', 123).one('buildings', 456).get();
Will Restangular send a single GET to the server or two? Is Restangular smart enough to know that there is already a request out to this resource when the second get() is issued?
Edit:
Chrome shows only one request go out, but I'm still not sure if this is implemented in Restangular or in the browser?
If you add a Cache, it'll also work on IE10 as that URL and its response would be cached.
After further investigation is seems that Restangular does not implement a feature to limit requests for same resource. The number of requests that go out to the server is dependent on the browser: Chrome only sends out only one GET request, IE 10 sends out two.