I am currently trying to learn nghttp2 and was trying to execute the client code which is provided at the bottom of this page:
https://nghttp2.org/documentation/tutorial-client.html
I executed the above C code doing the following:
./libevent-client URL
My Server is Windows IIS 10.0 and i want to see the http2-settings frame in the output of the header. As of now it is showing the following output:
Connected
Request headers:
:method: GET
:scheme: https
:authority: MY URL
:path: /
Response headers from stream ID=1:
:status: 200
content-type: text/html
last-modified: Mon, 01 Jul 2019 17:57:17 GMT
accept-ranges: bytes
etag: "c7c5406c3630d51:0"
server: Microsoft-IIS/10.0
date: Mon, 08 Jul 2019 16:02:27 GMT
content-length:51
All headers received
<html><head>Hello</head><html>
I need to know what should i need in the code to see whether the http-settings are getting passed on with the request. I know that following function does the work of sending the SETTINGS frames with the request:
static void send_client_connection_header(http2_session_data *session_data) {
nghttp2_settings_entry iv[1] = {
{NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100}};
int rv;
/* client 24 bytes magic string will be sent by nghttp2 library */
rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
ARRLEN(iv));
if (rv != 0) {
errx(1, "Could not submit SETTINGS: %s", nghttp2_strerror(rv));
}
}
I also don't know what is the tag we use for HTTP-Settings in http2 protocol just like for method we have ":method", for scheme ":scheme" etc. I couldn't find it even in the RFC.
The HTTP/2 settings frame is not an HTTP Header - it is a separate message sent at the beginning of connections. Therefore it is not possible to display it like as if it was an HTTP Header.
HTTP/2 contains many such non-visible control frames:
SETTINGS frame to define how the connection is used
WINDOW_UPDATE frame to implement flow control
PRIORITY frame to reprioritise streams (and therefore responses).
...etc.
Typically browsers and servers do not show these Control messages to the user or even in developer tools. Chrome allows you to see them using the chrome://net-export URL, or you can use a network sniffing tool like Wireshark to see them.
One of the easiest ways however, and a very good way to learn HTTP/2 by examining the raw frames, is to use the nghttpd tool (which is part of the nghttp2 suite) to create an HTTP/2 server that can log any messages sent to it when run in verbose mode, like this:
nghttpd -v 443 server.key server.crt
I discuss how to do this in more depth in my book, which you can preview online for free for 5 mins a day at: https://livebook.manning.com/#!/book/http2-in-action/chapter-4/176
One thing I should say is that when connecting over non-encrypted HTTP/1.1 (as opposed to HTTPS) and then upgrading to HTTP/2 then the Settings are sent in an HTTP Header (called HTTP2-Settings) but this is a special case, and when this is sent the message is an HTTP/1.1 message. Additionally browsers only support HTTP/2 over HTTPS (for good reasons) and I see you are using HTTPS too. So I would ignore this and only mentioning it for completeness sake.
I have MVC application which is accessed from external domain "https://example.com/ABC". There is code in application which construct URL for JQuery call to controller action method using "Request.Url.Host". "Request.Url.Host" picks internal domain which is "https://example-internal.com/ABC". Due to this, i am facing Preflight issue. I have so many such occurrences in code. I tried Translate URL "Application body" option but it didn't work. Is there any option to resolve this issue?
Below are some possible solutions:
Option 1: Custom Domains
Use the Custom Domain feature of the Azure AD Application Proxy, so you can use the same domain name and no changes to applications or headers are required. Thus the origin will continue to stay the same.
Option 2: Publish Parent Directory
Publish the parent directory of both applications. This works particularly well if you only have two applications on the web servers. Instead of publishing each application separately, you could publish the parent directory, that would result in the same origin.
App Published Individually
Instead publish the parent directory
The resulting URLs would be as below, effectively resolving your CORS issues.
https://corswebclient-allmylab.msappproxy.net/CORSWebService
https://corswebclient-allmylab.msappproxy.net/CORSWebClient
Option 3: Update HTTP Headers
Add the HTTP Response Header on the Web Service to match the Origin request. For example, below is how you could set it up for the websites running on the IIS.
This would also not require any change to the Code. You can also verify this in the Fiddler traces.
Post the Header Addition
HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: text/plain; charset=utf-8
Expires: -1
Vary: Accept-Encoding
Server: Microsoft-IIS/8.5 Microsoft-HTTPAPI/2.0
Access-Control-Allow-Origin: https://corswebclient-allmylab.msappproxy.net
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Content-Length: 17
Option 4: App Modification
Modify your application to add support for the CORS by adding the Access-Control-Allow-Origin header with appropriate values. The way to do this will depend on the language in which you wrote the application. This is the least recommended option since it requires more effort.
Reference: Understanding CORS Issues
When caching item in App Engine's memcache I used gzip compression to save space and get below the 1MB limit for some files.
Since I also put rendered pages into the memcache, I though it would be nice and much quicker to directly return the gzipped body to the client, if it accepts gzip encdoing.
Unfortunately the request's Accept-Encoding only has the value identity (using the AE dev server with Go), which to me means I have to return the body as-is (i.e. plain html).
Is one not supposed to gzip contents themselves? Or could I always return gzipped content with the appropriate headers, and the AE infrastructure would decompress this when the client does not support compression?
After all I hope to get even better response times by caching the response in its output state.
For caching the response, if your response is public (same copy for all users), you can make use of Google's edge cache by setting the proper HTTP headers, for example:
Cache-Control: public,max-age=86400
Expires: Sat, 16 May 2015 07:23:15 +0000
About compression, as far as I know, Google automatically compress the content in HTTP response whenever possible. There is no need to handle this manually.
In the APP Engine API, it is mentioned that, if the request comes with "Accept-Encoding" set, then it will automatically compress the response.
But when I look at the request, the header is not there. but at the browser, it is set. when I try to explicitly set the header(with JQuery ajax function), there is a message:
Refused to set unsafe header "Accept-Encoding"
But this situation is not occurring when working in local host - request has the "Accept-Encoding" header. this happens only after publishing. but not allowing to set the "Accept-Encoding" explicitly happens always.
I searched everywhere, but couldn't find a explanation to the problem. It would be really helpful if someone can explain...
You have two different problems:
App Engine does not compress reply. GAE uses a number of factors to determine if response needs to be compressed. It takes content type and user agent into account when deciding. See the answer by Nick Johnson (from GAE team).
jQuery refuses to set "Accept-Encoding" header. Note that this is a jQuery issue and has nothing to do with GAE. See this: Is it possible to force jQuery to make AJAX calls for URLs with gzip/deflate enabled?
I have a similar problem as in the HTTPRequest header, "Accept-Encoding" is null. As GAE has explained it looks for Accept-Encoding and User-Agent headers, if it wants to compress. but in my case there is no way the GAE can recognize whether to compress.
From the browser, then header is set, but in the request header, it is not.
tl;dr; About the Same Origin Policy
I have a Grunt process which initiates an instance of express.js server. This was working absolutely fine up until just now when it started serving a blank page with the following appearing in the error log in the developer's console in Chrome (latest version):
XMLHttpRequest cannot load https://www.example.com/
No 'Access-Control-Allow-Origin' header is present on the requested
resource. Origin 'http://localhost:4300' is therefore not allowed access.
What is stopping me from accessing the page?
tl;dr — When you want to read data, (mostly) using client-side JS, from a different server you need the server with the data to grant explicit permission to the code that wants the data.
There's a summary at the end and headings in the answer to make it easier to find the relevant parts. Reading everything is recommended though as it provides useful background for understanding the why that makes seeing how the how applies in different circumstances easier.
About the Same Origin Policy
This is the Same Origin Policy. It is a security feature implemented by browsers.
Your particular case is showing how it is implemented for XMLHttpRequest (and you'll get identical results if you were to use fetch), but it also applies to other things (such as images loaded onto a <canvas> or documents loaded into an <iframe>), just with slightly different implementations.
The standard scenario that demonstrates the need for the SOP can be demonstrated with three characters:
Alice is a person with a web browser
Bob runs a website (https://www.example.com/ in your example)
Mallory runs a website (http://localhost:4300 in your example)
Alice is logged into Bob's site and has some confidential data there. Perhaps it is a company intranet (accessible only to browsers on the LAN), or her online banking (accessible only with a cookie you get after entering a username and password).
Alice visits Mallory's website which has some JavaScript that causes Alice's browser to make an HTTP request to Bob's website (from her IP address with her cookies, etc). This could be as simple as using XMLHttpRequest and reading the responseText.
The browser's Same Origin Policy prevents that JavaScript from reading the data returned by Bob's website (which Bob and Alice don't want Mallory to access). (Note that you can, for example, display an image using an <img> element across origins because the content of the image is not exposed to JavaScript (or Mallory) … unless you throw canvas into the mix in which case you will generate a same-origin violation error).
Why the Same Origin Policy applies when you don't think it should
For any given URL it is possible that the SOP is not needed. A couple of common scenarios where this is the case are:
Alice, Bob, and Mallory are the same person.
Bob is providing entirely public information
… but the browser has no way of knowing if either of the above is true, so trust is not automatic and the SOP is applied. Permission has to be granted explicitly before the browser will give the data it has received from Bob to some other website.
Why the Same Origin Policy applies to JavaScript in a web page but little else
Outside the web page
Browser extensions*, the Network tab in browser developer tools, and applications like Postman are installed software. They aren't passing data from one website to the JavaScript belonging to a different website just because you visited that different website. Installing software usually takes a more conscious choice.
There isn't a third party (Mallory) who is considered a risk.
* Browser extensions do need to be written carefully to avoid cross-origin issues. See the Chrome documentation for example.
Inside the webpage
Most of the time, there isn't a great deal of information leakage when just showing something on a webpage.
If you use an <img> element to load an image, then it gets shown on the page, but very little information is exposed to Mallory. JavaScript can't read the image (unless you use a crossOrigin attribute to explicitly enable request permission with CORS) and then copy it to her server.
That said, some information does leak so, to quote Domenic Denicola (of Google):
The web's fundamental security model is the same origin policy. We
have several legacy exceptions to that rule from before that security
model was in place, with script tags being one of the most egregious
and most dangerous. (See the various "JSONP" attacks.)
Many years ago, perhaps with the introduction of XHR or web fonts (I
can't recall precisely), we drew a line in the sand, and said no new
web platform features would break the same origin policy. The existing
features need to be grandfathered in and subject to carefully-honed
and oft-exploited exceptions, for the sake of not breaking the web,
but we certainly can't add any more holes to our security policy.
This is why you need CORS permission to load fonts across origins.
Why you can display data on the page without reading it with JS
There are a number of circumstances where Mallory's site can cause a browser to fetch data from a third party and display it (e.g. by adding an <img> element to display an image). It isn't possible for Mallory's JavaScript to read the data in that resource though, only Alice's browser and Bob's server can do that, so it is still secure.
CORS
The Access-Control-Allow-Origin HTTP response header referred to in the error message is part of the CORS standard which allows Bob to explicitly grant permission to Mallory's site to access the data via Alice's browser.
A basic implementation would just include:
Access-Control-Allow-Origin: *
… in the response headers to permit any website to read the data.
Access-Control-Allow-Origin: http://example.com
… would allow only a specific site to access it, and Bob can dynamically generate that based on the Origin request header to permit multiple, but not all, sites to access it.
The specifics of how Bob sets that response header depend on Bob's HTTP server and/or server-side programming language. Users of Node.js/Express.js should use the well-documented CORS middleware. Users of other platforms should take a look at this collection of guides for various common configurations that might help.
NB: Some requests are complex and send a preflight OPTIONS request that the server will have to respond to before the browser will send the GET/POST/PUT/Whatever request that the JS wants to make. Implementations of CORS that only add Access-Control-Allow-Origin to specific URLs often get tripped up by this.
Obviously granting permission via CORS is something Bob would only do only if either:
The data was not private or
Mallory was trusted
How do I add these headers?
It depends on your server-side environment.
If you can, use a library designed to handle CORS as they will present you with simple options instead of having to deal with everything manually.
Enable-Cors.org has a list of documentation for specific platforms and frameworks that you might find useful.
But I'm not Bob!
There is no standard mechanism for Mallory to add this header because it has to come from Bob's website, which she does not control.
If Bob is running a public API then there might be a mechanism to turn on CORS (perhaps by formatting the request in a certain way, or a config option after logging into a Developer Portal site for Bob's site). This will have to be a mechanism implemented by Bob though. Mallory could read the documentation on Bob's site to see if something is available, or she could talk to Bob and ask him to implement CORS.
Error messages which mention "Response for preflight"
Some cross-origin requests are preflighted.
This happens when (roughly speaking) you try to make a cross-origin request that:
Includes credentials like cookies
Couldn't be generated with a regular HTML form (e.g. has custom headers or a Content-Type that you couldn't use in a form's enctype).
If you are correctly doing something that needs a preflight
In these cases then the rest of this answer still applies but you also need to make sure that the server can listen for the preflight request (which will be OPTIONS (and not GET, POST, or whatever you were trying to send) and respond to it with the right Access-Control-Allow-Origin header but also Access-Control-Allow-Methods and Access-Control-Allow-Headers to allow your specific HTTP methods or headers.
If you are triggering a preflight by mistake
Sometimes people make mistakes when trying to construct Ajax requests, and sometimes these trigger the need for a preflight. If the API is designed to allow cross-origin requests but doesn't require anything that would need a preflight, then this can break access.
Common mistakes that trigger this include:
trying to put Access-Control-Allow-Origin and other CORS response headers on the request. These don't belong on the request, don't do anything helpful (what would be the point of a permissions system where you could grant yourself permission?), and must appear only on the response.
trying to put a Content-Type: application/json header on a GET request that has no request body the content of which to describe (typically when the author confuses Content-Type and Accept).
In either of these cases, removing the extra request header will often be enough to avoid the need for a preflight (which will solve the problem when communicating with APIs that support simple requests but not preflighted requests).
Opaque responses (no-cors mode)
Sometimes you need to make an HTTP request, but you don't need to read the response. e.g. if you are posting a log message to the server for recording.
If you are using the fetch API (rather than XMLHttpRequest), then you can configure it to not try to use CORS.
Note that this won't let you do anything that you require CORS to do. You will not be able to read the response. You will not be able to make a request that requires a preflight.
It will let you make a simple request, not see the response, and not fill the Developer Console with error messages.
How to do it is explained by the Chrome error message given when you make a request using fetch and don't get permission to view the response with CORS:
Access to fetch at 'https://example.com/' from origin 'https://example.net' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
Thus:
fetch("http://example.com", { mode: "no-cors" });
Alternatives to CORS
JSONP
Bob could also provide the data using a hack like JSONP which is how people did cross-origin Ajax before CORS came along.
It works by presenting the data in the form of a JavaScript program that injects the data into Mallory's page.
It requires that Mallory trust Bob not to provide malicious code.
Note the common theme: The site providing the data has to tell the browser that it is OK for a third-party site to access the data it is sending to the browser.
Since JSONP works by appending a <script> element to load the data in the form of a JavaScript program that calls a function already in the page, attempting to use the JSONP technique on a URL that returns JSON will fail — typically with a CORB error — because JSON is not JavaScript.
Move the two resources to a single Origin
If the HTML document the JS runs in and the URL being requested are on the same origin (sharing the same scheme, hostname, and port) then the Same Origin Policy grants permission by default. CORS is not needed.
A Proxy
Mallory could use server-side code to fetch the data (which she could then pass from her server to Alice's browser through HTTP as usual).
It will either:
add CORS headers
convert the response to JSONP
exist on the same origin as the HTML document
That server-side code could be written & hosted by a third party (such as CORS Anywhere). Note the privacy implications of this: The third party can monitor who proxies what across their servers.
Bob wouldn't need to grant any permissions for that to happen.
There are no security implications here since that is just between Mallory and Bob. There is no way for Bob to think that Mallory is Alice and to provide Mallory with data that should be kept confidential between Alice and Bob.
Consequently, Mallory can only use this technique to read public data.
Do note, however, that taking content from someone else's website and displaying it on your own might be a violation of copyright and open you up to legal action.
Writing something other than a web app
As noted in the section "Why the Same Origin Policy only applies to JavaScript in a web page", you can avoid the SOP by not writing JavaScript in a webpage.
That doesn't mean you can't continue to use JavaScript and HTML, but you could distribute it using some other mechanism, such as Node-WebKit or PhoneGap.
Browser extensions
It is possible for a browser extension to inject the CORS headers in the response before the Same Origin Policy is applied.
These can be useful for development but are not practical for a production site (asking every user of your site to install a browser extension that disables a security feature of their browser is unreasonable).
They also tend to work only with simple requests (failing when handling preflight OPTIONS requests).
Having a proper development environment with a local development server
is usually a better approach.
Other security risks
Note that SOP / CORS do not mitigate XSS, CSRF, or SQL Injection attacks which need to be handled independently.
Summary
There is nothing you can do in your client-side code that will enable CORS access to someone else's server.
If you control the server the request is being made to: Add CORS permissions to it.
If you are friendly with the person who controls it: Get them to add CORS permissions to it.
If it is a public service:
Read their API documentation to see what they say about accessing it with client-side JavaScript:
They might tell you to use specific URLs
They might support JSONP
They might not support cross-origin access from client-side code at all (this might be a deliberate decision on security grounds, especially if you have to pass a personalized API Key in each request).
Make sure you aren't triggering a preflight request you don't need. The API might grant permission for simple requests but not preflighted requests.
If none of the above apply: Get the browser to talk to your server instead, and then have your server fetch the data from the other server and pass it on. (There are also third-party hosted services that attach CORS headers to publically accessible resources that you could use).
Target server must allowed cross-origin request. In order to allow it through express, simply handle http options request :
app.options('/url...', function(req, res, next){
res.header('Access-Control-Allow-Origin', "*");
res.header('Access-Control-Allow-Methods', 'POST');
res.header("Access-Control-Allow-Headers", "accept, content-type");
res.header("Access-Control-Max-Age", "1728000");
return res.sendStatus(200);
});
As this isn't mentioned in the accepted answer.
This is not the case for this exact question, but might help others that search for that problem
This is something you can do in your client-code to prevent CORS errors in some cases.
You can make use of Simple Requests.
In order to perform a 'Simple Requests' the request needs to meet several conditions. E.g. only allowing POST, GET and HEAD method, as well as only allowing some given Headers (you can find all conditions here).
If your client code does not explicit set affected Headers (e.g. "Accept") with a fix value in the request it might occur that some clients do set these Headers automatically with some "non-standard" values causing the server to not accept it as Simple Request - which will give you a CORS error.
This is happening because of the CORS error. CORS stands for Cross Origin Resource Sharing. In simple words, this error occurs when we try to access a domain/resource from another domain.
Read More about it here: CORS error with jquery
To fix this, if you have access to the other domain, you will have to allow Access-Control-Allow-Origin in the server. This can be added in the headers. You can enable this for all the requests/domains or a specific domain.
How to get a cross-origin resource sharing (CORS) post request working
These links may help
This CORS issue wasn't further elaborated (for other causes).
I'm having this issue currently under different reason.
My front end is returning 'Access-Control-Allow-Origin' header error as well.
Just that I've pointed the wrong URL so this header wasn't reflected properly (in which i kept presume it did). localhost (front end) -> call to non secured http (supposed to be https), make sure the API end point from front end is pointing to the correct protocol.
I got the same error in Chrome console.
My problem was, I was trying to go to the site using http:// instead of https://. So there was nothing to fix, just had to go to the same site using https.
This bug cost me 2 days. I checked my Server log, the Preflight Option request/response between browser Chrome/Edge and Server was ok. The main reason is that GET/POST/PUT/DELETE server response for XHTMLRequest must also have the following header:
access-control-allow-origin: origin
"origin" is in the request header (Browser will add it to request for you). for example:
Origin: http://localhost:4221
you can add response header like the following to accept for all:
access-control-allow-origin: *
or response header for a specific request like:
access-control-allow-origin: http://localhost:4221
The message in browsers is not clear to understand: "...The requested resource"
note that:
CORS works well for localhost. different port means different Domain.
if you get error message, check the CORS config on the server side.
In most housing services just add in the .htaccess on the target server folder this:
Header set Access-Control-Allow-Origin 'https://your.site.folder'
I had the same issue. In my case i fixed it by adding addition parameter of timestamp to my URL. Even this was not required by the server I was accessing.
Example yoururl.com/yourdocument?timestamp=1234567
Note: I used epos timestamp
"Get" request with appending headers transform to "Options" request. So Cors policy problems occur. You have to implement "Options" request to your server. Cors Policy about server side and you need to allow Cors Policy on your server side. For Nodejs server:details
app.use(cors)
For Java to integrate with Angular:details
#CrossOrigin(origins = "http://localhost:4200")
You should enable CORS to get it working.