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webRequest

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Add event listeners for the various stages of making an HTTP request, which includes websocket requests on ws:// and wss://. The event listener receives detailed information about the request and can modify or cancel the request.

Each event is fired at a particular stage of the request. The sequence of events is like this:

Order of requests is onBeforeRequest, onBeforeSendHeader, onSendHeaders, onHeadersReceived, onResponseStarted, and onCompleted. The onHeadersReceived can cause an onBeforeRedirect and an onAuthRequired. Events caused by onHeadersReceived start at the beginning onBeforeRequest. Events caused by onAuthRequired start at onBeforeSendHeader.

However, not all of these events might be observed by an extension. For example, onBeforeRedirect might not be followed by onBeforeRequest when the redirect target doesn’t match the event filter.urls. This can be because the URLs in the filter are narrowly defined, or the redirect target can’t be observed by an extension, such as when it redirects to a data: URL.

{{WebExtAPIRef("webRequest.onErrorOccurred", "onErrorOccurred")}}  can fire at any time during the request. Also, note that sometimes the sequence of events may differ from this. For example, in Firefox, on an HSTS upgrade, the onBeforeRedirect event is triggered immediately after onBeforeRequest. onErrorOccurred is also fired if Firefox Tracking Protection blocks a request.

All events – except onErrorOccurred – can take three arguments to addListener():

The listener function is passed a details object containing information about the request. This includes a request ID, which is provided to enable an add-on to correlate events associated with a single request. It is unique within a browser session and the add-on’s context. It stays the same throughout a request, even across redirections and authentication exchanges.

To use the webRequest API for a given host, an extension must have the "webRequest" API permission and the host permission for that host. To use the "blocking" feature, the extension must also have the "webRequestBlocking" API permission.

To intercept resources loaded by a page (such as images, scripts, or stylesheets), the extension must have the host permission for the resource as well as for the main page requesting the resource. For example, if a page at https://developer.mozilla.org loads an image from https://mdn.mozillademos.org, then an extension must have both host permissions if it is to intercept the image request.

Modifying requests

On some of these events, you can modify the request. Specifically, you can:

To do this, you need to pass an option with the value "blocking" in the extraInfoSpec argument to the event’s addListener(). This makes the listener synchronous.

In the listener, you can then return a {{WebExtAPIRef("webRequest.BlockingResponse", "BlockingResponse")}}  object, which indicates the modification you need to make: for example, the modified request header you want to send.

Requests at browser startup

When a listener is registered with the "blocking" option and is registered during the extension startup, if a request is made during the browser startup that matches the listener the extension starts early. This enables the extension to observe the request at browser startup. If you don’t take these steps, requests made at startup could be missed.

Speculative requests

The browser can make speculative connections, where it determines that a request to a URI may be coming soon. This type of connection does not provide valid tab information, so request details such as tabId, frameId, parentFrameId, etc. are inaccurate. These connections have a {{WebExtAPIRef("webRequest.ResourceType")}}  of speculative.

Accessing security information

In the {{WebExtAPIRef("webRequest.onHeadersReceived", "onHeadersReceived")}}  listener you can access the TLS properties of a request by calling {{WebExtAPIRef("webRequest.getSecurityInfo()", "getSecurityInfo()")}} . To do this you must also pass “blocking” in the extraInfoSpec argument to the event’s addListener().

You can read details of the TLS handshake, but can’t modify them or override the browser’s trust decisions.

Modifying responses

To modify the HTTP response bodies for a request, call {{WebExtAPIRef("webRequest.filterResponseData")}} , passing it the ID of the request. This returns a {{WebExtAPIRef("webRequest.StreamFilter")}}  object that you can use to examine and modify the data as it is received by the browser.

To do this, you must have the "webRequestBlocking" API permission as well as the "webRequest" API permission and the host permission for the relevant host.

Types

Properties

Methods

Events

Browser compatibility

{{Compat}} 

Additional notes on Chrome incompatibilities.

{{WebExtExamples("h2")}} 

[!NOTE] This API is based on Chromium’s chrome.webRequest API. This documentation is derived from web_request.json in the Chromium code.

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