webRequest
==========

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 typical 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.](/en-US/docs/Mozilla/Add-ons/WebExtensions/API/webRequest/webrequest-flow.png)

`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 itself
* a `filter` object, so you can only be notified for requests made to particular URLs or for particular types of resource
* an optional `extraInfoSpec` object. You can use this to pass additional event-specific instructions.

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:

* cancel the request in:
    + `onBeforeRequest`
    + `onBeforeSendHeaders`
    + `onAuthRequired`
* redirect the request in:
    + `onBeforeRequest`
    + `onHeadersReceived`
* modify request headers in:
    + `onBeforeSendHeaders`
* modify response headers in:
    + `onHeadersReceived`
* supply authentication credentials in:
    + `onAuthRequired`

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 `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 `webRequest.ResourceType` of `speculative`.

Accessing security information
------------------------------

In the `onHeadersReceived` listener you can access the TLS properties of a request by calling `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 `webRequest.filterResponseData`, passing it the ID of the request. This returns a `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
-----

`webRequest.BlockingResponse`

An object of this type is returned by event listeners that have set `"blocking"` in their `extraInfoSpec` argument. By setting particular properties in `BlockingResponse`, the listener can modify network requests.

`webRequest.CertificateInfo`

An object describing a single X.509 certificate.

`webRequest.HttpHeaders`

An array of HTTP headers. Each header is represented as an object with two properties: `name` and either `value` or `binaryValue`.

`webRequest.RequestFilter`

An object describing filters to apply to `webRequest` events.

`webRequest.ResourceType`

Represents a particular kind of resource fetched in a web request.

`webRequest.SecurityInfo`

An object describing the security properties of a particular web request.

`webRequest.StreamFilter`

An object that can be used to monitor and modify HTTP responses while they are being received.

`webRequest.UploadData`

Contains data uploaded in a URL request.

Properties
----------

`webRequest.MAX_HANDLER_BEHAVIOR_CHANGED_CALLS_PER_10_MINUTES`

The maximum number of times that `handlerBehaviorChanged()` can be called in a 10 minute period.

Methods
-------

`webRequest.handlerBehaviorChanged()`

This method can be used to ensure that event listeners are applied correctly when pages are in the browser's in-memory cache.

`webRequest.filterResponseData()`

Returns a `webRequest.StreamFilter` object for a given request.

`webRequest.getSecurityInfo()`

Gets detailed information about the TLS connection associated with a given request.

Events
------

`webRequest.onBeforeRequest`

Fired when a request is about to be made, and before headers are available. This is a good place to listen if you want to cancel or redirect the request.

`webRequest.onBeforeSendHeaders`

Fired before sending any HTTP data, but after HTTP headers are available. This is a good place to listen if you want to modify HTTP request headers.

`webRequest.onSendHeaders`

Fired just before sending headers. If your add-on or some other add-on modified headers in ``onBeforeSendHeaders``, you'll see the modified version here.

`webRequest.onHeadersReceived`

Fired when the HTTP response headers associated with a request have been received. You can use this event to modify HTTP response headers.

`webRequest.onAuthRequired`

Fired when the server asks the client to provide authentication credentials. The listener can do nothing, cancel the request, or supply authentication credentials.

`webRequest.onResponseStarted`

Fired when the first byte of the response body is received. For HTTP requests, this means that the status line and response headers are available.

`webRequest.onBeforeRedirect`

Fired when a server-initiated redirect is about to occur.

`webRequest.onCompleted`

Fired when a request is completed.

`webRequest.onErrorOccurred`

Fired when an error occurs.

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

Additional notes on Chrome incompatibilities.

Example extensions
------------------

* http-response
* root-cert-stats
* stored-credentials
* user-agent-rewriter

**Note:** This API is based on Chromium's `chrome.webRequest` API. This documentation is derived from `web_request.json` in the Chromium code.