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 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.

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:
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

Additional notes on Chrome incompatibilities.

Example extensions

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