Add event listeners for the various stages of making an HTTP request, which includes websocket requests on
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:
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
onErrorOccurred is also fired if Firefox Tracking Protection blocks a request.
All events – except
onErrorOccurred – can take three arguments to
- the listener itself
filterobject, so you can only be notified for requests made to particular URLs or for particular types of resource
- an optional
extraInfoSpecobject. 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.
On some of these events, you can modify the request. Specifically, you can:
- cancel the request in:
- redirect the request in:
- modify request headers in:
- modify response headers in:
- supply authentication credentials in:
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.
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.
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
parentFrameId, etc. are inaccurate. These connections have a
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
You can read details of the TLS handshake, but can't modify them or override the browser's trust decisions.
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.
An object of this type is returned by event listeners that have set
extraInfoSpecargument. By setting particular properties in
BlockingResponse, the listener can modify network requests.
An object describing a single X.509 certificate.
An array of HTTP headers. Each header is represented as an object with two properties:
An object describing filters to apply to
Represents a particular kind of resource fetched in a web request.
An object describing the security properties of a particular web request.
An object that can be used to monitor and modify HTTP responses while they are being received.
Contains data uploaded in a URL request.
The maximum number of times that
handlerBehaviorChanged()can be called in a 10 minute period.
This method can be used to ensure that event listeners are applied correctly when pages are in the browser's in-memory cache.
webRequest.StreamFilterobject for a given request.
Gets detailed information about the TLS connection associated with a given request.
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.
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.
Fired just before sending headers. If your add-on or some other add-on modified headers in
, you'll see the modified version here.
Fired when the HTTP response headers associated with a request have been received. You can use this event to modify HTTP response headers.
Fired when the server asks the client to provide authentication credentials. The listener can do nothing, cancel the request, or supply authentication credentials.
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.
Fired when a server-initiated redirect is about to occur.
Fired when a request is completed.
Fired when an error occurs.
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