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:
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:
- 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.
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 theirextraInfoSpec
argument. By setting particular properties inBlockingResponse
, 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 eithervalue
orbinaryValue
. 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
, you'll see the modified version here.onBeforeSendHeaders
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
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.