Manifest

You must declare the "webRequest" permission in the extension manifest to use the web request API, along with host permissions for any hosts whose network requests you want to access. If you want to use the web request API in a blocking fashion, you need to request the "webRequestBlocking" permission in addition. For example:

{
  "name": "My extension",
  ...
  "permissions": [
    "webRequest",
    "*://*.google.com/"
  ],
  ...
}

Life cycle of requests

The web request API defines a set of events that follow the life cycle of a web request. You can use these events to observe and analyze traffic. Certain synchronous events will allow you to intercept, block, or modify a request.

The event life cycle for successful requests is illustrated here, followed by event definitions:

onBeforeRequest (optionally synchronous)

Fires when a request is about to occur. This event is sent before any TCP connection is made and can be used to cancel or redirect requests.

onBeforeSendHeaders (optionally synchronous)

Fires when a request is about to occur and the initial headers have been prepared. The event is intended to allow extensions to add, modify, and delete request headers (*). The onBeforeSendHeaders event is passed to all subscribers, so different subscribers may attempt to modify the request; see the Implementation details section for how this is handled. This event can be used to cancel the request.

onSendHeaders

Fires after all extensions have had a chance to modify the request headers, and presents the final (*) version. The event is triggered before the headers are sent to the network. This event is informational and handled asynchronously. It does not allow modifying or cancelling the request.

onHeadersReceived (optionally synchronous)

Fires each time that an HTTP(S) response header is received. Due to redirects and authentication requests this can happen multiple times per request. This event is intended to allow extensions to add, modify, and delete response headers, such as incoming Set-Cookie headers.

onAuthRequired (optionally synchronous)

Fires when a request requires authentication of the user. This event can be handled synchronously to provide authentication credentials. Note that extensions may provide invalid credentials. Take care not to enter an infinite loop by repeatedly providing invalid credentials.

onBeforeRedirect

Fires when a redirect is about to be executed. A redirection can be triggered by an HTTP response code or by an extension. This event is informational and handled asynchronously. It does not allow you to modify or cancel the request.

onResponseStarted

Fires when the first byte of the response body is received. For HTTP requests, this means that the status line and response headers are available. This event is informational and handled asynchronously. It does not allow modifying or cancelling the request.

onCompleted

Fires when a request has been processed successfully.

onErrorOccurred

Fires when a request could not be processed successfully.

(*) Note that the web request API presents an abstraction of the network stack to the extension. Internally, one URL request can be split into several HTTP requests (for example to fetch individual byte ranges from a large file) or can be handled by the network stack without communicating with the network. For this reason, the API does not provide the final HTTP headers that are sent to the network. For example, all headers that are related to caching are invisible to the extension.

The following headers are currently not provided to the onBeforeSendHeaders event. This list is not guaranteed to be complete nor stable.

• Authorization
• Cache-Control
• Connection
• Content-Length
• Host
• If-Modified-Since
• If-None-Match
• If-Range
• Partial-Data
• Pragma
• Proxy-Authorization
• Proxy-Connection
• Transfer-Encoding

The webRequest API only exposes requests that the extension has permission to see, given its host permissions. Moreover, only the following schemes are accessible: http://, https://, ftp://, file://, or chrome-extension://. In addition, even certain requests with URLs using one of the above schemes are hidden, e.g., chrome-extension://other_extension_id where other_extension_id is not the ID of the extension to handle the request, https://www.google.com/chrome, and others (this list is not complete). Also synchronous XMLHttpRequests from your extension are hidden from blocking event handlers in order to prevent deadlocks. Note that for some of the supported schemes the set of available events might be limited due to the nature of the corresponding protocol. For example, for the file: scheme, only onBeforeRequest, onResponseStarted, onCompleted, and onErrorOccurred may be dispatched.

Concepts

As the following sections explain, events in the web request API use request IDs, and you can optionally specify filters and extra information when you register event listeners.

Request IDs

Each request is identified by a request ID. This ID is unique within a browser session and the context of an extension. It remains constant during the the life cycle of a request and can be used to match events for the same request. Note that several HTTP requests are mapped to one web request in case of HTTP redirection or HTTP authentication.

Registering event listeners

To register an event listener for a web request, you use a variation on the usual addListener() function. In addition to specifying a callback function, you have to specify a filter argument and you may specify an optional extra info argument.

The three arguments to the web request API's addListener() have the following definitions:

var callback = function(details) {...};
var filter = {...};
var opt_extraInfoSpec = [...];

Here's an example of listening for the onBeforeRequest event:

chrome.webRequest.onBeforeRequest.addListener(
  callback, filter, opt_extraInfoSpec);

Each addListener() call takes a mandatory callback function as the first parameter. This callback function is passed a dictionary containing information about the current URL request. The information in this dictionary depends on the specific event type as well as the content of opt_extraInfoSpec.

If the optional opt_extraInfoSpec array contains the string 'blocking' (only allowed for specific events), the callback function is handled synchronously. That means that the request is blocked until the callback function returns. In this case, the callback can return a $ref:webRequest.BlockingResponse that determines the further life cycle of the request. Depending on the context, this response allows cancelling or redirecting a request (onBeforeRequest), cancelling a request or modifying headers (onBeforeSendHeaders, onHeadersReceived), or providing authentication credentials (onAuthRequired).

The $ref:webRequest.RequestFilter filter allows limiting the requests for which events are triggered in various dimensions:

URLs

URL patterns such as *://www.google.com/foo*bar.

Types

Request types such as main_frame (a document that is loaded for a top-level frame), sub_frame (a document that is loaded for an embedded frame), and image (an image on a web site). See $ref:webRequest.RequestFilter.

Tab ID

The identifier for one tab.

Window ID

The identifier for a window.

Depending on the event type, you can specify strings in opt_extraInfoSpec to ask for additional information about the request. This is used to provide detailed information on request's data only if explicitly requested.

Implementation details

Several implementation details can be important to understand when developing an extension that uses the web request API:

Conflict resolution

In the current implementation of the web request API, a request is considered as cancelled if at least one extension instructs to cancel the request. If an extension cancels a request, all extensions are notified by an onErrorOccurred event. Only one extension is allowed to redirect a request or modify a header at a time. If more than one extension attempts to modify the request, the most recently installed extension wins and all others are ignored. An extension is not notified if its instruction to modify or redirect has been ignored.

Caching

Chrome employs two caches — an on-disk cache and a very fast in-memory cache. The lifetime of an in-memory cache is attached to the lifetime of a render process, which roughly corresponds to a tab. Requests that are answered from the in-memory cache are invisible to the web request API. If a request handler changes its behavior (for example, the behavior according to which requests are blocked), a simple page refresh might not respect this changed behavior. To make sure the behavior change goes through, call handlerBehaviorChanged() to flush the in-memory cache. But don't do it often; flushing the cache is a very expensive operation. You don't need to call handlerBehaviorChanged() after registering or unregistering an event listener.

Timestamps

The timestamp property of web request events is only guaranteed to be internally consistent. Comparing one event to another event will give you the correct offset between them, but comparing them to the current time inside the extension (via (new Date()).getTime(), for instance) might give unexpected results.

Examples

The following example illustrates how to block all requests to www.evil.com:

chrome.webRequest.onBeforeRequest.addListener(
  function(details) {
    return {cancel: details.url.indexOf("://www.evil.com/") != -1};
  },
  {urls: ["<all_urls>"]},
  ["blocking"]);

As this function uses a blocking event handler, it requires the "webRequest" as well as the "webRequestBlocking" permission in the manifest file.

The following example achieves the same goal in a more efficient way because requests that are not targeted to www.evil.com do not need to be passed to the extension:

chrome.webRequest.onBeforeRequest.addListener(
  function(details) { return {cancel: true}; },
  {urls: ["*://www.evil.com/*"]},
  ["blocking"]);

The following example illustrates how to delete the User-Agent header from all requests:

chrome.webRequest.onBeforeSendHeaders.addListener(
  function(details) {
    for (var i = 0; i < details.requestHeaders.length; ++i) {
      if (details.requestHeaders[i].name === 'User-Agent') {
        details.requestHeaders.splice(i, 1);
        break;
      }
    }
    return {requestHeaders: details.requestHeaders};
  },
  {urls: ["<all_urls>"]},
  ["blocking", "requestHeaders"]);

For more example code, see the web request samples.