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This event is triggered before sending any HTTP data, but after all HTTP headers are available. This is a good place to listen if you want to modify HTTP request headers.
To have the request headers passed into the listener along with the rest of the request data, pass "requestHeaders"
in the extraInfoSpec
array.
To modify the headers, pass "blocking"
in extraInfoSpec
, then in your event listener, return an object with a property named requestHeaders
, whose value is the set of request headers to send.
If you use "blocking"
, you must have the "webRequestBlocking" API permission in your manifest.json.
It is possible for add-ons to conflict here. If two add-ons listen to onBeforeSendHeaders
for the same request, then the second listener will see modifications made by the first listener, and will be able to undo any changes made by the first listener. For example, if the first listener adds a Cookie
header, and the second listener strips all Cookie
headers, then the first listener's modifications will be lost. If you want to see the headers that are actually sent, without the risk that another add-on will subsequently alter them, use onSendHeaders
, although you can't modify headers on this event.
Not all headers actually sent are always included in requestHeaders
. In particular, headers related to caching (for example, Cache-Control
, If-Modified-Since
, If-None-Match
) are never sent. Also, behavior here may differ across browsers.
Syntax
chrome.webRequest.onBeforeSendHeaders.addListener( function( // function details // object ) {...}, filter, // object extraInfoSpec // optional array of strings ) chrome.webRequest.onBeforeSendHeaders.removeListener(listener) chrome.webRequest.onBeforeSendHeaders.hasListener(listener)
This API is also available as browser.webRequest.onBeforeSendHeaders.*
.
Events have three functions:
addListener(callback, filter, extraInfoSpec)
- Adds a listener to this event.
removeListener(listener)
- Stop listening to this event. The
listener
argument is the listener to remove. hasListener(listener)
- Check whether
listener
is registered for this event. Returnstrue
if it is listening,false
otherwise.
addListener syntax
Parameters
callback
-
Function that will be called when this event occurs. The function will be passed the following arguments:
details
object
. Details of the request. This will include request headers if you have included"requestHeaders"
inextraInfoSpec
.
Returns:
webRequest.BlockingResponse
. If"blocking"
is specified in theextraInfoSpec
parameter, the event listener should return aBlockingResponse
object, and can set itsrequestHeaders
property. filter
webRequest.RequestFilter
. A set of filters that restricts the events that will be sent to this listener.extraInfoSpec
Optionalarray
ofstring
. Extra options for the event. You can pass any of the following values:-
"blocking"
: make the request synchronous, so you can modify request headers"requestHeaders"
: include the request headers in thedetails
object passed to the listener
Additional objects
details
requestId
string
. The ID of the request. Request IDs are unique within a browser session, so you can use them to relate different events associated with the same request.url
string
. Target of the request.method
string
. Standard HTTP method: for example, "GET" or "POST".frameId
integer
. Zero if the request happens in the main frame; a positive value is the ID of a subframe in which the request happens. If the document of a (sub-)frame is loaded (type
ismain_frame
orsub_frame
),frameId
indicates the ID of this frame, not the ID of the outer frame. Frame IDs are unique within a tab.parentFrameId
integer
. ID of the frame that contains the frame which sent the request. Set to -1 if no parent frame exists.tabId
integer
. ID of the tab in which the request takes place. Set to -1 if the request isn't related to a tab.type
webRequest.ResourceType
. The type of resource being requested: for example, "image", "script", "stylesheet".timeStamp
number
. The time when this event fired, in milliseconds since the epoch.originUrl
string
. URL of the resource that triggered this request. Note that this may not be the same as the URL of the page into which the requested resource will be loaded. For example, if a document triggers a load in a different window through the target attribute of a link, or a CSS document includes an image using theurl()
functional notation, then this will be the URL of the original document or of the CSS document, respectively.requestHeaders
OptionalwebRequest.HttpHeaders
. The HTTP request headers that will be sent with this request.
Browser compatibility
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Chrome | Edge | Firefox | Firefox for Android | Opera | |
---|---|---|---|---|---|
Basic support | Yes | Yes | 45.0 | 48.0 | 33 |
Compatibility notes
Chrome
- 'originUrl' is not supported.
Firefox
- 'originUrl' is supported from version 48.
Opera
- 'originUrl' is not supported.
Examples
This code changes the "User-Agent" header so the browser identifies itself as Opera 12.16, but only when visiting pages under https://useragentstring.com/".
"use strict"; // This is the page for which we want to rewrite // the User-Agent header. var targetPage = "https://useragentstring.com/*"; // Set UA string to Opera 12 var ua = "Opera/9.80 (X11; Linux i686; Ubuntu/14.10) Presto/2.12.388 Version/12.16"; // Rewrite the User-Agent header to "ua". function rewriteUserAgentHeader(e) { for (var header of e.requestHeaders) { if (header.name == "User-Agent") { header.value = ua; } } return {requestHeaders: e.requestHeaders}; } // Add rewriteUserAgentHeader as a listener // to onBeforeSendHeaders, only for the target page. // Make it "blocking" so we can modify the headers. chrome.webRequest.onBeforeSendHeaders.addListener( rewriteUserAgentHeader, {urls: [targetPage]}, ["blocking", "requestHeaders"] );
Example add-ons
This API is based on Chromium's chrome.webRequest
API. This documentation is derived from web_request.json
in the Chromium code.
Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.
// Copyright 2015 The Chromium Authors. All rights reserved. // // Redistribution and use in source and binary forms, with or without // modification, are permitted provided that the following conditions are // met: // // * Redistributions of source code must retain the above copyright // notice, this list of conditions and the following disclaimer. // * Redistributions in binary form must reproduce the above // copyright notice, this list of conditions and the following disclaimer // in the documentation and/or other materials provided with the // distribution. // * Neither the name of Google Inc. nor the names of its // contributors may be used to endorse or promote products derived from // this software without specific prior written permission. // // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.