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  7. webRequest.onBeforeSendHeaders

webRequest.onBeforeSendHeaders

In This Article
  1. Syntax
  2. addListener syntax
    1. Parameters
  3. Additional objects
    1. details
  4. Browser compatibility
  5. Examples
    1. Example extensions

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 synchronously: pass "blocking" in extraInfoSpec, then in your event listener, return a BlockingResponse with a property named requestHeaders, whose value is the set of request headers to send.

To modify the headers asynchronously: pass "blocking" in extraInfoSpec, then in your event listener, return a Promise which is resolved with a BlockingResponse.

If you use "blocking", you must have the "webRequestBlocking" API permission in your manifest.json.

It is possible for extensions to conflict here. If two extensions 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 extension 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.

According to the specification, header names are case-insensitive. This means that be to sure of matching a particular header, the listener should lowercase the name before comparing it:

for (let header of e.requestHeaders) {
  if (header.name.toLowerCase() === desiredHeader) {
    // process header
  }
}

The browser preserves the original case of the header name as generated by the browser. If the extension's listener changes the case, this change will not be kept.

Syntax

browser.webRequest.onBeforeSendHeaders.addListener(
  listener,             //  function
  filter,               //  object
  extraInfoSpec         //  optional array of strings
)
browser.webRequest.onBeforeSendHeaders.removeListener(listener)
browser.webRequest.onBeforeSendHeaders.hasListener(listener)

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. Returns true 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" in extraInfoSpec.

Returns: webRequest.BlockingResponse. If "blocking" is specified in the extraInfoSpec parameter, the event listener should return a BlockingResponse object, and can set its requestHeaders property.

filter
webRequest.RequestFilter. A set of filters that restricts the events that will be sent to this listener.
extraInfoSpecOptional
array of string. 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 the details 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 is main_frame or sub_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 the url() functional notation, then this will be the URL of the original document or of the CSS document, respectively.
requestHeadersOptional
webRequest.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.

ChromeEdgeFirefoxFirefox for AndroidOpera
Basic supportYes 1Yes 145 248 2Yes 1
originUrlNoNo4848No
1. Asynchronous event listeners are not supported.
2. Asynchronous event listeners are supported from version 52.

Examples

This code changes the "User-Agent" header so the browser identifies itself as Opera 12.16, but only when visiting pages under https://httpbin.org/".

"use strict";
/*
This is the page for which we want to rewrite the User-Agent header.
*/
var targetPage = "https://httpbin.org/*";
/*
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.toLowerCase() === "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.
*/
browser.webRequest.onBeforeSendHeaders.addListener(
  rewriteUserAgentHeader,
  {urls: [targetPage]},
  ["blocking", "requestHeaders"]
);

This code is exactly like the previous example, except that the listener is asynchronous, returning a Promise which is resolved with the new headers:

"use strict";
/*
This is the page for which we want to rewrite the User-Agent header.
*/
var targetPage = "https://httpbin.org/*";
/*
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 rewriteUserAgentHeaderAsync(e) {
  var asyncRewrite = new Promise((resolve, reject) => {
    window.setTimeout(() => {
      for (var header of e.requestHeaders) {
        if (header.name.toLowerCase() === "user-agent") {
          header.value = ua;
        }
      }
      resolve({requestHeaders: e.requestHeaders});
    }, 2000);
  });
  return asyncRewrite;
}
/*
Add rewriteUserAgentHeader as a listener to onBeforeSendHeaders,
only for the target page.
Make it "blocking" so we can modify the headers.
*/
browser.webRequest.onBeforeSendHeaders.addListener(
  rewriteUserAgentHeaderAsync,
  {urls: [targetPage]},
  ["blocking", "requestHeaders"]
);

Example extensions

  • user-agent-rewriter

Acknowledgements

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.

Document Tags and Contributors

Tags: 
  • Add-ons
  • API
  • Event
  • Extensions
  • Non-standard
  • onBeforeSendHeaders
  • Reference
  • WebExtensions
  • webRequest
 Contributors to this page: andrewtruongmoz, wbamberg, Makyen
 Last updated by: andrewtruongmoz, Jul 14, 2017, 11:15:08 AM
See also
  1. Browser extensions
  2. Getting started
    1. What are extensions?
    2. Your first extension
    3. Your second extension
    4. Anatomy of an extension
    5. Example extensions
  3. How to
    1. Intercept HTTP requests
    2. Modify a web page
    3. Add a button to the toolbar
    4. Implement a settings page
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  5. Concepts
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    2. Content scripts
    3. Match patterns
    4. Internationalization
    5. Content Security Policy
    6. Native messaging
  6. Porting
    1. Porting a Google Chrome extension
    2. Porting a legacy Firefox extension
    3. Embedded WebExtensions
    4. Comparison with the Add-on SDK
    5. Comparison with XUL/XPCOM extensions
    6. Chrome incompatibilities
    7. Differences between desktop and Android
  7. Firefox workflow
    1. Temporary Installation in Firefox
    2. Debugging
    3. Developing for Firefox for Android
    4. Getting started with web-ext
    5. web-ext command reference
    6. Extensions and the Add-on ID
    7. Publishing your extension
  8. JavaScript APIs
    1. Browser support for JavaScript APIs
    2. alarms
    3. bookmarks
    4. browserAction
    5. browsingData
    6. commands
    7. contextMenus
    8. contextualIdentities
    9. cookies
    10. devtools.inspectedWindow
    11. devtools.network
    12. devtools.panels
    13. downloads
    14. events
    15. extension
    16. extensionTypes
    17. history
    18. i18n
    19. identity
    20. idle
    21. management
    22. notifications
    23. omnibox
    24. pageAction
    25. permissions
    26. privacy
    27. proxy
    28. runtime
    29. sessions
    30. sidebarAction
    31. storage
    32. tabs
    33. topSites
    34. types
    35. webNavigation
    36. webRequest
      1. Methods
        1. handlerBehaviorChanged()
      2. Properties
        1. MAX_HANDLER_BEHAVIOR_CHANGED_CALLS_PER_10_MINUTES
      3. Types
        1. BlockingResponse
        2. HttpHeaders
        3. RequestFilter
        4. ResourceType
        5. UploadData
      4. Events
        1. onAuthRequired
        2. onBeforeRedirect
        3. onBeforeRequest
        4. onBeforeSendHeaders
        5. onCompleted
        6. onErrorOccurred
        7. onHeadersReceived
        8. onResponseStarted
        9. onSendHeaders
    37. windows
  9. Manifest keys
    1. applications
    2. author
    3. background
    4. browser_action
    5. chrome_settings_overrides
    6. chrome_url_overrides
    7. commands
    8. content_scripts
    9. content_security_policy
    10. default_locale
    11. description
    12. developer
    13. devtools_page
    14. homepage_url
    15. icons
    16. incognito
    17. manifest_version
    18. name
    19. omnibox
    20. optional_permissions
    21. options_ui
    22. page_action
    23. permissions
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    25. short_name
    26. sidebar_action
    27. version
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  15. Legacy add-ons
  16. Legacy technologies
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    3. Bootstrapped extensions
    4. Overlay extensions