The webRequest API can be used to modify the User Agent header.
Note: The Network tab at the Developer tools show the old headers. I’ve verified that the headers are set correctly, using netcat (nc -l 127.0.0.1 -p 6789).
In the example below, the code activates on all tabs. Adjust the request filter to meet your requirements. Add tabId to limit the functionality to this filter, with the tabId of your tabs (obtainable through various APIs, chrome.tabs in particular).
background.js
chrome.webRequest.onBeforeSendHeaders.addListener(
function(info) {
// Replace the User-Agent header
var headers = info.requestHeaders;
headers.forEach(function(header, i) {
if (header.name.toLowerCase() == 'user-agent') {
header.value="Spoofed UA";
}
});
return {requestHeaders: headers};
},
// Request filter
{
// Modify the headers for these pages
urls: [
"http://stackoverflow.com/*",
"http://127.0.0.1:6789/*"
],
// In the main window and frames
types: ["main_frame", "sub_frame"]
},
["blocking", "requestHeaders"]
);
manifest.json
{
"name": "WebRequest UA test",
"version": "1.0",
"permissions": ["webRequest", "webRequestBlocking", "http://*/*"],
"background": {
"scripts": ["background.js"]
},
"manifest_version": 2
}