Rules Presets / Tooling

Changing the User-Agent Header

The User-Agent HTTP Header lets servers identify the application, operating system, vendor, and/or version of the client that sends the request. Usually, each application sends its own specific User-Agent string value, but in some cases developers might want to quickly test server responses based on multiple different user agents.

When Fiddler issues a request through its Composer it automatically set its own User-Agent value. You might observe a similar behavior when using other third-party proxy tools chained to Fiddler.

You can easily create a rule that changes the User-Agent for any HTTP request that goes through Fiddler while using the Fiddler's rules.

The following examples show how to create a few different rules named "Change User-Agent" that effectively mock the Chrome browser on Windows and macOS.

Creating a "Set User-Agent" Rule for a Chrome Browser on Windows

Create a "Set User-Agent Chrome Windows" rule by setting the following actions through the Rules Builder.

  1. Create an action that sets the HTTP Request header user-agent to the following value:

    Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36`.

  2. Create an action that sets the HTTP Request header sec-ch-ua-platform to "Windows".

  3. Create an Update Response Body action that sets the following regular expression match:

    (<script(?:(?!src).)*?>)

    and update as:

    $1!function e(t,r){if(navigator.__defineGetter__?navigator.__defineGetter__("userAgent",function(){return r}):Object.defineProperty&&Object.defineProperty(navigator,"userAgent",{get:function(){return r}}),t.navigator.userAgent!==r){var n={get:function(){return r}};try{Object.defineProperty(t.navigator,"userAgent",n)}catch(i){t.navigator=Object.create(navigator,{userAgent:n})}}}(window,"Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36");

This sample Fiddler rule matches all sessions and explicitly changes the user agent.

Creating "Set User-Agent Chrome Windows" rule

Once the rule is created, enable the Rules tab, toggle the rule switch, and start capturing traffic.

Activating the "Set User-Agent Chrome Windows" rule

Download a ready-to-use "Set User-Agent Chrome Windows" rule as a FARX file, which you can import through the Rules toolbar.

Creating a "Set User-Agent" Rule for a Chrome Browser on macOS

Create a "Set User-Agent Chrome Mac" rule by setting the following actions through the Rules Builder.

  1. Create an action that sets the HTTP Request header user-agent to the following value: 

    Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36

  2. Create an action that sets the HTTP Request header sec-ch-ua-platform to "Mac".

  3. Create an Update Response Body action that sets the following regular expression match:

    (<script(?:(?!src).)*?>)

    and update as:

    $1!function e(t,r){if(navigator.__defineGetter__?navigator.__defineGetter__("userAgent",function(){return r}):Object.defineProperty&&Object.defineProperty(navigator,"userAgent",{get:function(){return r}}),t.navigator.userAgent!==r){var n={get:function(){return r}};try{Object.defineProperty(t.navigator,"userAgent",n)}catch(i){t.navigator=Object.create(navigator,{userAgent:n})}}}(window,"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36");

This sample Fiddler rule matches all sessions and explicitly changes the user agent to mock a Chrome browser on macOS.

Creating "Set User-Agent Chrome Mac" rule

Once the rule is created, enable the Rules tab, toggle the rule switch, and start capturing traffic.

Activating the "Set User-Agent Chrome Mac" rule

Download a ready-to-use "Set User-Agent Chrome Mac" rule as a FARX file, which you can import through the Rules toolbar.

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