Configure JavaScript Challenge


This guide provides instructions on how to configure the javascript challenge for the application traffic. The javascript challenge functionality enables the load balancer to perform client browser compatibility test by redirecting to a page with javascript. For more information on the loadbalancer, see Load Balancing and Service Mesh.

Enabling this functionality allows only those clients that are capable of executing javascript(mostly browsers) to complete the HTTP request.

When load balancer is configured to execute javascript challenge, it redirects the browser to a HTML page embedded with javascript for every new HTTP request. The Load balancer chooses a set of random numbers for every new client and sends these numbers along with an encrypted answer with the request such that it embed these numbers as input in the javascript.

Javascript runs on the browser of requestor and performs a complex mathematical operation. The script submits the answer to the load balancer. Load balancer validates the answer by comparing the calculated answer with the decrypted answer (which was encrypted when it was sent back as reply) and allows the request to the upstream server only if the answer is correct.

The load balancer tags response header with a cookie to avoid javascript challenge for subsequent requests.

The javascript challenge serves following purposes:

  • Validate that the request is coming from a browser that is capable of running javascript
  • Force the browser to run a complex operation that requires it to spend a large number of CPU cycles. This slows down a potential DoS attack by making it difficult to launch a large request flood without having to spend even larger CPU cost at their end.


The following prerequisites apply:


Enabling the javascript challenge requires you to optionally prepare a custom message page encoded in Base64 format and apply it in the HTTP load balancer configuration. This example shows preparing a custom page and enabling the javascript challenge with it.

Configuration Sequence

Enabling javascript challenge requires you to perform the following sequence of actions.

Phase Description
Prepare Custom Page for Redirection. Prepare a page to which the load balancer redirects.
Enable JavaScript Challenge Apply the custom page in load balancer configuration.

Prepare Custom Page for Redirection

The load balancer redirects to a page with default message or a custom message while performing the javascript challenge. The custom message can be in plain text or a HTML element such as a paragraph and applied to the load balancer through its configuration option. This example shows preparing a custom message in HTML paragraph.

Perform the following to prepare a custom message.

Step 1: Create a file with your custom message.

Create a file with your message in plain text or embedded in a HTML element such as paragraph. This example shows a sample message in a paragraph element stored in the test-redirect file.


Hello !!! Please wait 

Step 2: Apply Base64 encoding to the file.

Convert the file using Base64 encoding and store the output string for later use.

openssl base64 -in test-redirect


Enable JavaScript Challenge

Step 1: Start updating the HTTP load balancer in VoltConsole.

Log into the VoltConsole and change to your namespace. Select Manage -> Load Balancers -> from the configuration menu and HTTP Load Balancers in the options. Click ... -> Edit against your load balancer to open its configuration edit form.

Figure: Edit Configuration of Load Balancer

Step 2: Enable security options and select javascript challenge.
  • Scroll down to Security Configuration section and enable Show Advanced Fields. Select the Javascript Challenge option from the list of options for the Select Type of Challenge field.

Figure: Load Balancer Javascript Challenge Option

  • Click Configure to open javascript challenge configuation form.
Step 3: Set javascript challenge fields.

Enter the configuration values in the javascript challenge form as per the following guidelines:

  • Enter a value in the Javascript Delay field in milliseconds. This example sets 2000 milliseconds.
  • Enter value in the Cookie Expiry period in seconds. The load balancer performs javascript challenge again after cookie expiry.

Note: Once javascript challenge is performed, the subsequent requests do not undergo the javascript challenge till the cookie expiry.

  • Enter the Base64 encoded string in the Custom Message for Javascript Challenge field. The string must be entered in the string:///<Base64 String> format.

Note: Use the Base64 string generated in the Prepare Custom Page for Redirection chapter and prepend the string:/// string to it.

Figure: Javascript Challenge Configuration Fields

Step 4: Complete enabling javascript challenge.

Click Apply to enable the javascript configuration. Click Save and Exit to apply the configuration to load balancer.

Verify JavaScript Challenge Functionality

After enabling javascript challenge functionality, you can send request to the application domain from a browser and check if the load balancer redirects to the page you configured.

Step 1: Send request from the browser to your application domain.

Send a request by loading the domain specified in the load balancer configuration. This example shows the request for the hipster application deployed.

Figure: Javascript Challenge Redirection Message Page

Step 2: Verify the javascript challenge.

Check if the custom page you configured is displayed. After the configured delay, verify if the load balancer redirects to your application site. This example shows that after the delay, the load balancer loads the requested hipster application page.

Figure: Javascript Challenge Redirection Message Page

Note: You can check the status of the javascript challenge functionality for any issues using the ...->Show Status option of the load balancer.


API References