Synthetic Monitoring allows you to generate variables from JavaScript scripts so you can define custom authentications or encode parameters.

HTTP test with HMAC authentication

This guide walks you through how to create an HTTP test with an HMAC signature, using variables from script.

Note: There is no standard HMAC authentication, your own HMAC authentication may be slighty different. For instance, it may use a different header name.


Create the building blocks of HMAC authentication using local variables

Create a Synthetic HTTP test and click Create a Local Variable to add the following variables:

The UTF-8 encoded key that is used to sign the message (which can also be imported from a global variable).
The request body (which is set in the Request Body) and is used to compute the HMAC authentication.
A parameter to compute the HMAC signature. You can create this as a local variable or create and export this inside the variable from script script with dd.variable.set('DATETIME', new Date().toISOString()).

Define a test URL and request body

Define the URL and the request type for the HTTP test. Then, click Advanced Options > Request Body to add the {{ BODY }} variable as the request body.

A local variable set as the request body for an HTTP test

Compute the HMAC Signature with JavaScript

Click Variable From Script to generate the HMAC signature for your HTTP request.

A local variable generated with JavaScript
  • To import variables into your script, use dd.variable.get("<variable_name>").
  • To define a variable, use either dd.variable.set("<variable_name>", <value>) or dd.variable.setObfuscated("<variable_name>", <value>).

You also have access to helper functions, such as:

  • Most of the std library, accessible with std.*. For example, to call the function encodeHex defined in <std>/encoding/hex.ts, use std.encoding.hex.encodeHex.
  • Standard JavaScript APIs, such as the Web Crypto API.

Note: Some of these APIs are disabled for security reasons.

For example:

Variable from Script

const datetime = new Date().toISOString();
// Set a "date" HTTP header using DATETIME as its value in the UI
dd.variable.set("DATETIME", datetime);

const message = "Hello, World!";
// Use BODY as the request body in the UI
dd.variable.set("BODY", message);

const secretKeyUtf8 = dd.variable.get("MY_SECRET_KEY");
const key = await crypto.subtle.importKey(
  new TextEncoder().encode(secretKeyUtf8),
  { name: "HMAC", hash: "SHA-256" },

const rawSignature = await crypto.subtle.sign(
  { name: "HMAC" },
  new TextEncoder().encode(datetime + "." + message)

// Set an "authentication" HTTP header using SIGNATURE as its value in the UI
dd.variable.set("SIGNATURE", std.encoding.hex.encodeHex(rawSignature));

// Alternative:
dd.variable.set("SIGNATURE_BASE64", std.encoding.base64.encode(rawSignature));

Add the HMAC signature to the request header

Use the exported SIGNATURE variable to build the HTTP request header.

Under the Request Options tab, add a header with Name set to Authentication and Value set to {{ SIGNATURE }}, and another one with Name set to Date and Value set to {{ DATETIME }}. You can define a different header such as Authorization.

Configure the rest of your HTTP test, and click Create to save.

Further Reading