Tools for testing the Dropbox Sign API and callbacks locally

To test the Dropbox Sign API locally, you'll have to use a service to tunnel a public url to localhost (your local machine, as opposed to a publicly accessible url). Here are few examples of web services that allow you to do so:

ngrok

localtunnel

pagekite

forwardhq

These types of tools will give you a url to your localhost port, which you can use as the account and/or app callback URL while you're testing.