Testing PayPal with Rails

I need to test a new PayPal integration with my application. I use PayPal sandbox in development env.

In order to test the entire interface, I figured out that ngrok might solve the problem of messages from PayPal.

I installed ngrok using apt-get install ngrok-client on my new Ubuntu 16.04 instalation.

When I try to run it (ngrok 3000, ngrok 80) I get the following error:

Invalid address server_addr 'ngrokd.ngrok.com:443': lookup ngrokd.ngrok.com: no such host

Any idea?
Any other solution to test PayPal integration?

==================== EDIT ==================

Very important - if youi use Rails 4.2.0, have a look at this tutorial, it might save you some hours of frustration:
https://www.youtube.com/watch?v=YXKNxEv35HU

Try to Run ./ngrok http 3000 in the Directory where your Ngrok executable file is placed.

I ran into this when trying to install ngrok with apt-get on Ubuntu 14.

The packaged version (1.6) no longer works, because service for ngrok 1.x was discontinued in April 2016.

The ngrok 1.X service has shut down and all users are encouraged to upgrade to ngrok 2.0.

The easiest way to continue using ngrok is to upgrade to 2.0. ngrok 2.0 has been available for nearly a year and it is a faster, more stable and more powerful tool. I hope that you'll love it even more than the original.

Ubuntu 16 also ships ngrok 1.6, unfortunately.

The current version available for download at ngrok.com is 2.1.3, which works fine. ngrok is a single executable file, so all you need to do to install it is unzip and put it somewhere on your PATH.

I supposed you installed ngrok from package manager with this command