Question: I am building web applications for my customer's company. At the server side, there will be 2 kinds of server to server network communication.

1. Separated REST API servers making requests among each other.
2. Communication from application load balancers (AWS ALB specifically) to their auto-scaling EC2 instances.

Currently all of these communications use HTTP protocol. Only the user-facing nodes (such as the load balancer or the web server reverse proxy) will serve HTTPS with valid certificates.

The customer ask us to change them all to HTTPS as thet believe that it is the modern best practice to always use HTTPS instead of HTTP anywhere.

I would like to dispute with the customer but I am no security expert. Please help review my understanding below and correct me if I am wrong.

---

In my view, I think the purpose of HTTPS protocol is for being a trusted channel in an untrusted environment (such as the Internet). So I cannot see any benefit of changing the already trusted channel to HTTPS. Further more, having to install certificates to all servers make it difficult to maintain, chances are, the customer will find their application servers broken someday in the future because some server has certificate expired and no one know.

Another problem, if we have to config all the application server, apache for example, behind the load balance to serve HTTPS, then what is the `ServerName` to put inside the `VirtualHost`? Currently we have no problem using the domain name such as `my-website.example.com` for HTTP `VirtualHost`. But if it were to be HTTPS we have to install certificate of `my-website.example.com` to all instances behind the load-balancer? I think it's weird because then we have many servers claiming to be `my-website.example.com`.

Answer: 
Internal networks are not secure
--------------------------------

In general, internal networks are more secure than public-facing systems, but they should not be considered as completely secure. A significant portion of attacks come from the inside - spearphishing, social engineering and insider attacks are all popular vectors which start with a foothold inside of your network.

So there's no good reason for unencrypted traffic of secret or private information even over your internal networks. You don't necessarily need public names or CA hierarchy - if you have well-defined bilateral communications channels, then it may be simpler to have an explicit trust relationship where your load-balancers are configured to trust a particular self-signed certificate of your backend servers and nothing else.
"https" not only secures the communications as it passes over the network, but also verifies the *certificate* that is presented by the server. This enables you to know that you really are talking to the correct site. And this, in my opinion, is the real advantage of "https."

"letsencrypt.org," which issues signed certificates for free, has a lot of material on their web-site which discusses these benefits. They argue ... quite rightly, I think ... that "everything should be https, whether the material is actually sensitive or not."

(`mod_ssl` *et al* are also capable of enforcing certificate possession on the *client* side, too, although this is rarely done. For a secure internal application, however, you might want to do such a thing. The server is then able to restrict which computers are capable of connecting to it at all, restricting them by the credentials *they* must possess.)