1. Application relay concept

Application relays, also known as "proxies", are software programs that relay requests from one perimeter to another, in addition to controlling access. However, unlike a firewall operating at the network layer (3) of the OSI model, an application relay works at the application layer (7). Consequently, unlike a firewall, an application relay cannot work with any network service, but is specialized for a particular service. The most frequently encountered proxies are of course DNS and SMTP, which are native, but also FTP, HTTP, HTTPS, NNTP and so on.

Because of this feature, many network services based on encryption cannot be relayed to the application level. Indeed, if encryption is end-to-end, the relay between client and server will not be able to decrypt the information, and therefore relay it.

However, the application relay has advantages...