FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes the best practice for Explicit Proxy.

- For explicit proxies, when configuring limits on the number of concurrent users, it is necessary to allow for the number of users based on their authentication method.
Otherwise, run out of user resources prematurely.

- Each session-based authenticated user is counted as a single user using their authentication membership (RADIUS, LDAP, FSSO, local database etc.) to match users in other sessions.
So one authenticated user in multiple sessions is still one user.

- For all other situations, the source IP address is used to determine a user.
All sessions from a single source address are assumed to be from the same user.

- Set the explicit web proxy and explicit FTP proxy Default Firewall Policy Action to Deny.
This means that a firewall policy is required to use these explicit proxies, allowing to control access and impose security features.

Do not enable the explicit web or FTP proxy on an interface connected to the Internet.
This is a security risk because anyone on the Internet who finds the proxy could use it to hide their source address.
If it is necessary to enable the proxy on such an interface, make sure authentication is required to use the proxy.