In networking, ports are like numbered doors on a server. SSH typically uses port 22, and RDP uses port 3389. When these ports are "open" on your firewall and directed to your servers, it means the servers are actively listening for connection attempts on those ports.
Without a VPN, these ports are directly accessible from the entire internet. Anyone, anywhere in the world, can try to connect to them. This is like leaving the front door of your house not only unlocked but wide open for anyone to walk in.
Attackers use automated tools called port scanners to scan vast ranges of IP addresses, looking for open ports. These scanners can quickly identify servers that have SSH or RDP exposed.
By using a VPN, you close the "front gate" (hide the SSH and RDP ports). The only way to access those services is through the VPN "tunnel," which requires authentication and provides encryption, significantly enhancing security.
A) Implementing RDP with Network Level Authentication (NLA) for all servers and restricting SSH access to a single jump server. While NLA adds a layer of security to RDP and a jump server helps isolate SSH, it doesn't fully address the risk of exposing these services to the internet. Vulnerabilities in RDP or the jump server itself could still be exploited.
B) Enabling SSH access on Linux servers and RDP on Windows servers, both directly accessible from the internet with strong passwords. This is the least secure option. Exposing RDP and SSH directly to the internet, even with strong passwords, makes them prime targets for brute-force attacks, botnets, and exploits of known vulnerabilities.
C) Using SSH tunneling for all administrative tasks, disabling RDP entirely, and enforcing key-based authentication for SSH. While SSH with key-based authentication and tunneling is a very secure method for Linux servers, completely disabling RDP might not be reasonable in a hybrid environment where Windows servers require management. Furthermore, while SSH tunneling can be used to tunnel RDP, it adds complexity and doesn't provide the centralized access control and network-level security of a VPN.