On 4/29/20 9:24 AM, Mukund Sivaraman wrote:
If there's a lock on my door, and someone tries to pick it, you can call me at fault for having a lock on my door facing outside all you want. But the thief picking it has no business doing so, and will be guilty of a crime if caught.
This is a good start to an analogy. Let's build on it, courtesy to YouTube's "Lock Picking Lawyer". In a video, the host shows how to improve the security of a common easily-picked home lock: drill holes in the lock body, such that if someone picks the lock and tries to turn the keyway, the pins will fall into those carefully-placed holes and foil The Bad Guy(tm).
In the networking world, we use an Access Control List to limit access to the service. Unlike the simple modification shown in LPL's video, the "lock" is still usable by users from authorized IP addresses. Or, we require the use of certificates to validate access within the SSHD server itself.
Here's the deal: just blocking access or requiring certificate-based access is intrusion prevention. Having a log event when there are unsuccessful probes is intrusion [attempt] detection. Sure, the ne'er-do-well is kept out in the prevention cycle, but a persistent cracker lives by the axiom "if at first you don't succeed, try something else." You really want to stop an attacker from making a large number of attempts, such as with a Joe script.
I turn off root SSH access, pinhole 22/tcp to a limited number of IP addresses, and monitor failed SUDO attempts. As I build up my new firewall, I'll turn off public SSH access completely, and instead use a robust VPN implementation. (Which has its own issues.)