Brute force attackers seem to switch targets rapidly if you block them
Like everyone else, we have a constant stream of attackers trying brute force password guessing against us using SSH or authenticated SMTP, from a variety of source IPs. Some of the source IPs attack us at a low rate (although there can be bursts when a lot of them are trying), but some of them do so at a relatively high rate, high enough to be annoying. When I notice such IPs (ones making hundreds of attempts an hour, for example), I tend to put them in our firewall blocks. After recently starting to pay attention to what happens next, what I've discovered is that at least currently, most such high volume IPs give up almost immediately. Within a few minutes of being blocked their activity typically drops to nothing.
Once I thought about it, this behavior feels like an obvious thing for attackers to do. Attackers clearly have a roster of hosts they've obtained access to and a whole collection of target machines to try brute force attacks against, with very low expectations of success for any particular attack or target machine; to make up for the low success rate, they need to do as much as possible. Wasting resources on unresponsive machines cuts down the number of useful attacks they can make, so over time attackers have likely had a lot of motivation to move on rapidly when their target stops responding. If the target machine comes back some day, well, they have a big list, they'll get around to trying it again sometime.
The useful thing about this attacker behavior is that if attackers are going to entirely stop using an IP to attack you (at least for a reasonable amount of time) within a few minutes of it being blocked, you only need to block attacker IPs for those few minutes. After five or ten or twenty minutes, you can remove the IP block again. Since the attackers use a lot of IPs and their IPs may get reused later for innocent purposes, this is useful for keeping the size of firewall blocks down and limiting the potential impact of a mis-block.
(A traditional problem with putting IPs in your firewall blocks is that often you don't have a procedure to re-assess them periodically and remove them again. So once you block an IP, it can remain blocked for years, even after it gets turned over to someone completely different. This is especially the case with cloud provider IPs, which are both commonly used for attacks and then commonly turn over. Fast and essentially automated expiry helps a lot here.)
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