I've set up a site on Digital Ocean without a domain yet, so there is only the IP.
Despite telling no-one of its existence or advertising it, I get hundreds of notices from fail2ban that various IP's are trying to hack my SSL port or are looking for PHP files.

But how do they know that I do exist? Where do they get the IP from? 

You can't hide your IP address on the internet. They aren't secret.

Pretty much what @DeerHunter said. It's trivial to scan the entire internet. If they want, they can target all-known digital ocean droplets that are online. 

They can do this on a timer so that when you go offline, or online, it will just keep trying as those may be high-value targets that could become vulnerable at a moment's notice.

Let me give you a very rough coding example. Let's pretend your IP address is 104.16.25.255. Let's get the IP address of www.digitalocean.com so we can easily check for associated IP addresses. www.digitalocean.com returns 104.16.25.4. Let's scan everything: 104.16.25.*



Scanning is incredibly easy from a programming standpoint

Let's assume we want to try and find all nearby associated IP addreses. Assume programs can handle numbers and patterns very well. Here's an example of an integer being incremented:

i++;


This increments the current value of i by 1. Let's assume i starts off as 1. After i++, you'll get 2. Check out this painfully simple loop:

for (int i = 1; i < 256; i++)
{
    scanIpAddress("104.16.25." + i);
}


An alternative one-line bash variant would be as follows:

for ip in `seq 1 255`; do scan_thingy_command 192.168.0.$ip --options -oG lol.txt; done


You just scanned 104.16.25.1, and changed i from 0 to 1. As the whole loop continues, it will go from 104.16.25.0 to 104.16.25.255. I don't have time to scan and look right now, however, it's possible that this tiny block doesn't just belong to digitalocean. 

To find more targets on DigitalOcean, a programmer may change the numbers even more. For example, introduce another loop that nests the aforementioned loop on the inside, and add j: scanIpAddress("104.16." + j + "." + i);. This will allow them to scan 104.16.1-255.1-255. 

From there, they can keep going backwards and nesting for loops until they get the entire internet. There are other, more efficient ways to do this, such as masscan, but this is the most basic way.

Again, this could also be done on the command line with one line:

for oct1 in `seq 1 255`; do for oct2 in `seq 1 255`; do for oct3 in `seq 1 255`; do for oct4 in `seq 1 255`; do scan $oct1.$oct2.$oct3.$oct4 --stuff; done; done; done; done 




Other methods

The above example was a really rough example. They may be doing more, their code might be different, and they may be using entirely different methods and/or programs. However, the concept is pretty much the same.

It's also possible that the programs in question are just targeting everyone en masse. 



So how can I hide my stuff online?

If it's online, whatever you are hiding, they will find it... or try to find it. 

However, depending on your web server, you can try http access controls such as .htaccess. If you're using access controls - again, this depends on your web server - then it's likely that you'll be able to prevent others from viewing/accessing pages. 

That won't protect you against non-website login attempts, though. And if you're denying them access to non-existent webpages, they now know you're really online, and can focus their attacks more easily! However, it's still good practice. 

Here's an example .htaccess deny for Apache (2.4 and later):

Require ip 192.168.1.100


In the above example, you're denying everyone access to that folder, except your IP address. Keep in mind, 192.168.1.100 is a local IP address. You'll have to replace that with your public IP address. 

Also, keep in mind that if your attacker is running a proxy/VPN on your machine, they can still access those pages. If your attacker already has access to the website, they can either edit the .htaccess or remove it. Nothing's 100%.

Just don't put anything online if you aren't ready to be scanned. Everyone has a plan until they get port-scanned in the mouth.
