I have an Ubuntu VPS that I use for practice and deploying prototypes as I am a python developer.

I recently started teaching my self sys admin tasks, like installing OpenLDAP. I happened to turn off the ufw firewall for just a minute, and when I ran an netstat command, I saw a foreign ip connected to ssh that I traced to china. I'd like to know a few things:

1) Is there any good network intrusion detection software, such that if any IP that's outside a specific range connects to the VPN, I can be notified? -- I am thinking about scripting this, but I'm pretty sure there's something useful out there and I believe in the wisdom of crowds.

2) How did this person gain access to my server? Is it because my firewall was down? Or is it because they browsed my LDAP directory and from there figured out a way to connect (there was a clear text password in the tree but it wasn't one used by the server's sshd)?