It's not uncommon to see a lot of entries in your logs. i have BFD installed and my logs show many. Just make sure you use a really hard password numbers, and chars.
If you really want to make it a little harder, change the root shell port to anything other than 22 and create 2 logins. a dummy login and then the root login. It wont make your server impenitrable, but it can make it a little harder for intruders.
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