Allow unmetrered connections on networking is so risky. Attacker can use the brute-force attacks to comprosise your service (or simple DOS).
Linux has a cool firewall to hanlde this, via ip-tables. But it's so complicated to remember all the rule and syntax. That's why UFW was born to save us. :D
You can use simple command to manage your firewall
$ ufw default deny incoming # deny any incoming port, should be run before allow any port
$ ufw default allow outgoing # allow any outgoing port
$ ufw allow 80 # allow port 80
$ ufw deny 53/udp # allow udp protocol to port 53
$ ufw disable # disable firewall
$ ufw enable # enable firewall
$ ufw status # check all the rules
$ ufw delete [num] # delete the rule by its order in status result
$ ufw reload # reload all rule
$ ufw limit ssh/tcp # finnaly, limit ssh (port 22 tcp), deny connections if an IP address has attempted to initiate 6 or more connections in the last 30 seconds