UFW is Uncomplicated Firewall. It is an easy way to setup iptables firewall rules on your Linux system. If it is not already installed, it can be installed on Debian-based systems (like Ubuntu) like this:
sudo apt-get install ufw
Check the status of current firewall rules.
sudo ufw status
Here's the output on one of my systems:
Status: active To Action From -- ------ ---- 22 ALLOW Anywhere 80 ALLOW Anywhere 10000/tcp ALLOW Anywhere 443/tcp ALLOW Anywhere 22 (v6) ALLOW Anywhere (v6) 80 (v6) ALLOW Anywhere (v6) 10000/tcp (v6) ALLOW Anywhere (v6) 443/tcp (v6) ALLOW Anywhere (v6)
If you haven't set it up before, you won't see all those rules. To start configuring your firewall, add the default rules.
sudo ufw default deny incoming sudo ufw default allow outgoing
If you're SSH'd into the machine, be sure to add a rule to allow it, otherwise you could lose access to your system.
sudo ufw allow 22/tcp
When you've finished adding rules, enable the firewall.
sudo ufw enable
Other useful commands:
sudo ufw status verbose sudo ufw disable sudo ufw reset
Once you have your rules set up, they will be persistent across reboots.