A properly configured firewall is one of the most important layers of security for any internet-facing server. UFW (Uncomplicated Firewall) is a user-friendly front-end for managing iptables rules that ships pre-installed on Ubuntu. Its defaults are sensible: block all incoming connections, allow all outgoing connections. No outside traffic reaches your server unless you explicitly open a port. This guide covers how to configure a UFW firewall on Ubuntu 18.04, from setting default policies and application profiles to writing allow and deny rules for specific ports, IPs, and subnets. Configure UFW Firewall on Ubuntu: Defaults, SSH, and Application Profiles If UFW is not installed, add it with: Check the current status: Application profiles. When you install a service with apt, it registers a profile in /etc/ufw/applications.d/. List all available profiles: Use a profile name or a port number interchangeably when writing rules. To see what ports a profile covers: Allow SSH before enabling UFW. This is the most critical step. If you enable UFW without an SSH rule and you are on a remote server, you lose access immediately: If your SSH daemon runs on a non-standard port, use the port number instead, for example sudo ufw allow 4422/tcp. Enable UFW: Type y at the prompt to confirm. Allow Port Ranges, Specific IPs, Subnets, and Network Interface Rules Common...

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