As you may know, IPtables and NetFilter combine to make the most popular firewall solution in Linux. Given there's only a native command-line interface (CLI) for the two, though, there can be a learning curve. The good news, however, is that there are many graphical user interfaces (GUIs) you can use with Linux. Let's look at some of the most powerful yet easy-to-use options available.
Firewall Builder (fwbuilder): This is a flexible and powerful tool that allows you to build firewall configurations and policies for multiple firewalls/machines based on iptables, ipfilter, OpenBSD pf, Cisco ASA & PIX, or Cisco FWSM. fwbuilder uses objects and functionality like drag-and-drop and search-and-replace to help ease configurations. You can install the fwbuilder on a separate machine and then either manually transfer the configurations or securely deploy via SSH or SCP.
Firestarter: A simple but feature-rich GUI that you'd install on the server or workstation you'd like to manage the IPTables firewall. It has a wizard for initial setup but also allows you to manually create inbound and outbound policies and define a whitelist or blacklist. You can also view firewall events in real-time and get stats on active network connections, including any traffic routed through the firewall. It can also perform Internet connection sharing with a built-in DHCP server.
Gufw Firewall: This is the GUI for the Uncomplicated Firewall (UFW), which Ubuntu uses as its default. It was designed to be — well, as the name says — uncomplicated. However, it still provides the basic inbound and outbound policies and has some great functionality, such as preconfigured rules for various applications and multiple profile support. If you're running something other than Ubuntu, UFW can be easily downloaded via links on their website to packages in repositories from Debian, Mint, openSUSE, Arch, and Salix.
PeerGuardian Linux: Self proclaimed as a privacy-oriented firewall application, it blocks inbound and outbound connections to hosts specified in large blacklists you can choose from containing thousands or millions of IP ranges. It's designed for protecting you from aggressive IPs while you use P2P sharing. The blacklists contain IPs that point to "known bad" computers and servers, including those used for advertising or spyware and those that have been "hacked." Though there are PeerGuardian firewalls for Mac OS X and Windows, the Linux edition is the only one currently maintained.
FirewallD: This is a dynamically-managed firewall solution that's included in Red Hat Enterprise Linux (RHEL) 7, so changes can be applied without restarting the whole firewall, and it offers integration with other applications. It supports different network/firewall zones (public, work, home, etc) to categorize trust levels of network connections and interfaces. FirewallD supports both IPv4 and IPv6.
Vuurmuur Firewall: A powerful firewall solution that can be managed via the Ncurses GUI in console with no X required, and via SSH. It offers real-time monitoring of connections and bandwidth usage. It supports traffic shaping and anti-spoofing features, and it works with Suricata IPS and Snort.
Many desktop Linux distros actually come preloaded with a firewall GUI, some of which are discussed here, but you could use a different one if you'd like.
For great flexibility and for managing multiple machines, consider Firewall Builder (fwbuilder). For a simple firewall, check out Firestarter or Gufw.
If regular firewall changes or third-party integration are needed, consider FirewallD. If you're looking for a GUI that can run on non-X servers, check out Vuurmuur. For privacy while using P2P, there's PeerGuardian Linux.
Eric Geier is a freelance tech writer -- keep up with his writings on Facebook. He's also the founder of NoWiresSecurity, a cloud-based Wi-Fi security service, and On Spot Techs, an on-site computer services company.