Category: Networking

Using NFQUEUE to build your own firewall

Before we dive in, let me preface by stating that there are obviously pre-built firewalls (both free and commercial) which will be much more performant and secure. However, this should not stop you from building your own firewall: it’s both fun and educational! I’ll be using a Raspberry Pi to build the firewall, but you can use any Linux based OS.

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Port probing and scanning with netcat

Netcat is built into most if not all Linux distributions. It is an incredibly useful tool for quick network tests: to see if a device is reachable or a service is running. Seeing a “Connection refused” in netcat is in itself already valuable information as it could indicate that the remote host is reachable, but the service just isn’t running.

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Subnet calculator

As a first simple tool I’ve put up a basic IPv4 subnet calculator. It’s fully clientside and accepts the common input methods ( or An IPv6 one is also in the works.

Taking a useful packet capture using tcpdump

When troubleshooting an issue on any device where network connectivity is impacted, a packet capture is still king. This is not to say every 403 Forbidden error needs to be examined in Wireshark, but when a user reports their browsing saying the connection was reset or a timeout was hit, it might be time to fire up tcpdump.

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