ipsets with UFW

This is an example that lets you use UFW (https://wiki.ubuntu.com/UncomplicatedFirewall) and ipsets to dynamically whitelist IPs for a common host-based firewall. 

First you need a systemd script that creates an ipset on startup and then have a script that integrates this ipset with UFW to allow specific traffic (like an SSH knock), you'll need to create two main components:

1. A systemd service script that creates and loads the ipset at startup.
2. A UFW application profile or direct rules that utilize the ipset for allowing specific traffic.

1. Systemd Service for Ipset

Create a systemd service that will run at startup to create and populate your ipset. Here's how you can create this service:

Step 1: Create the ipset Script

First, create a script that will define and load your ipset. Let's call this script create_ipset.sh.

/usr/local/bin/create_ipset.sh:

#!/bin/bash 
# Name of the ipset 
IPSET_NAME="knocknoc"
# Flush if exists and create
ipset ipset -exist flush $IPSET_NAME
ipset -exist create $IPSET_NAME hash:ip
# Optionally, add IPs here or via separate mechanism
# ipset add $IPSET_NAME 192.168.1.100

Make sure to make it executable:

Step 2: Create the Systemd Service File

Create a new systemd service file called ipset.service.

/etc/systemd/system/ipset.service:

[Unit] 
Description=Create ipset for SSH Knocking
Before=ufw.service
After=network.target


[Service]
Type=oneshot
ExecStart=/usr/local/bin/create_ipset.sh

[Install]
WantedBy=multi-user.target

This script sets up the service to run before UFW (as indicated by Before=ufw.service) but after the network is available (After=network.target).

Enable and start the service:

2. Integrating Ipset with UFW

For integrating the ipset with UFW, you will typically use direct iptables rules. However, UFW does not directly support ipset in its regular configuration files, so you'll have to add these rules in the after.rules file for them to be applied with UFW.

Add Rules to UFW's after.rules

Edit the /etc/ufw/after.rules file to add your custom iptables rules referencing the ipset.

This rule will allow SSH connections on port 22 for IP addresses that are in the knocknoc ipset.

Notes and Considerations:

• Testing: Always test firewall and startup scripts in a controlled environment before deploying them on a production server. "Cutting your hands off" is too easy if you are not careful!
• Order of Execution: The systemd script needs to execute before UFW starts, hence the Before=ufw.service directive. Ensure that the network is available before the script runs.

With this setup, your system will create and populate the ipset at startup, and UFW will utilize these sets to allow traffic as specified. This approach provides a balance between the simplicity of UFW and the power of ipset and iptables. Always ensure to test and verify your configuration in a safe environment before applying it to a live system.