Netfilter::Ruby

gem 'netfilter-ruby'

$ bundle

$ gem install netfilter-ruby

require "netfilter"

firewall = Netfilter.new("example") do |eb, ip4, ip6|
  ip4.table :filter do |t|
    t.chain "INPUT" do |c|
      # dont lock yourself
      c.filter protocol: :tcp, dport: 22, jump: :accept
      # allow dhcp requests
      c.filter protocol: :udp, source: "0.0.0.0", sport: 68, destination: "255.255.255.255", dport: 67, jump: :accept

      # drop everything else
      # c.filter jump: :drop
    end

    # capture all outgoing traffic (all traffic to the interface)
    t.chain :input do |c|
      c.filter in_interface: "eth0", jump: "CUSTOM_CHAIN"
    end

    t.chain :forward do |c|
      c.filter in_interface: "eth0", jump: "CUSTOM_CHAIN"
    end
  end
end

# activate rules
# test your rules before put on production
firewall.up

# print applied rules
firewall.pp