Send an email when a new host joins the local network

notify-new-hosts

#!/usr/bin/env ruby

# Scans the network and sends an email whenever new hosts are detected. Uses MAC address to identify hosts.
# Published here: https://gist.github.com/f1sherman/1c6c5e5f31c1fab33bbd1028c15d88e1

EMAIL = 'you@example.com'

require 'set'
require 'yaml'
require 'net/smtp'

def send_alert(arpscan_info, nslookup_info)
  subject = "WARNING - A New Host Has Joined The Network"

  message = "#{arpscan_info}\n\n#{nslookup_info}"

  Net::SMTP.start('localhost', 25) do |smtp|
    mail_message = <<~EOS
      From: Notify New Hosts <#{EMAIL}>
      To: #{EMAIL}
      Subject: #{subject}
      #{message}"
    EOS

    smtp.send_message mail_message, EMAIL, EMAIL
  end
end

data_file = "#{ENV['HOME']}/.notify-new-hosts.yml"

known_hosts = File.exists?(data_file) ? YAML.load_file(data_file) : Set.new

arpscan_output = %x(arp-scan --localnet)

raise "arp-scan failed: #{arpscan_output}" unless $?.to_i == 0

host_lines = arpscan_output.split("\n").grep(/^\d+\.\d+\.\d+\.\d+/)

host_lines.each_with_object([]) do |line|
  ip, mac = line.split("\t")
  next if known_hosts.include? mac

  nslookup_output = %x(nslookup #{ip})
  send_alert line, nslookup_output
  known_hosts << mac
end

File.open data_file, 'w' do |out|
  YAML.dump known_hosts, out
end