Simple DNSSD FTPS client/server written as an example of a secure file transfer system of peer-to-peer nature with autodiscovery, ala https://pay.reddit.com/r/ruby/comments/1y13h4/secure_peertopeer_in_ruby/

client.rb

require 'double_bag_ftps'
require 'dnssd'

browser = DNSSD::Service.new
services = {}

browser.browse '_ftps._tcp' do |reply|
  puts reply.fullname
  DNSSD::Service.new.resolve(reply) do |r|
    puts "Listing files available on #{r.name} on #{r.target}:#{r.port}"
    ftp = DoubleBagFTPS.new
    ftp.ssl_context = DoubleBagFTPS.create_ssl_context(
                          :verify_mode => OpenSSL::SSL::VERIFY_NONE)
    ftp.passive = true
    ftp.connect r.target, r.port
    ftp.login
    puts ftp.list
  end
end

Gemfile

source 'https://rubygems.org'

gem 'dnssd'
gem 'double-bag-ftps'
gem 'ftpd'

gen_cert.sh

#!/bin/bash

#this is untested, just pulled from my history

openssl genrsa 4096 > priv
chmod 400 priv
openssl req -new -x509 -nodes -sha1 -days 3650 -key priv > cert.pem
cat priv cert.pem > cert2.pem

#note that we don't care at all about identity here, just that we're encrypting what goes over the wire

server.rb

require 'ftpd'
require 'tmpdir'
require 'dnssd'
require 'logger'

module Ftpd
  class Server
    def raw_socket
      @server_socket.to_io
    end
  end
end

class Driver
  include Ftpd::InsecureCertificate

  def initialize(temp_dir)
    @temp_dir = temp_dir
  end

  def authenticate(user, password)
    true
  end

  def file_system(user)
    Ftpd::DiskFileSystem.new(@temp_dir)
  end

end

Dir.mktmpdir do |temp_dir|
  driver = Driver.new(temp_dir)
  server = Ftpd::FtpServer.new(driver)
  server.interface = "0.0.0.0"
  server.port = 8721
  server.certfile_path = "cert2.pem"
  server.tls = :explicit
  server.log = Logger.new STDOUT
  server.start
  DNSSD.announce server.raw_socket, 'Ruby FTPS DNSSD Example', 'ftps'
  puts "Server listening on port #{server.bound_port}"
  puts "Files can go in #{temp_dir}"
  gets
end