FTP servers are supposed to respond to a PASV command by sending their respective passive port and IP address. Sometimes a misconfigured server that resides behind a NAT will respond with its private IP rather than a public one, which means your passive connection will fail. This little hack saved the day for me.

ruby_passive_ftp.rb

class IPAddr
  def is_private?
    return between?('172.16.0.0','172.31.255.255') || between?('10.0.0.0','10.255.255.255') || between?('192.168.0.0','192.168.255.255')
  end
end

class Net::FTP
  alias_method :parse227_original, :parse227
  alias_method :connect_original, :connect
  def connect(host, port = FTP_PORT)
    @_true_hostname = host
    connect_original(host, port)
  end
  private
  def parse227(resp)
    host, port = parse227_original(resp)
    begin
      ip = IPAddr.new(host)
      if ip.is_private?
        puts "Fixing bogus PASV data address from #{host} to #{@_true_hostname}" if @debug_mode
        host = @hostname
      end
    end
    return host, port
  end
end