本猫以前写asm和C的时候常常不忘“逆向”一把，后来写驱动的时候也用VM之类的搭建“双机”调试环境进行调试；也对于一些小的软件crack cd-key神马的不亦乐乎。自从使用鸟所谓的高级动态语言ruby之后，这种黑逆的心态貌似逐渐减弱了...不过逮到机会还是难免心痒痒啊。

    ruby+linux的开源方式早已不要向bin码一样还要dis asm，不过有时候想要搞清楚一些功能还是要用点小技巧的，下面就解决一个小的问题给大家展示下这些东东吧

    ntp是一个时钟同步协议用在服务器和路由器上，ruby也有很多相关的gem，比如net-ntp，在gem install net-ntp之后可以使用如下代码获取ntp服务器的标准时间：

#!/usr/bin/ruby
require 'net/ntp'

def get_ntp_time(srv_addr)
	puts Net::NTP.get(srv_addr).time
end

get_ntp_time(ARGV[0])

wisy@wisy-ThinkPad-X61:~/src/ruby_src$ ./dzh.rb pool.ntp.org
2014-12-04 14:06:20 +0800
wisy@wisy-ThinkPad-X61:~/src/ruby_src$ ./dzh.rb time.nist.gov
2014-12-04 14:07:00 +0800

我简单分析了下ntp协议，发现如果自己实现可以用tcp或是udp的方式向ntp服务器端口123(ntp服务端口)发送一些报文，然后接收返回即可。我开始以为报文是任意的，因为以前记得用telnet ip 123也返回了时间字符串（现在觉得可能是记错了啊！）于是有了我的第一次尝试：

#!/usr/bin/ruby
require 'net/ntp'

def get_ntp_time_udp(srv_addr,msg)
	s = UDPSocket.new
	s.connect srv_addr,123

	s.send msg,0
	response,address = s.recvfrom 1024
	puts [response,address]
      s.close
end

def get_ntp_time(srv_addr)
	puts Net::NTP.get(srv_addr).time
end

if ARGV.count == 1
	get_ntp_time(ARGV[0])
else
	get_ntp_time_udp(ARGV[0],ARGV[1])
end

运行要带2个参数，第二个参数是要发送的报文：./ut.rb time.nist.gov hi , 但是运行后长时间挂起，貌似ntp服务器没有返回啊！分析了一下，ntp报文可能不是随便发的，要有一定格式，但到底是啥格式呢？百度了一下，格式比较复杂，转换成代码较麻烦啊！不如看一下net-ntp的实现代码不是更好吗？虽然是net-ntp是开源的，但是源文件在哪呢？怎么找呢？不如先用ruby的调试模式debug一下呗，ruby在运行时加上 -r debug（或是-rdebug）可以实现调试的功能,然后用n指令实现单步，用s指令实现单步步入跟踪：

wisy@wisy-ThinkPad-X61:~/src/ruby_src$ ruby -rdebug ut.rb time.nist.gov
Debug.rb
Emacs support available.

/usr/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:57:        RUBYGEMS_ACTIVATION_MONITOR.enter
(rdb:1) n
/usr/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:143:    RUBYGEMS_ACTIVATION_MONITOR.exit
(rdb:1) n
dzh.rb:2:require 'net/ntp'
(rdb:1) n
dzh.rb:4:def get_ntp_time_udp(srv_addr,msg)
(rdb:1) n
dzh.rb:14:def get_ntp_time(srv_addr)
(rdb:1) n
dzh.rb:18:if ARGV.count == 1
(rdb:1) n
dzh.rb:19:	get_ntp_time(ARGV[0])
(rdb:1) s
dzh.rb:15:	puts Net::NTP.get(srv_addr).time
(rdb:1) s
/var/lib/gems/2.1.0/gems/net-ntp-2.1.2/lib/net/ntp/ntp.rb:67:      sock = UDPSocket.new

这样起码可以看到Net::NTP.get的源代码在哪了。打开ntp.rb，并不复杂，一共200多行代码。其中注释很多都是#:nodoc:,是不是这样实现作者也没有找到依据文档呢？ntp.rb全部源代码如下：

require 'socket'
require 'timeout'

module Net #:nodoc:
  module NTP
    TIMEOUT = 60         #:nodoc:
    NTP_ADJ = 2208988800 #:nodoc:
    NTP_FIELDS = [ :byte1, :stratum, :poll, :precision, :delay, :delay_fb,
                   :disp, :disp_fb, :ident, :ref_time, :ref_time_fb, :org_time,
                   :org_time_fb, :recv_time, :recv_time_fb, :trans_time,
                   :trans_time_fb ]

    MODE = {
      0 => 'reserved',
      1 => 'symmetric active',
      2 => 'symmetric passive',
      3 => 'client',
      4 => 'server',
      5 => 'broadcast',
      6 => 'reserved for NTP control message',
      7 => 'reserved for private use'
    }

    STRATUM = {
      0 => 'unspecified or unavailable',
      1 => 'primary reference (e.g., radio clock)'
    }

    2.upto(15) do |i|
      STRATUM[i] = 'secondary reference (via NTP or SNTP)'
    end

    16.upto(255) do |i|
      STRATUM[i] = 'reserved'
    end

    REFERENCE_CLOCK_IDENTIFIER = {
      'LOCL' => 'uncalibrated local clock used as a primary reference for a subnet without external means of synchronization',
      'PPS'  => 'atomic clock or other pulse-per-second source individually calibrated to national standards',
      'ACTS' => 'NIST dialup modem service',
      'USNO' => 'USNO modem service',
      'PTB'  => 'PTB (Germany) modem service',
      'TDF'  => 'Allouis (France) Radio 164 kHz',
      'DCF'  => 'Mainflingen (Germany) Radio 77.5 kHz',
      'MSF'  => 'Rugby (UK) Radio 60 kHz',
      'WWV'  => 'Ft. Collins (US) Radio 2.5, 5, 10, 15, 20 MHz',
      'WWVB' => 'Boulder (US) Radio 60 kHz',
      'WWVH' => 'Kaui Hawaii (US) Radio 2.5, 5, 10, 15 MHz',
      'CHU'  => 'Ottawa (Canada) Radio 3330, 7335, 14670 kHz',
      'LORC' => 'LORAN-C radionavigation system',
      'OMEG' => 'OMEGA radionavigation system',
      'GPS'  => 'Global Positioning Service',
      'GOES' => 'Geostationary Orbit Environment Satellite'
    }

    LEAP_INDICATOR = {
      0 => 'no warning',
      1 => 'last minute has 61 seconds',
      2 => 'last minute has 59 seconds)',
      3 => 'alarm condition (clock not synchronized)'
    }

    ###
    # Sends an NTP datagram to the specified NTP server and returns
    # a hash based upon RFC1305 and RFC2030.
    def self.get(host="pool.ntp.org", port="ntp", timeout=TIMEOUT)
      sock = UDPSocket.new
      sock.connect(host, port)

      client_localtime      = Time.now.to_f
      client_adj_localtime  = client_localtime + NTP_ADJ
      client_frac_localtime = frac2bin(client_adj_localtime)

      ntp_msg = (['00011011']+Array.new(12, 0)+[client_localtime, client_frac_localtime.to_s]).pack("B8 C3 N10 B32")

      sock.print ntp_msg
      sock.flush

      read, write, error = IO.select [sock], nil, nil, timeout
      if read[0]
        client_time_receive = Time.now.to_f
        data, _ = sock.recvfrom(960)
        Response.new(data, client_time_receive)
      else
        # For backwards compatibility we throw a Timeout error, even
        # though the timeout is being controlled by select()
        raise Timeout::Error
      end
    end

    def self.frac2bin(frac) #:nodoc:
      bin  = ''

      while bin.length < 32
        bin += ( frac * 2 ).to_i.to_s
        frac = ( frac * 2 ) - ( frac * 2 ).to_i
      end

      bin
    end
    private_class_method :frac2bin

    class Response

      attr_reader :client_time_receive
      
      def initialize(raw_data, client_time_receive)
        @raw_data             = http://www.mamicode.com/raw_data>
可以看到net-ntp的get方法使用的是TCP的方式,而且其实现的报文格式正常也是怎么也猜不到哦(),其中写了若干方法实现net字节顺序到本机字节顺序的相互转换。为了简单我们把它改写为UDP的方式吧，我们也不要啥类了，直接写全局方法吧，改写后的方法如下：
#!/usr/bin/ruby
require 'net/ntp'

MY_NTP_ADJ = 2208988800 #:nodoc:
MY_NTP_FIELDS = [ :byte1, :stratum, :poll, :precision, :delay, :delay_fb,
                   :disp, :disp_fb, :ident, :ref_time, :ref_time_fb, :org_time,
                   :org_time_fb, :recv_time, :recv_time_fb, :trans_time,
                   :trans_time_fb ]

def bin2frac(bin) #:nodoc:
        frac = 0
        bin.reverse.split("").each {|b|frac = ( frac + b.to_i ) / 2.0}
        frac
end

def frac2bin(frac) #:nodoc:
      bin  = ''
      while bin.length < 32
         bin += ( frac * 2 ).to_i.to_s
        frac = ( frac * 2 ) - ( frac * 2 ).to_i
      end
      bin
end

def packet_data_by_field(raw_data) #:nodoc:
          packetdata = http://www.mamicode.com/raw_data.unpack("a C3   n B16 n B16 H8   N B32 N B32   N B32 N B32")>
看一下运行结果吧：
wisy@wisy-ThinkPad-X61:~/src/ruby_src$ ./dzh.rb pool.ntp.org 1
ip:["AF_INET", 123, "202.112.29.82", "202.112.29.82"]
bin:**************************************************
!?
 _?z??*????_ST???`? ?*?x0??=?*?x0??
******************************************************
bin:@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
{:byte1=>"\x1C", :stratum=>2, :poll=>3, :precision=>233, :delay=>0, :delay_fb=>"0000110100100001", :disp=>0, :disp_fb=>"0000101000100000", :ident=>"5fde7ad2", 
:ref_time=>3626664177, :ref_time_fb=>"11010000110111000101111101010011", :org_time=>1417675511, :org_time_fb=>"10111000011000001001101000100000", 
:recv_time=>3626664312, :recv_time_fb=>"00110000100100001001110000111101", :trans_time=>3626664312, :trans_time_fb=>"00110000100110101101101011011
001"}
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
bin:@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
{:byte1=>"\x1C", :stratum=>2, :poll=>3, :precision=>233, :delay=>0, :delay_fb=>"0000110100100001", :disp=>0, :disp_fb=>"0000101000100000", :ident=>"5fde7ad2", 
:ref_time=>3626664177, :ref_time_fb=>"11010000110111000101111101010011", :org_time=>1417675511, :org_time_fb=>"10111000011000001001101000100000", 
:recv_time=>3626664312, :recv_time_fb=>"00110000100100001001110000111101", :trans_time=>3626664312, :trans_time_fb=>"00110000100110101101101011011
001"}
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
GET TIME IS : 2014-12-04 14:45:12 +0800
开源自有开源的好处，否则为了分析这点功能，只有载入IDA pro之类的重型武器了。还有一种办法是用抓包器看net-ntp发送的数据格式，看后照搬！正如看了《星际穿越》后觉得懂得了点神马一样：任何问题都有解决办法，无论什么样的问题！而且解决办法肯定不止一种！
从ruby实现时间服务器ntp同步功能也谈“逆向工程”