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/rubyrequire 'net/ntp'def get_ntp_time(srv_addr) puts Net::NTP.get(srv_addr).timeendget_ntp_time(ARGV[0])运行结果如下：
[email protected]:~/src/ruby_src$ ./dzh.rb pool.ntp.org2014-12-04 14:06:20 +0800[email protected]:~/src/ruby_src$ ./dzh.rb time.nist.gov2014-12-04 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/rubyrequire '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.closeenddef get_ntp_time(srv_addr) puts Net::NTP.get(srv_addr).timeendif 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：
[email protected]:~/src/ruby_src$ ruby -rdebug ut.rb time.nist.govDebug.rbEmacs 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) ndzh.rb:2:require 'net/ntp'(rdb:1) ndzh.rb:4:def get_ntp_time_udp(srv_addr,msg)(rdb:1) ndzh.rb:14:def get_ntp_time(srv_addr)(rdb:1) ndzh.rb:18:if ARGV.count == 1(rdb:1) ndzh.rb:19: get_ntp_time(ARGV[0])(rdb:1) sdzh.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 = raw_data @client_time_receive = client_time_receive @packet_data_by_field = nil end def leap_indicator @leap_indicator ||= (packet_data_by_field[:byte1].bytes.first & 0xC0) >> 6 end def leap_indicator_text @leap_indicator_text ||= LEAP_INDICATOR[leap_indicator] end def version_number @version_number ||= (packet_data_by_field[:byte1].bytes.first & 0x38) >> 3 end def mode @mode ||= (packet_data_by_field[:byte1].bytes.first & 0x07) end def mode_text @mode_text ||= MODE[mode] end def stratum @stratum ||= packet_data_by_field[:stratum] end def stratum_text @stratum_text ||= STRATUM[stratum] end def poll_interval @poll_interval ||= packet_data_by_field[:poll] end def precision @precision ||= packet_data_by_field[:precision] - 255 end def root_delay @root_delay ||= bin2frac(packet_data_by_field[:delay_fb]) end def root_dispersion @root_dispersion ||= packet_data_by_field[:disp] end def reference_clock_identifier @reference_clock_identifier ||= unpack_ip(packet_data_by_field[:stratum], packet_data_by_field[:ident]) end def reference_clock_identifier_text @reference_clock_identifier_text ||= REFERENCE_CLOCK_IDENTIFIER[reference_clock_identifier] end def reference_timestamp @reference_timestamp ||= ((packet_data_by_field[:ref_time] + bin2frac(packet_data_by_field[:ref_time_fb])) - NTP_ADJ) end def originate_timestamp @originate_timestamp ||= (packet_data_by_field[:org_time] + bin2frac(packet_data_by_field[:org_time_fb])) end def receive_timestamp @receive_timestamp ||= ((packet_data_by_field[:recv_time] + bin2frac(packet_data_by_field[:recv_time_fb])) - NTP_ADJ) end def transmit_timestamp @transmit_timestamp ||= ((packet_data_by_field[:trans_time] + bin2frac(packet_data_by_field[:trans_time_fb])) - NTP_ADJ) end def time @time ||= Time.at(receive_timestamp) end # As described in http://tools.ietf.org/html/rfc958 def offset @offset ||= (receive_timestamp - originate_timestamp + transmit_timestamp - client_time_receive) / 2.0 end protected def packet_data_by_field #:nodoc: if [email protected]_data_by_field @packetdata = @raw_data.unpack("a C3 n B16 n B16 H8 N B32 N B32 N B32 N B32") @packet_data_by_field = {} NTP_FIELDS.each do |field| @packet_data_by_field[field] = @packetdata.shift end end @packet_data_by_field end def bin2frac(bin) #:nodoc: frac = 0 bin.reverse.split("").each do |b| frac = ( frac + b.to_i ) / 2.0 end frac end def unpack_ip(stratum, tmp_ip) #:nodoc: if stratum < 2 [tmp_ip].pack("H8").unpack("A4").bytes.first else ipbytes = [tmp_ip].pack("H8").unpack("C4") sprintf("%d.%d.%d.%d", ipbytes[0], ipbytes[1], ipbytes[2], ipbytes[3]) end end end endend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/rubyrequire '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} fracenddef frac2bin(frac) #:nodoc: bin = '' while bin.length < 32 bin += ( frac * 2 ).to_i.to_s frac = ( frac * 2 ) - ( frac * 2 ).to_i end binenddef packet_data_by_field(raw_data) #:nodoc: packetdata = raw_data.unpack("a C3 n B16 n B16 H8 N B32 N B32 N B32 N B32") packet_data_by_field = {} MY_NTP_FIELDS.each do |field| packet_data_by_field[field] = packetdata.shift end puts "bin:"+"@"*50 puts packet_data_by_field puts "@"*54 packet_data_by_fieldenddef receive_timestamp(raw_data) (packet_data_by_field(raw_data)[:recv_time] + bin2frac(packet_data_by_field(raw_data)[:recv_time_fb])) - MY_NTP_ADJenddef get_ntp_time_udp(srv_addr) s = UDPSocket.new s.connect srv_addr,123 client_localtime = Time.now.to_f client_adj_localtime = client_localtime + Net::NTP::NTP_ADJ client_frac_localtime = frac2bin(client_adj_localtime) bin = (['00011011']+Array.new(12, 0)+[client_localtime, client_frac_localtime.to_s]).pack("B8 C3 N10 B32") s.send bin,0 response,address = s.recvfrom(1024) puts "ip:#{address}" puts "bin:"+"*"*50 puts response puts "*"*54 puts "GET TIME IS : #{Time.at(receive_timestamp(response))}"enddef get_ntp_time(srv_addr) puts Net::NTP.get(srv_addr).timeendif ARGV.count == 1 get_ntp_time(ARGV[0])else get_ntp_time_udp(ARGV[0])end看一下运行结果吧：
[email protected]:~/src/ruby_src$ ./dzh.rb pool.ntp.org 1ip:["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=>"00110000100110101101101011011001"}@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@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=>"00110000100110101101101011011001"}@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@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！