DHT是什么

• DHT全称叫分布式哈希表(Distributed Hash Table)，是一种分布式存储方法。在不需要服务器的情况下，每个客户端负责一个小范围的路由，并负责存储一小部分数据，从而实现整个DHT网络的寻址和存储。
• 各种博客已经有很多关于DHT介绍，这里就不详细列举了

加入DHT网络可以做什么

1. 老司机开车(神秘代码)
2. P2P资源共享

DHT网络的其中一种协议实现(Kademlia)

• 要加入一个DHT网络，需要首先知道这个网络中的任意一个节点。如何获得这个节点？在一些开源的P2P软件中，会提供一些节点地址
• 主要协议

1. ping(用于确定某个节点是否在线。这个请求主要用于辅助路由表的更新)
2. find_node(用于查找某个节点，以获得其地址信息。)
3. get_peer(通过资源的infohash获得资源对应的peer列表。)
4. announce_peer(通知其他节点自己开始下载某个资源,announce_peer中会携带get_peer回应消息里的token。)

DHT爬虫的工作原理

1. 通过其他节点的announce_peer发来的infohash确认网络中有某个资源可被下载
2. 通过从网络中获取这个资源的种子文件，来获得该资源的描述
3. 不停的认识新节点,让远程节点保存自身到远程的路由表中

爬虫做的工作

1. BOOTSTRAP过程,加入DHT网络(主动认识DHT网络的其中一个节点)
2. 加入进DHT网络后。远端节点会主动告诉我们它认识哪些节点
3. 认识远端节点认识的节点
4. 当远端成功保存自身节点到远端路由表中的时候，目的达成
5. 等待远端的announce_peer消息
6. 成功获取远端的下载hash
7. 开车！(雾

使用Python实现的爬虫:

# coding: utf-8import socket
from hashlib import sha1
from random import randint
from struct import unpack, pack
from socket import inet_aton, inet_ntoa
from bisect import bisect_left
from threading import Timer
# from time import sleepfrom bencode import bencode, bdecodeBOOTSTRAP_NODES = [("router.bittorrent.com", 6881),("dht.transmissionbt.com", 6881),("router.utorrent.com", 6881)
]
TID_LENGTH = 4
KRPC_TIMEOUT = 10
REBORN_TIME = 5 * 60
K = 8def entropy(bytes):s = ""for i in range(bytes):s += chr(randint(0, 255))return s# """把爬虫"伪装"成正常node, 一个正常的node有ip, port, node ID三个属性, 因为是基于UDP协议,     # 所以向对方发送信息时, 即使没"明确"说明自己的ip和port时, 对方自然会知道你的ip和port,     # 反之亦然. 那么我们自身node就只需要生成一个node ID就行, 协议里说到node ID用sha1算法生成,     # sha1算法生成的值是长度是20 byte, 也就是20 * 8 = 160 bit, 正好如DHT协议里说的那范围: 0 至 2的160次方,     # 也就是总共能生成1461501637330902918203684832716283019655932542976个独一无二的node.     # ok, 由于sha1总是生成20 byte的值, 所以哪怕你写SHA1(20)或SHA1(19)或SHA1("I am a 2B")都可以,     # 只要保证大大降低与别人重复几率就行. 注意, node ID非十六进制,     # 也就是说非FF5C85FE1FDB933503999F9EB2EF59E4B0F51ECA这个样子, 即非hash.hexdigest(). """
def random_id():hash = sha1()hash.update( entropy(20) )return hash.digest()def decode_nodes(nodes):n = []length = len(nodes)if (length % 26) != 0: return nfor i in range(0, length, 26):nid = nodes[i:i+20]ip = inet_ntoa(nodes[i+20:i+24])port = unpack("!H", nodes[i+24:i+26])[0]n.append( (nid, ip, port) )return ndef encode_nodes(nodes):strings = []for node in nodes:s = "%s%s%s" % (node.nid, inet_aton(node.ip), pack("!H", node.port))strings.append(s)return "".join(strings)def intify(hstr):#"""这是一个小工具, 把一个node ID转换为数字. 后面会频繁用到.""" return long(hstr.encode('hex'), 16) #先转换成16进制, 再变成数字def timer(t, f):Timer(t, f).start()class BucketFull(Exception):passclass KRPC(object):def __init__(self):self.types = {"r": self.response_received,"q": self.query_received}self.actions = {"ping": self.ping_received,"find_node": self.find_node_received,"get_peers": self.get_peers_received,"announce_peer": self.announce_peer_received,}self.socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)self.socket.bind(("0.0.0.0", self.port))def find_node_handler(self,msg):passdef response_received(self, msg, address):self.find_node_handler(msg)def query_received(self, msg, address):try:self.actions[msg["q"]](msg, address)except KeyError:passdef send_krpc(self, msg, address):try:self.socket.sendto(bencode(msg), address)except:passclass Client(KRPC):def __init__(self, table):self.table = tabletimer(KRPC_TIMEOUT, self.timeout)timer(REBORN_TIME, self.reborn)KRPC.__init__(self)def find_node(self, address, nid=None):print "find node:",addressnid = self.get_neighbor(nid) if nid else self.table.nidtid = entropy(TID_LENGTH)msg = {"t": tid,"y": "q","q": "find_node","a": {"id": nid, "target": random_id()}}self.send_krpc(msg, address)def find_node_handler(self, msg):try:nodes = decode_nodes(msg["r"]["nodes"])for node in nodes:(nid, ip, port) = nodeif len(nid) != 20: continueif nid == self.table.nid: continueself.find_node( (ip, port), nid )except KeyError:passdef joinDHT(self):for address in BOOTSTRAP_NODES: self.find_node(address)def timeout(self):if len( self.table.buckets ) < 2:self.joinDHT()timer(KRPC_TIMEOUT, self.timeout)def reborn(self):self.table.nid = random_id()self.table.buckets = [ KBucket(0, 2**160) ]timer(REBORN_TIME, self.reborn)def start(self):self.joinDHT()while True:try:(data, address) = self.socket.recvfrom(65536)msg = bdecode(data)self.types[msg["y"]](msg, address)except Exception:passdef get_neighbor(self, target):return target[:10]+random_id()[10:]class Server(Client):def __init__(self, master, table, port):self.table = tableself.master = masterself.port = portClient.__init__(self, table)def ping_received(self, msg, address):try:nid = msg["a"]["id"]msg = {"t": msg["t"],"y": "r","r": {"id": self.get_neighbor(nid)}}self.send_krpc(msg, address)self.find_node(address, nid)except KeyError:passdef find_node_received(self, msg, address):try:target = msg["a"]["target"]neighbors = self.table.get_neighbors(target)nid = msg["a"]["id"]msg = {"t": msg["t"],"y": "r","r": {"id": self.get_neighbor(target), "nodes": encode_nodes(neighbors)}}self.table.append(KNode(nid, *address))self.send_krpc(msg, address)self.find_node(address, nid)except KeyError:passdef get_peers_received(self, msg, address):try:infohash = msg["a"]["info_hash"]neighbors = self.table.get_neighbors(infohash)nid = msg["a"]["id"]msg = {"t": msg["t"],"y": "r","r": {"id": self.get_neighbor(infohash), "nodes": encode_nodes(neighbors)}}self.table.append(KNode(nid, *address))self.send_krpc(msg, address)self.master.log(infohash)self.find_node(address, nid)except KeyError:passdef announce_peer_received(self, msg, address):try:infohash = msg["a"]["info_hash"]nid = msg["a"]["id"]msg = { "t": msg["t"],"y": "r","r": {"id": self.get_neighbor(infohash)}}self.table.append(KNode(nid, *address))self.send_krpc(msg, address)self.master.log(infohash)self.find_node(address, nid)except KeyError:pass
# 该类只实例化一次.
class KTable(object):# 这里的nid就是通过node_id()函数生成的自身node ID. 协议里说道, 每个路由表至少有一个bucket,     #  还规定第一个bucket的min=0, max=2^160次方, 所以这里就给予了一个buckets属性来存储bucket, 这个是列表.def __init__(self, nid):self.nid = nidself.buckets = [ KBucket(0, 2**160) ]def append(self, node):index = self.bucket_index(node.nid)try:bucket = self.buckets[index]bucket.append(node)except IndexError:returnexcept BucketFull:if not bucket.in_range(self.nid): returnself.split_bucket(index)self.append(node)# 返回与目标node ID或infohash的最近K个node.    # 定位出与目标node ID或infohash所在的bucket, 如果该bucuck有K个节点, 返回.     # 如果不够到K个节点的话, 把该bucket前面的bucket和该bucket后面的bucket加起来, 只返回前K个节点.    # 还是不到K个话, 再重复这个动作. 要注意不要超出最小和最大索引范围.    # 总之, 不管你用什么算法, 想尽办法找出最近的K个节点.    def get_neighbors(self, target):nodes = []if len(self.buckets) == 0: return nodesif len(target) != 20 : return nodesindex = self.bucket_index(target)try:nodes = self.buckets[index].nodesmin = index - 1max = index + 1while len(nodes) < K and ((min >= 0) or (max < len(self.buckets))):if min >= 0:nodes.extend(self.buckets[min].nodes)if max < len(self.buckets):nodes.extend(self.buckets[max].nodes)min -= 1max += 1num = intify(target)nodes.sort(lambda a, b, num=num: cmp(num^intify(a.nid), num^intify(b.nid)))return nodes[:K] #K是个常量, K=8 except IndexError:return nodesdef bucket_index(self, target):return bisect_left(self.buckets, intify(target))# 拆表    # index是待拆分的bucket(old bucket)的所在索引值.     # 假设这个old bucket的min:0, max:16. 拆分该old bucket的话, 分界点是8, 然后把old bucket的max改为8, min还是0.     # 创建一个新的bucket, new bucket的min=8, max=16.    # 然后根据的old bucket中的各个node的nid, 看看是属于哪个bucket的范围里, 就装到对应的bucket里.     # 各回各家,各找各妈.    # new bucket的所在索引值就在old bucket后面, 即index+1, 把新的bucket插入到路由表里. def split_bucket(self, index):old = self.buckets[index]point = old.max - (old.max - old.min)/2new = KBucket(point, old.max)old.max = pointself.buckets.insert(index + 1, new)for node in old.nodes[:]:if new.in_range(node.nid):new.append(node)old.remove(node)def __iter__(self):for bucket in self.buckets:yield bucketclass KBucket(object):__slots__ = ("min", "max", "nodes")# min和max就是该bucket负责的范围, 比如该bucket的min:0, max:16的话,     # 那么存储的node的intify(nid)值均为: 0到15, 那16就不负责, 这16将会是该bucket后面的bucket的min值.     # nodes属性就是个列表, 存储node. last_accessed代表最后访问时间, 因为协议里说到,     # 当该bucket负责的node有请求, 回应操作; 删除node; 添加node; 更新node; 等这些操作时,     # 那么就要更新该bucket, 所以设置个last_accessed属性, 该属性标志着这个bucket的"新鲜程度". 用linux话来说, touch一下.    # 这个用来便于后面说的定时刷新路由表.    def __init__(self, min, max):self.min = minself.max = maxself.nodes = []# 添加node, 参数node是KNode实例.    # 如果新插入的node的nid属性长度不等于20, 终止.    # 如果满了, 抛出bucket已满的错误, 终止. 通知上层代码进行拆表.    # 如果未满, 先看看新插入的node是否已存在, 如果存在, 就替换掉, 不存在, 就添加,    # 添加/替换时, 更新该bucket的"新鲜程度".    def append(self, node):if node in self:self.remove(node)self.nodes.append(node)else:if len(self) < K:self.nodes.append(node)else:raise BucketFulldef remove(self, node):self.nodes.remove(node)def in_range(self, target):return self.min <= intify(target) < self.maxdef __len__(self):return len(self.nodes)def __contains__(self, node):return node in self.nodesdef __iter__(self):for node in self.nodes:yield nodedef __lt__(self, target):return self.max <= targetclass KNode(object):# """    #        nid就是node ID的简写, 就不取id这么模糊的变量名了. __init__方法相当于别的OOP语言中的构造方法,     #        在python严格来说不是构造方法, 它是初始化, 不过, 功能差不多就行.    #        """ __slots__ = ("nid", "ip", "port")def __init__(self, nid, ip, port):self.nid = nidself.ip = ipself.port = portdef __eq__(self, other):return self.nid == other.nid#using example
class Master(object):def __init__(self, f):self.f = fself.hashArr = []def log(self, infohash):nhash = infohash.encode("hex")if nhash not in self.hashArr:self.hashArr.append(nhash)self.f.write(+"\n")self.f.flush()
try:print "start DHT Spider"f = file("hash.txt","a+")m = Master(f)s = Server(Master(f), KTable(random_id()), 6881)s.start()
except KeyboardInterrupt:s.socket.close()f.close()