        /* This is how you trigger a search for a torrent hash.  If port (the second argument) is non-zero, it also performs an announce.

           Since peers expire announced data after 30 minutes, it's a good idea to reannounce every 28 minutes or so. */

        if(searching) {

            //m_dht.dht_random_bytes((void*)hashList[2],20);

            if(m_soListen >= )

                m_dht.dht_search(hashList[], , AF_INET, DHT_callback, this);

            if(s6 >= )

                m_dht.dht_search(hashList[], , AF_INET6, DHT_callback, this);

            searching = ;

        }

搜索到对方返回的IP信息和端口号后,大家可以分析dht.c里面的函数代码dht_periodic(const void *buf, size_t buflen,const struct sockaddr *fromAddr, int fromlen,time_t *tosleep,dht_callback *callback, void *closure)函数里面的ANNOUNCE_PEER返回请求里面带有对方表明自己此BT种子对应的认证码peerid.

dht_periodic(const void *buf, size_t buflen,const struct sockaddr *fromAddr, int fromlen,time_t *tosleep,dht_callback *callback, void *closure)

函数里面的ANNOUNCE_PEER

        case ANNOUNCE_PEER:

            _dout("Announce peer!From IP:%s:%d\n",inet_ntoa(tempip->sin_addr),tempip->sin_port);

            new_node(id, fromAddr, fromlen, );

            if(id_cmp(info_hash, zeroes) == )

            {

                _dout("Announce_peer with no info_hash.\n");

                send_error(fromAddr, fromlen, tid, tid_len,, "Announce_peer with no info_hash");

                break;

            }

            if(!token_match(token, token_len, fromAddr)) {

                _dout("Incorrect token for announce_peer.\n");

                send_error(fromAddr, fromlen, tid, tid_len,, "Announce_peer with wrong token");

                break;

            }

            if(port == ) {

                _dout("Announce_peer with forbidden port %d.\n", port);

                send_error(fromAddr, fromlen, tid, tid_len,, "Announce_peer with forbidden port number");

                break;

            }

            if(callback)

            {

                (*callback)(closure, DHT_EVENT_ANNOUNCE_PEER_VALUES, info_hash,(void *)fromAddr, port,id);//此ID就是peerid,

            }

知道了对应的IP,端口号,还有种子ID号,就可以向对方发送请求了.

获取HASH是通过UDP网络,但下载BT种子是通过TCP来处理,相当于别人是TCP服务器,我们连接过去,直接下载对应PEERID的种子就行了.

BT种子在DHT网络下载流程

先看http://www.bittorrent.org/beps/bep_0010.html协议介绍,我们必须先握手

handshake message

此包构造比较简单,按照格式进行组装就行了,然后发送出去,对方就会回应自己是什么客户端的软件提供种子下载.

void CH31BTMgr::Encode_handshake()

{

	//a byte with value 19 (the length of the string that follows);

	//the UTF-8 string "BitTorrent protocol" (which is the same as in ASCII);

	//eight reserved bytes used to mark extensions;

	//the 20 bytes of the torrent info hash;

	//the 20 bytes of the peer ID.

	char btname[256];

	memset(btname,0,sizeof(btname));

	sprintf(btname,"BitTorrent protocol");

	char msg[1280];

	memset(msg,0,sizeof(msg));

	msg[0]=19;

	memcpy(&msg[1],btname,19);

	char ext[8];

	memset(ext,0,sizeof(ext));

	ext[5]=0x10;

	memcpy(&msg[20],ext,8);

	memcpy(&msg[28],m_hash,20);

	memcpy(&msg[48],m_peer_id,20);

	int res1=Write(msg, 68);//TCP发送消息

}

在发送握手后,我们可以接着发送种子数据请求包,需要学习http://www.bittorrent.org/beps/bep_0009.html 里面的内容:　　

extension header

The metadata extension uses the extension protocol (specified in BEP  ) to advertize its existence. It adds the "ut_metadata" entry to the "m" dictionary in the extension header hand-shake message. This identifies the message code used for this message. It also adds "metadata_size" to the handshake message (not the "m" dictionary) specifying an integer value of the number of bytes of the metadata.

Example extension handshake message:

{'m': {'ut_metadata', }, 'metadata_size': }

extension message

The extension messages are bencoded. There are  different kinds of messages:

0 request

1 data

2 reject

The bencoded messages have a key "msg_type" which value is an integer corresponding to the type of message. They also have a key "piece", which indicates which part of the metadata this message refers to.

In order to support future extensability, an unrecognized message ID MUST be ignored.

这就需要会bencode的相关代码,这个大家可以网上搜索进行编译,如果实现搞不定,可以留下邮箱我将此类代码发送给你,其实也是网上收集整理的.

void CH31BTMgr::Encode_Ext_handshake()

{

    entry m;

    m["ut_metadata"] = ;

    entry e;

    e["m"]=m;

    char msg[];

    char* header = msg;

    char* p = &msg[];

    int len = bencode(p, e);

    int total_size =  + len;

    namespace io = detail;

    io::write_uint32(total_size, header);

    io::write_uint8(, header);

    io::write_uint8(, header);

    int res1=Write(msg, len + );

}

如果别人回应的是2,那就直接退出吧,说明别人拒绝了你.

如果回应是1,则返回的是数据区,每块是16K大小,最后一包不是.

data

The data message adds another entry to the dictionary, "total_size". This key has the same semantics as the "metadata_size" in the extension header. This is an integer.

The metadata piece is appended to the bencoded dictionary, it is not a part of the dictionary, but it is a part of the message (the length prefix MUST include it).

If the piece is the last piece of the metadata, it may be less than 16kiB. If it is not the last piece of the metadata, it MUST be 16kiB.

Example:

{'msg_type': , 'piece': , 'total_size': }

d8:msg_typei1e5:piecei0e10:total_sizei34256eexxxxxxxx...

The x represents binary data (the metadata).

下面给出如何进行提交我需要第几包的数据代码:

void CH31BTMgr::write_metadata_packet(int type, int piece)

{

    ASSERT(type >=  && type <= );

    ASSERT(piece >= );

    entry e;

    e["msg_type"] = type;

    e["piece"] = piece;

    char const* metadata = ;

    int metadata_piece_size = ;

    if (type == )

    {

        e["total_size"] = ;

        int offset = piece *  * ;

        //metadata = m_tp.metadata().begin + offset;

        metadata_piece_size = (std::min)(int( - offset),  * );

    }

    char msg[];

    char* header = msg;

    char* p = &msg[];

    int len = bencode(p, e);

    int total_size =  + len + metadata_piece_size;

    namespace io = detail;

    io::write_uint32(total_size, header);

    io::write_uint8(, header);

    io::write_uint8(m_message_index, header);

    int res1=Write(msg, len + );

}

在接收到一包请求后我们才可以继续下一包的请求,下面给了我们如何解析这一包的问题代码:

// 处理一个完整的包数据

bool CH31BTMgr::DeCodeFrameData(char * buffer,int buflen)

{

    char * p = (char *)mhFindstr(buffer, buflen, "ut_metadatai", );

    if(p)

    {

        m_message_index=atoi(&p[]);

        if(m_message_index==)

        {

            return false;

        }

        write_metadata_packet(,);

        char filename[];

        memset(filename,,sizeof(filename));

        sprintf(filename,"%s\\torrent.txt",m_workPath);

        DelFile(filename);

    } 

    p = (char *)mhFindstr(buffer, buflen, "metadata_sizei", );

    if(p)

    {

        m_metadata_size=atoi(&p[]);

        m_fileCnt=(int)(m_metadata_size/)+;

    } 

    p = (char *)mhFindstr(buffer, buflen, "msg_typei", );

    if(p)

    {

        int type1=atoi(&p[]);

        if(type1==)

        {

            p = (char *)mhFindstr(buffer, buflen, "piecei", );

            if(p)

            {

                int piece=atoi(&p[]);

                p = (char *)mhFindstr(buffer, buflen, "total_sizei", );

                if(p)

                {

                    int total_size=atoi(&p[]);

                    p = (char *)mhFindstr(buffer, buflen, "ee", );

                    if(p)

                    {

                        //保存数据

                        FILE* pfile=NULL;

                        char filename[];

                        memset(filename,,sizeof(filename));

                        sprintf(filename,"%s\\torrent.txt",m_workPath);

                        char openmethod[]="a";

                        if(piece==)

                            sprintf(openmethod,"w");

                        if((pfile=fopen(filename,openmethod))!=NULL)

                        {

                            if((piece+)**<total_size)

                            {

                                fseek(pfile,(piece)**,SEEK_SET);

                                fwrite(&p[],,*,pfile);

                                write_metadata_packet(,piece+);

                                fclose(pfile);

                            }

                            else

                            {

                                fwrite(&p[],,total_size-(piece)**,pfile);

                                fclose(pfile);

                                ManageTorrentFileToRealFile(filename);

                            }

                        }

                    }

                }

            }

        }

        else if(type1==)

        {

            return false;

        }

    } 

    return true;

}

void * mhFindstr(const void *haystack, size_t haystacklen,const void *needle, size_t needlelen)
{
const char *h =(const char *) haystack;
const char *n =(const char *) needle;
size_t i;

/* size_t is unsigned */
if(needlelen > haystacklen)
return NULL;

for(i = 0; i <= haystacklen - needlelen; i++) {
if(memcmp(h + i, n, needlelen) == 0)
return (void*)(h + i);
}
return NULL;
}

接下来说下如何进行快速调试的问题:

第一次调试也很天真的等着DHT网络上的数据过来,需要等很久,而且调试总是发现别人不回应,要么就是拒绝,经过一段时间后,

问朋友总是不对问题,结果是协议没有构造对.下面就需要注意的地方总结下:

1.一定要接收到别的人PEERID后才能够与别人交流,不然别人肯定不理你;

2.构造协议调试不能够在外网络上调试,最好大家将mono-monotorrent源代码下载回来,调试分析下,本地开启服务器;

3.通过本地与mono-monotorrent进行调试,你就可以分析出是哪里不对的问题,是不是协议哪些封装得不对的问题.

4.通过DHT网络下载回来的种子肯定是最新的,WEB下载的可能还没有呢..

5.通过协议下载回来的种子好像没有announce-list,不知道为什么不提供一些内容,可能还有些什么关键地方没有下载,分析mono-monotorrent代码里面就是不提供下载,希望高手指点.

6.TCPClient接收数据区需要开到16K以上,这样方便处理,当然如果会前后拼接包就更好.

7.如果需要bencode相关的编码C++代码,可以在此留言或者给h31h31#163.com发邮件.

如果此文章看不太明白,请先看看之前的文章,分析调试下代码,再来学习此文章可能就比较懂一些.

希望有了解的朋友更好的交流和进步.在此留言学习讨论.

希望大家多多推荐哦...大家的推荐才是下一篇介绍的动力...

码农公寓

DHT系列文章:

看懂此文章需要提前看明白上面的系列文章,还需要你有TCP网络编程和bencode编码方法基础上,如果都看不明白,可以到娱乐区http://www.sosobta.com 去看看,休息下...

目前我们下载BT种子有两种方式:

BT种子在DHT网络下载流程

接下来说下如何进行快速调试的问题:

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