使用crtmpserver作为rtmp服务器进行直播或点播时，经常遇到直播或点播失败的问题，严重时可能会存在二次中有一次失败的现象，所以对代码进行详细分析，发现是握手协议存在bug.
握手协议简述
第一步: Client->Server: C0 + C1
第二步: Server->Client: S0 + S1 + S2
第三部: Client->Server: C2
生成S0 + S1 + S2时存在bug分析及解决
InboundRTMPProtocol::PerformHandshake负责生成S1 + S2响应报文。
S1和S2报文的长度均为1536，所有生成的buffer大小应该1536 + 1536 = 3072。
S1和S2的具体结构可参考crtmpserver提供的文档:
crtmpserver/docs/RTMPEHandshake.pdf。
bool InboundRTMPProtocol::PerformHandshake(IOBuffer &buffer, bool encrypted) {
if (!ValidateClient(buffer)) {
if (encrypted || _pProtocolHandler->ValidateHandshake()) {
FATAL("Unable to validate client");
return false;
} else {
WARN("Client not validated");
_validationScheme = 0;
}
}
//get the buffers
uint8_t *pInputBuffer = GETIBPOINTER(buffer);
if (_pOutputBuffer == NULL) {
_pOutputBuffer = new uint8_t[3072];
} else {
delete[] _pOutputBuffer;
_pOutputBuffer = new uint8_t[3072];
}
// timestamp(前4个字节，时间戳)
EHTONLP(_pOutputBuffer, (uint32_t) time(NULL));
// version(第5字节到第8字节，版本)
EHTONLP(_pOutputBuffer + 4, (uint32_t) 0x00000000);
// generate random data (第9到3072字节用随机数据填充)
for (uint32_t i = 8; i < 3072; i++) {
_pOutputBuffer[i] = rand() % 256;
}
// 随机打上印记
for (uint32_t i = 0; i < 10; i++) {
uint32_t index = rand() % (3072 - HTTP_HEADERS_SERVER_US_LEN);
memcpy(_pOutputBuffer + index, HTTP_HEADERS_SERVER_US, HTTP_HEADERS_SERVER_US_LEN);
}
//**** FIRST 1536 bytes from server response ****//
//compute DH key position
uint32_t serverDHOffset = GetDHOffset(_pOutputBuffer, _validationScheme);
uint32_t clientDHOffset = GetDHOffset(pInputBuffer, _validationScheme);
//generate DH key
DHWrapper dhWrapper(1024);
if (!dhWrapper.Initialize()) {
FATAL("Unable to initialize DH wrapper");
return false;
}
if (!dhWrapper.CreateSharedKey(pInputBuffer + clientDHOffset, 128)) {
FATAL("Unable to create shared key");
return false;
}
if (!dhWrapper.CopyPublicKey(_pOutputBuffer + serverDHOffset, 128)) {
FATAL("Couldn't write public key!");
return false;
}
if (encrypted) {
uint8_t secretKey[128];
if (!dhWrapper.CopySharedKey(secretKey, sizeof (secretKey))) {
FATAL("Unable to copy shared key");
return false;
}
_pKeyIn = new RC4_KEY;
_pKeyOut = new RC4_KEY;
InitRC4Encryption(
secretKey,
(uint8_t*) & pInputBuffer[clientDHOffset],
(uint8_t*) & _pOutputBuffer[serverDHOffset],
_pKeyIn,
_pKeyOut);
//bring the keys to correct cursor
uint8_t data[1536];
RC4(_pKeyIn, 1536, data, data);
RC4(_pKeyOut, 1536, data, data);
}
//generate the digest
uint32_t serverDigestOffset = GetDigestOffset(_pOutputBuffer, _validationScheme);
uint8_t *pTempBuffer = new uint8_t[1536 - 32];
memcpy(pTempBuffer, _pOutputBuffer, serverDigestOffset);
memcpy(pTempBuffer + serverDigestOffset, _pOutputBuffer + serverDigestOffset + 32,
1536 - serverDigestOffset - 32);
uint8_t *pTempHash = new uint8_t[512];
HMACsha256(pTempBuffer, 1536 - 32, genuineFMSKey, 36, pTempHash);
//put the digest in place
memcpy(_pOutputBuffer + serverDigestOffset, pTempHash, 32);
//cleanup
delete[] pTempBuffer;
delete[] pTempHash;
//**** SECOND 1536 bytes from server response ****//
//Compute the chalange index from the initial client request
uint32_t keyChallengeIndex = GetDigestOffset(pInputBuffer, _validationScheme);
//compute the key
pTempHash = new uint8_t[512];
HMACsha256(pInputBuffer + keyChallengeIndex, //pData
32, //dataLength
BaseRTMPProtocol::genuineFMSKey, //key
68, //keyLength
pTempHash //pResult
);
//generate the hash
uint8_t *pLastHash = new uint8_t[512];
HMACsha256(_pOutputBuffer + 1536, //pData
1536 - 32, //dataLength
pTempHash, //key
32, //keyLength
pLastHash //pResult
);
//put the hash where it belongs
memcpy(_pOutputBuffer + 1536 * 2 - 32, pLastHash, 32);
//cleanup
delete[] pTempHash;
delete[] pLastHash;
//***** DONE BUILDING THE RESPONSE ***//
//wire the response
if (encrypted)
_outputBuffer.ReadFromByte(6);
else
_outputBuffer.ReadFromByte(3);
_outputBuffer.ReadFromBuffer(_pOutputBuffer, 3072);
//final cleanup
delete[] _pOutputBuffer;
_pOutputBuffer = NULL;
if (!buffer.IgnoreAll()) {
FATAL("Unable to ignore input buffer");
return false;
}
//signal outbound data
if (!EnqueueForOutbound()) {
FATAL("Unable to signal outbound data");
return false;
}
//move to the next stage in the handshake
_rtmpState = RTMP_STATE_SERVER_RESPONSE_SENT;
return true;
}
以上代码中有一段在buffer随机打上印记的代码:
// 随机打上印记
for (uint32_t i = 0; i < 10; i++) {
uint32_t index = rand() % (3072 - HTTP_HEADERS_SERVER_US_LEN);
memcpy(_pOutputBuffer + index, HTTP_HEADERS_SERVER_US, HTTP_HEADERS_SERVER_US_LEN);
}
其中HTTP_HEADERS_SERVER_US和HTTP_HEADERS_SERVER_US_LEN定义如下:
#define HTTP_HEADERS_SERVER_US "C++ RTMP Server (http://www.rtmpd.com)"
#define HTTP_HEADERS_SERVER_US_LEN 38
HTTP_HEADERS_SERVER_US_LEN为HTTP_HEADERS_SERVER_US的长度。
以上代码的主要作用是: 在S1+S1(1~3072字节)中随机插入10个C++ RTMP Server (http://www.rtmpd.com)字符串，作为标示。
执行完上述代码后，内存示意如下:
**随机插入10段`C++ RTMP Server (http://www.rtmpd.com)`之后的内存示意**
这里存在的bug是，因为是随机插入，所以有时会覆盖掉前8个字节(即timestamp+version)，这样会导致直播点播失败。
解决的方法:
int nHeaderLen = strlen(HTTP_HEADERS_SERVER_US);
for (uint32_t i = 0; i < 10; i++)
{
uint32_t index = rand() % (3072 - 8 - nHeaderLen);
memcpy(_pOutputBuffer + 8 + index, HTTP_HEADERS_SERVER_US, nHeaderLen);
}
即插入10个随机字符串时，避开前8个字节。
其实还有一种更简单的方法, 就是将原有随机插入10个字符串的代码直接注释掉，那个只是提供个标示，其实木有啥卵用。