stun udp

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https://cloud.tencent.com/developer/article/2215771

p2p server

需要通过stun穿透协议定时与穿透服务通信，即在定时器线程里面发送SStunLogin，再在epoll里面udp接收SStunLoginResponse表示登录成功，同时epoll里面udp需要解析SStunTransfer穿透命令，收到穿透命令后需要回复对方一个字节的包。

一个字节的穿透包不一定能发出去，比如当p2p client发起打洞标记时请求的p2p server port和p2p server外部端口对不上，那打的标记就没用，或者p2p client不是严格意义上的nat 3网络（外部端口会隔段时间变化），或者是nat 4对称型网络，基本打洞不成功。

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// 定时器线程：每隔30秒发送STUN登录请求
std::thread timer_thread([&]()
                            {
    while (true) {
        static unsigned int s_seq = 0;
        int size = sizeof(SStunLogin) + g_strDeviceId.size() - 1;
        char* buf = new char[size];
        SStunLogin* p = (SStunLogin*)buf;
        p->head.len = htons(size);
        p->head.cmd = htons(STUN_CMD_LOGIN);
        p->head.seq = htonl(++s_seq);
        p->nat = g_finalNatType;
        memcpy_s(p->uid, size - sizeof(SStunLogin) + 1, g_strDeviceId.c_str(), g_strDeviceId.size());
        sendto(sockfd, buf, size, 0, (const sockaddr*)&server_addr, sizeof(server_addr));
        delete[] buf;
        printf("udp stun alive seq = %u\n", s_seq);
        sleep(30);
    } });

// 创建epoll
int epoll_fd = epoll_create(1);
if (epoll_fd < 0)
{
    std::cerr << "Failed to create epoll" << std::endl;
    return 1;
}

// 添加UDP套接字到epoll
struct epoll_event event;
event.events = EPOLLIN;
event.data.fd = sockfd;
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, sockfd, &event) < 0)
{
    std::cerr << "Failed to add socket to epoll" << std::endl;
    return 1;
}

// epoll等待事件
const int MAX_EVENTS = 10;
struct epoll_event events[MAX_EVENTS];
while (true)
{
    int num_events = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
    if (num_events < 0)
    {
        std::cerr << "Error in epoll_wait" << std::endl;
        return 1;
    }

    for (int i = 0; i < num_events; ++i)
    {
        if (events[i].data.fd == sockfd && events[i].events & EPOLLIN)
        {
            char buf[1500];
            struct sockaddr_in client_addr;
            socklen_t addr_len = sizeof(client_addr);
            ssize_t num_bytes = recvfrom(sockfd, &buf, sizeof(buf), 0,
                                            (struct sockaddr *)&client_addr, &addr_len);
            if (num_bytes < 0)
            {
                std::cerr << "Error in recvfrom" << std::endl;
                continue;
            }

            printf("recvfrom %s_%d size = %ld\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port), num_bytes);

            if (sizeof(SStunLoginResponse) == num_bytes)
            {
                // 尝试解析成穿透响应
                SStunLoginResponse *slr = (SStunLoginResponse *)buf;
                if (ntohs(slr->head.cmd) == STUN_CMD_LOGIN && ntohs(slr->head.len) == num_bytes)
                {
                    printf("recv stun alive result = %d,seq = %u,ip = %s,port = %d\n", (int)slr->result, ntohl(slr->head.seq),
                            IpInt2Str(slr->ip).c_str(), ntohs(slr->port));
                }
            }
            if (sizeof(SStunTransfer) == num_bytes)
            {
                // 尝试解析成穿透命令
                SStunTransfer *st = (SStunTransfer *)buf;
                if (ntohs(st->head.cmd) == STUN_CMD_TRANSFER && ntohs(st->head.len) == num_bytes)
                {
                    SStunResponse response;
                    response.head.cmd = st->head.cmd;
                    response.head.seq = st->head.seq;
                    response.head.len = htons(sizeof(response));
                    response.result = 0;
                    int tmpRet = sendto(sockfd, (const char *)&response, sizeof(response), 0, (sockaddr *)&client_addr, addr_len);
                    if (tmpRet != sizeof(response))
                    {
                        printf("sendto %s_%d ret = %d,error = %d\n", IpInt2Str(client_addr.sin_addr.s_addr).c_str(), ntohs(client_addr.sin_port), tmpRet,
                                errno);
                    }
                    client_addr.sin_addr.s_addr = st->ip;
                    client_addr.sin_port = st->port;
                    tmpRet = sendto(sockfd, "0", 1, 0, (sockaddr *)&client_addr, addr_len);
                    if (tmpRet != 1)
                    {
                        printf("sendto %s_%d ret = %d,error = %d\n", IpInt2Str(client_addr.sin_addr.s_addr).c_str(), ntohs(client_addr.sin_port), tmpRet,
                                errno);
                    }
                    printf("stun to %s_%d\n", IpInt2Str(client_addr.sin_addr.s_addr).c_str(), ntohs(client_addr.sin_port));
                }
            }
            else
            {
                std::cout << "receive data " << string(buf, num_bytes) << std::endl;
            }
        }
    }
}

p2p client

首先尝试发送一个字节与p2p节点通信（可能通信不上，但会在本地路由器留下打洞标记），再向穿透服务器stun发送穿透请求，最后接收udp的响应，如果是来自穿透服务器stun的响应，根据返回码判断是否穿透请求成功，如果是收到p2p节点的1字节打洞包，说明穿透打洞成功。穿透打洞成功后再和p2p节点进行正常的udp通信，例如kcp同步建连。

p2p client穿透请求成功，只能代表p2p server正常在线，需要收到p2p2 server一个字节打洞包后，才能真正说明是打洞成功了，收到打洞包后，可以根据对方的ip和端口修改本地记录，真正请求数据时肯定按这个ip和端口进行通信。

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// 向P2P节点发送一个字节的数据（打洞标记）
struct sockaddr_in p2p_node_addr;
p2p_node_addr.sin_family = AF_INET;
p2p_node_addr.sin_port = htons(p2pPort);                    // P2P节点的端口号
inet_pton(AF_INET, p2pIp.c_str(), &p2p_node_addr.sin_addr); // P2P节点的IP地址

int ret = 0;
ret = sendto(sockfd, "0", 1, 0,
                (struct sockaddr *)&p2p_node_addr, sizeof(p2p_node_addr));
std::cout << "sendto p2p node " << ret << std::endl;

// 向STUN服务器发送穿透请求
struct sockaddr_in stun_server_addr;
stun_server_addr.sin_family = AF_INET;
stun_server_addr.sin_port = htons(STUN_SERVER_PORT);
inet_pton(AF_INET, p2pStunServer.c_str(), &stun_server_addr.sin_addr); // STUN服务器的IP地址

// 生成穿透请求数据
char buf[256];
SStunRequest *stunReq = (SStunRequest *)buf;
stunReq->head.cmd = htons(STUN_CMD_REQUEST);
stunReq->head.len = htons(sizeof(SStunRequest) - 1 + p2pUid.size());
stunReq->nat = 1;
memcpy(stunReq->uid, p2pUid.c_str(), p2pUid.size());
ret = sendto(sockfd, buf, ntohs(stunReq->head.len), 0,
                (struct sockaddr *)&stun_server_addr, sizeof(stun_server_addr));
std::cout << "sendto stun server " << ret << std::endl;

// 判断响应是否来自STUN服务器
if (remote_addr.sin_addr.s_addr == stun_server_addr.sin_addr.s_addr &&
    remote_addr.sin_port == stun_server_addr.sin_port)
{
    // 穿透服务器的响应
    SStunResponse *stun = (SStunResponse *)buffer;
    printf("stun result %d\n", stun->result);
    if (0 == stun->result)
    {
        // 穿透请求成功了
        std::cout << "穿透请求成功！" << std::endl;
        // 进行正常的UDP通信
        // ...
    }
    else
    {
        std::cout << "穿透请求失败。" << std::endl;
    }
}
else if (1 == num_bytes)
{
    std::cout << "收到P2P节点的打洞包，穿透成功！" << std::endl;
    // 穿透包，可能端口或ip不一致，改一下
    if (remote_addr.sin_addr.s_addr != p2p_node_addr.sin_addr.s_addr)
    {
        string oldIp = inet_ntoa(p2p_node_addr.sin_addr);
        string newIp = inet_ntoa(remote_addr.sin_addr);
        printf("p2p_node ip change %s -> %s\n", oldIp.c_str(), newIp.c_str());
        p2p_node_addr.sin_addr.s_addr = remote_addr.sin_addr.s_addr;
    }
    if (remote_addr.sin_port != p2p_node_addr.sin_port)
    {
        printf("p2p_node port change %d -> %d\n", ntohs(p2p_node_addr.sin_port), ntohs(remote_addr.sin_port));
        p2p_node_addr.sin_port = remote_addr.sin_port;
    }
    // 进行正常的UDP通信
    const char *sendData = "hello world";
    int ret = sendto(sockfd, sendData, strlen(sendData), 0, (struct sockaddr *)&p2p_node_addr, sizeof(p2p_node_addr));
    std::cout << "sendto p2p node " << ret << std::endl;
}

stun server

收到p2p server的SStunLogin后，根据uid记录对方的nat、addr和ts等信息放到map中，并回复对方SStunLoginResponse，带上p2p server的外部ip和端口，p2p server收到响应后，就知道自己的外部ip和端口了。

收到p2p client的STUN_CMD_REQUEST穿透请求后，查询当前map是否有该p2p server uid的心跳记录，如果没有，说明对方p2p server节点并未上线，如果有，转发STUN_CMD_TRANSFER穿透请求到该p2p server节点，stun收到该p2p server节点的响应后，再向p2p client响应穿透请求转发成功。

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