使用 getaddrinfo 连接套接字超时答案

【问题标题】：Connecting a socket, using getaddrinfo, times out使用 getaddrinfo 连接套接字超时
【发布时间】：2018-11-29 15:36:32
【问题描述】：

我花了几个小时试图找出我的问题，只是在我写问题的时候找到了解决方案（当你需要正式化你的问题并解释它时，它总是有帮助的）。我贴出来了，希望对大家有帮助。

使用 getaddrinfo，如果我尝试将套接字连接到我的服务器，执行（我认为的）正是在大量网站以及 getaddrinfo 的手册页示例代码中解释的内容，它会失败并显示“连接超时”错误信息：（简化代码更简洁）

void connect_UsingGetAddrInfo_Wrong (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams.  

    addrinfo hints, *addr;
    //fine-tune hints according to which socket you want to open
    hints.ai_family = domain; 
    hints.ai_socktype = socketType; 
    hints.ai_protocol = 0;           // no enum : possible value can be read in /etc/protocols
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_ADDRCONFIG;

    x =  getaddrinfo(hostname, NULL, &hints, &addr);
    //shall rather loop on addr linked list, but this is not the topic here.

    socketfd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);
    x = connect(socketfd, addr->ai_addr, addr->ai_addrlen);
}

但是，我能够使用 gethostbyname 方法将套接字连接到同一台服务器。

void connect_UsingGetHostByName_Deprecated (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams.  

    struct hostent DNS, *r;
    char buf[1024];
    x = gethostbyname_r(hostname.c_str(), & DNS, buf, sizeof(buf), & r, & err));
    socketfd = socket(domain, socketType, 0);

    //server.
    sockaddr_in server;
    memset(&server, 0x00, sizeof(server));
    server.sin_family=domain;
    server.sin_port=htons(port);
    memcpy(& server.sin_addr.s_addr, DNS.h_addr, (size_t) DNS.h_length);
    x = connect(socketfd, (struct sockaddr *) & server, sizeof(server));
}

运行代码显示两个版本都正确检索了服务器的有效 IP 地址。第一个仍然无法连接并且会超时。为什么？

【问题讨论】：

这些变量x、domain、socketType、hostname、fd是从哪里来的？
"//为简洁而简化的循环和错误处理" 我还应该添加 int 和 error var 声明。由于我想发布 4 段类似的代码，我试图只保留最低限度以了解差异并保持简洁。我本可以粘贴所有在此处没有帮助的痕迹、循环和错误。那么让我修正一些声明
我终于为每个 var 添加了问题和答案函数原型和类型声明（希望我没有错过一个）。

标签： c sockets getaddrinfo gethostbyname

【解决方案1】：

它一直失败的原因是：为了检索 addrinfo，我将字段“服务”设置为 NULL。它仍然会返回成功并为您提供一个地址（您可以使用 getnameinfo 将其映射到正确的 IP 地址）。该地址仍然无法连接您的套接字！

我在这里找到了两种方法的混合版本： https://codereview.stackexchange.com/questions/17863/socket-connect-realization-gethostbyname-or-getnameinfo 这个是功能性的，但我不买铸件

void connect_UsingGetAddrInfo_HYBRID (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams. 

    addrinfo hints, *addr;
    //fine-tune hints according to which socket you want to open
    hints.ai_family = domain; 
    hints.ai_socktype = socketType; 
    hints.ai_protocol = 0;           // no enum : possible value can be read in /etc/protocols
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_ADDRCONFIG;

    x =  getaddrinfo(host, NULL, &hints, &addr);
    socketfd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);

    //here is the hybrid part
    sockaddr_in servAddr;
    memset(&servAddr, 0, sizeof(servAddr));
    servAddr.sin_family = addr->ai_family;
    servAddr.sin_addr.s_addr = *((uint32_t*) & (((sockaddr_in*)addr->ai_addr)->sin_addr));
    servAddr.sin_port        = htons(port);

    x=connect(socketfd, (struct sockaddr*) &servAddr, sizeof(servAddr));
}

最后，它帮我找到了根本原因：

void connect_UsingGetAddrInfo_FIXED (std::string host, unsigned short int port, int& socketfd)
{
    //simplified loops & error handling for concision
    int x;

    int domain = AF_INET;         // IP_v4
    int socketType = SOCK_STREAM; // Sequenced, reliable, connection-based byte streams.  

    addrinfo hints, *addr;
    //fine-tune hints according to which socket you want to open
    hints.ai_family = domain; 
    hints.ai_socktype = socketType; 
    hints.ai_protocol = 0;           // no enum : possible value can be read in /etc/protocols
    hints.ai_flags = AI_CANONNAME | AI_ALL | AI_ADDRCONFIG;

    //Precise here the port !
    const char* service = std::to_string(port).c_str();

    x =  getaddrinfo(host, service, &hints, &addr);
    socketfd = socket(addr->ai_family, addr->ai_socktype, addr->ai_protocol);
    x = connect(socketfd, addr->ai_addr, addr->ai_addrlen);
}

希望有一天这会对某人有所帮助！

【讨论】：