连接套接字时使用getaddrinfo，超时了

Question

我花了好几个小时来寻找问题的解决方法，当我写问题描述并且需要说明时，终于找到了解决方案。我把解决方案发布出来，希望能够帮助到其他人。

使用getaddrinfo函数，如果我尝试连接到我的服务器，按照很多网站以及getaddrinfo函数的手册里面提供的示例代码所述，做（我认为的）正确的操作，但它失败了，并显示“连接超时”的错误消息： （为了更加简洁明了，下面是简化后的代码）

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);
}

然而，使用gethostbyname方法我能够连接到同一服务器上的套接字。

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获取了服务器的有效IP地址。但是第一个版本无法连接，会超时。为什么？

- Pellekrino

---

这些变量 x、domain、socketType、hostname、fd 是从哪里来的？ - Tormund Giantsbane

"//简化循环和错误处理以增加简洁性。我应该还添加int和error变量声明。由于我想要发布4个类似的代码片段，我尝试只保留最少的内容以了解差异并保持简洁。我本可以粘贴带有跟踪、循环和错误的所有内容，但这对此处没有帮助。让我先修复一些声明。" - Pellekrino

我最终为每个变量添加了问题和答案函数原型和类型声明（希望我没有漏掉任何一个）。 - Pellekrino

Answer 1

它一直失败的原因是：为了检索addrinfo，我将字段“service”保留为空。它仍然会返回成功并提供一个地址（您可以使用getnameinfo将其映射到正确的IP地址）。但是该地址无法用于连接套接字！

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 这个版本是可行的，但我不相信强制转换。

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));
}

最终，它帮助我找到了根本原因：

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);
}

希望这对某个人有所帮助！

- Pellekrino