使用 Winsock 将客户端 TCP 套接字绑定到特定的本地端口时，SO_REUSEADDR 没有任何影响 [英] When binding a client TCP socket to a specific local port with Winsock, SO_REUSEADDR does not have any effect

问题描述

我将一个客户端 TCP 套接字绑定到一个特定的本地端口.为了处理套接字保持在 TIME_WAIT 状态一段时间的情况，我在套接字上使用 setsockopt() 和 SO_REUSEADDR.

I'm binding a client TCP socket to a specific local port. To handle the situation where the socket remains in TIME_WAIT state for some time, I use setsockopt() with SO_REUSEADDR on a socket.

它适用于 Linux，但不适用于 Windows，当前一个连接仍处于 TIME_WAIT 时，我在 connect() 调用中得到 WSAEADDRINUSE代码>.

It works on Linux, but does not work on Windows, I get WSAEADDRINUSE on connect() call when the previous connection is still in TIME_WAIT.

MSDN 并不清楚客户端套接字会发生什么:

MSDN is not exactly clear what should happen with client sockets:

[...] 对于需要将多个套接字绑定到同一端口号的服务器应用程序，请考虑使用 setsockopt (SO_REUSEADDR).客户端应用程序通常根本不需要调用 bind——connect 会自动选择一个未使用的端口.[...]

[...] For server applications that need to bind multiple sockets to the same port number, consider using setsockopt (SO_REUSEADDR). Client applications usually need not call bind at all—connect chooses an unused port automatically. [...]

如何避免这种情况?

推荐答案

当您使用 socket() 创建套接字时，它只有一个类型和一个协议族.理想的做法是将其 bind() 也bind() 绑定到本地地址:端口.

When you create a socket with socket(), it has only a type and a protocol family. The ideal is to bind() it to a local address:port too.

您提到的错误通常发生在上次连接到同一主机时:端口没有正常关闭(FIN/ACK FIN/ACK).在这些情况下，套接字会在一段时间内保持 TIME_WAIT 状态(取决于操作系统，但可调整).

The error you mentioned normally happens when the last connection to the same host:port didn't have a graceful shutdown (FIN/ACK FIN/ACK). In these cases, the socket stays in TIME_WAIT state for a certain period of time (OS dependent, but adjustable).

然后发生的事情是，当您尝试 connect() 到同一主机和同一端口时，它使用默认套接字的名称/地址/端口/等，但这种组合已被使用你的 zombie 插座.为避免这种情况，您可以在创建套接字后通过调用 bind() 来更改用于建立连接的本地地址:端口，提供填充本地地址的 sockaddr 结构和一个随机端口.

What happens then is when you try to connect() to the same host and same port, it uses the default socket's name/address/port/etc, but this combination is already in use by your zombie socket. To avoid this, you can change the local address:port used to establish the connection by calling bind() after the socket creation, providing the sockaddr struct filled with your local address and a random port.

int main() {
    int ret, fd;
    struct sockaddr_in sa_dst;
    struct sockaddr_in sa_loc;
    char buffer[1024] = "GET / HTTP/1.1
Host: www.google.com

";

    fd = socket(AF_INET, SOCK_STREAM, 0);

    // Local
    memset(&sa_loc, 0, sizeof(struct sockaddr_in));
    sa_loc.sin_family = AF_INET;
    sa_loc.sin_port = htons(LOCAL_RANDOM_PORT);
    sa_loc.sin_addr.s_addr = inet_addr(LOCAL_IP_ADDRESS);

    ret = bind(fd, (struct sockaddr *)&sa_loc, sizeof(struct sockaddr));
    assert(ret != -1);

    // Remote
    memset(&sa_dst, 0, sizeof(struct sockaddr_in));
    sa_dst.sin_family = AF_INET;
    sa_dst.sin_port = htons(80);
    sa_dst.sin_addr.s_addr = inet_addr("64.233.163.104"); // google :)

    ret = connect(fd, (struct sockaddr *)&sa_dst, sizeof(struct sockaddr));
    assert(ret != -1);

    send(fd, buffer, strlen(buffer), 0);
    recv(fd, buffer, sizeof(buffer), 0);
    printf("%s
", buffer);
}

UPDATE:由于需要使用特定的本地端口，请考虑将 SO_LINGER 设置为 l_onoff=1 和 l_linger=0 所以你的套接字不会在 close/closesocket 时阻塞，它只会忽略排队的数据并(希望)关闭 fd.作为最后的手段，您可以通过更改此注册表项的值来调整 TIME_WAIT 延迟(非常不鼓励！):

UPDATE: As using a specific local port is a requirement, consider setting SO_LINGER with l_onoff=1 and l_linger=0 so your socket won't block upon close/closesocket, it will just ignore queued data and (hopefully) close the fd. As a last resort you can adjust the TIME_WAIT delay by changing the value of this registry key (highly discouraged!):

HKLMSYSTEMCurrentControlSetServicesTcpipParametersTcpTimedWaitDelay

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