linux - 关于 INADDR_ANY 的问题

Question

The constant INADDR_ANY is the so-called IPv4 wildcard address. The wildcard IP address is useful for applications that bind Internet domain sockets on multihomed hosts. If an application on a multihomed host binds a socket to just one of its host’s IP addresses, then that socket can receive only UDP datagrams or TCP connection requests sent to that IP address. However, we normally want an application on a multihomed host to be able to receive datagrams or connection requests that specify any of the host’s IP addresses, and binding the socket to the wildcard IP address makes this possible.

struct sockaddr_in server_address;
int server_sockfd = socket(AF_INET, SOCK_STREAM, 0);
memset(&server_address, 0, sizeof(struct sockaddr_in));
server_address.sin_family = AF_INET;
server_address.sin_addr.s_addr = htonl(INADDR_ANY); // here is my quesion
server_address.sin_port = htons(9734);

bind(server_sockfd, (struct sockaddr*)&server_address, sizeof(server_address));

问题>

如果我们将套接字绑定(bind)到特定的 IP 地址，则套接字只能接收发送到该 IP 地址的 UPD/TCP 请求。

如我在上面的代码中所示，现在套接字 server_sockfd 已与 INADDR_ANY 绑定(bind)。如果套接字可以接收互联网上的任何请求，我只是在这里 b/c 感到困惑，它如何仍然可以正常工作。互联网上有大量的 UDP/TCP 请求，如果套接字响应每个人，，怎么还能用呢？

//更新客户端代码//

int
main(int argc, char *argv[])
{
    struct sockaddr_in6 svaddr;
    int sfd, j;
    size_t msgLen;
    ssize_t numBytes;
    char resp[BUF_SIZE];

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s host-address msg...\n", argv[0]);

    /* Create a datagram socket; send to an address in the IPv6 somain */

    sfd = socket(AF_INET6, SOCK_DGRAM, 0);      /* Create client socket */
    if (sfd == -1)
        errExit("socket");

    memset(&svaddr, 0, sizeof(struct sockaddr_in6));
    svaddr.sin6_family = AF_INET6;
    svaddr.sin6_port = htons(PORT_NUM);
    if (inet_pton(AF_INET6, argv[1], &svaddr.sin6_addr) <= 0)
        fatal("inet_pton failed for address '%s'", argv[1]);

    /* Send messages to server; echo responses on stdout */

    for (j = 2; j < argc; j++) {
        msgLen = strlen(argv[j]);
        if (sendto(sfd, argv[j], msgLen, 0, (struct sockaddr *) &svaddr,
                    sizeof(struct sockaddr_in6)) != msgLen)
            fatal("sendto");

        numBytes = recvfrom(sfd, resp, BUF_SIZE, 0, NULL, NULL);
        if (numBytes == -1)
            errExit("recvfrom");

        printf("Response %d: %.*s\n", j - 1, (int) numBytes, resp);
    }

    exit(EXIT_SUCCESS);
}

//为服务器端代码更新

int
main(int argc, char *argv[])
{
    struct sockaddr_in6 svaddr, claddr;
    int sfd, j;
    ssize_t numBytes;
    socklen_t len;
    char buf[BUF_SIZE];
    char claddrStr[INET6_ADDRSTRLEN];

    /* Create a datagram socket bound to an address in the IPv6 somain */

    sfd = socket(AF_INET6, SOCK_DGRAM, 0);
    if (sfd == -1)
        errExit("socket");

    memset(&svaddr, 0, sizeof(struct sockaddr_in6));
    svaddr.sin6_family = AF_INET6;
    svaddr.sin6_addr = in6addr_any;                     /* Wildcard address */
    svaddr.sin6_port = htons(PORT_NUM);

    if (bind(sfd, (struct sockaddr *) &svaddr,
                sizeof(struct sockaddr_in6)) == -1)
        errExit("bind");

    /* Receive messages, convert to uppercase, and return to client */

    for (;;) {
        len = sizeof(struct sockaddr_in6);
        numBytes = recvfrom(sfd, buf, BUF_SIZE, 0,
                            (struct sockaddr *) &claddr, &len);
        if (numBytes == -1)
            errExit("recvfrom");

        /* Display address of client that sent the message */

        if (inet_ntop(AF_INET6, &claddr.sin6_addr, claddrStr,
                    INET6_ADDRSTRLEN) == NULL)
            printf("Couldn't convert client address to string\n");
        else
            printf("Server received %ld bytes from (%s, %u)\n",
                    (long) numBytes, claddrStr, ntohs(claddr.sin6_port));

        for (j = 0; j < numBytes; j++)
            buf[j] = toupper((unsigned char) buf[j]);

        if (sendto(sfd, buf, numBytes, 0, (struct sockaddr *) &claddr, len) !=
                numBytes)
            fatal("sendto");
    }
}

//更新了如何运行此服务器/客户端程序。

$ ./server_program &
[1] 31047
$ ./client_program ::1 ciao // Send to server on local host
Server received 4 bytes from (::1, 32770)
Response 1: CIAO

Answer 1

它不会获取对互联网上每个 IP 地址的请求^(a)，它会获取对其服务的每个 IP 地址的请求。例如，它可能有多个 NIC，每个 NIC 都有一个单独的 IP 地址，或者它可能有一个 NIC 能够管理多个 IP 地址(它甚至可能有多个 NIC，每个 NIC 都能够处理多个 IP 地址。

要查看的关键片段是:

... we normally want an application on a multi-homed host to be able to receive datagrams or connection requests that specify any of the host’s IP addresses (my italics).

换句话说，您可能有一个多宿主设置，其中您的机器服务于 10.0.0.15 和 10.0.0.16。使用 INADDR_ANY 将允许您获取这两个地址的流量，而不获取 10.0.0.17 的请求，它可能是长凳的另一端(或地球的另一边)。

下表，顶行是请求目的地，左列是您正在收听的地址，显示您是否会收到请求 (Y) 或 (N):

Request to>  10.0.0.15  10.0.0.16  10.0.0.17
Bind to:    *-------------------------------
10.0.0.15   |    Y          N          N
10.0.0.16   |    N          Y          N
INADDR_ANY  |    Y          Y          N

---

^(a) 它甚至看不到网络上的绝大多数请求。绝大多数甚至没有到达离您最近的路由器(甚至可能是您的 ISP)。即使那些确实到达了离您最近的路由器，您的特定机器也可能看不到它们是否发往本地网段上的另一台机器(尽管是混杂模式)。