c - 查询服务器并获取 addrinfo 结构体的结果

Question

我想查询特定的服务器，并以与我们通过 getaddrinfo 获取结果相同的方式获取结果。我想要一个addrinfo struct，这样我就可以获得 ip、端口和指向下一个结果的指针。

我正在使用下面的代码，它查询我想要的服务器并获取结果。但每个结果都位于另一个结构中，并且它们不指向彼此(不在列表中)。

这是代码:

static int my_getaddrinfo(const char *dns_server_s, const char *node, const char *service, const struct addrinfo *hints, struct addrinfo **res) {
    int retValue = 1;
    struct __res_state result;
    char ip[16];
    memset(ip, '\0', sizeof(ip));

    res_ninit(&result);
    struct in_addr addr;
    inet_aton(dns_server_s, &addr);

    result.nsaddr_list[0].sin_addr = addr;
    result.nsaddr_list[0].sin_family = AF_INET;
    result.nsaddr_list[0].sin_port = htons(NS_DEFAULTPORT);
    result.nscount = 1;

    u_char answer[NS_PACKETSZ];
    int len = res_nquery(&result, node, ns_c_in, ns_t_a, answer, sizeof(answer));
    ns_msg handle;
    ns_initparse(answer, len, &handle);

    if(ns_msg_count(handle, ns_s_an) > 0) {
        ns_rr rr;
        if(ns_parserr(&handle, ns_s_an, 0, &rr) == 0) {
            strcpy(ip, inet_ntoa(*(struct in_addr *)ns_rr_rdata(rr)));
            getaddrinfo(ip, service, hints, res);
            retValue = 0;
        }
    }

    return retValue;
}

是否可以按照我想要的方式获得结果？类似的东西addrinfo 结构？

编辑:我可以看到我得到了三个答案 ns_msg_count(handle, ns_s_an) = 3要访问每个答案，我应该调用 ns_parserr(&handle, ns_s_an, answer_index, &rr)但正如我所说，我想以列表的形式获得这些答案，就像我通过调用 getaddrinfo 获得它们一样.

Answer 1

getaddrinfo返回的不仅仅是 IP 地址，它还会将服务名称解析为端口号，并且它可以支持不同的协议(protocol)，主要是 tcp 和 udp。因此，您需要通过调用 getservbyname_r 来解析服务名称并手动构建结果addrinfo对于 IP、端口和协议(protocol)的每种组合。这是一个解析 ip 和端口的简单版本，但它只返回 TCP 的 addrinfo。您可以轻松地将其扩展为包含 UDP。

另一个功能是 getaddrinfo还支持IPV6，在这种情况下您需要调用res_nquery与 T_AAAA而不是T_A解析 IPV6 地址。

这是一个示例，注意它返回 struct addrinfo 的链表就像 getaddrinfo确实如此，所以结果应该是免费的 freeaddrinfo当你完成时。

static int my_getaddrinfo(const char *dns_server,
        const char *node, const char *service,
        const struct addrinfo *hints, struct addrinfo **res) {

    int ret;

    // get dns server sockaddr
    struct addrinfo hint = {0};
    struct addrinfo *ai = NULL;

    hint.ai_family = AF_INET;
    hint.ai_socktype = SOCK_DGRAM;
    hint.ai_protocol = IPPROTO_UDP;
    ret = getaddrinfo(dns_server, "domain", &hint, &ai);
    if (ret != 0) {
        puts("getaddrinfo dns error");
        return 1;
    }

    if (!ai) {
        printf("getaddrinfo returned no result\n");
        return 1;
    }

    freeaddrinfo(ai);

    int port = 0;
    // get service port
    if (service) {
        struct servent srv, *sres;
        char buf[1024];
        ret = getservbyname_r(service, NULL, &srv, buf, sizeof buf, &sres);
        if (ret != 0) {
            printf("getservbyname error\n");
            return 1;
        }
        port = sres->s_port;
    }

    struct __res_state p = {0};
    res_state state = &p;
    unsigned char ans[NS_MAXMSG];
    ns_msg msg;
    ns_rr rr;
    char line[1024];
    int len ;

    ret = res_ninit(state);
    if (ret != 0) {
        printf("res_ninit error\n");
        return 1;
    }

    state->nscount = 1;
    memset(state->nsaddr_list, 0, sizeof state->nsaddr_list);
    memcpy(state->nsaddr_list, ai->ai_addr, sizeof state->nsaddr_list[0]);

    ret = res_nquery(state, node, C_IN, T_A, ans, sizeof ans);
    if (ret < 0) {
        printf("res_nquery error\n");
        return 1;
    }

    len = ret;
    ret = ns_initparse(ans, len, &msg);
    if (ret != 0) {
        printf("ns_initparse error\n");
        return 1;
    }

    len = ns_msg_count(msg, ns_s_an);

    if (len == 0) {
        printf("no address found\n");
        return 0;
    }

    struct addrinfo *head = NULL;
    struct addrinfo *cur = NULL;
    struct addrinfo *prev = NULL;
    struct sockaddr_in *sin;

    for (int i = 0; i < len; i++) {
        ret = ns_parserr(&msg, ns_s_an, i, &rr);
        if (ret != 0) {
            printf("ns_parserr error\n");
        }

        if (ns_rr_rdlen(rr) != NS_INADDRSZ) {
            continue;
        }

        cur = malloc(sizeof *cur + sizeof(struct sockaddr_in));
        memset(cur, 0, sizeof *cur);
        cur->ai_family = AF_INET;
        cur->ai_socktype = SOCK_STREAM;
        cur->ai_protocol = IPPROTO_TCP;
        cur->ai_addrlen = sizeof(struct sockaddr_in);
        cur->ai_addr = (void*)(cur + 1);
        cur->ai_canonname = NULL;
        cur->ai_next = NULL;
        sin = (struct sockaddr_in*)(cur->ai_addr);
        sin->sin_family = cur->ai_family;
        sin->sin_port = port;
        memcpy(&sin->sin_addr, ns_rr_rdata(rr), sizeof sin->sin_addr);
        if (prev)
            prev->ai_next = cur;
        if (head == NULL)
            head = cur;
        prev = cur;
    }
    *res = head;

    return 0;
}

int main(int argc, char *argv[])
{
    const char *node = "bing.com";
    struct addrinfo *res = NULL;
    if (argc == 2)
        node = argv[1];
    int ret = my_getaddrinfo("8.8.8.8", node, "http", NULL, &res);
    if (ret != 0) {
        puts("getaddrinfo error");
        return 1;
    }

    // do stuff with res
    struct addrinfo *rp;
    struct sockaddr_in *sin;
    char p[1024];
    for (rp = res; rp != NULL; rp = rp->ai_next) {
        sin = (struct sockaddr_in*)rp->ai_addr;
        const char *s = inet_ntop(rp->ai_family,
                &sin->sin_addr, p, sizeof p);
        printf("Got %s: %d\n", s, ntohs(sin->sin_port));
    }
    freeaddrinfo(res);
    return 0;
}

示例:

$ ./a.out bing.com
Got 204.79.197.200: 80
Got 13.107.21.200: 80
$ ./a.out google.com
Got 172.217.24.14: 80