Question

我想查询特定服务器，并以与通过getaddrinfo获取结果相同的方式获取结果。我想获得一个addrinfo结构，所以我可以拥有ip，port和指向下一个结果的指针。

我正在使用下面的代码，它会查询我想要的服务器，并获得结果。但是每个结果都在另一个结构中，并且它们不会相互指向（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;
}

是否有可能以我想要的方式获得结果？类似的东西 addrinfo struct？

修改我可以看到我得到三个答案ns_msg_count(handle, ns_s_an) = 3 并且要访问每个答案，我应该致电ns_parserr(&handle, ns_s_an, answer_index, &rr) 但正如我所说，我希望将这些答案作为一个列表，就像我通过调用getaddrinfo得到它们一样。

Answer 1

getaddrinfo返回的不仅仅是ip地址，还会将服务名称解析为端口号，它可以支持不同的协议，主要是tcp和udp。因此，您需要通过调用getservbyname_r来解析服务名称，并为每个ip，端口和协议组合手动构造结果addrinfo。这是一个解析ip和port的简单版本，但它只返回TCP的addrinfo。您可以将其扩展为包含UDP。

另一个功能是getaddrinfo也支持IPV6，在这种情况下，您需要使用res_nquery而不是T_AAAA来调用T_A来解析IPV6地址。

以下是一个示例，请注意它会返回struct addrinfo的链接列表，就像getaddrinfo一样，所以当您完成后，结果应该是免费的freeaddrinfo。

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

查询服务器并将结果作为addrinfo结构获取

1 个答案: