通过RTM_GETROUTE 获取路由表，写出一个简单的测试程序，需要获取网关地址

好的，以下是一个简单的测试程序，它使用RTM_GETROUTE消息获取路由表中的网关地址，并输出到终端上。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <arpa/inet.h>

int main() {
    int netlink_fd;
    struct sockaddr_nl src_addr;

    memset(&src_addr, 0, sizeof(src_addr));
    src_addr.nl_family = AF_NETLINK;

    if ((netlink_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    if (bind(netlink_fd, (struct sockaddr *)&src_addr, sizeof(src_addr)) < 0) {
        perror("bind");
        close(netlink_fd);
        exit(EXIT_FAILURE);
    }

    struct {
        struct nlmsghdr nlh;
        struct rtmsg    rt;
    } req;

    memset(&req, 0, sizeof(req));

    req.nlh.nlmsg_len    = NLMSG_LENGTH(sizeof(struct rtmsg));
    req.nlh.nlmsg_type   = RTM_GETROUTE;
    req.nlh.nlmsg_flags  = NLM_F_REQUEST | NLM_F_DUMP;
    req.nlh.nlmsg_seq    = 1;
    req.rt.rtm_family    = AF_INET;
    req.rt.rtm_table     = RT_TABLE_MAIN;
    req.rt.rtm_protocol = RTPROT_UNSPEC;

    struct msghdr msg;
    struct iovec iov;

    memset(&msg, 0, sizeof(msg));
    memset(&iov, 0, sizeof(iov));

    iov.iov_base = &req;
    iov.iov_len  = req.nlh.nlmsg_len;
    msg.msg_name = &src_addr;
    msg.msg_namelen = sizeof(src_addr);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    if (sendmsg(netlink_fd, &msg, 0) < 0) {
        perror("sendmsg");
        close(netlink_fd);
        exit(EXIT_FAILURE);
    }

    char buf[4096];
    struct nlmsghdr *nlh;

    while (1) {
        int len = recv(netlink_fd, buf, sizeof(buf), 0);
        if (len < 0) {
            perror("recv");
            close(netlink_fd);
            exit(EXIT_FAILURE);
        }

        for (nlh = (struct nlmsghdr *)buf; NLMSG_OK(nlh, len); nlh = NLMSG_NEXT(nlh, len)) {
            if (nlh->nlmsg_type == NLMSG_ERROR) {
                perror("NLMSG_ERROR");
                close(netlink_fd);
                exit(EXIT_FAILURE);
            }

            if (nlh->nlmsg_type == NLMSG_DONE)
                break;

            struct rtmsg *rtm;
            struct rtattr *rta;
            int rt_len;

            rtm = (struct rtmsg *) NLMSG_DATA(nlh);
            rt_len = nlh->nlmsg_len - NLMSG_LENGTH(sizeof(struct rtmsg));

            for (rta = RTM_RTA(rtm); RTA_OK(rta, rt_len); rta = RTA_NEXT(rta, rt_len)) {
                if (rta->rta_type == RTA_GATEWAY) {
                    char gateway[INET_ADDRSTRLEN];
                    inet_ntop(AF_INET, RTA_DATA(rta), gateway, INET_ADDRSTRLEN);
                    printf("Gateway: %s\n", gateway);
                }
            }
        }

        if (nlh->nlmsg_type == NLMSG_DONE)
            break;
    }

    close(netlink_fd);
    return 0;
}

运行程序后，它会输出路由表中的网关地址。请注意，需要以root权限运行该程序。