通过RTM_GETROUTE 获取路由表

好的，您想了解如何通过RTM_GETROUTE获取路由表。RTM_GETROUTE是Linux内核中用于获取路由表的一种Netlink消息类型。要使用它，您需要使用Socket编程接口中的Netlink Socket，向内核发送RTM_GETROUTE消息，并解析内核返回的路由表信息。

以下是获取路由表的基本步骤：

1. 创建Netlink Socket：

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);
}

2. 构造RTM_GETROUTE消息：

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;

3. 发送RTM_GETROUTE消息并接收内核返回的消息：

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)) {
        // 处理内核返回的消息
    }

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

4. 解析内核返回的路由表信息：

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)) {
    switch (rta->rta_type) {
        case RTA_DST:
            // 目的地址
            break;
        case RTA_GATEWAY:
            // 网关地址
            break;
        case RTA_OIF:
            // 输出接口
            break;
        case RTA_PRIORITY:
            // 优先级
            break;
        // 其他属性...
    }
}

以上是基本的流程，您可以根据具体需求修改RTM_GETROUTE消息的参数，以获取特定的路由表信息。希望能够帮到您！