前言

这篇文章写了获取本机的ip地址和子网掩码：Linux c语言获取本机 ip、子网掩码

一、获取本机网关 ip 地址

1.1 Netlink套接字简介

关于Netlink套接字请参考：Linux 网络之netlink 简介

Linux Netlink套接字是一种用于在Linux内核和用户空间之间进行通信的机制。它是Linux内核中的一种通信协议，用于让用户空间程序与内核进行交互。使用Netlink套接字，用户空间程序可以向内核发送请求，以获取系统信息、控制系统行为或获取有关网络接口、路由表、套接字等的信息。

Netlink套接字提供了一种可扩展的、可靠的、异步的机制，用于在内核和用户空间之间传输大量的网络相关信息。它还支持多播，可以同时向多个进程发送消息。

Netlink套接字有多个协议族，每个协议族都有不同的目的和用途。其中最常见的是NETLINK_ROUTE协议族，它用于与网络配置和路由相关的操作。

1.2 代码示例

下面是使用Netlink套接字从Linux内核的路由表中检索默认网关IP地址的代码例程

代码如下：

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

#define BUFSIZE 8192

struct nlreq {
    struct nlmsghdr hdr;
    struct rtmsg msg;
};

int main(void) 
{
    int sockfd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sockfd == -1) {
        perror("socket error");
        exit(1);
    }

    struct sockaddr_nl sa;
    memset(&sa, 0, sizeof(sa));
    sa.nl_family = AF_NETLINK;
    if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
        perror("bind error");
        exit(1);
    }
    
    struct nlreq req;
    memset(&req, 0, sizeof(req));
    req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
    req.hdr.nlmsg_type = RTM_GETROUTE;
    req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
    req.msg.rtm_family = AF_INET;
    req.msg.rtm_table = RT_TABLE_MAIN;
    req.msg.rtm_protocol = RTPROT_UNSPEC;
    req.msg.rtm_scope = RT_SCOPE_UNIVERSE;
    req.msg.rtm_type = RTN_UNICAST;

    struct iovec iov;
    memset(&iov, 0, sizeof(iov));
    iov.iov_base = &req;
    iov.iov_len = req.hdr.nlmsg_len;

    struct msghdr msg;
    memset(&msg, 0, sizeof(msg));
    msg.msg_name = &sa;
    msg.msg_namelen = sizeof(sa);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;
    
    char buf[BUFSIZE];
    memset(buf, 0, sizeof(buf));
    struct nlmsghdr *hdr;
    int len;
    if (sendmsg(sockfd, &msg, 0) == -1) {
        perror("sendmsg error");
        exit(1);
    }
    while ((len = recv(sockfd, buf, sizeof(buf), 0)) > 0) {
        for (hdr = (struct nlmsghdr *) buf; NLMSG_OK(hdr, len); hdr = NLMSG_NEXT(hdr, len)) {
            if (hdr->nlmsg_type == NLMSG_DONE) {
                goto finish;
            }
            if (hdr->nlmsg_type == NLMSG_ERROR) {
                perror("NLMSG_ERROR");
                exit(1);
            }
            struct rtmsg *rt = (struct rtmsg *) NLMSG_DATA(hdr);
            if (rt->rtm_family != AF_INET || rt->rtm_table != RT_TABLE_MAIN || rt->rtm_type != RTN_UNICAST) {
                continue;
            }
            struct rtattr *attr;
            int attrlen;
            for (attr = (struct rtattr *) RTM_RTA(rt), attrlen = RTM_PAYLOAD(hdr); RTA_OK(attr, attrlen); attr = RTA_NEXT(attr, attrlen)) {
                if (attr->rta_type == RTA_GATEWAY) {
                    char gw_addr[INET_ADDRSTRLEN];
                    struct in_addr addr;
                    memcpy(&addr, RTA_DATA(attr), sizeof(addr));
                    if (inet_ntop(AF_INET, &addr, gw_addr, sizeof(gw_addr)) == NULL) {
                        perror("inet_ntop error");
                        continue;
                    }
                    printf("Gateway address: %s\n", gw_addr);
                    goto finish;
                }
            }
        }
    }
finish:
    close(sockfd);
    return 0;
}

首先，它使用socket（）函数创建一个Netlink套接字，并使用bind（）将其绑定到本地地址。然后，它使用nlreq结构体变量初始化Netlink消息的参数。消息请求使用RTM_GETROUTE类型检索路由表，并指定要检索的路由的地址族、表、协议、范围和类型。消息还指定了NLM_F_DUMP标志以检索所有可用路由。

消息使用sendmsg（）发送到内核，并使用recv（）接收。接收到的数据通过一个循环解析缓冲区中的Netlink消息。该循环跳过与IPv4单播路由和主表无关的消息。对于每个相关消息，它使用另一个循环迭代路由属性，并检索网关地址（如果存在）。网关地址使用printf（）打印到控制台，并使用goto语句退出循环。

1.3 代码详解介绍

（1）使用socket函数创建一个Netlink套接字

int sockfd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
if (sockfd == -1) {
    perror("socket error");
    exit(1);
}

/* Protocol families.  */
#define	PF_NETLINK	16

/* Address families.  */
#define	AF_NETLINK	PF_NETLINK

/* Types of sockets.  */
enum __socket_type
{
  SOCK_RAW = 3,			/* Raw protocol interface.  */
#define SOCK_RAW SOCK_RAW
};

#define NETLINK_ROUTE		0	/* Routing/device hook				*/

NETLINK_ROUTE：用于与网络配置和路由相关的操作，如获取和修改网络接口、路由表和ARP缓存等。

（2）绑定套接字到本地地址

struct sockaddr_nl sa;
memset(&sa, 0, sizeof(sa));
sa.nl_family = AF_NETLINK;
if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) == -1) {
    perror("bind error");
    exit(1);
}

绑定套接字到本地地址。这是为了确保接收到内核发送的Netlink消息。

（3）创建一个Netlink消息

struct nlreq req;
memset(&req, 0, sizeof(req));
req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
req.hdr.nlmsg_type = RTM_GETROUTE;
req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
req.msg.rtm_family = AF_INET;
req.msg.rtm_table = RT_TABLE_MAIN;
req.msg.rtm_protocol = RTPROT_UNSPEC;
req.msg.rtm_scope = RT_SCOPE_UNIVERSE;
req.msg.rtm_type = RTN_UNICAST;

创建一个Netlink消息，用于向内核请求路由表。我们设置消息头部的长度、类型和标志，以及rtmsg结构体的成员。这里我们只请求主路由表中的单播路由表项，以获取默认网关的地址。

（4）将req结构体打包成一个iovec结构体

struct iovec iov;
memset(&iov, 0, sizeof(iov));
iov.iov_base = &req;
iov.iov_len = req.hdr.nlmsg_len;

struct msghdr msg;
memset(&msg, 0, sizeof(msg));
msg.msg_name = &sa;
msg.msg_namelen = sizeof(sa);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;

将req结构体打包成一个iovec结构体，用于在消息中发送。我们还创建了一个msghdr结构体，指定发送和接收消息的参数。

（5）使用sendmsg函数将消息发送到内核

if (sendmsg(sockfd, &msg, 0) == -1) {
    perror("sendmsg error");
    exit(1);
}

（6）使用recv函数从内核接收消息

char buf[BUFSIZE];
memset(buf, 0, sizeof(buf));
struct nlmsghdr *hdr;
int len;
if (sendmsg(sockfd, &msg, 0) == -1) {
    perror("sendmsg error");
    exit(1);
}
while ((len = recv(sockfd, buf, sizeof(buf), 0)) > 0) {
    for (hdr = (struct nlmsghdr *) buf; NLMSG_OK(hdr, len); hdr = NLMSG_NEXT(hdr, len)) {
        if (hdr->nlmsg_type == NLMSG_DONE) {
            goto finish;
        }
        if (hdr->nlmsg_type == NLMSG_ERROR) {
            perror("NLMSG_ERROR");
            exit(1);
        }
        // 解析路由表项
    }
}

使用recv函数从内核接收消息，并使用NLMSG_OK、NLMSG_NEXT和NLMSG_DATA宏来循环遍历消息中的所有路由表项。

（7）获取默认网关的地址

struct rtmsg *rt = (struct rtmsg *) NLMSG_DATA(hdr);
if (rt->rtm_family != AF_INET || rt->rtm_table != RT_TABLE_MAIN || rt->rtm_type != RTN_UNICAST) {
    continue;
}
struct rtattr *attr;
int attrlen;
for (attr = (struct rtattr *) RTM_RTA(rt), attrlen = RTM_PAYLOAD(hdr); RTA_OK(attr, attrlen); attr = RTA_NEXT(attr, attrlen)) {
    if (attr->rta_type == RTA_GATEWAY) {
        char gw_addr[INET_ADDRSTRLEN];
        struct in_addr addr;
        memcpy(&addr, RTA_DATA(attr), sizeof(addr));
        if (inet_ntop(AF_INET, &addr, gw_addr, sizeof(gw_addr)) == NULL) {
            perror("inet_ntop error");
            continue;
        }
        printf("Gateway address: %s\n", gw_addr);
        goto finish;
    }
}
finish:
    close(sockfd);
    return 0;
}

/* Routing message attributes */

enum rtattr_type_t {
	RTA_GATEWAY,
};

检查每个路由表项的类型和成员，以确定是否找到了默认网关的地址。如果找到了，使用inet_ntop函数将地址转换为可读形式，并打印它。

二、使用Netlink套接字实时监控网络事件

2.1 简介

可以使用Netlink套接字实时监控网络事件。Netlink是一种基于套接字的接口，用于与Linux内核通信。内核中的各个子系统都使用Netlink与用户空间应用程序通信，包括网络子系统。

网络子系统使用Netlink在某些网络事件发生时向用户空间应用程序发送消息，例如添加或删除网络接口、更改网络路由和更改网络地址。这些消息可以被用户空间应用程序用于实时监控网络事件。

要使用Netlink实时监控网络事件，您需要创建一个Netlink套接字，将其绑定到特定的Netlink协议，然后使用recv函数从内核接收Netlink消息。然后，您可以解析消息以提取有关已发生的网络事件的信息。

2.2 示例代码

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

#define BUFFER_SIZE 4096

int main(int argc, char *argv[]) {
    int netlinkSocket = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (netlinkSocket < 0) {
        perror("创建Netlink套接字失败");
        exit(EXIT_FAILURE);
    }

    struct sockaddr_nl addr;
    memset(&addr, 0, sizeof(addr));
    addr.nl_family = AF_NETLINK;
    addr.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR;

    if (bind(netlinkSocket, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
        perror("将Netlink套接字绑定到地址失败");
        close(netlinkSocket);
        exit(EXIT_FAILURE);
    }

    char buffer[BUFFER_SIZE];
    struct iovec iov = { buffer, sizeof(buffer) };
    struct msghdr msg = { (void *)&addr, sizeof(addr), &iov, 1, NULL, 0, 0 };

    while (1) {
        ssize_t len = recvmsg(netlinkSocket, &msg, 0);
        if (len < 0) {
            perror("接收Netlink消息失败");
            close(netlinkSocket);
            exit(EXIT_FAILURE);
        }

        struct nlmsghdr *nlh;
        for (nlh = (struct nlmsghdr *)buffer; NLMSG_OK(nlh, len); nlh = NLMSG_NEXT(nlh, len)) {
            switch(nlh->nlmsg_type) {
                case RTM_NEWLINK:
                    printf("检测到新链接\n");
                    break;
                case RTM_DELLINK:
                    printf("链接已删除\n");
                    break;
                case RTM_NEWADDR:
                    printf("检测到新地址\n");
                    break;
                case RTM_DELADDR:
                    printf("地址已删除\n");
                    break;
                default:
                    printf("未知的消息类型 (%d)\n", nlh->nlmsg_type);
                    break;
            }
        }
    }

    close(netlinkSocket);
    return 0;
}

在一个终端上编译程序并运行。

在另一个终端上输入以下命令来更改网络接口的状态：

sudo ifconfig eth0 down

sudo ifconfig eth0 up

这只是一个简单的示例，可以使用Netlink监控许多其他类型的网络事件。

rtnetlink.h头文件提供了所有可能的Netlink消息类型的列表，可以用于监控其他类型的网络事件。
如下所示：

/****
 *		Routing/neighbour discovery messages.
 ****/

/* Types of messages */

enum {
	RTM_BASE	= 16,
#define RTM_BASE	RTM_BASE

	RTM_NEWLINK	= 16,
#define RTM_NEWLINK	RTM_NEWLINK
	RTM_DELLINK,
#define RTM_DELLINK	RTM_DELLINK
	RTM_GETLINK,
#define RTM_GETLINK	RTM_GETLINK
	RTM_SETLINK,
#define RTM_SETLINK	RTM_SETLINK

	RTM_NEWADDR	= 20,
#define RTM_NEWADDR	RTM_NEWADDR
	RTM_DELADDR,
#define RTM_DELADDR	RTM_DELADDR
	RTM_GETADDR,
#define RTM_GETADDR	RTM_GETADDR

	RTM_NEWROUTE	= 24,
#define RTM_NEWROUTE	RTM_NEWROUTE
	RTM_DELROUTE,
#define RTM_DELROUTE	RTM_DELROUTE
	RTM_GETROUTE,
#define RTM_GETROUTE	RTM_GETROUTE

	RTM_NEWNEIGH	= 28,
#define RTM_NEWNEIGH	RTM_NEWNEIGH
	RTM_DELNEIGH,
#define RTM_DELNEIGH	RTM_DELNEIGH
	RTM_GETNEIGH,
#define RTM_GETNEIGH	RTM_GETNEIGH

	RTM_NEWRULE	= 32,
#define RTM_NEWRULE	RTM_NEWRULE
	RTM_DELRULE,
#define RTM_DELRULE	RTM_DELRULE
	RTM_GETRULE,
#define RTM_GETRULE	RTM_GETRULE

	RTM_NEWQDISC	= 36,
#define RTM_NEWQDISC	RTM_NEWQDISC
	RTM_DELQDISC,
#define RTM_DELQDISC	RTM_DELQDISC
	RTM_GETQDISC,
#define RTM_GETQDISC	RTM_GETQDISC

	RTM_NEWTCLASS	= 40,
#define RTM_NEWTCLASS	RTM_NEWTCLASS
	RTM_DELTCLASS,
#define RTM_DELTCLASS	RTM_DELTCLASS
	RTM_GETTCLASS,
#define RTM_GETTCLASS	RTM_GETTCLASS

	RTM_NEWTFILTER	= 44,
#define RTM_NEWTFILTER	RTM_NEWTFILTER
	RTM_DELTFILTER,
#define RTM_DELTFILTER	RTM_DELTFILTER
	RTM_GETTFILTER,
#define RTM_GETTFILTER	RTM_GETTFILTER

	RTM_NEWACTION	= 48,
#define RTM_NEWACTION   RTM_NEWACTION
	RTM_DELACTION,
#define RTM_DELACTION   RTM_DELACTION
	RTM_GETACTION,
#define RTM_GETACTION   RTM_GETACTION

	RTM_NEWPREFIX	= 52,
#define RTM_NEWPREFIX	RTM_NEWPREFIX

	RTM_GETMULTICAST = 58,
#define RTM_GETMULTICAST RTM_GETMULTICAST

	RTM_GETANYCAST	= 62,
#define RTM_GETANYCAST	RTM_GETANYCAST

	RTM_NEWNEIGHTBL	= 64,
#define RTM_NEWNEIGHTBL	RTM_NEWNEIGHTBL
	RTM_GETNEIGHTBL	= 66,
#define RTM_GETNEIGHTBL	RTM_GETNEIGHTBL
	RTM_SETNEIGHTBL,
#define RTM_SETNEIGHTBL	RTM_SETNEIGHTBL

	RTM_NEWNDUSEROPT = 68,
#define RTM_NEWNDUSEROPT RTM_NEWNDUSEROPT

	RTM_NEWADDRLABEL = 72,
#define RTM_NEWADDRLABEL RTM_NEWADDRLABEL
	RTM_DELADDRLABEL,
#define RTM_DELADDRLABEL RTM_DELADDRLABEL
	RTM_GETADDRLABEL,
#define RTM_GETADDRLABEL RTM_GETADDRLABEL

	RTM_GETDCB = 78,
#define RTM_GETDCB RTM_GETDCB
	RTM_SETDCB,
#define RTM_SETDCB RTM_SETDCB

	RTM_NEWNETCONF = 80,
#define RTM_NEWNETCONF RTM_NEWNETCONF
	RTM_GETNETCONF = 82,
#define RTM_GETNETCONF RTM_GETNETCONF

	RTM_NEWMDB = 84,
#define RTM_NEWMDB RTM_NEWMDB
	RTM_DELMDB = 85,
#define RTM_DELMDB RTM_DELMDB
	RTM_GETMDB = 86,
#define RTM_GETMDB RTM_GETMDB

	RTM_NEWNSID = 88,
#define RTM_NEWNSID RTM_NEWNSID
	RTM_DELNSID = 89,
#define RTM_DELNSID RTM_DELNSID
	RTM_GETNSID = 90,
#define RTM_GETNSID RTM_GETNSID

	RTM_NEWSTATS = 92,
#define RTM_NEWSTATS RTM_NEWSTATS
	RTM_GETSTATS = 94,
#define RTM_GETSTATS RTM_GETSTATS

	__RTM_MAX,
#define RTM_MAX		(((__RTM_MAX + 3) & ~3) - 1)
};