思路：

　　1、socket 建立一个数据报套接字。

       2、定义一个struct ifreq ifr 结构体。将网络名称如“eth0” 赋值给ifr结构体的ifr.ifr_name。

       3、调用ioctl(sockfd, SIOCGIFFLAGS, &ifr) 获取网络标识。

如需设置网络标识，更改ifr结构体调用ioctl(sockfd, SIOCSIFFLAGS, &ifr) 进行设置。

       4、比对网络标识来确定网络是否正在运行。ifr.ifr_ifru.ifru_flags &IFF_RUNNING。

int GetNetStatus(const char *ifname)
{
    struct ifreq ifr;
    int sockfd;

    if((sockfd = Socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        printf("socket create error!\n");
        return - 1;
    }

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, ifname, IFNAMSIZ);

    if(ioctl(sockfd, SIOCGIFFLAGS, &ifr) < 0)
    {
        Close(sockfd);
        return - 1;
    }
    Close(sockfd);
    if(ifr.ifr_ifru.ifru_flags &IFF_RUNNING)
    {
        return 1;
    }
    else
    {
        return 0;
    }
   
}

struct ifreq 结构体的详细介绍可参考。

struct ifreq学习和实例 - 走看看 (zoukankan.com)

 这个结构定义在/usr/include/net/if.h，用来配置和获取ip地址，掩码，MTU等接口信息的。

/* Interface request structure used for socket ioctl's. All interface
ioctl's must have parameter definitions which begin with ifr_name.
The remainder may be interface specific. */

struct ifreq
{
# define IFHWADDRLEN 6
# define IFNAMSIZ IF_NAMESIZE
union
{
char ifrn_name[IFNAMSIZ]; /* Interface name, e.g. "en0". */
} ifr_ifrn;

union
{
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
struct sockaddr ifru_broadaddr;
struct sockaddr ifru_netmask;
struct sockaddr ifru_hwaddr;
short int ifru_flags;
int ifru_ivalue;
int ifru_mtu;
struct ifmap ifru_map;
char ifru_slave[IFNAMSIZ]; /* Just fits the size */
char ifru_newname[IFNAMSIZ];
__caddr_t ifru_data;
} ifr_ifru;
};
# define ifr_name ifr_ifrn.ifrn_name /* interface name */
# define ifr_hwaddr ifr_ifru.ifru_hwaddr /* MAC address */
# define ifr_addr ifr_ifru.ifru_addr /* address */
# define ifr_dstaddr ifr_ifru.ifru_dstaddr /* other end of p-p lnk */
# define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */
# define ifr_netmask ifr_ifru.ifru_netmask /* interface net mask */
# define ifr_flags ifr_ifru.ifru_flags /* flags */
# define ifr_metric ifr_ifru.ifru_ivalue /* metric */
# define ifr_mtu ifr_ifru.ifru_mtu /* mtu */
# define ifr_map ifr_ifru.ifru_map /* device map */
# define ifr_slave ifr_ifru.ifru_slave /* slave device */
# define ifr_data ifr_ifru.ifru_data /* for use by interface */
# define ifr_ifindex ifr_ifru.ifru_ivalue /* interface index */
# define ifr_bandwidth ifr_ifru.ifru_ivalue /* link bandwidth */
# define ifr_qlen ifr_ifru.ifru_ivalue /* queue length */
# define ifr_newname ifr_ifru.ifru_newname /* New name */
# define _IOT_ifreq _IOT(_IOTS(char),IFNAMSIZ,_IOTS(char),16,0,0)
# define _IOT_ifreq_short _IOT(_IOTS(char),IFNAMSIZ,_IOTS(short),1,0,0)
# define _IOT_ifreq_int _IOT(_IOTS(char),IFNAMSIZ,_IOTS(int),1,0,0)

实例一，获取网卡的IP地址：
#include <string.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <stdio.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int main()
{
int inet_sock;
struct ifreq ifr;
inet_sock = socket(AF_INET, SOCK_DGRAM, 0);

strcpy(ifr.ifr_name, "eth0");
//SIOCGIFADDR标志代表获取接口地址
if (ioctl(inet_sock, SIOCGIFADDR, &ifr) < 0)
perror("ioctl");
printf("%s
", inet_ntoa(((struct sockaddr_in*)&(ifr.ifr_addr))->sin_addr));
return 0;
}
实例二，实现简单ifconfig功能：
/**
* file getifstat.c
* author wzj
* rief 访问这个struct ifconf 修改，查询状态
* version
*
ote
* date: 2012年08月11日星期六22:55:25
*/
#include <net/if.h> /* for ifconf */
#include <linux/sockios.h> /* for net status mask */
#include <netinet/in.h> /* for sockaddr_in */
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <stdio.h>

#define MAX_INTERFACE (16)

void port_status(unsigned int flags);

/* set == 0: do clean , set == 1: do set! */
int set_if_flags(char *pif_name, int sock, int status, int set)
{
struct ifreq ifr;
int ret = 0;

strncpy(ifr.ifr_name, pif_name, strlen(pif_name) + 1);
ret = ioctl(sock, SIOCGIFFLAGS, &ifr);
if(ret)
return -1;
/* set or clean */
if(set)
ifr.ifr_flags |= status;
else
ifr.ifr_flags &= ~status;
/* set flags */
ret = ioctl(sock, SIOCSIFFLAGS, &ifr);
if(ret)
return -1;

return 0;
}

int get_if_info(int fd)
{
struct ifreq buf[MAX_INTERFACE];
struct ifconf ifc;
int ret = 0;
int if_num = 0;

ifc.ifc_len = sizeof(buf);
ifc.ifc_buf = (caddr_t) buf;

ret = ioctl(fd, SIOCGIFCONF, (char*)&ifc);
if(ret)
{
printf("get if config info failed");
return -1;
}
/* 网口总数 ifc.ifc_len 应该是一个出入参数 */
if_num = ifc.ifc_len/sizeof(struct ifreq);
printf("interface num is interface = %d
", if_num);
while(if_num-- > 0)
{
printf("net device: %s
", buf[if_num].ifr_name);
/* 获取第n个网口信息 */
ret = ioctl(fd, SIOCGIFFLAGS, (char*)&buf[if_num]);
if(ret)
continue;

/* 获取网口状态 */
port_status(buf[if_num].ifr_flags);

/* 获取当前网卡的ip地址 */
ret = ioctl(fd, SIOCGIFADDR, (char*)&buf[if_num]);
if(ret)
continue;
printf("IP address is:
%s
", inet_ntoa(((struct sockaddr_in *)(&buf[if_num].ifr_addr))->sin_addr));

/* 获取当前网卡的mac */
ret = ioctl(fd, SIOCGIFHWADDR, (char*)&buf[if_num]);
if(ret)
continue;

printf("%02x:%02x:%02x:%02x:%02x:%02x

",
(unsigned char)buf[if_num].ifr_hwaddr.sa_data[0],
(unsigned char)buf[if_num].ifr_hwaddr.sa_data[1],
(unsigned char)buf[if_num].ifr_hwaddr.sa_data[2],
(unsigned char)buf[if_num].ifr_hwaddr.sa_data[3],
(unsigned char)buf[if_num].ifr_hwaddr.sa_data[4],
(unsigned char)buf[if_num].ifr_hwaddr.sa_data[5]
);
}
}

void port_status(unsigned int flags)
{
if(flags & IFF_UP)
{
printf("is up
");
}
if(flags & IFF_BROADCAST)
{
printf("is broadcast
");
}
if(flags & IFF_LOOPBACK)
{
printf("is loop back
");
}
if(flags & IFF_POINTOPOINT)
{
printf("is point to point
");
}
if(flags & IFF_RUNNING)
{
printf("is running
");
}
if(flags & IFF_PROMISC)
{
printf("is promisc
");
}
}

int main()
{
int fd;

fd = socket(AF_INET, SOCK_DGRAM, 0);
if(fd > 0)
{
get_if_info(fd);
close(fd);
}

return 0;
}
运行结果：
interface num is interface = 2
net device: eth0
is up
is broadcast
is running
IP address is:
192.168.100.200
54:be:f7:33:57:26
net device: lo
is up
is loop back
is running
IP address is:
127.0.0.1
00:00:00:00:00:00
参考转载博客地址：
http://blog.csdn.net/dsg333/article/details/7525634
http://blog.csdn.net/kulung/article/details/6442597
http://blog.csdn.net/joker0910/article/details/7855998