【转】Linux C编程获取本地网卡设备信息以及域名解析DNS

一、Linux 中ifreq 结构体原型：

struct ifreq 
{
#define IFHWADDRLEN 6
 union
 {
  char ifrn_name[IFNAMSIZ];  /* if 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 ifru_flags;
  int ifru_ivalue;
  int ifru_mtu;
  struct  ifmap ifru_map;
  char ifru_slave[IFNAMSIZ]; /* Just fits the size */
  char ifru_newname[IFNAMSIZ];
  void __user * ifru_data;
  struct if_settings ifru_settings;
 } 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 ifr_settings ifr_ifru.ifru_settings		/* Device/proto settings*/

基本介绍：

ifreq结构定义在/usr/include/net/if.h，用来配置ip地址，激活 接口，配置MTU等接口信息的。其中包含了一个接口的名字和具体内容——（是个共用体，有可能是IP地址，广播地址，子网掩码，MAC号，MTU或其他内 容）。ifreq包含在ifconf结构中。而ifconf结构通常是用来保存所有接口的信息的。

举例说明：

在Linux系统中，ifconfig命令是通过ioctl接口与内核通信，例如，当系统管理员输入如下命令来改变接口eth0的MTU大小：

        ifconfig eth0 mtu 1250

ifconfig命令首先打开一个socket，然后通过系统管理员输入的参数初始化一个数据结构，并通过ioctl调用将数据传送到内核。SIOCSIFMTU是命令标识符。

	struct ifreq data;
	fd = socket(PF_INET, SOCK_DGRAM, 0);
	<... initialize "data" ...>
	err = ioctl(fd, SIOCSIFMTU, &data);

二、获取本地ip，netmask和mac地址

#include 
#include         //gethostbyaddr,gethostbyname
//#include        //比如read,write函数
#include    //地址簇相关,比如sockaddr_in
#include     //inet_ntoa
#include 
#include 
#include    //比如文件描述符fd_set
#include 
#include 

struct in_addr myip,mymask;
int fd_arp;     		//socket fd for receive packets
struct ifreq ifr;		//ifr struct

int main()
{
	char device[32];  //ethernet device name
	struct sockaddr from,to;
	struct sockaddr_in *sin_ptr;
	u_char *ptr;
	

	if((fd_arp=socket(AF_INET,SOCK_DGRAM,0))<0)
	{
		perror("arp socket error");
		exit(-1);
	}
	
	strcpy(device,"eth0");
	strcpy(ifr.ifr_name,device);
	
	/**** get ip address of my interface****/
	if(ioctl(fd_arp,SIOCGIFADDR,&ifr)<0)
	{
		perror("ioctl SIOCGIFADDR error");
		exit(-1);
	}
	sin_ptr = (struct sockaddr_in *)&ifr.ifr_addr;
	myip = sin_ptr->sin_addr;
	printf("request IP  %s\n",inet_ntoa(myip));
	/**** get network mask of my interface****/
	if(ioctl(fd_arp,SIOCGIFNETMASK,&ifr)<0)
	{
		perror("ioctl SIOCGIFNETMASK error");
		exit(-1);
	}
	sin_ptr = (struct sockaddr_in *)&ifr.ifr_addr;
	mymask = sin_ptr->sin_addr;
	printf("request netmask  %s\n",inet_ntoa(mymask));
	/**** get mac address of my interface****/
	if(ioctl(fd_arp,SIOCGIFHWADDR,&ifr)<0)
	{
		perror("ioctl SIOCGIFHWADDR error");
		exit(-1);
	}
	ptr = (u_char *)&ifr.ifr_ifru.ifru_hwaddr.sa_data[0];
	printf("request mac %02x:%02x:%02x:%02x:%02x:%02x\n",
		*ptr,*(ptr+1),*(ptr+2),*(ptr+3),*(ptr+4),*(ptr+5));	
	/**** ---end--- ****/
	return 0;
}

三、域名解析 DNS

 gethostbyname()和gethostbyaddr()：这两个函数可以获得DNS 信息。

使用gethostbyname()函数，不能使用perror() 来输出错误信息，因为错误代码存储在h_error中而不是error中。所以，需要调用herror()函数。