Get my IP from network card

這個範例顯示如何取得網卡的IP:
eth0.c

#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <arpa/inet.h>
#include <net/if.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc, char *argv[])
{
 int fd;
 char IPbuf[30];
 struct ifreq ifr;
 struct sockaddr_in *myAddr;

 strcpy(ifr.ifr_name, "eth0");
 fd = socket(AF_INET, SOCK_STREAM, 0);

 if(ioctl(fd, SIOCGIFFLAGS, &ifr)>=0)
 {
   if (ifr.ifr_flags & IFF_UP)
    puts("eth0 is UP");
   else
   {
    puts("eth0 is DOWN");
    return 1;
  }
 }

 if(ioctl(fd, SIOCGIFADDR, &ifr)>=0)
 {
  myAddr = (struct sockaddr_in*)&ifr.ifr_addr;
  sprintf(IPbuf,inet_ntoa(myAddr->sin_addr));
   printf("IP:%s\n",IPbuf);
 }

 close(fd);

 return 0;
}
 

執行結果:
[root@ecken01 net]# gcc -Wall eth0.c -o eth0
[root@ecken01 net]# ./eth0
eth0 is UP
IP:192.168.0.51

程式中struct ifreq 定義:

struct ifreq {
    char    ifr_name[IFNAMSIZ];/* Interface name */
    union {
            struct sockaddrifr_addr;
            struct sockaddrifr_dstaddr;
            struct sockaddrifr_broadaddr;
            struct sockaddrifr_netmask;
            struct sockaddrifr_hwaddr;
            short   ifr_flags;
            int     ifr_ifindex;
            int     ifr_metric;
            int     ifr_mtu;
            struct ifmapifr_map;
            char    ifr_slave[IFNAMSIZ];
            char    ifr_newname[IFNAMSIZ];
            char *  ifr_data;
    };
};

struct ifconf {
    int ifc_len;    /* size of buffer */
    union {    
            char *  ifc_buf; /* buffer address */
            struct ifreq *ifc_req; /* array of structures */
    };
};

ioctl的指令:
SIOCGIFNAME:回傳網路卡名稱
SIOCGIFINDEX:取得網路卡索引值
SIOCGIFFLAGS, SIOCSIFFLAGS:取得或設定裝置的Enable旗標

















Device flags
IFF_UP
Interface is running.
IFF_BROADCAST
Valid broadcast address set.
IFF_DEBUG
Internal debugging flag.
IFF_LOOPBACK
Interface is a loopback interface.
IFF_POINTOPOINT
Interface is a point-to-point link.
IFF_RUNNING
Resources allocated.
IFF_NOARP
No arp protocol, L2 destination address not set.
IFF_PROMISC
Interface is in promiscuous mode.
IFF_NOTRAILERS
Avoid use of trailers.
IFF_ALLMULTI
Receive all multicast packets.
IFF_MASTER
Master of a load balancing bundle.
IFF_SLAVE
Slave of a load balancing bundle.
IFF_MULTICAST
Supports multicast
IFF_PORTSEL
Is able to select media type via ifmap.
IFF_AUTOMEDIA
Auto media selection active.
IFF_DYNAMIC
The addresses are lost when the interface goes down. 

SIOCGIFMTU, SIOCSIFMTU:傳回或設定最大的傳輸單位MTU
SIOCGIFHWADDR, SIOCSIFHWADDR:取得或設定網路卡的硬體位址
SIOCSIFHWBROADCAST:設定網路卡的廣播位址
SIOCGIFMAP, SIOCSIFMAP:取得或設定網路卡的硬體參數，參數使用ifr_map
    struct ifmap {
      unsigned long mem_start;
      unsigned long mem_end;
      unsigned short base_addr;
      unsigned char irq;
      unsigned char dma;
      unsigned char port;
      };

SIOCADDMULTI, SIOCDELMULTI:增加或刪除網路卡的位址(link layer using ifr_hwaddr)
SIOCGIFTXQLEN, SIOCSIFTXQLEN:取得或設定傳輸佇列的長度
SIOCSIFNAME:改變網路卡名稱
SIOCGIFADDR:取得網路卡位址
SIOCGIFCONF:傳回網路卡位址列表(IPV4)

參考資料:
netdevice(7) - Linux man page