Question

I am developing an application on Linux. Linux executes the command:

ifconfig eth0 192.168.10.15 netmask 255.255.255.0

at the boot process with systemd.

When I try to learn my ip address with ifconfig here below the result:

eth0      Link encap:Ethernet  HWaddr 00:18:31:E0:44:C6  
          inet addr:192.168.10.15  Bcast:192.168.10.255  Mask:255.255.255.0
          inet6 addr: fe80::218:31ff:fee0:44c6/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:24799 errors:0 dropped:1 overruns:0 frame:0
          TX packets:13753 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:5475289 (5.2 MiB)  TX bytes:1403459 (1.3 MiB)

On the other hand when I try to learn my ip address via program below, I get 192.168.10.42.

void GetMyIp(char caIP[INET_ADDRSTRLEN])
{
  int s;
  struct ifconf ifconf;
  struct ifreq ifr[50];
  int ifs;
  int i;    

  s = socket(AF_INET, SOCK_STREAM, 0);
  if (s < 0) 
  {
      #ifdef _DEBUG_PROCESS
        if(EDebugProcess<GetDebugLevelOfPMC())
        {
        PrintToLogFile("Socket error\n");
        }
      #endif
  }

  ifconf.ifc_buf = (char *) ifr;
  ifconf.ifc_len = sizeof ifr;

  if (ioctl(s, SIOCGIFCONF, &ifconf) == -1) 
  {
    #ifdef _DEBUG_PROCESS
        if(EDebugProcess<GetDebugLevelOfPMC())
        {
        PrintToLogFile("ioctl error\n");
        }
      #endif
  }

  ifs = ifconf.ifc_len / sizeof(ifr[0]);
    struct sockaddr_in *s_in = (struct sockaddr_in *) &ifr[ifs-1].ifr_addr;
    if (!inet_ntop(AF_INET, &s_in->sin_addr, gcaMyIP, sizeof(gcaMyIP))) 
    {
    #ifdef _DEBUG_PROCESS
        if(EDebugProcess<GetDebugLevelOfPMC())
        {
        PrintToLogFile("inet_ntop error\n");
        }
    #endif
    }


    close(s);
    memcpy(caIP,gcaMyIP,INET_ADDRSTRLEN);
}

What is the source of the difference?

Answer 1

You can try that if you want:

http://www.binarytides.com/get-local-ip-in-c-on-linux/