Cómo configurar la dirección IP desde C en Linux

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>           // close()
#include <string.h>           // strcpy, memset(), and memcpy()

#include <netdb.h>            // struct addrinfo
#include <sys/types.h>        // needed for socket(), uint8_t, uint16_t
#include <sys/socket.h>       // needed for socket()
#include <netinet/in.h>       // IPPROTO_RAW, INET_ADDRSTRLEN
#include <netinet/ip.h>       // IP_MAXPACKET (which is 65535)
#include <arpa/inet.h>        // inet_pton() and inet_ntop()
#include <sys/ioctl.h>        // macro ioctl is defined
#include <bits/ioctls.h>      // defines values for argument "request" of ioctl.
#include <net/if.h>           // struct ifreq
#include <linux/if_ether.h>   // ETH_P_ARP = 0x0806
#include <linux/if_packet.h>  // struct sockaddr_ll (see man 7 packet)
#include <net/ethernet.h>

#include <errno.h>            // errno, perror()

#include <netinet/in.h>
#include <net/route.h>    


 /**
    * Create socket function
    */
    int create_socket() {

      int sockfd = 0;

      sockfd = socket(AF_INET, SOCK_DGRAM, 0);
      if(sockfd == -1){
        fprintf(stderr, "Could not get socket.\n");
        return -1;
      }

      return sockfd;

    }

    /**
    * Generic ioctrlcall to reduce code size
    */
    int generic_ioctrlcall(int sockfd, u_long *flags, struct ifreq *ifr) {

      if (ioctl(sockfd, (long unsigned int)flags, &ifr) < 0) {
        fprintf(stderr, "ioctl: %s\n", (char *)flags);
        return -1;
      }
      return 1;
    }

    /**
    * Set route with metric 100
    */
    int set_route(int sockfd, char *gateway_addr,  struct sockaddr_in *addr) {
      struct rtentry route;
      int err = 0;
      memset(&route, 0, sizeof(route));
      addr = (struct sockaddr_in*) &route.rt_gateway;
      addr->sin_family = AF_INET;
      addr->sin_addr.s_addr = inet_addr(gateway_addr);
      addr = (struct sockaddr_in*) &route.rt_dst;
      addr->sin_family = AF_INET;
      addr->sin_addr.s_addr = inet_addr("0.0.0.0");
      addr = (struct sockaddr_in*) &route.rt_genmask;
      addr->sin_family = AF_INET;
      addr->sin_addr.s_addr = inet_addr("0.0.0.0");
      route.rt_flags = RTF_UP | RTF_GATEWAY;
      route.rt_metric = 100;
      err = ioctl(sockfd, SIOCADDRT, &route);
      if ((err) < 0) {
        fprintf(stderr, "ioctl: %s\n",  "mahdi MOAHMMADI Error");
        return -1;
      }
      return 1;
    }

    /**
    * Set ip function
    */
    int set_ip(char *iface_name, char *ip_addr, char *gateway_addr)
    {
      if(!iface_name)
        return -1;
      struct ifreq ifr;
      struct sockaddr_in sin;
      int sockfd = create_socket();

      sin.sin_family = AF_INET;

      // Convert IP from numbers and dots to binary notation
      inet_aton(ip_addr,&sin.sin_addr.s_addr);

      /* get interface name */
      strncpy(ifr.ifr_name, iface_name, IFNAMSIZ);

      /* Read interface flags */
      generic_ioctrlcall(sockfd, (u_long *)"SIOCGIFFLAGS", &ifr);
      /*
      * Expected in <net/if.h> according to
      * "UNIX Network Programming".
      */
      #ifdef ifr_flags
      # define IRFFLAGS       ifr_flags
      #else   /* Present on kFreeBSD */
      # define IRFFLAGS       ifr_flagshigh
      #endif
      // If interface is down, bring it up
      if (ifr.IRFFLAGS | ~(IFF_UP)) {
        ifr.IRFFLAGS |= IFF_UP;
        generic_ioctrlcall(sockfd, (u_long *)"SIOCSIFFLAGS", &ifr);
      }
      // Set route
      set_route(sockfd, gateway_addr    ,  &sin);
      memcpy(&ifr.ifr_addr, &sin, sizeof(struct sockaddr)); 
      // Set interface address
      if (ioctl(sockfd, SIOCSIFADDR, &ifr) < 0) {
        fprintf(stderr, "Cannot set IP address. ");
        perror(ifr.ifr_name);
        return -1;
      }             
      #undef IRFFLAGS 

      return 0;
    }

uso:

set_ip("eth0", "192.168.181.128", "192.168.181.1");

La forma "correcta" de hacerlo es generar una copia del programa "ip" iproute2 (en /sbin/ip ) con los parámetros relevantes.

la interfaz ioctl generalmente está obsoleta y no le permite configurar todos los parámetros (por ejemplo, alias de IP sin nombre).

Incluso demonios como dhcpcd que necesitan cambiar la dirección IP, generalmente lo hacen generando un programa externo... no es como si lo hicieras muy a menudo.

La forma "correcta" de IPv4 sin magia +2:

struct sockaddr_in* addr = (struct sockaddr_in*)&ifr.ifr_addr;
inet_pton(AF_INET, "10.12.0.1", &addr->sin_addr);

Para usar IPv6, envíalo a sockaddr_in6

Hola:En la función set_ip me daba error. Tuve que cambiar de :

inet_aton(ip_addr, &sin.sin_addr.s_addr);

a

inet_aton(ip_addr, &sin.sin_addr);

y funcionó bien.