How to set the IP address from C in linux

Question

By using strace and ifconfig, I found that I can set the IP address this way:

#include <sys/ioctl.h> #include <arpa/inet.h> #include <net/if.h> #include <string.h>  int main(int argc, const char *argv[]) {     struct ifreq ifr;     const char * name = "eth1";     int fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP);      strncpy(ifr.ifr_name, name, IFNAMSIZ);      ifr.ifr_addr.sa_family = AF_INET;     inet_pton(AF_INET, "10.12.0.1", ifr.ifr_addr.sa_data + 2);     ioctl(fd, SIOCSIFADDR, &ifr);      inet_pton(AF_INET, "255.255.0.0", ifr.ifr_addr.sa_data + 2);     ioctl(fd, SIOCSIFNETMASK, &ifr);      ioctl(fd, SIOCGIFFLAGS, &ifr);     strncpy(ifr.ifr_name, name, IFNAMSIZ);     ifr.ifr_flags |= (IFF_UP | IFF_RUNNING);      ioctl(fd, SIOCSIFFLAGS, &ifr);      return 0; } 

But I am not very happy with this solution:

inet_pton(AF_INET, "10.12.0.1", ifr.ifr_addr.sa_data + 2); 

What is the "right" way of doing this?

Answer 1

The "correct" way for IPv4 without magic +2:

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

To use IPv6, cast it to sockaddr_in6

Answer 2

#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;     } 

usage:

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