How to set the IP address from C in linux - c

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");

Answer 3

The "correct" way to do it is to spawn a copy of the iproute2 "ip" program (in /sbin/ip ) with relevant parameters.
the ioctl interface is generally obsolescent and doesn't allow you to configure all parameters (for example, un-named IP aliases).
Even daemons like dhcpcd which need to change the IP address, typically do it by spawning an external program... it's not like you're going to do it very often.

Answer 4

Hello: In the set_ip function, it gave me an error.
I had to change from :
inet_aton(ip_addr, &sin.sin_addr.s_addr);
to
inet_aton(ip_addr, &sin.sin_addr);
and worked well.