Setting an IP address using SIOCSIFADDR ioctl

Question

I am trying to set and set an IP address using the IOCTL interface on Linux. I can successfully install and install it. When I set the ip address, ifconfig eth0 shows the correct IP address, but then the system shuts down. Those. the system is not pingable. Here is my code to set the IP address. Please let me know if I missed something.

 struct ifreq ifr; in_addr_t in_addr; struct sockaddr_in sin; memset(&ifr, 0, sizeof(struct ifreq)); memset(&sin, 0, sizeof(struct sockaddr_in)); sockfd = socket(AF_INET, SOCK_STREAM, 0); sprintf(ifr.ifr_name, "eth0"); in_addr = inet_addr("192.168.101.17"); sin.sin_addr.s_addr = in_addr; memcpy(&ifr.ifr_addr, &sin, sizeof(struct sockaddr)); io = ioctl(sockfd, SIOCSIFADDR, (char *)&ifr); 

Answer 1

This will work for interfaces or aliases. Use "strace" to verify proper operation:

 strace ./ifconfig socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 5 ioctl(5, SIOCSIFADDR, {ifr_name="eth0:8", ifr_addr={AF_INET, inet_addr("192.168.1.202")}}) = 0 ioctl(5, SIOCGIFFLAGS, {ifr_name="eth0:8", ifr_flags=IFF_UP|IFF_BROADCAST|IFF_RUNNING|IFF_MULTICAST}) = 0 ioctl(5, SIOCSIFFLAGS, {ifr_name="eth0:8", ifr_flags=IFF_UP|IFF_BROADCAST|IFF_RUNNING|IFF_MULTICAST}) = 0 close(5) = 0 

Full source code:

 #include <stdio.h> #include <stdlib.h> #include <string.h> #include <stddef.h> /* offsetof */ #include <net/if.h> #include <net/if.h> #include <linux/sockios.h> #include <netinet/in.h> #if __GLIBC__ >=2 && __GLIBC_MINOR >= 1 #include <netpacket/packet.h> #include <net/ethernet.h> #else #include <asm/types.h> #include <linux/if_ether.h> #endif #define IFNAME "eth0:2" #define HOST "192.168.1.204" #define ifreq_offsetof(x) offsetof(struct ifreq, x) int main(int argc, char **argv) { struct ifreq ifr; struct sockaddr_in sai; int sockfd; /* socket fd we use to manipulate stuff with */ int selector; unsigned char mask; char *p; /* Create a channel to the NET kernel. */ sockfd = socket(AF_INET, SOCK_DGRAM, 0); /* get interface name */ strncpy(ifr.ifr_name, IFNAME, IFNAMSIZ); memset(&sai, 0, sizeof(struct sockaddr)); sai.sin_family = AF_INET; sai.sin_port = 0; sai.sin_addr.s_addr = inet_addr(HOST); p = (char *) &sai; memcpy( (((char *)&ifr + ifreq_offsetof(ifr_addr) )), p, sizeof(struct sockaddr)); ioctl(sockfd, SIOCSIFADDR, &ifr); ioctl(sockfd, SIOCGIFFLAGS, &ifr); ifr.ifr_flags |= IFF_UP | IFF_RUNNING; // ifr.ifr_flags &= ~selector; // unset something ioctl(sockfd, SIOCSIFFLAGS, &ifr); close(sockfd); return 0; } 

Answer 2

Perhaps you forgot to configure the interface?

 ioctl(sockfd, SIOCGIFFLAGS, &ifr); ifr.ifr_flags |= IFF_UP | IFF_RUNNING; ioctl(sockfd, SIOCSIFFLAGS, &ifr);