How to detect IP address change programmatically in Linux?

Question

Is there a way to detect IP address changes on the local machine in Linux programmatically using C++?

Answer 1

here you go.. this does it without polling.

it only listens for RTM_NEWADDR but it should be easy to change to support RTM_DELADDR if you need

#include <stdio.h> #include <string.h> #include <netinet/in.h> #include <linux/netlink.h> #include <linux/rtnetlink.h> #include <net/if.h>  int main() {     struct sockaddr_nl addr;     int sock, len;     char buffer[4096];     struct nlmsghdr *nlh;      if ((sock = socket(PF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) == -1) {         perror("couldn't open NETLINK_ROUTE socket");         return 1;     }      memset(&addr, 0, sizeof(addr));     addr.nl_family = AF_NETLINK;     addr.nl_groups = RTMGRP_IPV4_IFADDR;      if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) == -1) {         perror("couldn't bind");         return 1;     }      nlh = (struct nlmsghdr *)buffer;     while ((len = recv(sock, nlh, 4096, 0)) > 0) {         while ((NLMSG_OK(nlh, len)) && (nlh->nlmsg_type != NLMSG_DONE)) {             if (nlh->nlmsg_type == RTM_NEWADDR) {                 struct ifaddrmsg *ifa = (struct ifaddrmsg *) NLMSG_DATA(nlh);                 struct rtattr *rth = IFA_RTA(ifa);                 int rtl = IFA_PAYLOAD(nlh);                  while (rtl && RTA_OK(rth, rtl)) {                     if (rth->rta_type == IFA_LOCAL) {                         char name[IFNAMSIZ];                         if_indextoname(ifa->ifa_index, name);                         char ip[INET_ADDRSTRLEN];                         inet_ntop(AF_INET, RTA_DATA(rth), ip, sizeof(ip));                         printf("interface %s ip: %s\n", name, ip);                     }                     rth = RTA_NEXT(rth, rtl);                 }             }             nlh = NLMSG_NEXT(nlh, len);         }     }     return 0; } 

Answer 2

In C, to get the current IP I use:

    int s;     struct ifreq ifr = {};      s = socket(PF_INET, SOCK_DGRAM, 0);      strncpy(ifr.ifr_name, "eth0", sizeof(ifr.ifr_name));      if (ioctl(s, SIOCGIFADDR, &ifr) >= 0)         printf("%s\n",           inet_ntoa(((struct sockaddr_in *)&ifr.ifr_addr)->sin_addr)); 

Replace "eth0" with the interface you're looking at. All you now need to do is poll for a change.