Detect add/delete IP address to interface on Linux

ipmond.c

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <linux/rtnetlink.h>
#include <linux/if.h>

static volatile sig_atomic_t terminate;

static int
process_rtm_addr (struct nlmsghdr *hdr) {
    struct ifaddrmsg *msg;
    size_t n;
    struct rtattr *rta;
    char local[INET6_ADDRSTRLEN] = {0}, *label = NULL, *dir;

    msg = NLMSG_DATA(hdr);
    n = hdr->nlmsg_len - NLMSG_LENGTH(sizeof(struct ifaddrmsg));
    for (rta = IFA_RTA(msg); RTA_OK(rta, n); rta = RTA_NEXT(rta, n)) {
        switch (rta->rta_type) {
        case IFA_LOCAL:
            inet_ntop(msg->ifa_family, RTA_DATA(rta), local, sizeof(local));
            break;
        case IFA_LABEL:
            label = (char *)RTA_DATA(rta);
            break;
        }
    }
    dir = (hdr->nlmsg_type == RTM_NEWADDR) ? "add" : "del";
    if (label) {
        fprintf(stderr, "%s: %s %s/%u\n", label, dir, local, msg->ifa_prefixlen);
    } else {
        fprintf(stderr, "(%d): %s %s/%u\n", msg->ifa_index, dir, local, msg->ifa_prefixlen);
    }
    return 0;
}

static void
on_signal (int signum) {
    (void)signum;
    terminate = 1;
}

int
main (int argc, char *argv[]) {
    struct sigaction sig;
    int soc;
    struct sockaddr_nl sa;
    char buf[4096];
    ssize_t n;
    struct nlmsghdr *hdr;

    (void)argc, (void)argv;
    memset(&sig, 0, sizeof(sig));
    sig.sa_handler = on_signal;
    if (sigaction(SIGINT, &sig, NULL) == -1) {
        perror("sigaction");
        return -1;
    }
    if (sigaction(SIGTERM, &sig, NULL) == -1) {
        perror("sigaction");
        return -1;
    }
    soc = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
    if (soc == -1) {
        perror("socket");
        return -1;
    }
    memset(&sa, 0, sizeof(sa));
    sa.nl_family = AF_NETLINK;
    sa.nl_pid = 0;
    sa.nl_groups = RTMGRP_IPV4_IFADDR;
    if (bind(soc, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
        perror("bind");
        close(soc);
        return -1;
    }
    while (!terminate) {
        n = recv(soc, buf, sizeof(buf), 0);
        if (n == -1) {
            if (errno == EINTR) {
                continue;
            }
            perror("recv");
            close(soc);
            return -1;
        }
        if (!n) {
            fprintf(stderr, "EOF\n");
            break;
        }
        for (hdr = (struct nlmsghdr *)buf; NLMSG_OK(hdr, n); hdr = NLMSG_NEXT(hdr, n)) {
            switch (hdr->nlmsg_type) {
            case RTM_NEWADDR:
            case RTM_DELADDR:
                process_rtm_addr(hdr);
                break;
            default:
                fprintf(stderr, "Unknown Type (%u)\n", hdr->nlmsg_type);
                break;
            }
        }
    }
    close(soc);
    return 0;
}

$ gcc -W -Wall -o ipmond ipmond.c
$ ./ipmond
enp4s0f0: del 10.15.2.165/24
enp4s0f0: add 10.15.2.165/24