Welcome to the Linux Foundation Forum!

How to use rtnetlink GETADDR query to retrieve a **single** address information?

pseusys Posts: 1
edited May 2 in Network Management

Good morning!

I am interested in one specific issue about rtnetlink library. I found a similar question on stackoverflow (unanswered), and now I am facing a similar problem.

Let's imagine I'm connected to internet via device with index 3 and name "wlo1" right now. I would like to use the following code snippet in order to receive the address information (such as prefixlength). In order to do that I'm trying to use the following code (taken from the stackoverflow question and modified):

#include <stdio.h>
#include <stdlib.h>
#include <linux/input.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <linux/if_addr.h>
#include <sys/socket.h>
#include <unistd.h>
#include <string.h>

struct packet {
    struct nlmsghdr hdr;
    struct ifaddrmsg msg;

int main() {
    struct sockaddr_nl addr;
    memset(&addr, 0, sizeof(struct sockaddr_nl));

    int sock = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    bind(sock, (struct sockaddr *) &addr, sizeof(struct sockaddr_nl));

    socklen_t len =  sizeof(struct sockaddr_nl);
    getsockname(sock, (struct sockaddr *) &addr, &len);

    struct packet pack;
    memset(&pack, 0, sizeof(struct packet));
    pack.hdr.nlmsg_len = sizeof(struct packet);
    pack.hdr.nlmsg_type = RTM_GETADDR;
    pack.hdr.nlmsg_flags = NLM_F_REQUEST;
    pack.msg.ifa_index = 3;  // This is ignored, why?
    write(sock, &pack, sizeof(struct packet));

    char buf[8000];
    memset(buf, 0, 8000);

    int x = read(sock, buf, (socklen_t) 8000);
    for (int i = 0; i < x; i++) printf("%i \t: %i\n",i, buf[i]);

Instead of the requested address information, I receive an error message (errno number is stored in 16th byte and is -95, which, I assume, means "operation not permitted").

Do you have any idea what's going on? If I add NLM_F_DUMP flag, I can get information about all addresses available, but I'm only interested in one. Is there any configuration I am missing? Do I do something wrong?..


Upcoming Training