Example which communicates with Linux Netlink subsystem to fetch interface information

get_link_info.c

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <sys/socket.h>
#include <linux/rtnetlink.h>
#include <linux/if_arp.h>
#include <linux/types.h>

#define MAX_PAYLOAD 1024
#define MAX_RECV_BUF_LEN 32768

int main(int argc, char *argv[])
{
    char *cmd_str;

    if (argc < 2) {
        printf("Please specify ifname\n");
        return 1;
    }

    cmd_str = argv[1];

    int fd = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
    if (fd < 0) {
        perror("Opening socket failed");
        return errno;
    }

    int pid = getpid();
    struct sockaddr_nl sa;
    memset(&sa, 0, sizeof(sa));
    sa.nl_family = AF_NETLINK;
    sa.nl_pid = pid;

    if (bind(fd, (struct sockaddr*)&sa, sizeof(sa)) < 0) {
        perror("Binding socket to address failed");
        goto handle_failure;
    }

    /*
     * Build Netlink message header
     */
    char *ifname = cmd_str;
    unsigned int ifindex = if_nametoindex(ifname);
    if (ifindex == 0) {
        perror("Finding ifindex failed");
        goto handle_failure;
    }
    struct ifinfomsg iim = {
        .ifi_family = AF_UNSPEC,
        .ifi_index = ifindex,
        .ifi_type = ARPHRD_ETHER,
        .ifi_change = 0xFFFFFFFF
    };

    __u16 netlink_flags = NLM_F_REQUEST | NLM_F_DUMP;
    struct nlmsghdr nh = {
        .nlmsg_len = NLMSG_LENGTH(sizeof(iim)),
        .nlmsg_type = RTM_GETLINK,
        .nlmsg_flags = netlink_flags,
        .nlmsg_seq = 1,
        .nlmsg_pid = pid
    };

    struct {
        struct nlmsghdr n;
        struct ifinfomsg i;
        char    buf[2014];
    } req = {
        .n = nh,
        .i = iim
    };

    struct iovec iov = { &req, req.n.nlmsg_len };
    struct msghdr msg = { &sa, sizeof(sa), &iov, 1, NULL, 0, 0 };
    if (sendmsg(fd, &msg, 0) < 0) {
        perror("Sending message failed");
        goto handle_failure;
    }

    /*
     * Receive message
     */
    /* Read message from kernel */
    char recv_buf[MAX_RECV_BUF_LEN];
    struct iovec riov = {
        .iov_base = &recv_buf,
        .iov_len = MAX_RECV_BUF_LEN
    };
	msg.msg_iov = &riov;
	msg.msg_iovlen = 1;

    int recv_len = recvmsg(fd, &msg, 0);
    if (recv_len < 0) {
        perror("Receiving message failed");
        goto handle_failure;
    }

    struct nlmsghdr *rnh;
    for (rnh = (struct nlmsghdr *) recv_buf; NLMSG_OK (rnh, recv_len); rnh = NLMSG_NEXT (rnh, recv_len)) {
        /* The end of multipart message */
        if (rnh->nlmsg_type == NLMSG_DONE) {
            printf("NLMSG_DONE detected\n");
            break;
        }
        if (rnh->nlmsg_type == NLMSG_ERROR) {
            perror("Receiving message failed");
            goto handle_failure;
        }

        printf("Received nlmsghdr\n");
        printf("\tnlhdr address: %p\n", rnh);
        printf("\tnlmsg_len: %d\n", rnh->nlmsg_len);
        printf("\tnlmsg_type: %d\n", rnh->nlmsg_type);
        printf("\tnlmsg_flags: %x\n", rnh->nlmsg_flags);
        printf("\tnlmsg_seq: %d\n", rnh->nlmsg_seq);
        printf("\tnlmsg_pid: %d\n", rnh->nlmsg_pid);

        struct ifinfomsg *riim = NLMSG_DATA(rnh);
        printf("Received ifinfomsg\n");
        printf("\tifi_type: %d\n", riim->ifi_type);
        printf("\tifi_index: %d\n", riim->ifi_index);
        printf("\tifi_flags: %x\n", riim->ifi_flags);
        printf("\tifi_change: %x\n", riim->ifi_change);
    }

    close(fd);
    return 0;

handle_failure:
    close(fd);
    return errno;
}

Testing with Ubuntu 20.04 running Linux 5.11.0...

I get "Operation not permitted" if I don't run this as root/sudo. But if I comment out line 35 sa.nl_pid = pid; I can run it fine as a non-root/sudo.

I get information for all interfaces, not just the one named on the command line. If I remove NLM_F_DUMP on line 58, then it gives information for just the named interface.