Netlink programming demo.

netlink_demo.c

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <memory.h>
#include <linux/types.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>

struct nl_req_s {
    struct nlmsghdr hdr;
    struct rtgenmsg gen;
};

typedef struct nl_req_s nl_req_t;

int get_faces() {
    int fd;
    fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);

    struct sockaddr_nl kernel;
    struct msghdr rtnl_msg;
    struct iovec io;
    nl_req_t req;
    memset(&rtnl_msg, 0, sizeof(rtnl_msg));
    memset(&kernel, 0, sizeof(kernel));
    memset(&req, 0, sizeof(req));
    kernel.nl_family = AF_NETLINK; /* fill-in kernel address (destination) */

    req.hdr.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtgenmsg));
    req.hdr.nlmsg_type = RTM_GETLINK;
    req.hdr.nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP; 
    req.hdr.nlmsg_seq = 1;
    req.hdr.nlmsg_pid = getpid();
    req.gen.rtgen_family = AF_PACKET; /*  no preferred AF, we will get *all* interfaces */

    io.iov_base = &req;
    io.iov_len = req.hdr.nlmsg_len;
    rtnl_msg.msg_iov = &io;
    rtnl_msg.msg_iovlen = 1;
    rtnl_msg.msg_name = &kernel;
    rtnl_msg.msg_namelen = sizeof(kernel);

    struct sockaddr_nl local;
    memset(&local, 0, sizeof(local));
    local.nl_family = AF_NETLINK;
    local.nl_pid = getpid(); /* can be obtained using getpid*/
    local.nl_groups = 0;

    if (bind(fd, (struct sockaddr *) &local, sizeof(local)) < 0) {
        perror("crap...");
    }

    sendmsg(fd, (struct msghdr *) &rtnl_msg, 0);

    char reply[1024]; /* a large buffer */
    int len;
    struct nlmsghdr *msg_ptr;    /* pointer to current part */
    struct msghdr rtnl_reply;    /* generic msghdr structure */
    struct iovec io_reply;
    memset(&io_reply, 0, sizeof(io_reply));
    memset(&rtnl_reply, 0, sizeof(rtnl_reply));

    io.iov_base = reply;
    io.iov_len = 1024;
    rtnl_reply.msg_iov = &io;
    rtnl_reply.msg_iovlen = 1;
    rtnl_reply.msg_name = &kernel;
    rtnl_reply.msg_namelen = sizeof(kernel);

    len = recvmsg(fd, &rtnl_reply, 0); /* read lots of data */

    for (msg_ptr = (struct nlmsghdr *) reply; NLMSG_OK(msg_ptr, len); msg_ptr = NLMSG_NEXT(msg_ptr, len))
    {
        if (msg_ptr->nlmsg_type == RTM_NEWLINK) {
            rtnl_print_link(msg_ptr);
        }
    }
    return 0;
}

void rtnl_print_link(struct nlmsghdr *h)
{
   struct ifinfomsg *iface;
   struct rtattr *attribute;
   int len;

   iface = NLMSG_DATA(h);
   len = h->nlmsg_len - NLMSG_LENGTH(sizeof(*iface));

   for (attribute = IFLA_RTA(iface); RTA_OK(attribute, len); attribute = RTA_NEXT(attribute, len))
   {
       switch(attribute->rta_type)
       {
           case IFLA_IFNAME: 
           printf("Interface %d : %s\n", iface->ifi_index, (char *) RTA_DATA(attribute));
           break;
                                                                                        }
    }
}

int main()
{
    get_faces();
    return 0;
}

This demo code does not use padding between header and message. It 'works', because the compiler does
the padding for struct nl_req_s.

This demo code does not use padding between header and message. It 'works', because the compiler does the padding for struct nl_req_s.

@igaw Hello, could you provide the correct way to pad nl_req_s? thank you

The raw RTNL interface is tricky. I would recomand to use libmnl & co. If you want/have to use the Kernel API check the examples in the man pages. Basically you want a buffer and use 'the position macros' (e.g. NLMSG_DATA) to place the write/read head. These macros get the alignment and padding right.

https://man7.org/linux/man-pages/man7/netlink.7.html