debug-nl80211.c

/*
sudo apt-get install libnl-genl-3-dev
gcc debug-nl80211.c $(pkg-config --cflags --libs libnl-genl-3.0) -o debug-nl80211
*/

#include <errno.h>
#include <netlink/netlink.h>
#include <netlink/genl/ctrl.h>
#include <netlink/genl/genl.h>
#include <linux/nl80211.h>
#include <net/if.h>

struct handler_args {  // For family_handler() and nl_get_multicast_id().
    const char *group;
    int id;
};

static int error_handler(struct sockaddr_nl *nla, struct nlmsgerr *err, void *arg) {
    // Callback for errors.
    printf("error_handler() called.\n");
    int *ret = arg;
    *ret = err->error;
    return NL_STOP;
}

static int finish_handler(struct nl_msg *msg, void *arg) {
    // Callback for NL_CB_FINISH.
    printf("finish_handler() called.\n");
    int *ret = arg;
    *ret = 0;
    return NL_SKIP;
}


static int ack_handler(struct nl_msg *msg, void *arg) {
    // Callback for NL_CB_ACK.
    printf("ack_handler() called.\n");
    int *ret = arg;
    *ret = 0;
    return NL_STOP;
}

static int family_handler(struct nl_msg *msg, void *arg) {
    // Callback for NL_CB_VALID within nl_get_multicast_id(). From http://sourcecodebrowser.com/iw/0.9.14/genl_8c.html.
    printf("family_handler() called.\n");
    struct handler_args *grp = arg;
    struct nlattr *tb[CTRL_ATTR_MAX + 1];
    struct genlmsghdr *gnlh = nlmsg_data(nlmsg_hdr(msg));
    struct nlattr *mcgrp;
    int rem_mcgrp;

    nla_parse(tb, CTRL_ATTR_MAX, genlmsg_attrdata(gnlh, 0), genlmsg_attrlen(gnlh, 0), NULL);

    if (!tb[CTRL_ATTR_MCAST_GROUPS]) return NL_SKIP;

    nla_for_each_nested(mcgrp, tb[CTRL_ATTR_MCAST_GROUPS], rem_mcgrp) {  // This is a loop.
        struct nlattr *tb_mcgrp[CTRL_ATTR_MCAST_GRP_MAX + 1];

        nla_parse(tb_mcgrp, CTRL_ATTR_MCAST_GRP_MAX, nla_data(mcgrp), nla_len(mcgrp), NULL);

        if (!tb_mcgrp[CTRL_ATTR_MCAST_GRP_NAME] || !tb_mcgrp[CTRL_ATTR_MCAST_GRP_ID]) continue;
        if (strncmp(nla_data(tb_mcgrp[CTRL_ATTR_MCAST_GRP_NAME]), grp->group,
                nla_len(tb_mcgrp[CTRL_ATTR_MCAST_GRP_NAME]))) {
            continue;
                }

        grp->id = nla_get_u32(tb_mcgrp[CTRL_ATTR_MCAST_GRP_ID]);
        break;
    }

    return NL_SKIP;
}

int nl_get_multicast_id(struct nl_sock *sock, const char *family, const char *group) {
    // From http://sourcecodebrowser.com/iw/0.9.14/genl_8c.html.
    struct nl_msg *msg;
    struct nl_cb *cb;
    int ret, ctrlid;
    struct handler_args grp = { .group = group, .id = -ENOENT, };

    msg = nlmsg_alloc();
    if (!msg) return -ENOMEM;

    cb = nl_cb_alloc(NL_CB_DEFAULT);
    if (!cb) {
        ret = -ENOMEM;
        goto out_fail_cb;
    }

    ctrlid = genl_ctrl_resolve(sock, "nlctrl");

    genlmsg_put(msg, 0, 0, ctrlid, 0, 0, CTRL_CMD_GETFAMILY, 0);

    ret = -ENOBUFS;
    NLA_PUT_STRING(msg, CTRL_ATTR_FAMILY_NAME, family);

    ret = nl_send_auto_complete(sock, msg);
    if (ret < 0) goto out;

    ret = 1;

    nl_cb_err(cb, NL_CB_CUSTOM, error_handler, &ret);
    nl_cb_set(cb, NL_CB_ACK, NL_CB_CUSTOM, ack_handler, &ret);
    nl_cb_set(cb, NL_CB_FINISH, NL_CB_CUSTOM, finish_handler, &ret);
    nl_cb_set(cb, NL_CB_VALID, NL_CB_CUSTOM, family_handler, &grp);

    while (ret > 0) nl_recvmsgs(sock, cb);

    if (ret == 0) ret = grp.id;

    nla_put_failure:
        out:
            nl_cb_put(cb);
        out_fail_cb:
            nlmsg_free(msg);
            return ret;
}

int do_scan_trigger(struct nl_sock *socket, int if_index, int driver_id) {
    printf("do_scan_trigger start\n");
    int mcid = nl_get_multicast_id(socket, "nl80211", "scan");
    printf("nl_get_multicast_id: %d\n", mcid);

    return 0;
}

int main() {
    int if_index = if_nametoindex("wlan0"); // Use this wireless interface for scanning.

    // Open socket to kernel.
    struct nl_sock *socket = nl_socket_alloc();  // Allocate new netlink socket in memory.
    genl_connect(socket);  // Create file descriptor and bind socket.
    int driver_id = genl_ctrl_resolve(socket, "nl80211");  // Find the nl80211 driver ID.

    // Issue NL80211_CMD_TRIGGER_SCAN to the kernel and wait for it to finish.
    int err = do_scan_trigger(socket, if_index, driver_id);
    if (err != 0) {
        printf("do_scan_trigger() failed with %d.\n", err);
        return err;
    }

    return 0;
}