Using nl80211.h to scan access points

Question

I'm trying to use nl80211.h for scanning access points for a simple WLAN manager. I can't find any example code and only documentation I can find is kerneldoc. I have been trying to study from iw and wpa_supplicant source but it's rather complex.

This is only documentation I can find:

NL80211_CMD_GET_SCAN get scan results

NL80211_CMD_TRIGGER_SCAN trigger a new scan with the given parameters NL80211_ATTR_TX_NO_CCK_RATE is used to decide whether to send the probe requests at CCK rate or not.

How can I scan access points with nl80211? I think I need to use enum nl80211_commands {NL80211_CMD_GET_SCAN NL80211_CMD_TRIGGER_SCAN}. How can I use them?

Answer 1

I know this is an old question but I ran across it while I was trying to do the same thing. Being new to C and libnl I struggled to get a simple C program to just spit out access points. I'm posting this here for others who were also trying to write a simple program. iw was a great reference but it was challenging following the code around since it does so much more than scan for access points.

/*
 * scan_access_points.c: Prints all detected access points with wlan0 using NL80211 (netlink).
 *
 * Only works on network interfaces whose drivers are compatible with Netlink. Test this by running `iw list`.
 *
 * Since only privileged users may submit NL80211_CMD_TRIGGER_SCAN, you'll have to run the compiled program as root.
 *
 * Build with: gcc $(pkg-config --cflags --libs libnl-genl-3.0) scan_access_points.c
 *
 * Raspbian prerequisites:
 *      sudo apt-get install libnl-genl-3-dev
 *
 * Resources:
 *      http://git.kernel.org/cgit/linux/kernel/git/jberg/iw.git/tree/scan.c
 *      http://stackoverflow.com/questions/21601521/how-to-use-the-libnl-library-to-trigger-nl80211-commands
 *      http://stackoverflow.com/questions/23760780/how-to-send-single-channel-scan-request-to-libnl-and-receive-single-
 *
 * Expected output (as root):
 *      NL80211_CMD_TRIGGER_SCAN sent 36 bytes to the kernel.
 *      Waiting for scan to complete...
 *      Got NL80211_CMD_NEW_SCAN_RESULTS.
 *      Scan is done.
 *      NL80211_CMD_GET_SCAN sent 28 bytes to the kernel.
 *      47:be:34:f0:bb:be, 2457 MHz, NETGEAR16
 *      6b:db:ed:85:ef:42, 2432 MHz, NETGEAR31
 *      d8:06:ef:a7:f9:80, 2412 MHz, ATT912
 *      a7:0d:af:0a:19:08, 2462 MHz, ATT185
 *
 * Expected output (without root):
 *      NL80211_CMD_TRIGGER_SCAN sent 36 bytes to the kernel.
 *      Waiting for scan to complete...
 *      error_handler() called.
 *      WARNING: err has a value of -1.
 *      ERROR: nl_recvmsgs() returned -28 (Operation not permitted).
 *      do_scan_trigger() failed with -28.
 *
 */
#include <errno.h>
#include <netlink/errno.h>
#include <netlink/netlink.h>
#include <netlink/genl/genl.h>
#include <linux/nl80211.h>


struct trigger_results {
    int done;
    int aborted;
};


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.
    int *ret = arg;
    *ret = 0;
    return NL_SKIP;
}


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


static int no_seq_check(struct nl_msg *msg, void *arg) {
    // Callback for NL_CB_SEQ_CHECK.
    return NL_OK;
}


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.
    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_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;
}


void mac_addr_n2a(char *mac_addr, unsigned char *arg) {
    // From http://git.kernel.org/cgit/linux/kernel/git/jberg/iw.git/tree/util.c.
    int i, l;

    l = 0;
    for (i = 0; i < 6; i++) {
        if (i == 0) {
            sprintf(mac_addr+l, "%02x", arg[i]);
            l += 2;
        } else {
            sprintf(mac_addr+l, ":%02x", arg[i]);
            l += 3;
        }
    }
}


void print_ssid(unsigned char *ie, int ielen) {
    uint8_t len;
    uint8_t *data;
    int i;

    while (ielen >= 2 && ielen >= ie[1]) {
        if (ie[0] == 0 && ie[1] >= 0 && ie[1] <= 32) {
            len = ie[1];
            data = ie + 2;
            for (i = 0; i < len; i++) {
                if (isprint(data[i]) && data[i] != ' ' && data[i] != '\\') printf("%c", data[i]);
                else if (data[i] == ' ' && (i != 0 && i != len -1)) printf(" ");
                else printf("\\x%.2x", data[i]);
            }
            break;
        }
        ielen -= ie[1] + 2;
        ie += ie[1] + 2;
    }
}


static int callback_trigger(struct nl_msg *msg, void *arg) {
    // Called by the kernel when the scan is done or has been aborted.
    struct genlmsghdr *gnlh = nlmsg_data(nlmsg_hdr(msg));
    struct trigger_results *results = arg;

    //printf("Got something.\n");
    //printf("%d\n", arg);
    //nl_msg_dump(msg, stdout);

    if (gnlh->cmd == NL80211_CMD_SCAN_ABORTED) {
        printf("Got NL80211_CMD_SCAN_ABORTED.\n");
        results->done = 1;
        results->aborted = 1;
    } else if (gnlh->cmd == NL80211_CMD_NEW_SCAN_RESULTS) {
        printf("Got NL80211_CMD_NEW_SCAN_RESULTS.\n");
        results->done = 1;
        results->aborted = 0;
    }  // else probably an uninteresting multicast message.

    return NL_SKIP;
}


static int callback_dump(struct nl_msg *msg, void *arg) {
    // Called by the kernel with a dump of the successful scan's data. Called for each SSID.
    struct genlmsghdr *gnlh = nlmsg_data(nlmsg_hdr(msg));
    char mac_addr[20];
    struct nlattr *tb[NL80211_ATTR_MAX + 1];
    struct nlattr *bss[NL80211_BSS_MAX + 1];
    static struct nla_policy bss_policy[NL80211_BSS_MAX + 1] = {
        [NL80211_BSS_TSF] = { .type = NLA_U64 },
        [NL80211_BSS_FREQUENCY] = { .type = NLA_U32 },
        [NL80211_BSS_BSSID] = { },
        [NL80211_BSS_BEACON_INTERVAL] = { .type = NLA_U16 },
        [NL80211_BSS_CAPABILITY] = { .type = NLA_U16 },
        [NL80211_BSS_INFORMATION_ELEMENTS] = { },
        [NL80211_BSS_SIGNAL_MBM] = { .type = NLA_U32 },
        [NL80211_BSS_SIGNAL_UNSPEC] = { .type = NLA_U8 },
        [NL80211_BSS_STATUS] = { .type = NLA_U32 },
        [NL80211_BSS_SEEN_MS_AGO] = { .type = NLA_U32 },
        [NL80211_BSS_BEACON_IES] = { },
    };

    // Parse and error check.
    nla_parse(tb, NL80211_ATTR_MAX, genlmsg_attrdata(gnlh, 0), genlmsg_attrlen(gnlh, 0), NULL);
    if (!tb[NL80211_ATTR_BSS]) {
        printf("bss info missing!\n");
        return NL_SKIP;
    }
    if (nla_parse_nested(bss, NL80211_BSS_MAX, tb[NL80211_ATTR_BSS], bss_policy)) {
        printf("failed to parse nested attributes!\n");
        return NL_SKIP;
    }
    if (!bss[NL80211_BSS_BSSID]) return NL_SKIP;
    if (!bss[NL80211_BSS_INFORMATION_ELEMENTS]) return NL_SKIP;

    // Start printing.
    mac_addr_n2a(mac_addr, nla_data(bss[NL80211_BSS_BSSID]));
    printf("%s, ", mac_addr);
    printf("%d MHz, ", nla_get_u32(bss[NL80211_BSS_FREQUENCY]));
    print_ssid(nla_data(bss[NL80211_BSS_INFORMATION_ELEMENTS]), nla_len(bss[NL80211_BSS_INFORMATION_ELEMENTS]));
    printf("\n");

    return NL_SKIP;
}


int do_scan_trigger(struct nl_sock *socket, int if_index, int driver_id) {
    // Starts the scan and waits for it to finish. Does not return until the scan is done or has been aborted.
    struct trigger_results results = { .done = 0, .aborted = 0 };
    struct nl_msg *msg;
    struct nl_cb *cb;
    struct nl_msg *ssids_to_scan;
    int err;
    int ret;
    int mcid = nl_get_multicast_id(socket, "nl80211", "scan");
    nl_socket_add_membership(socket, mcid);  // Without this, callback_trigger() won't be called.

    // Allocate the messages and callback handler.
    msg = nlmsg_alloc();
    if (!msg) {
        printf("ERROR: Failed to allocate netlink message for msg.\n");
        return -ENOMEM;
    }
    ssids_to_scan = nlmsg_alloc();
    if (!ssids_to_scan) {
        printf("ERROR: Failed to allocate netlink message for ssids_to_scan.\n");
        nlmsg_free(msg);
        return -ENOMEM;
    }
    cb = nl_cb_alloc(NL_CB_DEFAULT);
    if (!cb) {
        printf("ERROR: Failed to allocate netlink callbacks.\n");
        nlmsg_free(msg);
        nlmsg_free(ssids_to_scan);
        return -ENOMEM;
    }

    // Setup the messages and callback handler.
    genlmsg_put(msg, 0, 0, driver_id, 0, 0, NL80211_CMD_TRIGGER_SCAN, 0);  // Setup which command to run.
    nla_put_u32(msg, NL80211_ATTR_IFINDEX, if_index);  // Add message attribute, which interface to use.
    nla_put(ssids_to_scan, 1, 0, "");  // Scan all SSIDs.
    nla_put_nested(msg, NL80211_ATTR_SCAN_SSIDS, ssids_to_scan);  // Add message attribute, which SSIDs to scan for.
    nlmsg_free(ssids_to_scan);  // Copied to `msg` above, no longer need this.
    nl_cb_set(cb, NL_CB_VALID, NL_CB_CUSTOM, callback_trigger, &results);  // Add the callback.
    nl_cb_err(cb, NL_CB_CUSTOM, error_handler, &err);
    nl_cb_set(cb, NL_CB_FINISH, NL_CB_CUSTOM, finish_handler, &err);
    nl_cb_set(cb, NL_CB_ACK, NL_CB_CUSTOM, ack_handler, &err);
    nl_cb_set(cb, NL_CB_SEQ_CHECK, NL_CB_CUSTOM, no_seq_check, NULL);  // No sequence checking for multicast messages.

    // Send NL80211_CMD_TRIGGER_SCAN to start the scan. The kernel may reply with NL80211_CMD_NEW_SCAN_RESULTS on
    // success or NL80211_CMD_SCAN_ABORTED if another scan was started by another process.
    err = 1;
    ret = nl_send_auto(socket, msg);  // Send the message.
    printf("NL80211_CMD_TRIGGER_SCAN sent %d bytes to the kernel.\n", ret);
    printf("Waiting for scan to complete...\n");
    while (err > 0) ret = nl_recvmsgs(socket, cb);  // First wait for ack_handler(). This helps with basic errors.
    if (err < 0) {
        printf("WARNING: err has a value of %d.\n", err);
    }
    if (ret < 0) {
        printf("ERROR: nl_recvmsgs() returned %d (%s).\n", ret, nl_geterror(-ret));
        return ret;
    }
    while (!results.done) nl_recvmsgs(socket, cb);  // Now wait until the scan is done or aborted.
    if (results.aborted) {
        printf("ERROR: Kernel aborted scan.\n");
        return 1;
    }
    printf("Scan is done.\n");

    // Cleanup.
    nlmsg_free(msg);
    nl_cb_put(cb);
    nl_socket_drop_membership(socket, mcid);  // No longer need this.
    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;
    }

    // Now get info for all SSIDs detected.
    struct nl_msg *msg = nlmsg_alloc();  // Allocate a message.
    genlmsg_put(msg, 0, 0, driver_id, 0, NLM_F_DUMP, NL80211_CMD_GET_SCAN, 0);  // Setup which command to run.
    nla_put_u32(msg, NL80211_ATTR_IFINDEX, if_index);  // Add message attribute, which interface to use.
    nl_socket_modify_cb(socket, NL_CB_VALID, NL_CB_CUSTOM, callback_dump, NULL);  // Add the callback.
    int ret = nl_send_auto(socket, msg);  // Send the message.
    printf("NL80211_CMD_GET_SCAN sent %d bytes to the kernel.\n", ret);
    ret = nl_recvmsgs_default(socket);  // Retrieve the kernel's answer. callback_dump() prints SSIDs to stdout.
    nlmsg_free(msg);
    if (ret < 0) {
        printf("ERROR: nl_recvmsgs_default() returned %d (%s).\n", ret, nl_geterror(-ret));
        return ret;
    }

    return 0;
}

Answer 2

nl80211.h only provides these enums for you to use with the real wireless library (which is libnl). You can use libnl by downloading it and including it in your c program: http://www.carisma.slowglass.com/~tgr/libnl/

Then with nl80211.h included, you can use all the enums that are defined with the commands defined in libnl.