scan_access_points.c

/*
 * "Staging" code before porting it to Python. 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.
 *
 * I don't know C so it was hard to get a Python application to talk to the kernel by looking at C code, trying to
 * resolve header files and variable origins/values in my head.
 *
 * I'm "staging" my code here in C and once it's working I'll port it over to Python. Here I can insert printf
 * statements to learn what the values of some variables are, and so on.
 *
 * None of this code is used by the Python library/module.
 *
 * Raspbian prerequisites:
 *      sudo apt-get install libnl-genl-3-dev
 *
 * Build and execute:
 *      gcc -o program scan_access_points.c $(pkg-config --cflags --libs libnl-genl-3.0) && ./program
 *
 * 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-
 *      https://github.com/Robpol86/libnl/tree/master/example_c
 *
 * 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/genl/genl.h>
#include <linux/nl80211.h>


struct trigger_results {
    int done;
    int aborted;
};


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;
}


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 = genl_ctrl_resolve_grp(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;
}