Wifi Switcher

Goal and benefit for the costumer

In this guide we will describe how to setup the WiFi Switcher SDK script on a Netmodule router. This script will take care of automatically connecting to a WiFi Access Point when it is part of a list we defined. The list is in a seperate file and consists of SSIDs and associated Password. The benefit is that only these known access points can be connected to.

Application example

Background for this script is a customer who uses our routers in railway applications. The router in the train is supposed to connect to a WiFi network on each respective train station. Only WiFi access points that can be found in the list are used to connect to.

TRAIN        TRAIN        TRAIN          TRAIN
+--------+   +--------+   +--------+     +--------+
|WIFI AP1|   |WIFI AP2|   |WIFI AP3|     |WIFI APX|
|        |   |        |   |        |     |        |
+--------+   +--------+   +--------+ ... +--------+


Here we will describe step-by-step how to setup the router with the wifi switcher functionality.

First we need to make sure that, after setting up our WiFi client, WLAN1 is supposed to be in position 2 in link manager. This is neccessary so that the script queries the correct interface for it's status. The order of WAN interfaces can be changed under “INTERFACES → WAN → Link Management”. Also we set WLAN1 to permanent.

Setup Trigger

Here we setup a trigger that will start our SDK script later on. Since the script is supposed to run at all times we will set our trigger to “event-based → sdk-startup”. As soon as SDK is enabled our wifi switcher script will be running.

Setup Script

This is where we insert the code of our wifi-switcher script (can be found further down in this guide).

Setup Job

Finally we have to bring script and trigger together in a job so that the trigger will be applied to our script.

Setup WiFi Networks List

Unlike the wifi-switcher script the list of allowed wifi access points does not need to be connected with a job or trigger. It just needts to be available on the device. The reason why we use the GUI to insert this list is update security and also having this file be part of the router config. This would not be the case when we simply just copy it by hand.

It is important to use the correct name “wifi-networks.cfg” for this file, otherwise the WiFi Switcher script will not be able to find it. So just get the list example (can be found at the end of this guide) and adjust SSIDs and passwords to your scenario. The list is not limited to 7 entrys, it is possible to insert an arbitary number of SSIDs to the file.

After succesfull setup your Scripts Tab should look like this.



Here is a short description of the funtionality of the script.

You have the option to set DEBUG=1, this will give you some debug output visible in tail-log.

First the scirpt will import the list of access points it is supposed to connect to. Then the scirpt will keep on looking up the state of the WiFi (WLAN1 which is setup as WANLINK2).

In case of the state being “dialing” or “up” we do nothing and wait for 5 seconds before we check this status again. In caste the state is “down” and the last scan is at least 30 seconds in the past we trigger a new scan of the currently available Wifi networkss.

After the scan is finished we take the data and compare it to the list we imported earlier. When a known access point is found and the last WiFi config update is at least 120 seconds in the past, we update the wifi config so we can connect to the now available WiFi AP.

include "wifi-networks.cfg";
string enc_to_cfg(enc) {
    switch(enc) {
    case "WPA2-SAE":
    enc = "wpa3";
    case "WPA2-PSK":
    enc = "wpa2";
    enc = "wpa2";
    if(DEBUG) nb_syslog("wifi swticher: undefinded enc: %s, using wpa2...", enc);
    return enc;
while(1) {
    // we get the current state of wanlink2 (wlan)
    state=struct_get(wan_status, "WANLINK2_STATE");
    if(DEBUG) nb_syslog("wifi switcher: current state = %s", state);
    // in case the wlan connection is up or dialing we do not start a new scan
    if (state != "down") {
    // only scan networks if last scan is more than SCAN_TIME_WAIT in the past
    if(DEBUG) nb_syslog("wifi switcher: last scan was %ds ago", uptime() - SCAN_TIME);
    if (uptime() - SCAN_TIME > SCAN_TIME_WAIT){
        scan_res = nb_scan_networks("WLAN1");
        if(DEBUG) nb_syslog("wifi switcher: scanning WLAN networks took %ds", uptime() - SCAN_TIME);
    // get all the wifi information from the ap
    do {
        ssid=   struct_get(scan_res, sprintf("NETWORK%d_SSID", i));
        chipher=struct_get(scan_res, sprintf("NETWORK%d_CIPHER_1", i));
        enc=    struct_get(scan_res, sprintf("NETWORK%d_ENCRYPTION_1", i));
        /* if(DEBUG) nb_syslog("wifi switcher: position in do loop i = %d",i); */
        for (j=0; j<length(wifi_list); j++) {
            if (ssid == wifi_list[j][0]) {
                if(DEBUG) nb_syslog("wifi switcher: configuring wifi with ssid: %s", ssid);
                cfg=sprintf("wlan.0.client.0.ssid=%s ", ssid);
                cfg=strcat(cfg, sprintf("wlan.0.client.0.secmode=wpa-psk "));
                cfg=strcat(cfg, sprintf("wlan.0.client.0.secproto=%s ", enc_to_cfg(enc)));
                cfg=strcat(cfg, sprintf("wlan.0.client.0.cipher=%s ", tolower(chipher)));
                cfg=strcat(cfg, sprintf("wlan.0.client.0.minqual=2 "));
                cfg=strcat(cfg, sprintf("wlan.0.client.0.minrssi=-99 "));
                cfg=strcat(cfg, sprintf("wlan.0.client.0.psk=%s", wifi_list[j][1]));
                // only apply config if last config set is more tan CFG_TIME_WAIT in the past
                if(DEBUG) nb_syslog("wifi switcher: last config set was %ds ago", uptime() - CFG_TIME);
                if (uptime() - CFG_TIME > CFG_TIME_WAIT){
                    CFG_TIME = uptime();
                    if(DEBUG) nb_syslog("wifi switcher: new wlan config got setup");
        if(found) break;
    } while ( !is_void(ssid) );


    mkarray("SSID1", "wifipassword1"),
    mkarray("SSID2", "wifipassword2"),
    mkarray("SSID3", "wifipassword3"),
    mkarray("SSID4", "wifipassword4"),
    mkarray("SSID5", "wifipassword5"),
    mkarray("SSID6", "wifipassword6"),
    mkarray("SSID7", "wifipassword7")