Testing WiFi existence/credentials before dhclient called?

Question

I have the following script that disconnects from the current WiFi network and reconnects based on the content of wpa_supplicant.conf:

#!/bin/bash

if iwgetid; then
    sudo wpa_cli terminate
    sudo ip addr flush wlan0
    sudo ip link set dev wlan0 down
    sudo rm -r /var/run/wpa_supplicant > /dev/null
fi

sudo wpa_supplicant -B -i wlan0 -c /etc/wpa_supplicant/wpa_supplicant.conf > /dev/null 2>&1
sudo service ntp restart
sudo ip link set dev wlan0 down
sudo ip link set dev wlan0 up
sudo dhclient wlan0

The problem is, if the credentials in wpa_supplicant are not correct (these are based on user input in my app, so very possible), dhclient hangs for quite a while, trying to connect to a DHCP server and assign an IP address before eventually failing.

Is there a way to test that the credentials are correct before calling dhclient?

GAD3R · Answer 1 · 2018-03-14T07:29:37.453

This is a sample ./script including the wpa_passphrase command to set up The SSID and the password:

#!/bin/bash

if iwgetid; then
    wpa_cli terminate
    ip addr flush wlan0
    ip link set dev wlan0 down
    rm -r /var/run/wpa_supplicant > /dev/null
    ip link set dev wlan0 up
fi

wpa_supplicant -B -i wlan0 -c<(wpa_passphrase "$1" "$2") > /dev/null 2>&1
dhclient wlan0
service ntp restart
exit

Usage:

sudo ./script "My SSID" "My-Password"

Update

If the wpa_supplicant is successful authenticated with the given ssid and pass you will get WPA: Key negotiation completed on the log file.

The -f option added to the wpa_supplicant command to provid a log file.

You can check the exit status of grep before executing the dhclient command.

#!/bin/bash

if iwgetid; then
    wpa_cli terminate
    ip addr flush wlan0
    ip link set dev wlan0 down
    rm -r /var/run/wpa_supplicant > /dev/null
    ip link set dev wlan0 up
fi
# A minimal wpa_supplicant.conf configuration file.

echo -e "\nctrl_interface=/run/wpa_supplicant \nupdate_config=1\n" > /etc/wpa_supplicant/wpa_supplicant.conf

# A prompt for SSID and WPA passphrase. The -f option to get a log file.
# sleep command will be useful because the authentication will take a few 
# second before executing grep.

echo "" > logfile

wpa_supplicant -B -i wlan0 -c<(wpa_passphrase "$1" "$2") -f logfile

sleep 10

grep -c "WPA: Key negotiation completed" logfile

if [ $? -eq 0 ] 
   then
   echo "Key negotiation completed successfully" 
   timeout 15 dhclient wlan0
   exit 0
else
  echo "Authentication failed"
fi
exit 1

Usage:

sudo ./script "My SSID" "My-Password"

This almost works great. I can connect over and over with the correct credentials. I get the appropriate message when I enter the wrong credentials. However, when I input the wrong ones, then the right ones, I get Authentication Failed. — Ryan Bobrowski, Mar 13 '18 at 22:43
wlan0: CTRL-EVENT-SSID-REENABLED id=0 ssid="my_ssid" wlan0: Associated with some_ipv6_address wlan0: CTRL-EVENT-DISCONNECTED bssid=some_ipv6_address reason=0 locally_generated=1 wlan0: CTRL-EVENT-DISCONNECTED bssid=some_ipv6_address reason=0 locally_generated=1 wlan0: WPA: 4-Way Handshake failed - pre-shared key may be incorrect wlan0: WPA: 4-Way Handshake failed - pre-shared key may be incorrect wlan0: CTRL-EVENT-SSID-TEMP-DISABLED id=0 ssid="my_ssid" auth_failures=8 duration=116 reason=WRONG_KEY — Ryan Bobrowski, Mar 13 '18 at 22:44

score 1 · Answer 2 · answered Mar 08 '18 at 17:15

I would break it up into separate functions, allowing me to test the connection to wifi before reconfiguring the network.

#!/usr/bin/env bash
CONF="/etc/wpa_supplicant/wpa_supplicant.conf"

function connect_wifi() {
    wpa_supplicant -B -i wlan0 -c $CONF >/dev/null 2>&1
}

function connect_tcpip() {
    service ntp restart
    ip link set dev wlan0 down
    ip link set dev wlan0 up
    dhclient wlan0
}

FUNC_CW=$(declare -f connect_wifi)
FUNC_CT=$(declare -f connect_tcpip)

if (sudo bash -c "$FUNC_CW; connect_wifi"); then
    sudo bash -c "$FUNC_CT; connect_tcpip"
else
    echo "Error Connecting to Wifi.  Please check your credentials and try again."
fi

it seems to be entering the first if expression no matter if the credentials are correct in wpa_supplicant.conf or not — Ryan Bobrowski, Mar 08 '18 at 20:31

Testing WiFi existence/credentials before dhclient called?

2 Answers2