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I can't figure out how to properly bring up the wi-fi card on my laptop. When I turn it on and issue

$ sudo iwconfig wlan0 txpower auto
$ sudo iwlist wlan0 scan
wlan0     Interface doesn't support scanning : Network is down

it reports that the network is down. Trying to bring it up fails too:

$ sudo ifup wlan0
wlan0     no private ioctls.

Failed to bring up wlan0.

Apparently I'm missing some basic low-level iw... command.

When I issue dhclient on the interface:

$ sudo dhclient -v wlan0
Internet Systems Consortium DHCP Client 4.2.2
Copyright 2004-2011 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

^C$

and interrupt it, it brings the device up somehow and then scanning etc. works. I'd like to avoid this obviously superfluous step.

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3 Answers 3

up vote 5 down vote accepted

Indeed, try sudo ifconfig wlan0 up. To elaborate on the answer by Martin:

ifup and ifdown commands are part of ifupdown package, which now is considered a legacy frontend for network configuration, compared to newer ones, such as network manager.

Upon ifup ifupdown reads configuration settings from /etc/network/interfaces; it runs pre-up, post-up and post-down scripts from /etc/network, which include starting /etc/wpasupplicant/ifupdown.sh that processes additional wpa-* configuration options for wpa wifi, in /etc/network/interfaces (see zcat /usr/share/doc/wpasupplicant/README.Debian.gz for documentation). For WEP wireless-tools package plays similar role to wpa-supplicant. iwconfig is from wireless-tools, too.

ifconfig at the same time is a lower level tool, which is used by ifupdown and allows for more flexibility. For instance, there are 6 modes of wifi adapter functioning and IIRC ifupdown covers only managed mode (+ roaming mode, which formally isn't mode?). With iwconfig and ifconfig you can enable e.g. monitor mode of your wireless card, while with ifupdown you won't be able to do that directly.

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Try ifconfig wlan0 up instead of ifup.

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Here's what I use to connect to a WEP access point, on a daily basis:

#!/bin/bash

DEV=$(iw dev | awk '/Interface/ {interf=$2} END {print interf}')
PIDFILE=/var/run/dhcpcd-$DEV.pid


if [[ -f $PIDFILE ]] && kill -0 $(cat /var/run/dhcpcd-$DEV.pid)
then
    dhcpcd -k $DEV
fi

ifconfig $DEV down
sleep 1
iwconfig $DEV mode managed
iwconfig $DEV  key blabfoobar
ifconfig $DEV up
iwconfig $DEV essid 'Unindicted Co-conspirator'
iwconfig $DEV ap 00:0D:51:EB:E5:1E

sleep 5
dhcpcd --noipv6rs --noarp $DEV

You'll have to put in the ESSID and MAC for whatever access point you want.

It's a little harder for WPA encryption. You have to have a wpa_supplicant.conf file with an appropriate entry. Like this:

# Simple case: WPA-PSK, PSK as an ASCII passphrase, allow all valid ciphers
network={
    ssid="FaveCoffeeHouse"
    psk="cafe241800"
    priority=1
}

Once you have such a conf file, then you can run this script:

#!/bin/bash

DEV=$(iw dev | awk '/Interface/ {print $2}')

ifconfig $DEV down
iwconfig $DEV mode managed
ifconfig $DEV up
iwconfig $DEV essid 'FaveCoffeeHouse'
iwconfig $DEV ap 00:21:1e:3d:2a:80

echo now run: wpa_supplicant -Dnl80211 -i$DEV -c./wpa_supplicant.conf 
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