Configuring Wireless Access Point or Hotspot on a RaspberryPi using Edimax EW-7811UN

After you have setup raspbian on your raspberrypi, do the following:

sudo apt-get update #Bring 'em all

Since we don't want to depend on external DHCP server (by bridging network interfaces) to hand over leases, we will setup our own DHCP server for our private wifi network. Read about dnsmasq

sudo apt-get install dnsmasq

Let's setup dnsmasq configuration. Edit the file `/etc/dnsmasq.conf`

interface=wlan0
expand-hosts
domain=local
dhcp-range=192.168.60.10,192.168.60.255,24h
dhcp-option=3,192.168.60.1

The drivers that come with linux kernel do not support our Edimax device to behave as an Access Point by default. Realtek has been generous enough to provide the necessary drivers to do so. Visit http://www.realtek.com/downloads/ to download the driver for your particular device (Mine is RTL8188cus. Type the command lshw to know about yours). After downloading the driver, do the following:

sudo apt-get remove hostapd
wget ftp://WebUser:n8W9ErCy@95.130.192.218/cn/wlan/RTL8188C_8192C_USB_linux_v4.0.2_9000.20130911.zip
unzip RTL8188C_8192C_USB_linux_v4.0.2_9000.20130911.zip
cd RTL8188C_8192C_USB_linux_v4.0.2_9000.20130911
cd wpa_supplicant_hostapd/
tar -xf wpa_supplicant_hostapd-0.8_rtw_r7475.20130812.tar.gz
cd wpa_supplicant_hostapd-0.8_rtw_r7475.20130812
cd hostapd
sudo make install

Now we need to edit the file `/etc/hostapd.conf` to enter our configuration for wireless AP.

# Basic configuration
interface=wlan0
ssid=NoFreeLoaders
channel=1
#bridge=br0

# WPA and WPA2 configuration
macaddr_acl=0
auth_algs=1
ignore_broadcast_ssid=0
wpa=3
wpa_passphrase=8em10h_kc01reh8
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP

# Hardware configuration
driver=rtl871xdrv
ieee80211n=1
hw_mode=g
device_name=RTL8192CUS #Replace this with the name of your device.
manufacturer=Realtek

Next, create the file `/etc/init.d/hostapd` (it is needed so that we can control the hostapd program as a service) and make sure it's contents are:

#!/bin/sh

### BEGIN INIT INFO
# Provides:             hostapd
# Required-Start:       $remote_fs
# Required-Stop:        $remote_fs
# Should-Start:         $network
# Should-Stop:
# Default-Start:        2 3 4 5
# Default-Stop:         0 1 6
# Short-Description:    Advanced IEEE 802.11 management daemon
# Description:          Userspace IEEE 802.11 AP and IEEE 802.1X/WPA/WPA2/EAP
#                       Authenticator
### END INIT INFO

PATH=/sbin:/bin:/usr/sbin:/usr/bin
DAEMON_SBIN=/usr/local/bin/hostapd
DAEMON_CONF=/etc/hostapd.conf
NAME=hostapd
DESC="advanced IEEE 802.11 management"
PIDFILE=/var/run/hostapd.pid

[ -x "$DAEMON_SBIN" ] || exit 0
[ -n "$DAEMON_CONF" ] || exit 0

DAEMON_OPTS="-B -P $PIDFILE $DAEMON_OPTS $DAEMON_CONF"

. /lib/lsb/init-functions

case "$1" in
  start)
        log_daemon_msg "Starting $DESC" "$NAME"
        start-stop-daemon --start --oknodo --quiet --exec "$DAEMON_SBIN" \
                --pidfile "$PIDFILE" -- $DAEMON_OPTS >/dev/null
        log_end_msg "$?"
        ;;
  stop)
        log_daemon_msg "Stopping $DESC" "$NAME"
        start-stop-daemon --stop --oknodo --quiet --exec "$DAEMON_SBIN" \
                --pidfile "$PIDFILE"
        log_end_msg "$?"
        ;;
  reload)
        log_daemon_msg "Reloading $DESC" "$NAME"
        start-stop-daemon --stop --signal HUP --exec "$DAEMON_SBIN" \
                --pidfile "$PIDFILE"
        log_end_msg "$?"
        ;;
  restart|force-reload)
        $0 stop
        sleep 8
        $0 start
        ;;
  status)
        status_of_proc "$DAEMON_SBIN" "$NAME"
        exit $?
        ;;
  *)
        N=/etc/init.d/$NAME
        echo "Usage: $N {start|stop|restart|force-reload|reload|status}" >&2
        exit 1
        ;;
esac

exit 0

Then, we need to change make it executable and enable it (along with dnsmasq) to be executed every time the system starts:

chmod a+x /etc/init.d/hostapd
sudo update-rc.d dnsmasq defaults
sudo update-rc.d hostapd defaults

Next, edit the file `/etc/network/interfaces` and make it look something like this:

auto lo

iface lo inet loopback
iface eth0 inet dhcp

allow-hotplug wlan0
iface wlan0 inet static
address 192.168.60.1
network 192.168.60.0
netmask 255.255.255.0
broadcast 192.168.60.255

The above config states that the configuration for loopback interface 'lo' is automatic, interface 'eth0' requires a DHCP server for ip address, interface 'wlan0' (brought in by our Edimax EW-7811UN) has a static configuration, in which, it's ip address is 192.168.60.1, it lies in the subnet 192.168.60.0 with netmask 255.255.255.0 (i.e. 192.168.60.0/24 in CIDR format) and 192.168.60.255 is the broadcast ID for the network. Also, hot-plugging of this interface is allowed.
Next, we configure our rpi to support ipv4 forwarding. Open the file `/etc/sysctl.conf` add make sure the file has this line:

net.ipv4.ip_forward=1

Read about IP forwarding here. Make sure that the above change is applied.

sudo sysctl -p

Next, we need to add firewall rules so that NATting can take place nicely.

sudo echo -e "iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE\n\
iptables --append FORWARD --in-interface wlan0 -j ACCEPT" > /etc/network/if-up.d/router.sh
sudo chmod +x /etc/network/if-up.d/router.sh

Next, we need to make sure that our wlan0 interface is up and running and also the iptables rules are applied automatically if the system restarts. (We need to make them persistent). To do that, add the following two lines before 'exit 0' in `/etc/rc.local`

ifup wlan0
/etc/network/if-up.d/router.sh

Now, reboot and enjoy your rpi as linux wireless router! xD