Update 9 Nov: The tutorial has now been moved to: http://elinux.org/RPI-Wireless-Hotspot
Update 11/8/12: Tutorial updated to work with 10/28/12 Raspbian image.
Recently I needed a WiFi hotspot (AKA access point) for work. I had my Raspberry Pi and a WiFi USB dongle, so I set to work configuring it as a hotspot. My Pi is now set up where on boot, it connects to the Internet through ethernet, and shares that connection over WiFi. The following steps were performed on Raspbian but should be much the same on any Debian-based distro. Here's what I did:
1. Install the necessary software.
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sudo apt-get install hostapd udhcpd
2. Configure DHCP. Edit the file /etc/udhcpd.conf and configure it like this:
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start 192.168.2.2 # This is the range of IPs that the hostspot will give to client devices.
end 192.168.2.20
interface wlan0 # The device uDHCP listens on.
remaining yes
opt dns 8.8.8.8 4.2.2.2 # The DNS servers client devices will use.
opt subnet 255.255.255.0
opt router 192.168.2.1 # The Pi's IP address on wlan0 which we will set up shortly.
opt lease 864000 # 10 day DHCP lease time in seconds
Edit the file /etc/default/udhcpd.conf and change the line:
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DHCPD_ENABLED="no"
to
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#DHCPD_ENABLED="no"
You will need to give the Pi a static IP address with the following command:
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sudo ifconfig wlan0 192.168.2.1
To set this up automatically on boot, edit the file /etc/network/interfaces and replace the line "iface wlan0 inet dhcp" to:
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iface wlan0 inet static
address 192.168.2.1
netmask 255.255.255.0
If the line "iface wlan0 inet dhcp" is not present, add the above lines to the bottom of the file.
Change the lines:
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wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
iface default inet dhcp
to:
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#wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf
#iface default inet dhcp
3. Configure HostAPD. Edit the file /etc/hostapd/hostapd.conf (create it if it doesn't exist) and add the following lines:
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interface=wlan0
ssid=Your Network Name Here
hw_mode=g
channel=6 # Change this channel to one of your choosing, between 1 and 12. If you have connection issues, try changing the channel.
auth_algs=1 # This will create an open, unencrypted WiFi network. Use at your own risk.
wmm_enabled=0 # Some wireless adapters/drivers have issues with 802.11n, this disables it (basically).
Change ssid= and channel= to values of your choice. SSID is the hotspot's name which is broadcast to other devices, channel is what frequency the hotspot will run on. For many, many more options see the file /usr/share/doc/hostapd/examples/hostapd.conf.gz
Edit the file /etc/default/hostapd and change the line:
[code]#DAEMON_CONF=""[/code]
to:
[code]DAEMON_CONF="/etc/hostapd/hostapd.conf[/code]
4. Configure NAT. NAT (Network Address Translation) is a technique that allows several devices to use a single connection to the internet. Linux supports NAT using Netfilter (also known as iptables), and if fairly easy to set up. First, edit the file /etc/sysctl.conf and add the following line to the bottom of the file:
[code]net.ipv4.ip_forward=1[/code]
This will enable NAT in the kernel. Second, run the following commands:
[code]sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
sudo iptables -A FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT[/code]
Your Pi is now NAT-ing. To make this permanent so you don't have to run the commands after each reboot, run the following command:
[code]sudo iptables-save > /etc/iptables.ipv4.nat[/code]
Now edit the file /etc/network/interfaces and add the following line to the bottom of the file:
[code]up iptables-restore < /etc/iptables.ipv4.nat[/code]
5. Fire it up! Run the following commands to start the access point:
[code]sudo service hostapd start
sudo service udhcpd start[/code]
Your Pi should now be hosting a wireless hotspot. To get the hotspot to start on boot, run these additional commands:
[code] sudo update-rc.d hostapd enable
sudo update-rc.d udhcpd enable
At the completion of these instructions, your Pi should be providing a wireless network and allowing other devices to connect to the Internet. From my experience, the Pi makes a decent access point, although with cheaper WiFi dongles range will be fairly limited. I haven't stress tested this setup, but it seems to work fairly well and is handy when a "real" access point isn't available. I wrote most of the instructions from memory, if you find any errors/typos I'll correct them.
Note: A few errors have been fixed since the original post, if you have had issues please try again with the new instructions
, I tried it, but it doesn't work 
.