First thing to be done is to test whether your wireless card supports going into wireless access point mode. This test is for mac80211 framework based driver.

Install iw and run the command iw list:

sudo aptitude install iw
iw list

Look for supported interface section, where it should be a entry called AP like this:

Supported interface modes:
	 * IBSS
	 * managed
	 * AP
	 * AP/VLAN
	 * monitor
	 * mesh point

If your driver doesn't show this AP, it doesn't mean it can't create wireless hotspot. But those cards aren't in the scope of this tutorial.

Install hostapd (sudo aptitude install hostapd) and create the configuration file /etc/hostapd/hostapd.conf with a content like this:

interface=wlan0
driver=nl80211
ssid=name_of_network
hw_mode=g
channel=1
macaddr_acl=0
auth_algs=1
ignore_broadcast_ssid=0
wpa=3
wpa_passphrase=1234567890
wpa_key_mgmt=WPA-PSK
wpa_pairwise=TKIP
rsn_pairwise=CCMP

Things that you need to change here are:

• Change interface=wlan0 to your wireless card name. If you have one wireless card it should be wlan0.
• Change ssid=name_of_network with the name that you want to give to yout hosted network.
• Change wpa_passphrase=1234567890 with the password for accessing your network.

The name of the wifi interface can be found with a command like this:

lshw -quiet -c network | sed -n -e '/Wireless interface/,+12 p' | sed -n -e '/logical name:/p' | cut -d: -f2 | sed -e 's/ //g

The configuration above creates a WPA & WPA2 enabled access point in g mode. A more detailed instruction on how to build the configuration file can be found here: http://wireless.kernel.org/en/users/Documentation/hostapd

Before starting hostapd, we should also edit the file /etc/default/hostapd and modify the line of DAEMON_CONF like this:

DAEMON_CONF="/etc/hostapd/hostapd.conf"

Now we can start hostapd with the command sudo service hostapd start . It should start a wireless network. In your mobile device now you can see a wireless network and can authenticate. But the device won't get IP address. Stop it with the command sudo service hostapd stop .

Note: If you get any errors, maybe your card doesn't support g mode. Try with other modes (see: http://linuxwireless.org/en/users/Documentation/hostapd).

Install isc-dhcp-server (aptitude install isc-dhcp-server). Edit the file /etc/default/isc-dhcp-server and set INTERFACES like this:

INTERFACES="wlan0"

Modify the name of the wireless interface accordingly, as in the section above. Then modify the configuration file /etc/dhcp/dhcpd.conf like this:

1. Comment the following lines:

   #option domain-name “example.org”;
   #option domain-name-servers ns1.example.org, ns2.example.org;
   
   #default-lease-time 600;
   #max-lease-time 7200;
   

2. Append these lines as the end:

   subnet 10.10.0.0 netmask 255.255.255.0 {
   	range 10.10.0.2 10.10.0.16;
   	option domain-name-servers 8.8.4.4, 208.67.222.222;
   	option routers 10.10.0.1;
   }
   

Range describes how large the address pool will be. You need to adjust subnet value also. The config above can give IP up to 15 devices.

Now edit /etc/network/interfaces and add the following lines:

auto wlan0
iface wlan0 inet static
address 10.10.0.1
netmask 255.255.255.0

wlan0 is your wireless interface; change it accordingly.

Note: After reboot, the wireless will be shown as not managed. So you can't use any other wi-fi network. To get wireless with normal behaviour, put # before those newly added lines and execute sudo service networking restart

If you start now the services isc-dhcp-server and hostapd, your mobile device will see a network, authenticate to it and after authentication it will get as IP address something like 10.10.0.2 But still it can't connect to internet.

For Internet connection sharing we need IP forwarding and IP masquerading.

Enable IP forwarding like this:

echo 1 > /proc/sys/net/ipv4/ip_forward

Setup IP masquerading like this:

iptables -t nat -A POSTROUTING -s 10.10.0.0/16 -o ppp0 -j MASQUERADE

Here ppp0 is the logical interface name if you are using a dial up/usb modem connection to Internet. It may be eth0 if you are using an Ethernet cable connection. If you are connecting to Internet through an android device with USB tethering, then the interface name could be usb0.

You can find out the name of the logical interface that you are using to connect to the Internet with a command like this:

ip route | grep default | cut -d' ' -f5

I like to automate configuration steps whenever it is possible and it is something that can be reused. Here is a script on gist that can be used to install a wifi access point automatically: https://gist.github.com/dashohoxha/5767262

It should work well on Ubuntu 12.04 or its derivatives (for example Linux Mint). Let me know if there are any bugs on it or things that can be fixed or improved.

• http://askubuntu.com/questions/180733/how-to-setup-a-wi-fi-hotspot-access-point-mode/180734#180734
• https://help.ubuntu.com/community/SharingMobileBroadband
• http://askubuntu.com/questions/140126/how-do-i-configure-a-dhcp-server
• http://linuxwireless.org/en/users/Documentation/hostapd