How can I configure my headless server to connect to a wireless network automatically?

I'm setting up a headless server, but I don't have ethernet access where I want to put it, so I need it to automatically connect to my WiFi network when it boots up (it has a wireless card). I can connect to an ethernet connection to set it up, but I need it to automatically connect to my access point from then on.

How can I configure this?

After doing a lot of research I've gotten it working. Since I have an Intel wireless card I didn't have to install any extra drivers, but you might have to, depending on the card you have.

First you need to figure out what interface your wireless card is using. We use the iwconfig command for this:

iwconfig

In my case my wireless card is the wlan0 interface, so I will be using that. Now we need to scan for wireless networks:

iwlist wlan0 s

This should give lots of output, showing the details of the various wireless networks in your area. It's usually easier to filter by ESSID. grep helps us out here:

iwlist wlan0 s | grep ESSID

This will list the names of all the wireless networks in your area. Now it's time to connect to your network.

Unsecure and WEP networks

If your network is unsecured or is secured by the older WEP (time to upgrade your security or router!) connecting is relatively simple. If your network is unsecured you should be able to connect with this:

iwconfig wlan0 essid NAME_OF_NETWORK

If your network is WEP protected just add the key argument followed by your password, like this:

iwconfig wlan0 essid NAME_OF_NETWORK key PASSWORD

You might have to run

dhclient

To get your router to assign you an IP address.

WPA/WPA2

WPA/WPA2 is a bit more complicated. You will need to use wpa_supplicant. First create the config file in /etc:

sudo wpa_passphrase NETWORK_NAME NETWORK_PASSWORD > /etc/network/wpa_supplicant.conf

Now we need to connect:

sudo wpa_supplicant -B -iINTERFACE_NAME -cPATH_TO_CONFIG -DDRIVER

For example:

sudo wpa_supplicant -B -iwlan0 -c/etc/network/wpa_supplicant.conf -Dwext

-B runs wpa_supplicant in the background. The wext driver should work in most cases. To see other drivers run:

wpa_supplicant

For more information on getting connected see How to connect and disconnect to a network manually in terminal?

Get connected on startup

Now we need to edit /etc/network/interfaces. Open it in your favorite editor (vim, nano, etc); you'll need to use sudo.

Remove everything except:

auto lo
iface lo inet loopback

(the loopback device). Now add:

auto wlan0  
iface wlan0 inet dhcp  

pre-up <COMMAND>

Where wlan0 is your wireless interface and <COMMAND> is the command you use to connect to your network (see above). For example if you're network is unsecured you would add:

pre-up iwconfig wlan0 essid some_network_name

If your network is secured with WPA/2 you would add something like this:

pre-up sudo wpa_supplicant -B -iwlan0 -c/etc/network/wpa_supplicant.conf -Dwext

If you're using wpa_supplicant you should also add:

post-down sudo killall -q wpa_supplicant

Save the file, restart, unplug your ethernet and try running:

sudo apt-get update

If the command completes successfully congratulations! You're online! If the command does not complete successfully please add a comment below.

Examples and Explanations

If your network is unsecured or secured with WEP your /etc/network/interfaces should look similar to this now:

auto lo
iface lo inet loopback  

auto wlan0  
iface wlan0 inet dhcp  

pre-up iwconfig wlan0 essid some_network_name 
# note: if WEP secured you would also have a 'key' argument with your password

If your network is WPA/2 secured your /etc/network/interfaces should look similar to this:

auto lo
iface lo inet loopback  

auto wlan0  
iface wlan0 inet dhcp  

pre-up sudo wpa_supplicant -B -iwlan0 -c/etc/network/wpa_supplicant.conf -Dwext  

post-down sudo killall -q wpa_supplicant

Now an explanation.

auto wlan0: Starts the wlan0 interface automatically.
iface wlan0 inet dhcp: Gets us an IP address through DHCP
pre-up: Specifies the command(s) to get the connection going.
post-down: Specifies the command(s) to be used to clean up after ourselves (if necessary).

Sources:

A wireless, headless Linux box
How to connect and disconnect to a network manually in terminal?
Personal experience and lots of tinkering.

I suggest a static IP address so that you can easily ssh and ftp into the server. Also, you can significantly simplify your file:

auto lo
iface lo inet loopback  

auto wlan0  
iface wlan0 inet static
address 192.168.1.125  
netmask 255.255.255.0
gateway 192.168.1.1
dns-nameservers 8.8.8.8 192.168.1.1
wpa-ssid <your_network>
wpa-psk <your_key>

Be certain the address is outside the range used by the router for DHCP and, of course, substitute your appropriate details here.