Turn your laptop (running Linux) into a captive WI-FI hotspot
This article shows how to turn your laptop (running Linux) into a captive WI-FI hotspot.
I assume that the WI-FI card shipped with your laptop can be used as an access point. If this is not the case, then you can use a cheap WI-FI USB adapter (for example: EDUP EP-N8531 WI-FI USB adapter).
Please note that if you choose to use a WI-FI USB adapter, then you may have to adapt the command line examples given in this article. Depending on your laptop’s specifications, you may need to replace « wlan0 » by whatever wireless interface identifier that is right for you (it could be « wlan1 », « wlan2 »…).
Please also note that I use Mint 17 as operating system. This OS is based on Ubuntu 14.04 (kernel 3.13). If you are using another version of Linux, some details of the procedure may be different. However, the overall explanation is similar.
- A laptop running Linux Mint 17, or another recent flavour of Linux.
- A good WI-FI card that is supported by hostapd. If your laptop does not come with a good WI-FI card, then you can use this one: EDUP EP-N8531 WI-FI USB adapter.
- hostapd: this tool turns your WI-FI interface into an access point.
- dnsmasq: this tool provides DHCP and DNS services.
My laptop's OS:
$ lsb_release -da No LSB modules are available. Distributor ID: LinuxMint Description: Linux Mint 17.2 Rafaela Release: 17.2 Codename: rafaela
You may need to upgrade your system: sudo apt-get update; sudo apt-get upgrade
If you use a WI-FI USB adapter, then plug the adapter into an USB slot. Then, make sure that your WI-FI adapter is correctly integrated to the system. The following output comes from the EDUP EP-N8531 WI-FI USB adapter:
$ lsusb Bus 001 Device 004: ID 148f:5370 Ralink Technology, Corp. RT5370 Wireless Adapter Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp. Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Install hostapd and dnsmasq: sudo apt-get install hostapd dnsmasq
Note that, in case you want to remove it later: sudo apt-get --purge remove hostapd dnsmasq (don't do that now !)
Now, you must create the file /etc/hostapd/hostapd.conf.
To create an open (no authentication) access point, then put the following configuration into your file:
interface=wlan0 driver=nl80211 ssid=denis-hotspot channel=6
Notes:
- You should have an example for the hostapd configuration file:
/usr/share/doc/hostapd/examples/hostapd.conf.gz. This file contains lots of comments that explain all the configuration's variables. - To get the list of all files associated to the package, you can use the following command:
dpkg-query -L hostapd - You may have to change wlan0 by whatever identifier applies to your system.
Then, edit the file /etc/default/hostapd. You must tell hostapd that it must load a configuration file.
# Defaults for hostapd initscript # # See /usr/share/doc/hostapd/README.Debian for information about alternative # methods of managing hostapd. # # Uncomment and set DAEMON_CONF to the absolute path of a hostapd configuration # file and hostapd will be started during system boot. An example configuration # file can be found at /usr/share/doc/hostapd/examples/hostapd.conf.gz # DAEMON_CONF="/etc/hostapd/hostapd.conf" # Additional daemon options to be appended to hostapd command:- # -d show more debug messages (-dd for even more) # -K include key data in debug messages # -t include timestamps in some debug messages # # Note that -B (daemon mode) and -P (pidfile) options are automatically # configured by the init.d script and must not be added to DAEMON_OPTS. # #DAEMON_OPTS=""
Make a backup copy of the file /etc/dnsmasq.conf:
sudo cp /etc/dnsmasq.conf /etc/dnsmasq.conf.org
Create the file /etc/dnsmasq.conf.ap and set the following variables:
interface=wlan0except-interface=eth0dhcp-range=10.0.0.32,10.0.0.248,255.255.255.0,12h
Note: you may have to change wlan0 by whatever identifier applies to your system.
With the configuration above, the DNS service (on your laptop) is based on your laptop's "/etc/hosts" file.
Using dnsmasq you can build a "catch-all DNS" : all DNS queries will return the same IP address. This can be useful. Let's say that we want all DNS queries to return the IP address of the laptop (10.0.0.32). To do that, just add the following line into the file /etc/dnsmasq.conf.ap:
address=/#/10.0.0.32
Note: to get the list of all files associated to the package, you can use the following command: dpkg-query -L dnsmasq.
Configure your system so that wlan0 (or whatever identifier applies to your system) will be assigned a "well-known" static IP address.
Make a backup copy of the file /etc/network/interfaces:
sudo cp /etc/network/interfaces /etc/network/interfaces.org
Create the file /etc/network/wifi/interfaces.ap:
auto lo iface lo inet loopback ihwaddress ether cc:e7:df:03:89:11 auto wlan0 iface wlan0 inet static address 10.0.0.32 netmask 255.255.255.0
Note: you may have to change wlan0 by whatever identifier applies to your system.
Configure your system so that hostapd is NOT started automatically after boot: sudo update-rc.d hostapd disable
Configure your system so that dnsmasq is NOT started automatically after boot: sudo update-rc.d dnsmasq disable
Notes:
- Information about the "init" files: https://www.debian-administration.org/article/212/An_introduction_to_run-levels, https://wiki.debian.org/LSBInitScripts.
- Finding the current run level:
runlevel(it should be "N 2" - The 'N' stands for none, meaning there has been no run level change since powering up). - List the services automatically started at run level 2, in chronological order:
ls -l /etc/rc2.d/.
OPTIONAL: you MAY need to disable ifplugd (especially if you use a WI-FI adapter) for wlan0 (or whatever wireless interface identifier that is right for you). To do that, edit the file /etc/default/ifplugd and replace:
INTERFACES="auto" HOTPLUG_INTERFACES="all"
By:
INTERFACES="eth0" HOTPLUG_INTERFACES="eth0"
Note:
- See http://sirlagz.net/2013/02/10/how-to-use-the-raspberry-pi-as-a-wireless-access-pointrouter-part-3b/
- On my laptop, I do not have to disable ifplugd. However, if you experience difficulties, try to disable it.
- The file
/etc/default/ifplugdmay not exist on your file system. In this case, and if you experience difficulties, try to find a way to disable ifplugd for your wireless interface.
Create the script start-ap.sh that will turn the WI-FI interface into an access point.
stop network-manager cp /etc/network/wifi/interfaces.ap /etc/network/interfaces ifdown wlan0 ifup wlan0 cp /etc/dnsmasq.conf.ap /etc/dnsmasq.conf /etc/init.d/hostapd start /etc/init.d/dnsmasq start
Note: you may have to change wlan0 by whatever identifier applies to your system.
Create the script stop-ap.sh that will stop the access point and restore nominal operation mode.
/etc/init.d/hostapd stop /etc/init.d/dnsmasq stop cp /etc/network/interfaces.org /etc/network/interfaces cp /etc/dnsmasq.conf.org /etc/dnsmasq.conf ifdown wlan0 ifup wlan0 start network-manager
Note: you may have to change wlan0 by whatever identifier applies to your system.
Then restart your system: sudo shutdown -r now
Check that hostapd is NOT running: ps awx | grep hostapd
Check that dnsmasq is NOT running: ps awx | grep dnsmasq
Check the configuration of wlan0:
$ iwconfig wlan0
wlan0 IEEE 802.11bgn ESSID:off/any
Mode:Managed Access Point: Not-Associated Tx-Power=20 dBm
Retry short limit:7 RTS thr=2347 B Fragment thr:off
Encryption key:off
Power Management:off
$ ifconfig wlan0
wlan0 Link encap:Ethernet HWaddr 6c:71:d9:03:89:11
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Start the access point: sudo ./start-ap.sh.
Stop the access point: sudo ./stop-ap.sh.
